Skip to main content
NCBI home page
NIHPA Author Manuscripts logoLink to NIHPA Author Manuscripts
. Author manuscript; available in PMC: 2022 Feb 1.
Published in final edited form as: Exp Physiol. 2020 Dec 9;106(2):401–411. doi: 10.1113/EP089053

Ventricular Contraction and Relaxation Rates During Muscle Metaboreflex Activation in Heart Failure: Are they Coupled?

Joseph Mannozzi 1, Louis Moussad 1, Jasdeep Kaur 1, Matthew Coutsos 1, Donal S O’Leary 1
PMCID: PMC7855894  NIHMSID: NIHMS1650332  PMID: 33226720

Abstract

The relationship between contraction and relaxation rates of the left ventricle varies with exercise. In in-vitro models, this ratio was shown to be relatively unaltered by changes in sarcomere length, frequency of stimulation, and β adrenergic stimulation. We investigated whether the ratio of contraction to relaxation rate is maintained in the whole heart during exercise and muscle metaboreflex activation and whether heart failure alters these relationships. We observed that in healthy subjects the ratio of contraction to relaxation increases from rest to exercise as a result of a higher increase in contraction relative to relaxation. During muscle metaboreflex activation the ratio of contraction to relaxation is significantly reduced towards 1.0 due to a large increase in relaxation rate matching contraction rate. In heart failure, contraction and relaxation rates are significantly reduced and increases during exercise are attenuated. A significant increase in the ratio was observed from rest to exercise although baseline ratio values were significantly reduced close to 1.0 when compared to healthy subjects. There was no significant change observed between exercise and muscle metaboreflex activation nor was the ratio during muscle metaboreflex activation significantly different between heart failure and control. We conclude that heart failure reduces the muscle metaboreflex gain and contraction and relaxation rates. Furthermore, we observed that the ratio of the contraction and relaxation rates during muscle metaboreflex activation is not significantly different between control and heart failure, however, significant changes in the ratio in healthy subjects]due to increased relaxation rate were abolished in heart failure.

Keywords: Contraction, relaxation, metaboreflex, diastolic function, heart failure

INTRODUCTION:

Ventricular performance is a key determinant of the cardiac response to dynamic exercise and the maintenance of exercise workload. A key reflex mediating increases in cardiac output in response to exercise is the muscle metaboreflex - initiated via increases in activity of skeletal muscle afferents sensitive to increases in metabolite concentration (Adreani, Hill, & Kaufman, 1997; Ansorge et al., 2005; Coutsos et al., 2010; Crisafulli et al., 2011; Crisafulli et al., 2006; Crisafulli et al., 2003; Hammond et al., 2000; Kaufman, Longhurst, Rybicki, Wallach, & Mitchell, 1983; Kaufman, Rybicki, Waldrop, & Ordway, 1984; Kaur, Spranger, et al., 2015; Middlekauff & Sinoway, 2007; Mitchell, Kaufman, & Iwamoto, 1983; O’Leary & Augustyniak, 1998; O’Leary, Augustyniak, Ansorge, & Collins, 1999; O’Leary, Senador, & Augustyniak, 2019; Rotto & Kaufman, 1988; Sheriff, Augustyniak, & O’Leary, 1998; L. Sinoway et al., 1989; L.I. Sinoway, Rea, Mosher, Smith, & Mark, 1992; L. I. Sinoway et al., 1994; L. I. Sinoway, Wroblewski, et al., 1992). When activated this reflex causes significant increases in heart rate, stroke volume, ventricular performance, and central blood volume mobilization which increases cardiac output and thereby perfusion of the active skeletal muscle (Crisafulli et al., 2011; Crisafulli et al., 2006; Crisafulli et al., 2003; Hammond et al., 2000; Kaur, Spranger, et al., 2015; O’Leary & Augustyniak, 1998; O’Leary et al., 1999; O’Leary et al., 2019; Robert A. Augustyniak, Eric J. Ansorge, Noreen F. Rossi, & O’Leary, 2000; Sala-Mercado et al., 2006). After induction of heart failure, the ability of this reflex to raise ventricular performance and thus cardiac output is attenuated, and the reflex now elicits profound peripheral vasoconstriction including within the ventricular myocardium and even the active skeletal muscle, the tissue from which the reflex originates (Ansorge et al., 2005; Coutsos et al., 2013; Crisafulli et al., 2007; Hammond et al., 2000; Kaur et al., 2017; Kim et al., 2005; O’Leary et al., 2004; Sala-Mercado et al., 2007; Shoemaker, Kunselman, Silber, & Sinoway, 1998; Silber et al., 1998). Inasmuch as muscle metaboreflex control of central blood volume mobilization is maintained in heart failure (O’Leary et al., 2019), the mechanism for the reduction in the ability to raise cardiac output during muscle metaboreflex activation is likely impaired ventricular dynamics due to both the inherent dysfunction of the myocardium coupled with enhanced coronary vasoconstriction (Ansorge et al., 2005; Coutsos et al., 2010, 2013). Previously we have reported that left ventricular maximal elastance, preload recruitable stroke work, and dP/dt MAX and MIN are all reduced and exhibit attenuated responses during muscle metaboreflex activation (MMA) in heart failure (Ansorge et al., 2005; Coutsos et al., 2013; O’Leary et al., 2004; Sala-Mercado et al., 2007). Previous in-vitro studies of sarcomere dynamics determined that the ratio of contraction to relaxation rates are maintained in cardiac muscle in response to changes in frequency, length, and beta adrenergic stimulation (P. M. Janssen, 2010; P. M. L. Janssen, 2010). Whether or not aspects of single sarcomere dynamics will be reflected in the whole heart in-vivo has yet to be determined. We assessed whether ventricular contraction and relaxation rates as determined by dP/dt MAX and MIN in the canine model are coupled between rest and mild exercise and with pronounced further sympathetic activation induced by muscle metaboreflex activation. We also assessed whether heart failure affects these relationships. Secondly, we wished to determine the strength or gain of the muscle metaboreflex in the control of ventricular contraction and relaxation rates before and after the induction of heart failure. We hypothesized that muscle metaboreflex activation before or after heart failure will not impede contraction-relaxation coupling and heart failure will induce a significant reduction in the gain (strength) of ventricular contraction and relaxation rates.

METHODS:

Ethical Approval:

All procedures, surgical and experimental used in this study comply with the 8th edition of the National Institutes of Health Guide to the Care and Use of Laboratory Animals published in 2011 and were approved by the Wayne State University Institutional Animal Care and Use Committee (IACUC) (approval protocol number IUCAC 19-11-1493). The authors understand the ethical guidelines with which the Journal of Experimental Physiology operates, and this work abides by the checklist provided.

Experimental subjects.

The experiments were performed on 14 adult mongrel canines (1 male, 13 females) of approximately 18–25 kg were selected on their willingness to walk on a motor driven treadmill prior to acclimatization to a workload of 3.2 km/h 0% grade. The gender mix was determined by the availability of animals from the vendor (Marshall Bioresources). Previously we have shown that sex does not significantly impact the strength or mechanisms of the muscle metaboreflex (Laprad, Augustyniak, Hammond, & O’Leary, 1999). No experiments during frank estrus were included in this study. Animals were fasted 12 hours prior to surgical procedures and otherwise fed once daily an amount of food equal to 45–54 kcal/kg and had access to water at all times. All animals in this study underwent a 7–14-day acclimation period to the laboratory surroundings and personnel prior to volitional treadmill exercise. All animals exercised voluntarily during all experiments.

SURGICAL PROCEDURES

Anesthetic / Analgesic Management and Euthanasia

Animals underwent a series of surgical procedures spanning 4–6 weeks utilizing standard aseptic techniques. Prior to each surgical procedure in this study, animals were sedated with an intramuscular injection of acepromazine (0.4–0.5mg/kg) 30 minutes prior to induction of anesthesia. Induction was achieved through administration of ketamine (5 mg/kg intravenously) and diazepam (0.2–0.3 mg/kg intravenously). Anesthesia was maintained after induction preoperatively and during surgery with isoflurane gas (1–3%). Prior to performing incisions bupivacaine (1 mg/kg) was administered intramuscularly around the incision site. After all surgical sites were closed and sutured isoflurane gas was turned off and animals remained ventilated until they began to breath and swallow on their own. Preoperative analgesics included application of a Fentanyl patch on the lower lumbar (75–125 μg/h (72) transdermal delivery], buprenorphine (0.01–0.03 mg/kg intramuscularly), and carprofen (4.4 mg/kg intravenously). Post operatively buprenorphine (0.01–0.03 mg/kg) and acepromazine (0.3–0.3 mg/kg) were administered IV as needed for analgesic management and for seven days post operatively carprofen (4.4 mg/kg) was administered orally once daily. The antibiotic cephalexin (30 mg/kg) was administered intravenously acutely pre and post operatively and oral cephalexin (30 mg/kg) was administered every 12 hours to prevent microbial infection. After completion of all experiments for the study all animals were sedated with an intramuscular injection of acepromazine (0.4–0.5mg/kg) 30 minutes prior to intravenous administration of a lethal dose of sodium pentobarbital (90 mg/kg) for euthanasia.

The first procedure was a left thoracotomy thru the 3rd/4th intercostal space to access the heart. the pericardium was incised cranially to caudally to access the apex of the heart for insertion of a telemetric pressure transducer tip (TA11 PA-D70, DSI) to measure LV pressure, and to access the ascending aorta for placement of a blood flow transducer (20PAU, Transonic Systems) for measures of cardiac output. Four stainless steel pacing leads (0-Flexon, Ethicon) were attached to the right ventricle free wall for subsequent induction of heart failure via rapid ventricular pacing. 7 of 14 animals were surgically prepared with sonometric crystals in the long and short axis of the left ventricle, a blood flow transducer (3PSB, Transonic Systems) on the circumflex artery as an index of coronary blood flow, and vascular occluders (DocXS Biomedical Products) were placed on the superior and inferior vena cava for experimental questions unrelated to the current study. All cables, wires, and occluder lines were tunneled subcutaneously between the scapulae and animals recovered for 14 days prior to further experiments or surgical procedures.

In the second procedure a left retroperitoneal approach was performed to access the terminal aorta for placement of a 19-gauge polyvinyl catheter (Tygon, S54-HL, Norton) in a lumbar artery caudal to the left renal artery for measures of systemic arterial pressure. A blood flow transducer (10PAU Transonic Systems) was placed on the terminal aorta caudal to the catheter for measures of hindlimb blood flow, followed by two hydraulic vascular occulders (DocXS Biomedical Products) used to manipulate hindlimb blood flow during experiments. On the left renal artery, a blood flow transducer (4PSB Transonic Systems) was placed for studies unrelated to the present investigation. All cables, catheters, and occluders were tunneled subcutaneously to the scapula and animals then recovered for a minimum of 14 days prior to any experiments or additional surgeries.

Six out of fourteen animals in this study underwent a third procedure for placement of vascular occluders (DocXS Biomedical Products) bilaterally on the carotid arteries for experimental questions unrelated to the current study. These animals were allowed to recover for 10–14 days prior to experiments or surgeries.

DATA ACQUISITION

Prior to the experiments the animals were acclimated to the laboratory setting for 15–20 minutes. Animals were then led to the motor driven treadmill for connection of monitoring equipment. Arterial pressure was measured through a direct connection from the fluid filled 19-gauge polyvinyl catheter (Tygon, S54-HL, Norton) to a pressure transducer (Transpac IV, ICU Medical). The telemetric pressure tip placed in the left ventricle was connected and left ventricular pressure and heart rate were derived. All flow probes were connected to bench top flow meters (Transonic Sysytems). All in vivo hemodynamic measurements were continuously monitored in real time through passage of signals through an A-D converter (iWorx) and output into Labscribe Acquisition software (iWorx).

EXPERIMENTAL PROCEDURES

All in-vivo hemodynamics were measured under steady state conditions taken at rest, exercise (3.2km/h 0% grade), and during exercise with muscle metaboreflex activation. In 7 animals responses were observed with successive reductions in hindlimb blood flow to approximately 40–60% of free flow conditions. Reductions in hindlimb blood flow were achieved through inflation of vascular occluders (DocXS Biomedical Products). In 7 animals, the muscle metaboreflex was activated via a one-step reduction in hindlimb blood flow to values similar to the maximal occlusion used in the graded reduction protocol. Wyss et al. showed that there was no significant difference in the hemodynamic responses observed during graded vs. single step reductions regardless of workload (Wyss, Ardell, Scher, & Rowell, 1983). After control experiments were performed in all 14 animals, the ventricular pacing leads were connected to a pacemaker for rapid ventricular pacing (200 to 240 bpm, ~ 30 days) to achieve heart failure with reduced ejection fraction as described in previous studies (Ansorge et al., 2005; Coutsos et al., 2013; Hammond et al., 2000; Kaur et al., 2017; Mannozzi et al., 2020; O’Leary et al., 2004; O’Leary et al., 2019; Sala-Mercado et al., 2007). Heart failure was indicated by frank reduction in ventricular performance was apparent when the pacemaker was disconnected; e.g., substantial reductions in dP/dt MAX and MIN, tachycardia, reduced cardiac output, and hypotension at rest. After heart failure was induced the experiments were repeated. The pacemaker was removed approximately 30 minutes prior to each experiment and reconnected afterwards for maintenance of heart failure. Thus, each animal served as its own control in this longitudinally designed study.

ANALYSIS

In vivo hemodynamics (cardiac output, hindlimb blood flow, renal blood flow, mean arterial pressure, left ventricular pressure) were measured and recorded continuously for each experiment. Average one-minute steady state values for all variables were taken at rest, exercise (3.2km/h 0 % grade), and during each successive reduction in hindlimb blood flow during exercise before and after induction of heart failure. Peak reductions in hindlimb blood flow (HLBF) were used to assess peak muscle metaboreflex activation. All other variables were derived from in-vivo steady state hemodynamic measures. The gain of the muscle metaboreflex was determined as the slope of the relationship between the reduction in hindlimb blood flow and the given reflex response once hindlimb blood flow had been reduced beyond metaboreflex threshold (Ansorge et al., 2005; Coutsos et al., 2013; Hammond et al., 2001; Kaur, Machado, et al., 2015; Kaur et al., 2017; Robert A. Augustyniak et al., 2000; Spranger et al., 2017; Wyss et al., 1983). Multiple experiments were conducted on each animal in this study. Therefore, steady state values for each animal were averaged across all experiments from that individual animal. Average values for each animal were then averaged across all animals such that each animal contributed only once to the mean values.

STATISTICAL ANALYSIS

All statistics were performed with Systat Software (Systat 13). Data is reported is reported with error bars that show standard deviation with statistical significance determined by an α level of P < 0.05. A Two-Way ANOVA for repeated measures was used for analysis and when significant interactions were observed means were compared using a C-Matrix Test for Simple Effects. Observations of muscle metaboreflex gain and changes observed from exercise to muscle metaboreflex activation between control and heart failure were compared using a Students Paired T-Test.

RESULTS:

Figure 1 shows the average steady-state hemodynamic responses in 14 animals during rest, exercise (3.2 km/h 0% incline), and exercise with peak muscle metaboreflex activation before and after induction of heart failure. In normal subjects, significant increases in heart rate (HR), stroke volume (SV), cardiac output (CO), stroke work (SW, calculated as (MAP × SV)), and non-ischemic vascular conductance (NIVC) (conductance to all areas except the hindlimb calculated as (CO-HLBF)/MAP)) were observed from rest to exercise. With subsequent muscle metaboreflex activation, heart rate, stroke volume, cardiac output, mean arterial pressure, and effective arterial elastance (HR × (1/NIVC) all increased significantly. Non-ischemic vascular conductance approached significance (p=0.053) between exercise and muscle metaboreflex activation. After induction of heart failure, resting values of heart rate and effective arterial elastance were significantly increased and, stroke volume, cardiac output, stroke work, mean arterial pressure and hindlimb blood flow were significantly reduced and no difference was observed in non-ischemic vascular conductance. From rest to exercise stroke volume, cardiac output, mean arterial pressure, non-ischemic vascular conductance and stroke work significantly increased and all except non-ischemic vascular conductance were significantly attenuated compared to control. Heart rate significantly increased from rest to exercise but was significantly higher vs. control. In heart failure from exercise to muscle metaboreflex activation all variables significantly increased. However, all variables were significantly attenuated when compared to control with the exceptions of heart rate and effective arterial elastance which were significantly increased. Hindlimb blood flow was significantly reduced in heart failure at rest. Significant increases were observed in hindlimb blood flow from rest to exercise however the steady state level during exercise was significantly attenuated compared to control. Hindlimb blood flow was mechanically reduced 40–60% of free flow condition values as was done in healthy subjects to elicit the muscle metaboreflex after induction of heart failure.

FIGURE 1:

FIGURE 1:

Open in a new tab

Average one-minute steady state hemodynamic values taken at rest (REST), exercise (3.2 km/h 0% grade) (EX), and exercise with muscle metaboreflex activation (MMA) before (white) and after induction of heart failure via rapid ventricular pacing (gray). Standard deviation is shown on the bar graphs. Statistical significance against previous workload is shown as * P < 0.05. Comparisons between control and heart failure for a given workload shown as † P < 0.05. (N=14).

Figure 2 shows the average steady-state values in 14 animals during rest, exercise, and exercise with peak muscle metaboreflex activation before and after induction of heart failure. In control experiments significant increases were observed in dP/dt MAX from rest to exercise and from exercise to peak muscle metaboreflex activation. dP/dt MIN was not significantly changed from rest to exercise but was significantly increased from exercise to muscle metaboreflex activation. After induction of heart failure, dP/dt MAX and MIN were significantly reduced at rest and increased between rest and exercise and between exercise and muscle metaboreflex activation, however after induction of heart failure these responses were all attenuated.

FIGURE 2:

FIGURE 2:

Open in a new tab

Left average 1-minute steady state values of dP/dt MAX and MIN taken at rest (REST), exercise (3.2 km/h 0% grade) (EX), and exercise with muscle metaboreflex activation (MMA) before (white) and after induction of heart failure via rapid ventricular pacing (gray). Right shows the average change from EX to MMA in control and heart failure. Standard deviation is shown on the bar graphs. Statistical significance against previous workload is shown as * P < 0.05. Comparisons between control and heart failure for a given workload or change from previous workload shown as † P < 0.05. (N=14).

Figure 3 shows the relationship between dP/dt MAX and MIN and hindlimb blood flow (right column) and the slope (gain) of these relationships (left column) in 7 animals before and after induction of heart failure (data analyzed as described previously,(Coutsos et al., 2010, 2013; Hammond et al., 2000; Kaur et al., 2016; Kaur, Machado, et al., 2015; Kaur et al., 2017; Kaur, Spranger, et al., 2015; Sala-Mercado et al., 2007; Sala-Mercado et al., 2006). The graphs on the left show the average values during steady-state exercise (right-hand points), the average threshold of the reflex (middle points) and the values observed at peak metaboreflex activation (left hand points) before and after induction of heart failure. In heart failure, the baseline levels of hindlimb blood flow during mild exercise were lower and ventricular contractility was decreased. There was no change in the threshold level of hindlimb blood flow in heart failure and upon muscle metaboreflex activation the slope of the relationships between hindlimb blood flow and dP/dt MAX and MIN were significantly lower.

FIGURE 3:

FIGURE 3:

Open in a new tab

(Left) Line graph representation of the change in the slope of dP/dts before (solid circles) and after induction of heart failure via rapid ventricular pacing (open circles) should be observed from the right point (exercise) to the middle point (reflex threshold) to the final point left (peak muscle metaboreflex activation) as hindlimb blood flow is reduced. (Right) Assessment of muscle metaboreflex gain as judged by the slope of the line from threshold to maximal muscle metaboreflex activation before (white bars) and after induction of heart failure via rapid ventricular pacing (grey bars). Statistical significance between control and heart failure shown as † P < 0.05. (N=7).

Figure 4 shows the contraction/relaxation rate ratios at rest, exercise and metaboreflex activation (left) and the changes seen with metaboreflex activation (right) in 14 animals before and after induction of heart failure. In normal subjects, the contraction/relaxation ratio at rest is greater than 1.0 and significantly increases from rest to exercise which favors contraction. With muscle metaboreflex activation the ratio significantly reduces back towards 1.0 indicating a larger improvement in the relaxation rate relative to the contraction rate during muscle metaboreflex activation. After induction of heart failure, the ratio is significantly lower at rest, there is a small but statistically significant increase from rest to exercise but no further significant changes in the ratio during muscle metaboreflex activation. Thus, in normal animals metaboreflex activation results in a large reduction in the ratio whereas in heart failure there is little change.

FIGURE 4:

FIGURE 4:

Open in a new tab

Average one-minute steady state values of the ratio of dP/dt MAX divided by dP/dt MIN taken at rest (REST), exercise (3.2 km/h 0% grade) (EX), and exercise with muscle metaboreflex activation (MMA) before (white) and after induction of heart failure via rapid ventricular pacing (gray). Right shows the average change from EX to MMA in control and heart failure. Standard deviation is shown on the bar graphs. Statistical significance against previous workload is shown as * P < 0.05. Comparisons between control and heart failure for a given workload or change from previous workload shown as † P < 0.05. (N=14).

DISCUSSION:

This is the first study to demonstrate that muscle metaboreflex activation enhances ventricular relaxation rate relative to the contraction rate and that these increases are stifled in heart failure. Additionally, we observed that at rest the ventricular contraction and relaxation rates are maintained near a ratio of 1:1 in healthy and heart failure similar to what is observed in genetic models of rats (P. M. Janssen, 2010; P. M. L. Janssen, 2010). However in contrast to observations in-vitro wherein the 1:1 ratio is sustained during beta adrenergic receptor activation, we observed in-vivo this ratio does vary with exercise and metaboreflex activation in healthy subjects. After induction of heart failure, the rise in this ratio seen during exercise and subsequent decrease observed with muscle metaboreflex activation was abolished. Significant differences in contraction-relaxation ratios were observed in healthy subjects between rest, exercise and exercise with muscle metaboreflex activation. We observed that from rest to mild exercise there was a significant increase in the ratio favoring contraction rate. However, with subsequent muscle metaboreflex activation there was a reduction in the ratio to return closer to 1.0 implying significant greater increases in relaxation rate relative to contraction rate. This improved relaxation rate would favor improved ventricular filling time and thereby may aid in the ability to raise cardiac output. In heart failure the effect of metaboreflex activation on the contraction-relaxation ratio was abolished. This failure to substantially improve relaxation rate in heart failure may contribute to the impaired ability to raise cardiac output. In addition to the effects observed regarding contraction/relaxation ratios, this is the first study to quantify the gain of the muscle metaboreflex control of ventricular dP/dt MAX and MIN and the effects of heart failure on metaboreflex gain of dP/dt MAX and MIN. We observed that gain of the muscle metaboreflex control of both the ability to improve contraction and relaxation are substantially impaired in heart failure.

Increases in ventricular function during exercise are key to the ability to raise cardiac output (Ansorge et al., 2005; Coutsos et al., 2010, 2013; O’Leary & Augustyniak, 1998; Sala-Mercado et al., 2007; Sala-Mercado et al., 2006). During mild exercise (3.2 km/h 0% grade) increases in ventricular function (dP/dt MAX and MIN) in response to the exercise itself are small. However, the level of increase is not uniform and these small increases in ventricular function do not always reach statistical significance (Kaur et al., 2018; Mannozzi et al., 2020). During muscle metaboreflex activation at mild workload sympathetic drive to the heart is substantially increased causing highly significant large increases ventricular function. This occurs despite limits on coronary vasodilation due to activation of vascular alpha-adrenergic receptors and thus functional coronary vasoconstriction occurs restraining the ability to maximally raise ventricular function (Coutsos et al., 2010). In heart failure, frank coronary vasoconstriction occurs and further restrains already compromised ventricular performance (Ansorge et al., 2005; Coutsos et al., 2013). After blockade of alpha adrenergic receptors, this coronary vasoconstriction is reversed to coronary vasodilation during metaboreflex activation and coronary blood flow increases substantially and there are significant improvements in ventricular function (Coutsos et al., 2010, 2013). Furthermore, we have shown that ventricular-vascular coupling is also markedly altered in heart failure as much greater increases in arterial elastance occurs which are unaccompanied by increases in ventricular elastance thereby further uncoupling an already impaired ventricular-vascular relationship (Mannozzi et al., 2020). The higher the arterial elastance the less effective energy transfer becomes and this in turn reduces the ability to improve stroke work. Interestingly, even with reduced energy transfer and poor vascular accommodation, muscle metaboreflex-induced central blood volume mobilization in heart failure is sustained. (O’Leary et al., 2019). Thus, the inability to improve cardiac output in heart failure is likely not only a result of reduced ventricular contraction (Ansorge et al., 2005; Coutsos et al., 2013; O’Leary et al., 2004; Sala-Mercado et al., 2007) but also poor relaxation dynamics that may be affected by changes in coronary artery blood flow similar to contractile function as seen in our previous studies (Ansorge et al., 2005; Coutsos et al., 2010, 2013; O’Leary et al., 2007; Spranger et al., 2017). In-depth analyses of diastolic function during exercise and metaboreflex activation in heart failure are not well understood, however in the present study we have shown that relaxation rate is altered and this likely significantly impacts diastolic performance.

In-Vitro vs In-Vivo

Many in-vitro studies have quantified changes in sarcomere function through assessments of changes in sarcomere length, frequency of stimulation, calcium availability, and many other factors to better understand cardiac contraction and relaxation function (Borbely et al., 2009; Chung, Strunc, Oliver, & Kovacs, 2006; Hanft, Korte, & McDonald, 2008; P. M. Janssen, 2010; P. M. L. Janssen, 2010; King et al., 2011; ter Keurs et al., 2006). These models focus on preparations devoid of intrinsic and extrinsic cellular factors as well as systemic circulating factors. Thus, these models have a limited ability to determine how single sarcomere function impacts function of the entire ventricle. Whereas, our experiments lack the ability to assess minute sarcomere changes, they do incorporate the sum of the effects of changes in afterload, preload, vascular changes and sympathetic activation together on ventricular control of contraction and relaxation in an intact animal which thereby reflect the functional importance of these changes in integrative cardiovascular function. Previously we have been able to assess multiple aspects of ventricular function including changes in ventricular maximal elastance, contractility, the relationship between contractility and heart rate, and roles of central volume mobilization in the ability to raise cardiac output (Ansorge et al., 2005; Chen et al., 2010; Coutsos et al., 2010, 2013; Hammond et al., 2000; O’Leary, 1993; O’Leary & Augustyniak, 1998; O’Leary et al., 2004; Sala-Mercado et al., 2007; Sala-Mercado et al., 2006; Sala-Mercado et al., 2014). Inasmuch as the same increases in cardiac output are observed when heart rate is maintained constant as during normal settings (O’Leary & Augustyniak, 1998; Sheriff et al., 1998), increases in ventricular function directly due to increased sympathetic activity to the ventricular myocardium is the primary factor in the ability to raise cardiac output during metaboreflex activation. Our previous data has shown that heart failure alone is not solely responsible for the major deficits in the ability to improve ventricular function. We have observed that enhanced coronary vasoconstriction occurs during metaboreflex activation in heart failure that likely limits myocardial oxygen consumption and thereby further attenuates contractile function (Coutsos et al., 2013). Effects such as these are difficult to assess outside the context of whole animal physiology and thus are likely factors that contribute to differences in our results when compared with those of in-vitro models.

NEW FINDINGS:

What is the central question of this study?

Does the muscle metaboreflex affect the ratio of left ventricular contraction/relaxation rates and does heart failure impact this relationship.

What is the main finding and its importance?

The effect of muscle metaboreflex activation on the ventricular relaxation rate was significantly attenuated in heart failure. Heart failure attenuates the exercise and muscle metaboreflex-induced changes in the contraction/relaxation ratio. In heart failure, the reduced ability to raise cardiac output during muscle metaboreflex activation may not solely be due to attenuation of ventricular contraction but also alterations in ventricular relaxation and diastolic function.

Implications:

The muscle metaboreflex is a negative feedback reflex which acts to lessen metabolite accumulation in the skeletal muscle by increasing total systemic blood flow (e.g. cardiac output) as well as arterial O2 content via red blood cell mobilization (Coutsos et al., 2010; Crisafulli et al., 2011; Crisafulli et al., 2003; Donal S. O’Leary & Collins, 1999; M. Ichinose, Ichinose-Kuwahara, Kondo, & Nishiyasu, 2015; M. J. Ichinose et al., 2010; O’Leary & Augustyniak, 1998; Robert A. Augustyniak et al., 2000; Sala-Mercado et al., 2006; Sheriff, Wyss, Rowell, & Scher, 1987). In healthy subjects during relative ischemia when the levels of metabolites rise within the skeletal muscle, they activate metabosensitive afferents. These afferents activate cardiovascular centers within the brain to increase sympathetic outflow. This culminates in tachycardia and increased ventricular function thereby raising cardiac output and β2 mediated vasodilation via epinephrine release. Combined these enhance skeletal muscle blood flow and oxygen delivery which reduces the interstitial metabolite concentration (Ansorge et al., 2005; D. S. O’Leary & Sheriff, 1995; Donal S. O’Leary & Collins, 1999; O’Leary et al., 1999; Sheriff, 1997; Sheriff et al., 1998; Sheriff et al., 1987). Muscle metaboreflex activation also enhances central blood volume mobilization thereby maintaining ventricular filling pressure which thereby supports the enhanced ventricular performance in order to sustain the reflex increase in cardiac output. In heart failure, during exercise skeletal muscle blood flow is low and is already near, at, or beyond the threshold required to elicit activation of skeletal muscle afferents depending on the workload (Hammond et al., 2000; Hammond et al., 2001). Muscle metaboreflex activation causes vasoconstriction in inactive vascular beds and even within the active skeletal muscle - the vascular bed from which the reflex originates thereby exaggerating the original response developing a limiting positive feedback loop (Kaur, Machado, et al., 2015; Kaur et al., 2017). Although the muscle metaboreflex elicits tachycardia in heart failure, ventricular function is compromised and exaggerated coronary vasoconstriction (Coutsos et al., 2013) further limits reflex increases in cardiac output. Thus, the ability of the reflex to improve perfusion to the ischemic muscle is compromised. Although central blood volume mobilization is sustained in heart failure (O’Leary et al., 2019), we observed in the present study that ventricular relaxation rate is depressed at rest and during mild exercise and the metaboreflex-induced reflex increases in relaxation rate are attenuated To what extent these impairments in diastolic function are related to exaggerated metaboreflex-induced coronary vasoconstriction are unknown.

From a clinical perspective alteration in muscle metaboreflex function as a result of heart failure pose a significant detriment to exercise tolerance due to reduced skeletal muscle perfusion and exaggerated sympatho-activation. A reduced exercise capacity in turn limits cardiac rehabilitative regimes and may even pose further detriment to cardiac health. Our previous studies have shown that the majority of the effects observed in heart failure are due to an inability to increase ventricular function which is typically treated through the use of vasodilators, ionotropic and blood volume reducing agents. In this study we observed that in a model of dilated cardiomyopathy-systolic heart failure that detriments in cardiac relaxation exist which likely contribute to reduced exercise tollerance. To what extent other aspects of diastolic function also contribute to impaired ventricular function during exercise in heart failure have yet to be determined, however, changes in diastolic function are commonly seen in HFpEF and thus improving our understanding of diastolic function in the context of systolic failure will likely lead to mechanistic insights to assess, diagnose, and treat cardiovascular diseases.

ACKNOWLEDGEMENTS

The authors thank Audrey Nelson and Jody Helme-Day for expert technical assistance and animal care. The Authors additionally thank Charles Chung, Ph.D. for his expertise and advice.

GRANTS

This work was supported by the National Heart, Lung and, Blood Institute Grants HL-55473 and HL-126706, HL-120882.

Footnotes

DISCLOSURES

The authors have no conflicts of interest to disclose.

DATA AVAILABILITY STATEMENT

All data within this study is available upon request from the corresponding author.

References

  1. Adreani CM, Hill JM, & Kaufman MP (1997). Responses of group III and IV muscle afferents to dynamic exercise. J Appl Physiol (1985), 82(6), 1811–1817. doi:10.220.33.5 [DOI] [PubMed] [Google Scholar]
  2. Ansorge EJ, Augustyniak RA, Perinot ML, Hammond RL, Kim JK, Sala-Mercado JA, … O’Leary DS (2005). Altered muscle metaboreflex control of coronary blood flow and ventricular function in heart failure. Am J Physiol Heart Circ Physiol, 288(3), H1381–1388. doi: 10.1152/ajpheart.00985.2004 [DOI] [PubMed] [Google Scholar]
  3. Borbely A, Falcao-Pires I, van Heerebeek L, Hamdani N, Edes I, Gavina C, … Paulus WJ (2009). Hypophosphorylation of the Stiff N2B titin isoform raises cardiomyocyte resting tension in failing human myocardium. Circ Res, 104(6), 780–786. doi: 10.1161/CIRCRESAHA.108.193326 [DOI] [PubMed] [Google Scholar]
  4. Chen X, Sala-Mercado JA, Hammond RL, Ichinose M, Soltani S, Mukkamala R, & O’Leary DS (2010). Dynamic control of maximal ventricular elastance via the baroreflex and force-frequency relation in awake dogs before and after pacing-induced heart failure. Am J Physiol Heart Circ Physiol, 299(1), H62–69. doi: 10.1152/ajpheart.00922.2009 [DOI] [PMC free article] [PubMed] [Google Scholar]
  5. Chung CS, Strunc A, Oliver R, & Kovacs SJ (2006). Diastolic ventricular-vascular stiffness and relaxation relation: elucidation of coupling via pressure phase plane-derived indexes. Am J Physiol Heart Circ Physiol, 291(5), H2415–2423. doi: 10.1152/ajpheart.00257.2006 [DOI] [PubMed] [Google Scholar]
  6. Coutsos M, Sala-Mercado JA, Ichinose M, Li Z, Dawe EJ, & O’Leary DS (2010). Muscle metaboreflex-induced coronary vasoconstriction functionally limits increases in ventricular contractility. J Appl Physiol (1985), 109(2), 271–278. doi: 10.1152/japplphysiol.01243.2009 [DOI] [PMC free article] [PubMed] [Google Scholar]
  7. Coutsos M, Sala-Mercado JA, Ichinose M, Li Z, Dawe EJ, & O’Leary DS (2013). Muscle metaboreflex-induced coronary vasoconstriction limits ventricular contractility during dynamic exercise in heart failure. Am J Physiol Heart Circ Physiol, 304(7), H1029–1037. doi: 10.1152/ajpheart.00879.2012 [DOI] [PMC free article] [PubMed] [Google Scholar]
  8. Crisafulli A, Piras F, Filippi M, Piredda C, Chiappori P, Melis F, … Concu A (2011). Role of heart rate and stroke volume during muscle metaboreflex-induced cardiac output increase: differences between activation during and after exercise. J Physiol Sci, 61(5), 385–394. doi: 10.1007/s12576-011-0163-x [DOI] [PMC free article] [PubMed] [Google Scholar]
  9. Crisafulli A, Salis E, Pittau G, Lorrai L, Tocco F, Melis F, … Concu A (2006). Modulation of cardiac contractility by muscle metaboreflex following efforts of different intensities in humans. Am J Physiol Heart Circ Physiol, 291(6), H3035–3042. doi: 10.1152/ajpheart.00221.2006 [DOI] [PubMed] [Google Scholar]
  10. Crisafulli A, Salis E, Tocco F, Melis F, Milia R, Pittau G, … Concu A (2007). Impaired central hemodynamic response and exaggerated vasoconstriction during muscle metaboreflex activation in heart failure patients. Am J Physiol Heart Circ Physiol, 292(6), H2988–2996. doi: 10.1152/ajpheart.00008.2007 [DOI] [PubMed] [Google Scholar]
  11. Crisafulli A, Scott AC, Wensel R, Davos CH, Francis DP, Pagliaro P, … Piepoli MF (2003). Muscle metaboreflex-induced increases in stroke volume. Med Sci Sports Exerc, 35(2), 221–228; discussion 229. doi: 10.1249/01.MSS.0000048639.02548.24 [DOI] [PubMed] [Google Scholar]
  12. O’Leary DS, & Sheriff DD (1995). Is the muscle metaboreflex important in control of blood flow to ischemic active skeletal muscle in dogs? American Journal Physiology - Heart and Circulatory Physiology, 268(3), H980–H986. doi:10.220.33.5 [DOI] [PubMed] [Google Scholar]
  13. Donal S O’Leary RAA, Ansorge Eric J.,, & Collins HL (1999). Muscle metaboreflex improves O2 delivery to ischemic active skeletal muscle. Am J Physiol Heart Circ Physiol, 276(4), H1399–H1403. [DOI] [PubMed] [Google Scholar]
  14. Hammond RL, Augustyniak RA, Rossi NF, Churchill PC, Lapanowski K, & O’Leary DS (2000). Heart failure alters the strength and mechanisms of the muscle metaboreflex. Am J Physiol Heart Circ Physiol, 278(3), H818–828. Retrieved from https://www.ncbi.nlm.nih.gov/pubmed/10710350 [DOI] [PubMed] [Google Scholar]
  15. Hammond RL, Augustyniak RA, Rossi NF, Lapanowski K, Dunbar JC, & O’Leary DS (2001). Alteration of humoral and peripheral vascular responses during graded exercise in heart failure. Journal of Applied Physiology, 90(1), 55–61. doi:10.220.33.4 [DOI] [PubMed] [Google Scholar]
  16. Hanft LM, Korte FS, & McDonald KS (2008). Cardiac function and modulation of sarcomeric function by length. Cardiovasc Res, 77(4), 627–636. doi: 10.1093/cvr/cvm099 [DOI] [PubMed] [Google Scholar]
  17. Ichinose M, Ichinose-Kuwahara T, Kondo N, & Nishiyasu T (2015). Increasing blood flow to exercising muscle attenuates systemic cardiovascular responses during dynamic exercise in humans. Am J Physiol Regul Integr Comp Physiol, 309(10), R1234–1242. doi: 10.1152/ajpregu.00063.2015 [DOI] [PMC free article] [PubMed] [Google Scholar]
  18. Ichinose MJ, Sala-Mercado JA, Coutsos M, Li Z, Ichinose TK, Dawe E, & O’Leary DS (2010). Modulation of cardiac output alters the mechanisms of the muscle metaboreflex pressor response. Am J Physiol Heart Circ Physiol, 298(1), H245–250. doi: 10.1152/ajpheart.00909.2009 [DOI] [PMC free article] [PubMed] [Google Scholar]
  19. Janssen PM (2010). Myocardial contraction-relaxation coupling. Am J Physiol Heart Circ Physiol, 299(6), H1741–1749. doi: 10.1152/ajpheart.00759.2010 [DOI] [PMC free article] [PubMed] [Google Scholar]
  20. Janssen PML (2010). Kinetics of cardiac muscle contraction and relaxation are linked and determined by properties of the cardiac sarcomere. American Journal of Physiology-Heart and Circulatory Physiology, 299(4), H1092–H1099. doi: 10.1152/ajpheart.00417.2010 [DOI] [PMC free article] [PubMed] [Google Scholar]
  21. Kaufman MP, Longhurst JC, Rybicki KJ, Wallach JH, & Mitchell JH (1983). Effects of static muscular contraction on impulse activity of groups III and IV afferents in cats. J Appl Physiol Respir Environ Exerc Physiol, 55(1 Pt 1), 105–112. doi: 10.1152/jappl.1983.55.1.105 [DOI] [PubMed] [Google Scholar]
  22. Kaufman MP, Rybicki KJ, Waldrop TG, & Ordway GA (1984). Effect of ischemia on responses of group III and IV afferents to contraction. J Appl Physiol Respir Environ Exerc Physiol, 57(3), 644–650. doi: 10.1152/jappl.1984.57.3.644 [DOI] [PubMed] [Google Scholar]
  23. Kaur J, Alvarez A, Hanna HW, Krishnan AC, Senador D, Machado TM, … O’Leary DS (2016). Interaction between the muscle metaboreflex and the arterial baroreflex in control of arterial pressure and skeletal muscle blood flow. Am J Physiol Heart Circ Physiol, 311(5), H1268–H1276. doi: 10.1152/ajpheart.00501.2016 [DOI] [PMC free article] [PubMed] [Google Scholar]
  24. Kaur J, Krishnan AC, Senador D, Alvarez A, Hanna HW, & O’Leary DS (2018). Altered arterial baroreflex-muscle metaboreflex interaction in heart failure. Am J Physiol Heart Circ Physiol, 315(5), H1383–H1392. doi: 10.1152/ajpheart.00338.2018 [DOI] [PMC free article] [PubMed] [Google Scholar]
  25. Kaur J, Machado TM, Alvarez A, Krishnan AC, Hanna HW, Altamimi YH, … O’Leary DS (2015). Muscle metaboreflex activation during dynamic exercise vasoconstricts ischemic active skeletal muscle. Am J Physiol Heart Circ Physiol, 309(12), H2145–2151. doi: 10.1152/ajpheart.00679.2015 [DOI] [PMC free article] [PubMed] [Google Scholar]
  26. Kaur J, Senador D, Krishnan AC, Hanna HW, Alvarez A, Machado TM, & O’Leary DS (2017). Muscle Metaboreflex-Induced Vasoconstriction in the Ischemic Active Muscle is Exaggerated in Heart Failure. Am J Physiol Heart Circ Physiol, ajpheart 00375 02017. doi: 10.1152/ajpheart.00375.2017 [DOI] [PMC free article] [PubMed] [Google Scholar]
  27. Kaur J, Spranger MD, Hammond RL, Krishnan AC, Alvarez A, Augustyniak RA, & O’Leary DS (2015). Muscle metaboreflex activation during dynamic exercise evokes epinephrine release resulting in beta2-mediated vasodilation. Am J Physiol Heart Circ Physiol, 308(5), H524–529. doi: 10.1152/ajpheart.00648.2014 [DOI] [PMC free article] [PubMed] [Google Scholar]
  28. Kim JK, Sala-Mercado JA, Hammond RL, Rodriguez J, Scislo TJ, & O’Leary DS (2005). Attenuated arterial baroreflex buffering of muscle metaboreflex in heart failure. Am J Physiol Heart Circ Physiol, 289(6), H2416–2423. doi: 10.1152/ajpheart.00654.2005 [DOI] [PubMed] [Google Scholar]
  29. King NMP, Methawasin M, Nedrud J, Harrell N, Chung CS, Helmes M, & Granzier H (2011). Mouse intact cardiac myocyte mechanics: cross-bridge and titin-based stress in unactivated cells. Journal of General Physiology, 137(1), 81–91. doi: 10.1085/jgp.201010499 [DOI] [PMC free article] [PubMed] [Google Scholar]
  30. Laprad SL, Augustyniak RA, Hammond RL, & O’Leary DS (1999). Does gender influence the strength and mechanisms of the muscle metaboreflex during dynamic exercise in dogs? Am J Physiol, 276(4), R1203–1208. doi: 10.1152/ajpregu.1999.276.4.R1203 [DOI] [PubMed] [Google Scholar]
  31. Mannozzi J, Kaur J, Spranger MD, Al-Hassan MH, Lessanework B, Alvarez A, … O’Leary DS (2020). Muscle Metaboreflex-Induced Increases in Effective Arterial Elastance: Effect of Heart Failure. Am J Physiol Regul Integr Comp Physiol. doi: 10.1152/ajpregu.00040.2020 [DOI] [PMC free article] [PubMed] [Google Scholar]
  32. Middlekauff HR, & Sinoway LI (2007). Increased mechanoreceptor stimulation explains the exaggerated exercise pressor reflex seen in heart failure. J Appl Physiol (1985), 102(1), 492–494; discussion 496. doi: 10.1152/japplphysiol.00994.2006 [DOI] [PubMed] [Google Scholar]
  33. Mitchell JH, Kaufman MP, & Iwamoto GA (1983). The exercise pressor reflex: its cardiovascular effects, afferent mechanisms, and central pathways. Annu Rev Physiol, 45, 229–242. doi: 10.1146/annurev.ph.45.030183.001305 [DOI] [PubMed] [Google Scholar]
  34. O’Leary DS (1993). Autonomic mechanisms of muscle metaboreflex control of heart rate. J Appl Physiol (1985), 74(4), 1748–1754. doi: 10.1152/jappl.1993.74.4.1748 [DOI] [PubMed] [Google Scholar]
  35. O’Leary DS, & Augustyniak RA (1998). Muscle metaboreflex increases ventricular performance in conscious dogs. Am J Physiol, 275(1 Pt 2), H220–224. Retrieved from https://www.ncbi.nlm.nih.gov/pubmed/9688917 [DOI] [PubMed] [Google Scholar]
  36. O’Leary DS, Augustyniak RA, Ansorge EJ, & Collins HL (1999). Muscle metaboreflex improves O2 delivery to ischemic active skeletal muscle. Am J Physiol, 276(4), H1399–1403. doi: 10.1152/ajpheart.1999.276.4.H1399 [DOI] [PubMed] [Google Scholar]
  37. O’Leary DS, Sala-Mercado JA, Augustyniak RA, Hammond RL, Rossi NF, & Ansorge EJ (2004). Impaired muscle metaboreflex-induced increases in ventricular function in heart failure. Am J Physiol Heart Circ Physiol, 287(6), H2612–2618. doi: 10.1152/ajpheart.00604.2004 [DOI] [PubMed] [Google Scholar]
  38. O’Leary DS, Sala-Mercado JA, Hammond RL, Ansorge EJ, Kim JK, Rodriguez J, … Ichinose M (2007). Muscle metaboreflex-induced increases in cardiac sympathetic activity vasoconstrict the coronary vasculature. J Appl Physiol (1985), 103(1), 190–194. doi: 10.1152/japplphysiol.00139.2007 [DOI] [PubMed] [Google Scholar]
  39. O’Leary DS, Senador D, & Augustyniak RA (2019). Muscle metaboreflex-induced central blood volume mobilization in heart failure. Am J Physiol Heart Circ Physiol, 316(5), H1047–H1052. doi: 10.1152/ajpheart.00805.2018 [DOI] [PMC free article] [PubMed] [Google Scholar]
  40. Augustyniak Robert A., C HL, Ansorge Eric J., Rossi Noreen F., & O’Leary DS (2000). Severe exercise alters the strength and mechanisms of the muscle metaboreflex. Am J Physiol Heart Circ Physiol, 280(4), H1645–H1652. [DOI] [PubMed] [Google Scholar]
  41. Rotto DM, & Kaufman MP (1988). Effect of metabolic products of muscular contraction on discharge of group III and IV afferents. J Appl Physiol (1985), 64(6), 2306–2313. doi: 10.1152/jappl.1988.64.6.2306 [DOI] [PubMed] [Google Scholar]
  42. Sala-Mercado JA, Hammond RL, Kim JK, McDonald PJ, Stephenson LW, & O’Leary DS (2007). Heart failure attenuates muscle metaboreflex control of ventricular contractility during dynamic exercise. Am J Physiol Heart Circ Physiol, 292(5), H2159–2166. doi: 10.1152/ajpheart.01240.2006 [DOI] [PubMed] [Google Scholar]
  43. Sala-Mercado JA, Hammond RL, Kim JK, Rossi NF, Stephenson LW, & O’Leary DS (2006). Muscle metaboreflex control of ventricular contractility during dynamic exercise. Am J Physiol Heart Circ Physiol, 290(2), H751–757. doi: 10.1152/ajpheart.00869.2005 [DOI] [PubMed] [Google Scholar]
  44. Sala-Mercado JA, Moslehpour M, Hammond RL, Ichinose M, Chen X, Evan S, … Mukkamala R, (2014). Stimulation of the cardiopulmonary baroreflex enhances ventricular contractility in awake dogs: a mathematical analysis study. Am J Physiol Regul Integr Comp Physiol, 307(4), R455–464. doi: 10.1152/ajpregu.00510.2013 [DOI] [PMC free article] [PubMed] [Google Scholar]
  45. Sheriff DD (1997). Latency of muscle chemoreflex to vascular occlusion of active muscle during dynamic exercise. Am J Physiol, 272(4 Pt 2), H1981–1985. doi: 10.1152/ajpheart.1997.272.4.H1981 [DOI] [PubMed] [Google Scholar]
  46. Sheriff DD, Augustyniak RA, & O’Leary DS (1998). Muscle chemoreflex-induced increases in right atrial pressure. American Journal of Physiology-Heart and Circulatory Physiology, 275(3), H767–H775. Retrieved from <Go to ISI>://WOS:000075620800005 [DOI] [PubMed] [Google Scholar]
  47. Sheriff DD, Wyss CR, Rowell LB, & Scher AM (1987). Does inadequate oxygen delivery trigger pressor response to muscle hypoperfusion during exercise? Am J Physiol, 253(5 Pt 2), H1199–1207. doi: 10.1152/ajpheart.1987.253.5.H1199 [DOI] [PubMed] [Google Scholar]
  48. Shoemaker JK, Kunselman AR, Silber DH, & Sinoway LI (1998). Maintained exercise pressor response in heart failure. J Appl Physiol (1985), 85(5), 1793–1799. doi:10.220.32.247 [DOI] [PubMed] [Google Scholar]
  49. Silber DH, Sutliff G, Yang QX, Smith MB, Sinoway LI, & Leuenberger UA (1998). Altered mechanisms of sympathetic activation during rhythmic forearm exercise in heart failure. J Appl Physiol (1985), 84(5), 1551–1559. doi: 10.1152/jappl.1998.84.5.1551 [DOI] [PubMed] [Google Scholar]
  50. Sinoway L, Prophet S, Gorman I, Mosher T, Shenberger J, Dolecki M, … Zelis R, (1989). Muscle acidosis during static exercise is associated with calf vasoconstriction. J Appl Physiol (1985), 66(1), 429–436. doi: 10.1152/jappl.1989.66.1.429 [DOI] [PubMed] [Google Scholar]
  51. Sinoway LI, Rea RF, Mosher TJ, Smith MB, & Mark AL (1992). Hydrogen ion concentration is not the sole determinant of muscle metaboreceptor responses in humans. J Clin Invest, 89(6), 1875–1884. doi: 10.1172/JCI115792 [DOI] [PMC free article] [PubMed] [Google Scholar]
  52. Sinoway LI, Smith MB, Enders B, Leuenberger U, Dzwonczyk T, Gray K, … Moore RL (1994). Role of diprotonated phosphate in evoking muscle reflex responses in cats and humans. Am J Physiol, 267(2 Pt 2), H770–778. doi: 10.1152/ajpheart.1994.267.2.H770 [DOI] [PubMed] [Google Scholar]
  53. Sinoway LI, Wroblewski KJ, Prophet SA, Ettinger SM, Gray KS, Whisler SK, … Moore RL (1992). Glycogen depletion-induced lactate reductions attenuate reflex responses in exercising humans. Am J Physiol, 263(5 Pt 2), H1499–1505. doi: 10.1152/ajpheart.1992.263.5.H1499 [DOI] [PubMed] [Google Scholar]
  54. Spranger MD, Kaur J, Sala-Mercado JA, Krishnan AC, Abu-Hamdah R, Alvarez A, … O’Leary DS (2017). Exaggerated coronary vasoconstriction limits muscle metaboreflex-induced increases in ventricular performance in hypertension. Am J Physiol Heart Circ Physiol, 312(1), H68–H79. doi: 10.1152/ajpheart.00417.2016 [DOI] [PMC free article] [PubMed] [Google Scholar]
  55. ter Keurs HE, Wakayama Y, Sugai Y, Price G, Kagaya Y, Boyden PA, … Stuyvers BD (2006). Role of sarcomere mechanics and Ca2+ overload in Ca2+ waves and arrhythmias in rat cardiac muscle. Ann N Y Acad Sci, 1080, 248–267. doi: 10.1196/annals.1380.020 [DOI] [PubMed] [Google Scholar]
  56. Wyss CR, Ardell JL, Scher AM, & Rowell LB (1983). Cardiovascular responses to graded reductions in hindlimb perfusion in exercising dogs. Am J Physiol, 245(3), H481–486. doi: 10.1152/ajpheart.1983.245.3.H481 [DOI] [PubMed] [Google Scholar]

RESOURCES