Sympathetic Transduction in Type 2 Diabetes: Impact of Statin Therapy

Abstract

Approximately 60% of Type 2 diabetes (T2D) patients develop hypertension. Recent work also indicates greater blood pressure (BP) excursions throughout the day in T2D. Collectively, these findings suggest altered BP control in T2D. Although muscle sympathetic nerve activity (MSNA) recordings in T2D have provided equivocal results, quantification of MSNA alone does not account for ensuing vasoconstriction and BP response elicited by MSNA. Thus, we tested the hypothesis that T2D patients exhibit enhanced sympathetic transduction to BP. MSNA (microneurography) and beat-to-beat BP (Finometer) were measured at rest in 21 T2D and 13 age- and BMI-matched control (CON) subjects and signal-averaging was performed to quantify the mean arterial pressure (MAP) and total vascular conductance (TVC) responses to spontaneous bursts of MSNA. The peak MAP and TVC responses to spontaneous MSNA were similar between T2D and CON (both p>0.05). However, further analysis, separating T2D into those taking statins (n=13, T2D +statin) and not taking statins (n=8, T2D −statin), indicated that T2D −statin patients (4.2±0.6 mmHg) exhibited greater peak MAP responses compared to both T2D +statin patients (2.5±0.3 mmHg, p=0.01) and CON (CON: 2.8±0.3 mmHg, p=0.02). Likewise, nadir total vascular conductance responses to spontaneous MSNA bursts were greater in T2D −statin patients (T2D −statin: −3.3±0.6 mL/(min·mm Hg), T2D +statin: −1.6±0.3 mL/(min·mm Hg), P=0.03; control −2.2±0.3 mL/(min·mm Hg), P=0.08). Notably, T2D +statin patients exhibited similar peak MAP and TVC responses to MSNA compared to CON. Collectively, these findings demonstrate, for the first time, that T2D patients exhibited augmented sympathetic transduction and this effect appears to be offset by statin therapy.

Introduction

Currently, over 30 million adults in the United States have been diagnosed with type 2 diabetes (T2D), and the prevalence of T2D is expected to increase to nearly one-third of the population by the year 2050 (1). Approximately 60% of T2D patients also develop hypertension (HTN), which suggests alterations in blood pressure (BP) control in T2D (2). Notably, recent work has also suggested that T2D is associated with greater BP excursions throughout the day (3), which is an independent factor for cardiovascular risk (4). Although the mechanism(s) contributing to greater BP excursions in T2D remain unclear, a plausible link is greater sympathetic nervous system activation (5, 6). Indeed, the sympathetic nervous system dynamically regulates BP by modifying total vascular conductance (TVC), and thus contributes importantly to BP control.

Interestingly, direct recordings of muscle sympathetic nerve activity (MSNA) in T2D have provided equivocal results, wherein some report greater resting MSNA (7–9), and others no difference in MSNA (10, 11) compared to controls. Importantly, resting MSNA is only one aspect of sympathetic regulation, which alone does not account for the ensuing vascular smooth muscle contractile response, and ultimately the BP response to MSNA (i.e., sympathetic transduction). In this regard, it is plausible that an augmentation in the BP response to efferent sympathetic nerve activity contributes to the heightened BP excursions in T2D. However, to our knowledge this has not been directly tested. Recent animal studies have demonstrated enhanced α-adrenergic receptor expression in T2D rats (12), the primary receptors for norepinephrine released from sympathetic nerve terminals. Further, Hogikyan et al. (13) reported augmented vasoconstriction in response to intra-arterial infusion of norepinephrine in T2D patients, indicating that α-adrenergic receptors may be up-regulated and/or more sensitive in T2D. Overall, these data suggest vascular adaptations in T2D that may cause excessive vasoconstriction and greater BP responses to sympathetic outflow.

Approximately 53% of T2D patients are currently prescribed statin medication in the United States (14). This is important in terms of the sympathetic nervous system because statin medication has been shown to beneficially reduce sympathetic overactivity in several disease populations (15, 16). Moreover, in rats, statin therapy has been shown to reduce the magnitude of vasoconstriction in response to phenylephrine (α−1 receptor agonist), suggesting statin therapy may reduce the vascular smooth muscle contractile response to sympathetic nerve activity (17). In this regard, examining the effect of statin therapy on sympathetic transduction in T2D would be important.

With this background in mind, the purpose of our study was to examine resting sympathetic transduction to BP in T2D patients compared to age- and BMI-matched controls (CON) subjects. We tested the hypothesis that T2D patients would exhibit enhanced sympathetic transduction to BP compared to age- and BMI-matched CON. In addition, because of the high number of T2D patients taking statins (14), and recent rodent work identifying that statin treatment reduced vasoconstrictor responses to the α−1 agonist phenylephrine (i.e., sympathetic transduction) (17), further analysis was performed to examine the influence of statin therapy on sympathetic transduction. We tested the hypothesis that enhanced sympathetic transduction would be reduced in T2D patients receiving statin therapy.

METHODS

Subjects

Twenty one T2D patients (9 female) and 13 CON (9 female) subjects, matched for age and body mass index (BMI) were recruited for the present investigation. All subjects provided written-informed consent after explanation of the study procedures and experimental measures, which were approved by the Institutional Review Board at the University of Missouri and conformed to the Declaration of Helsinki. Of the 21 T2D patients, 13 were currently taking statin medication, which included simvastatin (n=5), atorvastatin (n=4), lovastatin (n=3), and rosuvastatin (n=1). Furthermore, 9 T2D patients had a concomitant medical diagnosis of HTN, of which 7 were taking statins. Other medications being taken by the T2D patients included: metformin (n=15), ACE inhibitors (n=7), exogenous insulin (n=7), diuretics (n=5), sulfonylureas (n=5), dipeptidyl peptidase-4 inhibitors (n=3), β-blockers (n=1), and Angiotensin II receptor blockers (n=1). T2D patients were only excluded if they took medications thought to directly affect central sympathetic outflow (e.g., clonidine). All CON subjects were free of medication. The resting MSNA data used for this study were retrospectively analyzed from previously published work (10, 11, 18); however, the analyses and hypotheses tested in the present investigation are independent from those previous studies. The data to support the findings of this study can be made available by the corresponding author upon reasonable request.

Experimental Measurements

Heart rate was measured using a lead II electrocardiogram (Quinton Q710, Bothell, WA). Beat-to-beat BP was measured using finger photoplethysmography (Finapres Medical Systems, Amsterdam, The Netherlands). The Finometer BP was verified by several automated sphygmomanometry (Welch Allyn; Skaneateles Falls, NY) measurements taken throughout the duration of the protocol. Post-ganglionic MSNA was directly recorded from the peroneal nerve as previously described (10, 19, 20). Briefly, a unipolar tungsten microelectrode was inserted percutaneously into the skin just below the fibular head. The electrode was then positioned into muscle fascicles within the peroneal nerve. The signal was amplified, band-pass filtered (700-2000 Hz), rectified, and integrated (0.1 second time constant) to provide a mean voltage neurogram (Nerve Traffic Analyzer, Bioengineering, University of Iowa). MSNA was identified by pulse synchronous bursts and confirmed with muscle afferent stimulation, in the absence of skin afferent stimulation. Respiration pattern was monitored using a strain-gauge pneumobelt secured around the abdomen (pneumotrace, UFI, Morro Bay, CA) to ensure consistent respiration during the recordings due to the known influence of respiration on MSNA (21, 22).

Experimental Protocol

Prior to the study, subjects were instructed to fast overnight, refrain from exercise and alcohol for 24 hours, and caffeine intake for 12 hours. Medications were held the morning of the study, until completion of all experimental measures. Upon arrival to the laboratory, a venous catheter was inserted into the subject’s antecubital or dorsal hand vein and blood was drawn and sent to a commercial blood processing laboratory for a complete metabolic panel and lipid profile. Then, subjects were instrumented for heart rate, BP, MSNA, and respiration. After an adequate MSNA signal was obtained, baseline measurements were recorded continuously for a minimum of 5 minutes (average duration: 9.5±0.2 min) in a dimly lit, temperature controlled room (22-24°C), and subjects were instructed to remain quiet and awake. All cardiovascular and neural measurements were acquired simultaneously at 1,000 Hz (PowerLab; ADInstruments, Bella Vista, Australia) and stored for later analysis.

Analysis

Insulin was measured with an enzyme immunoassay (ALPCO, Salem, NH) and the homeostatic model assessment of insulin resistance [HOMA-IR; (glucose X insulin)/22.5] was calculated to provide an index of insulin resistance. Insulin measurements were not obtained in 3 T2D patients and 1 CON subject.

All cardiovascular and neural measures were averaged across the duration of each baseline. Mean arterial pressure (MAP) was calculated as an average of the automated sphygmamonometer BPs obtained during the baseline. Stroke volume was estimated via the Modelflow method (23), and TVC was calculated as the product of stroke volume multiplied by heart rate, divided by MAP.

Customized LabVIEW (National Instruments, Austin, Texas) software was used to analyze the neurograms on a beat-to-beat basis as previously described (20, 24–26). All neurograms were analyzed by the same investigator (B.E.Y.). Bursts of sympathetic outflow were identified by pulse synchronicity, signal to noise ratio of 3:1, and morphology of the burst. Neurograms were analyzed for resting sympathetic nerve activity, quantified as burst incidence (bursts/100 heartbeats).

The transduction analysis of MSNA into MAP and TVC was performed as previously described in detail (19, 20, 24–26). Briefly, spike triggered averaging was performed in which bursts of MSNA acted as a trigger and then were followed for 15 subsequent cardiac cycles. All MSNA bursts are used in this signal averaging approach regardless of the proximity of other MSNA bursts. The change in MAP or TVC is defined as the instantaneous MAP/TVC at each given cardiac cycle subtracted by the MAP or TVC at time-point 0 (cardiac cycle in which the burst occurred). The average response over the 15 cardiac cycles for all bursts of MSNA was signal-averaged for each subject. Moreover, the MSNA bursts were further characterized into burst patterns of either single bursts (occurring in isolation) or multiple bursts (adjacent to at least one other burst) and signal-averaged. The peak MAP and TVC responses following bursts of MSNA were chosen within the first 10 cardiac cycles, given the peak response latency is consistently within 5-8 heart beats in humans (19, 20, 27). Finally, to assess the specificity of MAP/TVC responses to MSNA bursts, a white noise control was performed in which cardiac cycles were randomly selected and followed for 15 cardiac cycles. The number of cardiac cycles selected for the white noise control was equal to number of MSNA bursts in each individual recording keeping the number of events consistent within a subject.

In order to quantify the contribution of burst amplitude on the ensuing MAP and TVC responses, all bursts of MSNA were normalized as a percentage of the average peak voltage of the 3 largest bursts in the recording. The bursts were then divided into quartiles of burst height (smallest to largest) and signal-averaged such that Q1 represented the smallest 25% of bursts within the given recording and Q4 represented the largest 25%. Finally, the peak response was calculated and used to characterize the maximal response induced by the height quartiles (Q1-4).

Statistics

All data are presented as mean ± standard error (SE). The statistical analysis was conducted using the commercial statistical package Sigmaplot 13 (Systat Software; unpaired t-tests and one-way ANOVAs) or SPSS version 24 (mixed model ANOVAs only). Normality was assessed using the Shapiro-Wilk test and when appropriate non-parametric testing was performed. Baseline subject comparisons were made using unpaired t-tests (CON vs T2D; and T2D taking statins vs. T2D not taking statins). The peak response to bursts of MSNA compared between groups were made using one-way ANOVA with post-hoc comparisons made using Student-Newman-Keul’s test where appropriate. The beat-to-beat MAP and TVC responses to spontaneous bursts of MSNA were compared between groups using mixed model ANOVA with Bonferroni post-hoc correction to encompass the peak effector response of MSNA bursts. Likewise, the peak responses to quartiles of MSNA burst amplitude were compared between groups utilizing a mixed model ANOVA with Bonferroni post-hoc correction. For statistical analyses, 10 cardiac cycles following an MSNA burst were used rather than all 15 cardiac cycles since peak responses occurred prior to the 10^th cardiac cycle in all groups. In addition, to better understand the impact of common medications on the peak BP and TVC responses to MSNA, a multiple linear regression analysis was performed using SPSS (independent variables: statin, metformin, ACE inhibitor, and exogenous insulin). Significance was set at a P-value <0.05.

A power calculation was performed on the results of Hogikyan et al. (13), wherein they infused intra-arterial norepinephrine and examined blood flow responsiveness in T2D patients. Based on these results, in order to provide at least 80% power at an α=0.05, a minimal sample size of 26 subjects (T2D, n=13; CON, n=13) would be required.

Results

T2D patients and CON subjects, were well matched for age (T2D: 49±2 years, CON: 46±2 years, P=0.31) and BMI (T2D: 32±1 kg/m², CON: 32±1 kg/m²; P=0.93). Resting metabolic parameters for all groups are provided in Table 1. As expected, the T2D patients had a significantly greater fasting blood glucose, HOMA-IR, HbA1c, and triglyceride concentration compared to the CON subjects. There was no significant difference in fasting insulin between groups, and the CON subjects had a significantly greater cLDL (Table 1). The lower cLDL among T2D compared to CON subjects was likely attributable to the number of T2D patients on statin therapy. Resting MAP (T2D: 96±3 mmHg; CON: 94±3 mmHg, P=0.61), systolic (T2D: 127±3 mmHg; CON: 124±4 mmHg, P=0.57), and diastolic (T2D: 80±2 mmHg; CON: 79±2 mmHg, P=0.88) BP, as well as heart rate (T2D: 67±2 beats/min; CON: 67±2 beats/min, P=0.97) were similar between T2D patients and CON subjects. Likewise, MSNA burst incidence (T2D: median, 35 bursts/100 heartbeats; CON: median, 32 bursts/100 heartbeats, P=0.51) was not significantly different between groups.

Table 1:

Baseline Metabolic Measurements

Metabolic Parameters	CON	T2D	P Value
Glucose (mg/dL)	93±3	168±16	<0.001
Insulin (µIu/mL)	7.6±0.7	11.5±1.9	0.12
HOMA-IR	1.8±0.2	4.4±0.6	0.001
cLDL (mg/dL)	118±7	89±6	0.005
HbA1c (%)	5.4±0.1	7.9±0.4	<0.001
Triglyceride (mg/dL)	100±11	185±29	0.04
	T2D −Statin	T2D +Statin	P Value
Glucose (mg/dL)	201±31	147±16	0.10
Insulin (µIu/mL)	8.0±1.4	14.2±3.1	0.11
HOMA-IR	4.1±0.9	4.6±0.8	0.65
cLDL (mg/dL)	105±9	79±7	0.03
HbA1c (%)	8.9±0.8	7.2±0.4	0.04
Triglyceride (mg/dL)	249±61	145±26	0.08

Values are means ± SE; CON, control subjects (n=13); T2D, type 2 diabetes patients (n=21); T2D −Statin, T2D patients not taking statins (n=8); T2D +Statin, T2D patients taking statins (n=13). HOMA-IR, Homeostatic model assessment of insulin resistance.

In regards to statin therapy, T2D patients taking statins (T2D +statin) were significantly older than the T2D group not taking statins (T2D −Statin: 43±3 years; T2D +Statin: 54±2 years, P=0.006) with both groups having similar BMIs (T2D −Statin: 31±2 kg/m²; T2D +Statin: 32±1 kg/m², P=0.55). No difference in fasting blood glucose, insulin, or HOMA-IR were found between these groups (Table 1), whereas T2D +statin had significantly lower cLDL and HbA1c, as well as a trend for a lower triglyceride (P=0.08) concentration compared to T2D −statin. Similarly, there was no difference in resting MAP (T2D −Statin: 98±4 mmHg; T2D +Statin 95±3 mmHg, P=0.55), systolic BP (T2D −Statin: 127±6 mmHg; T2D +Statin: 126±3 mmHg, P=0.89), diastolic BP (T2D −Statin: 83±4 mmHg; T2D +Statin: 78±3 mmHg, P=0.27), or heart rate (T2D −Statin: 69±4 beats/min; T2D +Statin: 66±3 beats/min, P=0.50) between T2D −statin and T2D +statin patients. Likewise, MSNA burst incidence was not significantly different between groups (T2D −Statin: median, 29 bursts/100 heartbeats; T2D +Statin: median, 42 bursts/100 heartbeats, P=0.12).

Sympathetic Transduction in T2D

The beat-to-beat increase in MAP following a spontaneous burst of MSNA for T2D and CON groups is presented in Figure 1A. Both groups exhibited a clear and robust increase in MAP across the 10 cardiac cycles following spontaneous bursts of MSNA (Cardiac Cycle effect, P<0.001) that was not different between groups. Likewise, the peak increase in MAP following a burst of MSNA was not significantly different between T2D patients (3.2±0.3 mmHg) and CON subjects (2.8±0.3 mmHg, P=0.49), with both groups exhibiting a similar latency to the peak increase in MAP (T2D: 7.3±0.3 cardiac cycles; CON: 6.7±0.5 cardiac cycles, P=0.28). Also, peak increases in MAP were similar between groups when analyzed according to either single or multiple burst patterns (e.g., multiple bursts; T2D: 4.0±0.4 mmHg; CON: 3.6±0.4 mmHg, P=0.54). Figure 1C presents the peak increases in MAP following a spontaneous burst of MSNA divided into quartiles according to the relative size of the MSNA burst (i.e., burst amplitude). An increased MSNA burst amplitude caused a graded increase in MAP, which was not different between T2D patients and CON subjects. As expected, there were no changes in MAP for the white noise control condition (Cardiac Cycle effect, P>0.05; Figure 1A).

Figure 1:

Panels A and B: Summary data for Type 2 Diabetes (T2D; n=21) patients and control (CON; n=13) subjects showing beat-to-beat mean arterial pressure (MAP; A) and total vascular conductance (TVC; B) responses following spontaneous bursts of muscle sympathetic nerve activity (MSNA) or the same number of cardiac cycles randomly sampled (white noise). Panels C and D: Summary data for the peak changes in MAP (Panel C) and TVC (Panel D) in response to quartiles based on burst amplitude (Q1 smallest 25%; Q4 largest 25%).

The average beat-to-beat change in TVC following a spontaneous burst of MSNA is presented in Figure 1B. There were clear decreases in TVC that were not different between groups with peak reductions of −2.2±0.4 mL/min/mmHg in T2D patients and −2.2±0.3 mL/min/mmHg in CON subjects (P=0.88). Likewise, the latency to the peak reduction in TVC following a spontaneous burst of MSNA was similar between groups (T2D: 8.5±0.3 cardiac cycles; CON: 8.1±0.3 cardiac cycles, P=0.43). Finally, increased MSNA burst amplitude caused a graded reduction in TVC, which was also not different between groups (Figure 1D). As observed with MAP, no changes in TVC were found for the white noise control condition (Cardiac Cycle effect, P>0.05; Figure 1B).

Sympathetic Transduction in T2D: Influence of Statin Therapy

The beat-to-beat increase in MAP following a spontaneous burst of MSNA was significantly greater in the T2D −statin compared to T2D +statin patients and CON subjects (Figure 2A). Likewise, the increase in MSNA burst amplitude elicited a graded increase in MAP, which was greater in the T2D −statin compared to T2D +statin patients (P=0.012) and CON subjects (P=0.063; Figure 2C). Notably, both the beat-to-beat increase in MAP following a burst of MSNA, and the increase in MAP to graded increases in MSNA burst amplitude (P=1.0) were similar between T2D +statin patients and CON subjects. In terms of burst pattern, single bursts of MSNA elicited peak MAP responses that were not different between the three groups; however, in T2D −statin patients, multiple bursts of MSNA elicited significantly greater peak MAP responses compared to both T2D +statin patients (P=0.012) and CON subjects (P=0.02; Table 2). Notably, multiple bursts elicited similar peak MAP responses in T2D +statin patients compared to CON subjects (P=0.48; Table 2). No changes in MAP were observed for the white noise control condition (Cardiac Cycle effect, P>0.05; Figure 2A).

Figure 2:

Panels A and B: Summary data for Type 2 Diabetes not taking statins (T2D −Statin; n=8), Type 2 Diabetes patients taking statins (T2D +Statin; n=13), and control subjects (CON; n=13) showing beat-to-beat mean arterial pressure (MAP; A) and total vascular conductance (TVC; B) responses following spontaneous bursts of muscle sympathetic nerve activity (MSNA) or the same number of cardiac cycles randomly sampled (white noise). Panels C and D: Summary data for the peak changes in MAP (Panel C) and TVC (Panel D) in response to quartiles of burst amplitude (Q1 smallest 25%; Q4 largest 25%). * P<0.05, T2D −Statin vs T2D +Statin; † P<0.05, T2D −Statin vs. CON.

Table 2:

Peak Responses to Muscle Sympathetic Nerve Activity Based on Burst Pattern

Burst Distribution	CON	T2D −Statin	T2D +Statin	P Value
Peak ΔMAP (mmHg)
All Bursts	+2.8±0.3	+4.2±0.6†*	+2.5±0.3	0.014
Single Bursts	+1.2±0.3	+1.5±0.3	+0.9±0.3	0.358
Multiple Bursts	+3.6±0.4	+5.4±0.8†*	+3.2±0.4	0.014
Peak ∆TVC (mL/min/mmHg)
All Bursts	−2.2±0.3	−3.3-±0.6*	−1.6-±0.3	0.036
Single Bursts	−0.7±0.3	−1.1±0.4	−0.4±0.2	0.281
Multiple Bursts	−3.1±0.5	−4.3±0.9*	−2.1±0.4	0.042

Values are means ± SE; CON, control subjects (n=13); T2D, type 2 diabetes patients; T2D −Statin, T2D patients not taking statins (n=8); T2D +Statin, T2D patients taking statins (n=13); MAP, mean arterial pressure; TVC, total vascular conductance.

^*P<0.05, T2D −Statin vs T2D +Statin;

^†P<0.05, T2D −Statin vs CON.

The beat-to-beat reduction in TVC following spontaneous bursts of MSNA was significantly larger in the T2D −statin compared to T2D +statin patients (Figure 2B). The graded reduction in TVC to the increased MSNA burst amplitude was also significantly greater in T2D −statin compared to T2D +statin patients (P=0.03; Figure 2D). Although the beat-to-beat reduction in TVC following MSNA bursts (Figure 2B) and the graded reduction in TVC to increased MSNA burst amplitude (Figure 2D) appeared greater in T2D −statin patients compared to CON subjects, this did not reach statistical significance (P=0.34). No changes in TVC were found for the white noise control condition (Cardiac Cycle effect, P>0.05; Figure 2B).

Overall, when compared to both CON and T2D +statin patients, the T2D −statin patients exhibited significantly greater peak increases in MAP (Figure 3A) and greater reductions in TVC (Figure 3B) to all bursts of MSNA, while the T2D +statin patients exhibited similar peak MAP (P=0.54) and nadir TVC (P=0.31) responses compared to CON subjects. Notably, the latency to peak BP (T2D −Statin: 6.9±0.6 cardiac cycles; T2D +Statin: 7.6±0.4 cardiac cycles, CON: 6.7±0.5 cardiac cycles, P=0.35) and nadir TVC (T2D −Statin: 8.1±0.5 cardiac cycles; T2D +Statin: 8.7±0.5 cardiac cycles; CON: 8.1±0.3 cardiac cycles, P=0.40) were not significantly different between groups.

Figure 3:

Summary data for the peak changes in mean arterial pressure (MAP; Panel A) and total vascular conductance (TVC; Panel B) following spontaneous bursts of muscle sympathetic nerve activity in Type 2 Diabetic patients not taking statins (T2D −Statin; n=8), Type 2 Diabetic patients taking statins (T2D +Statin; n=13), and control subjects (CON; n=13). *P<0.05, T2D −Statin vs T2D +Statin; † P<0.05, T2D −Statin vs CON.

Results from multiple linear regression analysis indicated that statin use was a significant determinant of a reduced peak BP response to MSNA bursts (β=−0.621, P=0.010), consistent with results of the beat-to-beat analysis. In contrast, metformin (β=0.536, P=0.009) and exogenous insulin (β=0.306, P=0.048) were significant determinants of a higher peak MAP response to bursts of MSNA (model R² =0.38). Only statin use (P=0.023) was a significant determinant of a reduced peak TVC response to MSNA bursts (model R² =0.25). ACE inhibitors were not a significant determinant of the peak MAP response (P=0.78) or the peak TVC response (P=0.88) to bursts of MSNA.

Discussion

The primary novel finding of the present study was that T2D patients not taking statins exhibited augmented sympathetic transduction to BP compared to CON subjects, whereas T2D patients taking statins had similar sympathetic transduction compared to CON. Collectively, these findings indicate that T2D patients exhibit exaggerated BP responses to spontaneous bursts of MSNA, an effect that appears to be reduced by statin therapy.

In the present study, we found an augmented BP response to spontaneous bursts of resting MSNA in T2D patients not taking statins. These exaggerated responses were graded to the size of the sympathetic bursts, an index of the number of activated pre- and post-ganglionic fibers (28, 29), which likely represents the amount of norepinephrine release. Although the mechanism(s) for this greater BP responsiveness to MSNA remain unclear, there are several possibilities that warrant discussion. First, it is plausible that T2D exhibit greater peripheral α-adrenergic receptor sensitivity. In this regard, previous work has demonstrated greater vasoconstrictor responses to intra-arterial infusions of norepinephrine in T2D patients (13). Another possibility is a reduced plasma clearance of norepinephrine as well as a reduced neuronal reuptake of norepinephrine in T2D patients (9), which would increase the amount of norepinephrine available to bind to α-adrenergic receptors on post-synaptic vascular smooth muscle. Lastly, a reduced endothelium-dependent vasodilation among T2D patients not taking statin therapy may contribute to an augmented sympathetically-mediated vasoconstriction (30). Indeed, administration of the nitric oxide synthase (NOS) inhibitor N^G-monomethyl L-arginine to reduce nitric oxide-mediated vasodilation has been shown to potentiate the vasoconstriction induced by intra-arterial norepinephrine (31). Likewise, phenylephrine mediated constriction of isolated rat aortic rings is augmented after NOS inhibition (32). These data suggest that nitric oxide plays a role in restraining sympathetically-mediated vasoconstriction.

Notably, statin therapy appeared to offset the augmented BP response to MSNA bursts in T2D such that T2D patients on statin therapy had similar responses to CON subjects. While the mechanism for this effect of statins on sympathetic transduction is unknown, it is not likely related to the lowering of cholesterol alone. In a rodent model of obesity, greater vasoconstrictor responses to phenylephrine (i.e., sympathetic transduction) were reduced following statin treatment whereas similar reductions in cholesterol with other anti-cholesterol therapies had no impact on the vasoconstrictor response to phenylephrine (17). Therefore, it is likely that the reduction in sympathetic transduction to BP in T2D is due to a pleiotropic effect of statins. In this regard, statins are known to have anti-oxidant properties and to reduce the formation of reactive oxygen species (ROS). In support of a role for oxidative stress in augmenting sympathetic transduction, Fleischhacker et al. (33) have demonstrated that superoxide production mediated an increased contractile response to phenylephrine in isolated vascular smooth muscle cells from hypercholesterolemic patients, via alterations in intracellular calcium signaling. These data suggest that ROS may contribute to the greater BP response to MSNA in T2D, and reductions in ROS may be one putative mechanism for the reduction in sympathetic transduction following statin therapy.

There are some potential limitations of the current investigation that should be considered. First, our calculations of TVC were derived using stroke volume measurements estimated from the Modelflow methodology. Although used in numerous published studies, we are unaware of any studies that have validated this methodology in disease populations such as T2D patients. A second potential limitation to consider is our small sample size of T2D patients not on statin therapy. While the change in MAP following MSNA bursts was rather robust in this group with a large effect size and statistical power, future studies with a larger sample size are warranted. Likewise, additional studies that prospectively treat T2D patients with statins would be insightful to further consider the effect of statin therapy on sympathetic transduction as well as on resting MSNA. The latter becomes important since we did not observe a difference in resting MSNA between T2D patients taking and not taking statins, a finding that differs from other studies reporting statin-induced reductions in MSNA in other disease populations (15, 16).

Perspectives

An augmented BP response to spontaneous bursts of MSNA may represent a potential mechanism by which T2D patients experience greater BP excursions throughout the day (3), and augment their cardiovascular risk profile. Although the etiology of this exaggerated pressor response to MSNA remains incompletely understood in T2D, our findings that statin therapy reduced sympathetic transduction to BP similar to that of CON subjects is clinically relevant and warrants consideration for statin prescription in T2D patients. Notably, statin prescription in T2D rose from ~33%- to greater than 50% within the last two decades (14), and our findings may further support the use of statin therapy in T2D for cardiovascular protection. Interestingly, when other medications were added to the multiple linear regression model, only statin therapy predicted a reduced sympathetic transduction to BP. Nevertheless, it is important to consider that statins may exert negative metabolic actions, which could further impair glycemic control in some T2D patients (34, 35). A more complete understanding of the mechanisms by which statin therapy reduces the exaggerated sympathetic transduction in T2D would provide valuable information regarding the mechanism(s) by which these patients develop an exaggerated BP reactivity and may inform the development of future therapeutic approaches for mitigating cardiovascular risk in T2D.

In summary, we demonstrate for the first time that T2D patients exhibited an augmented sympathetic transduction to BP that appears to be offset by statin therapy.

Novelty and Significance.

What is New?

We demonstrate for the first time that patients with Type 2 Diabetes exhibit augmented blood pressure responses to spontaneous bursts of muscle sympathetic nerve activity, and this effect appears to be offset by statin therapy.

What is Relevant?

This is relevant because an augmented sympathetic transduction to blood pressure may represent a potential mechanism by which Type 2 Diabetes patients experience greater blood pressure excursions throughout the day, and increase their cardiovascular risk profile.

Summary

Collectively, our findings demonstrate an augmented sympathetic transduction to blood pressure in patients with Type 2 Diabetes, an effect that appears to be offset by statin therapy. These findings elucidate a potential source for the greater daily blood pressure excursions reported in Type 2 Diabetes Patients, which contributes to the augmented cardiovascular risk in this population, and also, highlight a possible cardiovascular benefit of statin therapy in Type 2 Diabetes.