Effects of aging and exercise training on skeletal muscle blood flow and resistance artery morphology

Abstract

With old age, blood flow to the high-oxidative red skeletal muscle is reduced and blood flow to the low-oxidative white muscle is elevated during exercise. Changes in the number of feed arteries perforating the muscle are thought to contribute to this altered hyperemic response during exercise. We tested the hypothesis that exercise training would ameliorate age-related differences in blood flow during exercise and feed artery structure in skeletal muscle. Young (6–7 mo old, n = 36) and old (24 mo old, n = 25) male Fischer 344 rats were divided into young sedentary (Sed), old Sed, young exercise-trained (ET), and old ET groups, where training consisted of 10–12 wk of treadmill exercise. In Sed and ET rats, blood flow to the red and white portions of the gastrocnemius muscle (Gast_Red and Gast_White) and the number and luminal cross-sectional area (CSA) of all feed arteries perforating the muscle were measured at rest and during exercise. In the old ET group, blood flow was greater to Gast_Red (264 ± 13 and 195 ± 9 ml·min⁻¹·100 g⁻¹ in old ET and old Sed, respectively) and lower to Gast_White (78 ± 5 and 120 ± 6 ml·min⁻¹·100 g⁻¹ in old ET and old Sed, respectively) than in the old Sed group. There was no difference in the number of feed arteries between the old ET and old Sed group, although the CSA of feed arteries from old ET rats was larger. In young ET rats, there was an increase in the number of feed arteries perforating the muscle. Exercise training mitigated old age-associated differences in blood flow during exercise within gastrocnemius muscle. However, training-induced adaptations in resistance artery morphology differed between young (increase in feed artery number) and old (increase in artery CSA) animals. The altered blood flow pattern induced by exercise training with old age would improve the local matching of O₂ delivery to consumption within the skeletal muscle.

aging is associated with a decline in maximal aerobic and exercise capacity (32, 46, 49, 54), resulting in a reduced capability for regular physical activity, which increases the risk of chronic disease. Age-associated alterations in exercising muscle blood flow and the resultant mismatching of O₂ delivery to O₂ consumption (6, 7, 11, 26) represent a key mechanism for this age-related functional decline. During submaximal exercise, blood flow to the working limbs is reduced in old vs. young subjects (4, 20, 37, 55, 67) and distribution of blood flow is altered among and within skeletal muscles (44). For example, in the gastrocnemius muscle, there is relative underperfusion of the high-oxidative red portion (Gast_Red) and overperfusion of the low-oxidative white portion (Gast_White) in old rats during exercise (44) due, in part, to old age-associated arterial rarefaction (8) and a diminished nitric oxide (NO) contribution to vasomotor control in highly oxidative muscle during exercise (28). Therefore, aging disrupts the normally tight coupling between O₂ delivery and the oxidative capacity of muscle during exercise. The altered blood flow pattern during exercise cannot be explained by differences in relative workloads (44) but is likely due to fiber type-specific alterations in vascular function (40, 41) and changes in resistance artery geometry (8) that occur with advancing age.

Exercise training induces peripheral vascular alterations that result in enhanced blood flow capacity due, in part, to intrinsic changes in the vascular endothelium (13, 17, 62–64). Furthermore, in young animals, endurance training alters the perfusion patterns among and within skeletal muscle during exercise (2), with high-oxidative motor units receiving a higher blood flow and low-oxidative muscles receiving a lower blood flow than in sedentary (i.e., untrained) rats. Given the similarities of skeletal muscle blood flow patterns during exercise in the sedentary vs. trained condition to that of the old vs. young, it is possible that exercise training normalizes exercising blood flow distribution in skeletal muscle with old age. In aged humans, endurance training enhances aerobic capacity (3, 9) and is associated with a substantial increase in exercising leg peak blood flow (4). Furthermore, endurance exercise training improves the matching of O₂ delivery to O₂ utilization in young (27) and aged subjects (38, 43). Whether this improved matching is due to functional vascular adaptations or involves structural alterations of resistance arteries is unknown.

Therefore, the purpose of this investigation was to test the hypotheses that 1) the pattern of blood flow distribution for greater O₂ delivery to low-oxidative muscle and lower perfusion of high-oxidative muscle with old age can be reversed with chronic aerobic exercise training and 2) endurance exercise training can increase the number of feed arteries perforating the gastrocnemius muscle, counteracting the arterial rarefaction observed with old age (8). To test these hypotheses, two studies were undertaken in young and old, sedentary and endurance-trained animals. The first study, which measured blood flow at rest and during exercise in the different portions of the gastrocnemius muscle, i.e., high-oxidative red (Gast_Red: 51% type I, 35% type IIA, and 13% type IIX fibers), mixed (Gast_Mixed: 3% type I, 6% type IIA, 34% type IIX, and 57% type IIB fibers), and low-oxidative white (Gast_White: 8% type IIX and 92% type IIB fibers) portions (14), demonstrated the less precise matching of O₂ delivery to muscle oxidative capacity with old age during exercise (44). The second study quantified the number and cross-sectional area (CSA) of all feed arteries perforating Gast_Red, Gast_Mixed, and Gast_White.

METHODS

All procedures were approved by the Institutional Animal Care and Use Committees at West Virginia University, Texas A & M University, and the University of Florida. All methods complied fully with the National Institutes of Health Guide for the Care and Use of Laboratory Animals (revised 1996). Male Fischer 344 rats were obtained at 4–5 and 22 mo of age to represent young (n = 36) and old (n = 25) animals, respectively. The rats were housed individually at 23°C and maintained on a 12:12-h light-dark cycle, with rat chow and water provided ad libitum. This strain was chosen because cardiovascular function decreases with age without the development of hypertension (33).

Endurance exercise training.

Young and old rats were randomly assigned to a sedentary (Sed) control group [young Sed (n = 20) and old Sed (n = 14)] and an exercise-trained (ET) group [young ET (n = 16) and old ET (n = 11)]. ET rats were habituated to treadmill exercise, during which each rat walked on a motor-driven treadmill at 15 m/min (0° incline) 5 min/day for 3 days. After the habituation period, the incline was raised to 15° for the duration of the training period while the 15 m/min speed was maintained. During the first 5 wk of training, the time of exercise was increased by 10 min/wk, until 60-min duration was reached by the 6th week. The ET rats continued to exercise 5 days/wk for 60 min/day for the remainder of the 10- to 12-wk training period.

Study 1: blood flow at rest and during exercise.

Blood flow to the gastrocnemius muscle in young Sed (n = 13), young ET (n = 9), old Sed (n = 7), and old ET (n = 5) animals at rest and during exercise was determined using the radionuclide-tagged microsphere technique, as previously described (12, 34, 44). Prior to the surgical procedure, Sed animals were familiarized with treadmill running. During the familiarization period (<2 wk), animals exercised 10 min/day at 15 m/min at a 15° incline two to three times per week. At ≥24 h after the last exercise bout during the familiarization or training period, animals were anesthetized with 2.5% isoflurane-balance O₂, and a catheter (Silastic, Dow Corning; 0.6 mm ID, 1.0 mm OD) filled with heparinized saline solution (Elkins-Sinn; 100 U/ml) was advanced into the ascending aorta via the right carotid artery. This catheter was used to infuse radiolabeled microspheres for tissue blood flow measurements and to monitor mean arterial pressure. The carotid catheter was externalized at the base of the neck and secured to the skin between the shoulder blades. A second polyurethane catheter (Braintree Scientific; 0.36 mm ID, 0.84 mm OD) was implanted in the caudal tail artery and externalized at the tail. This catheter was used to obtain a reference blood sample, which serves as an artificial organ for calculating tissue flows. After closure of the incisions, the animals were given ≥4 h to recover, as previous studies demonstrated that circulatory dynamics, regional blood flow, arterial blood gases, and acid-base status are stable in the awake rat 1–6 h after gas anesthesia (24).

After the recovery period, the rat was placed on the treadmill, and the tail artery catheter was connected to a 1-ml plastic syringe that was connected to an infusion-withdrawal pump (model 907, Harvard, Cambridge, MA), and the carotid artery catheter was connected to a blood pressure transducer (model MLT844, ADInstruments, Colorado Springs, CO). Exercise was initiated (15 m/min) at a 0° incline to ensure that all animals ran continuously during the testing period without changing gait patterns (e.g., stopping and starting running on the treadmill). After 4 min of total exercise time, blood withdrawal from the caudal artery at 0.25 ml/min was begun. The right carotid artery catheter was disconnected from the pressure transducer, and a distinct radiolabeled (⁴⁶Sc, ¹¹³Sn, ⁵⁷Co, or ⁸⁵Sr) microsphere (15 μm diameter; DuPont/NEN, Boston, MA) was infused (∼2.5 × 10⁵ in number) into the ascending aorta, which was flushed with warmed saline to ensure clearance of the beads. Blood withdrawal from the caudal artery continued for 45 s after microsphere infusion. After a 30-min recovery period, a second microsphere infusion was performed in conscious, standing animals following the procedures described above. This strategy was utilized to minimize the preexercise anticipatory response (1) and facilitates an accurate “resting” measurement. After the microsphere infusion, animals were euthanized with pentobarbital sodium (>100 mg/kg ip). The gastrocnemius muscle and kidneys were removed, and the gastrocnemius muscle was sectioned into the Gast_Red, Gast_Mixed, and Gast_White portions (14). The radioactivity level of the tissues was determined by a gamma scintillation counter (Cobra II Auto Gamma Counter, Packard, Downers Grove, IL) set to record the peak energy activity of each isotope for 1–5 min. Total blood flow to each tissue was calculated by the reference sample method (34, 44) and expressed in milliliters per minute per 100 g of tissue. To account for possible changes in perfusion due to alterations in arterial pressure, vascular conductance was calculated (i.e., blood flow/mean arterial pressure) and expressed in milliliters per minute per 100 g of tissue per mmHg. Only data for animals that displayed adequate mixing of the microspheres, as verified by a <20% difference in blood flow between the right and left kidney, were analyzed and reported.

Study 2: vascular morphology.

The number of feed arteries perforating the gastrocnemius muscle and the vessel CSA (i.e., πr², where r is radius) were determined in young Sed (n = 7), young ET (n = 7), old Sed (n = 7), and old ET (n = 6) animals. After the animals were euthanized with pentobarbital sodium (>100 mg/kg ip), the plantaris-gastrocnemius muscle group was carefully dissected free and placed in a 4°C filtered Ca²⁺-free physiological saline solution buffer (in mM: 147 NaCl, 4.7 KCl, 1.2 NaH₂PO₂, 1.17 MgSO₄, 5.0 glucose, 2.0 pyruvate, and 3.0 MOPS, pH 7.4). The feed arteries perforating the Gast_Red, Gast_Mixed, and Gast_White were identified and isolated using a stereomicroscope, removed from the muscle, and transferred to a Lucite chamber. Subsequently, both ends of the feed artery were cannulated with glass micropipettes filled with the filtered Ca²⁺-free physiological saline solution and secured to the pipettes with 11-0 ophthalmic suture. After cannulation, each vessel in the tissue chamber was transferred to the stage of an inverted microscope (model IX70, Olympus) coupled to a video camera (model BP310, Panasonic) and video caliper (Microcirculation Research Institute, Texas A & M University). Intraluminal pressure in the isolated artery was set at 75 cmH₂O, and 10⁻⁴ M sodium nitroprusside was added to the vessel chamber to prevent development of spontaneous tone and ensure maximal dilation. Leaks were detected by pressurizing the vessel and then closing the valves to the reservoirs and verifying that intraluminal diameter remained constant. Vessels free of leaks equilibrated for ≥1 h at 37°C, with the bathing solution replaced every 20 min during this period, before maximal diameter was measured for calculation of CSA.

Muscle oxidative enzyme activity.

Citrate synthase, a mitochondrial enzyme and marker of muscle oxidative potential, was measured in duplicate from Gast_White muscle homogenates, according to the method of Srere (65). Citrate synthase activity, expressed as micromoles per minute per gram wet weight, was measured spectrophotometrically using a Spectramax M5 microplate (Molecular Devices, Sunnyvale, CA) in 300-μl aliquots at 30°C to determine the efficacy of the training protocol.

Statistical analysis.

One-way ANOVA was used to determine differences in the number and CSA of feed arteries, body and muscle mass, and citrate synthase activity. A two-way ANOVA with repeated-measures design was used to compare within-group (resting and exercising) and between-group (young vs. old and Sed vs. ET) differences in arterial pressure, blood flow, and vascular conductance. When a significant F ratio was demonstrated, a Student-Newman-Keuls post hoc test was performed to determine differences between mean values. Where established literature values clearly justified a directional hypothesis [i.e., augmented flow to Gast_Red and diminished flow to Gast_White after training (2)], a one-tailed test was used. All values are means ± SE. P ≤ 0.05 was required for significance.

RESULTS

Body mass, muscle mass, citrate synthase activity, and arterial pressure.

Body mass, gastrocnemius mass, gastrocnemius-to-body weight ratio, and mean arterial pressure are described in Table 1. The efficacy of the training program was confirmed with an increase of ∼40% in citrate synthase activity of the Gast_White in young and old groups (Table 1).

Table 1.

Mean body mass, muscle characteristics, white gastrocnemius muscle citrate synthase activity, and mean arterial pressure responses of young and old sedentary and exercise-trained rats

	Sedentary		Exercise-Trained
	Young (n = 20)	Old (n = 14)	Young (n = 16)	Old (n = 11)
Body wt, g	374 ± 9	436 ± 7*	354 ± 3†	385 ± 6*†
Gastrocnemius wt, g	1.55 ± 0.06	1.52 ± 0.05	1.60 ± 0.04†	1.59 ± 0.03†
Gastrocnemius wt-to-body wt ratio, mg/g	4.16 ± 0.20	3.50 ± 0.15*	4.53 ± 0.14†	4.14 ± 0.13*†
Citrate synthase activity, μmol · min−1 · g wet wt−1	15.8 ± 1.5	15.0 ± 1.4	22.5 ± 1.5†	22.3 ± 1.6†
MAP, mmHg
Rest	135 ± 6	129 ± 5	138 ± 4	125 ± 6*
Exercise	156 ± 5‡	149 ± 4‡	159 ± 4‡	148 ± 3*‡

Values are means ± SE; n, number of rats. MAP, mean arterial pressure.

^*P < 0.05 vs. corresponding young group.

^†P < 0.05 vs. age-matched sedentary group.

^‡P < 0.05 vs. rest.

Blood flow at rest and during exercise.

There were no differences in blood flow to any portions of the gastrocnemius muscle among groups at rest (Fig. 1). Exercising hyperemia was lower in the Gast_Red (Fig. 2A) and higher in the Gast_White (Fig. 2C) of old Sed than young Sed rats. Vascular conductance during exercise paralleled blood flow (Fig. 3). Blood flow (Fig. 2B) and vascular conductance (Fig. 3B) during exercise were also higher in the Gast_Mixed of old Sed than young Sed animals. Exercise training of old rats resulted in an ∼36% increase in blood flow to the Gast_Red and an ∼35% decrease in perfusion to the Gast_White (Figs. 2 and 4). In the young group, there was a lower flow (∼20% decrease) to the Gast_White after training. Although there was ∼15% higher absolute exercising blood flow to the Gast_Red in young ET than young Sed animals (Fig. 4), it did not reach statistical significance (P = 0.055). Exercise training abolished age-associated differences in blood flow to the Gast_White (Fig. 2C). Exercise training did not affect exercising blood flow to the Gast_Mixed within age groups (Fig. 4).

Fig. 1.

Resting blood flow in red gastrocnemius (Gast_Red, A), mixed gastrocnemius (Gast_Mixed, B), and white gastrocnemius (Gast_White, C) of young and old sedentary (Sed) and exercise-trained (ET) rats.

Fig. 2.

Blood flow measured during steady state of exercise in Gast_Red (A), Gast_Mixed (B), and Gast_White (C) of young and old Sed and ET rats. *P < 0.05 vs. Sed of corresponding age group. †P < 0.05 vs. young group in the same condition. #P = 0.055 vs. young Sed.

Fig. 3.

Vascular conductance calculated during steady state of exercise in Gast_Red (A), Gast_Mixed (B), and Gast_White (C) of young and old Sed and ET rats. *P < 0.05 vs. Sed of corresponding age group. †P < 0.05 vs. young group in the same condition. #P = 0.055 vs. young Sed.

Fig. 4.

Percent differences in exercising muscle blood flow between Sed and ET young and old groups. Percent difference was calculated as follows: [(blood flow ET − blood flow Sed)/blood flow Sed] × 100.

Vascular morphology.

There were 1.3 fewer total feed arteries perforated the gastrocnemius muscle of old Sed than young Sed rats (Fig. 5), with fewer arteries perforating the highly oxidative Gast_Red and Gast_Mixed regions (Fig. 6B). Exercise training increased the total number of feed arteries perforating the gastrocnemius muscle in young ET vs. young Sed rats. Conversely, there was no change in the total number of arteries perforating the gastrocnemius muscle of the old ET rats (Fig. 5), resulting in an increased disparity in the number of resistance arteries between the two age groups.

Fig. 5.

Average number of feed arteries and average cross-sectional area (CSA) of feed arteries perforating gastrocnemius muscle from young and old Sed and ET (Ex-trained) animals. *P < 0.05 vs. Sed of corresponding age group. †P < 0.05 vs. young group in the same condition.

Fig. 6.

Average number of feed arteries and average CSA of feed arteries perforating Gast_Red (A), Gast_Mixed (B), and Gast_White (C) from young and old Sed and ET animals. *P < 0.05 vs. Sed of corresponding age group. †P < 0.05 vs. young group in the same condition. #P = 0.08 vs. young Sed.

Average CSA of feed arteries was greater in the Gast_Red and Gast_White in old than young Sed rats (Fig. 6, A and C). In old rats, exercise training increased the average CSA of feed arteries from the Gast_Red and Gast_White by 20% and 35%, respectively, relative to that in old Sed animals (Fig. 6, A and C). In the young group, exercise training did not result in an increase in CSA in arteries perfusing any muscle section, although there was a tendency (P = 0.08) for an increase in the Gast_Red.

DISCUSSION

With advancing age, there is a redistribution of skeletal muscle blood flow during exercise, such that high-oxidative regions of a muscle (e.g., Gast_Red) are relatively underperfused and low-oxidative portions (e.g., Gast_White) are relatively overperfused vs. younger animals (44). In addition to alterations in vasomotor function (13, 17, 18, 40, 41, 62–64), an old age-associated arterial rarefaction in high-oxidative muscles is thought to be a key mechanism for this redistribution of blood flow during exercise (8). Therefore, the primary purpose of this investigation was to determine whether endurance training could mitigate the old age-associated redistribution of skeletal muscle blood flow during exercise. The data demonstrate that exercise training in old animals enhances the matching of O₂ delivery to muscle oxidative capacity by increasing vascular conductance and blood flow to the Gast_Red and decreasing vascular conductance and perfusion of the Gast_White with no change in the Gast_Mixed (Figs. 2 and 3). The secondary purpose of this study was to investigate how exercise training affected the number and CSA of feed arteries perforating the gastrocnemius muscle. In young rats, there was an increase in the total number of arteries perforating the gastrocnemius muscle but no change in average CSA. Conversely, with old age, there was no increase in the number of perforating arterial vessels following training but an increase in feed artery CSA (Figs. 5). The training effect in the young rats to increase feed artery number occurred in the Gast_Mixed, while the training effect to increase CSA in feed arteries from the old rats occurred in the Gast_Red and Gast_White.

Muscle blood flow with aging and exercise training.

Skeletal muscle blood flow capacity was reduced with old age (20, 37, 56, 67). During submaximal exercise, leg blood flow is lower in sedentary older adults than younger individuals (4, 52). Even when whole limb blood flow during submaximal exercise is similar between young and old rats, the distribution profile within and among muscles is affected by age. Specifically, Musch et al. (44) demonstrated a significant blood flow reduction in high-oxidative muscles and muscle sections concomitant with an increased flow in the low-oxidative glycolytic muscles during submaximal exercise. Therefore, the available evidence demonstrates not only a reduction in limb blood flow during exercise in older adults but also an uncoupling of the typically precise matching of O₂ delivery to O₂ requirements in different sections of the working muscle, both of which would contribute to premature fatigue in the aged individual.

After exercise training, there is a substantial increase in submaximal leg blood flow in older men (4), even though exercising flow is still lower in endurance-trained older than younger men (56, 67). In the latter studies, although muscle blood flow was still lower in older than younger subjects posttraining, there is likely a more precise matching of O₂ delivery to O₂ requirements in the active muscle, as fractional extraction is increased in the older trained vs. sedentary subjects (39, 56). In the current study, after endurance training, there was an ∼36% increase in exercising blood flow to the Gast_Red (predominantly type I and IIA fibers) in the old group (Fig. 4). Furthermore, there was a significant reduction in exercising blood flow to the Gast_White after training in the old group (Fig. 4). Although our data do not reveal the precise mechanism(s) for the altered blood flow pattern after exercise training, it is likely due to a combination of alterations in vascular function (see below) and fiber recruitment patterns. With respect to the latter, the slightly higher relative work intensity in the old Sed group [as discussed by Musch et al. (44)] would theoretically result in a higher flow to the Gast_Red and Gast_White on the basis of the higher relative work rate (34). However, there is a decreased flow to the Gast_Red and a substantial increase in flow to the Gast_White with aging during exercise relative to the younger counterparts. Therefore, in the sedentary condition with old age, a diminished blood flow to the Gast_Red during exercise may force a greater reliance on the recruitment of muscle fibers from the Gast_White to sustain locomotion. However, after exercise training, the distribution pattern of blood flow during exercise is similar with age. This suggests that the elevated exercising blood flow to the Gast_Red with old age after training, through alterations in vascular function and/or structure, may result in a better matching of O₂ delivery to O₂ consumption in this muscle section and diminish the need to recruit the Gast_White after training (which is supported through the reduced exercising flow to this section in old ET vs. old Sed; Fig. 2C).

Functional and structural vascular modifications with age and exercise training.

Aging is associated with decrements in skeletal muscle endothelial function (13, 19, 31, 41, 61, 63) that can be improved with aerobic exercise training (22, 38, 47, 60, 62–64, 66). These training-induced adaptations in endothelial function with old age are due, in part, to an increased NO bioavailability (27, 62–64). Therefore, functional alterations in vasomotor regulation after exercise training appear to favor vasodilation vs. vasoconstriction. In the old group, these functional changes and the increase in resistance artery CSA (Fig. 6) likely contributed to the increased vascular conductance and hyperemia that occur in high-oxidative muscle such as the Gast_Red (Figs. 2A and 3A) after training. Conversely, in the Gast_White, the hyperemic response indicates an enhanced constriction of the resistance vasculature with age after exercise training. However, the literature does not support an upregulation of vasoconstrictor pathways in this muscle, as endurance training does not increase α-adrenergic-induced (17), endothelin-1-induced (18), or angiotensin II-induced (48) vasoconstriction. However, similar to previous findings in the coronary circulation (42), we have found an enhanced myogenic vasoconstriction in the Gast_White resistance arteries after training in old rats (unpublished observation), which, along with a putative reduced recruitment of type IIB fibers, could contribute to the reduced hyperemic response during exercise.

In old rats, there is a structural remodeling of the resistance vasculature (e.g., arterial rarefaction) in skeletal muscle (8). It appears that the old age-related decrements in physical activity and, thus, attenuated muscular activation and blood flow contribute to this vascular remodeling (8). Specifically, a reduced physical activity with old age would diminish the daily hyperemic response (and associated shear stress) in resistance arteries and metabolite production in skeletal muscle. As endothelium-derived NO is a major mediator of vascular remodeling (58), a reduction in NO bioavailability with old age may destabilize vascular networks in oxidative muscle (53) and preferentially increase flow through one network and decreases flow through another. Over time, this would likely lead to a recession of some networks and an increase in luminal diameter of others. Exercise is one of the most powerful stimulants of capillary growth (i.e., angiogenesis) in active tissue, and if the exercise stimulus is chronic, an increase in the number of arteries (i.e., arteriogenesis) can occur (68). Therefore, we hypothesized that chronic exercise training and the subsequent increased NO bioavailability (22, 62, 63), together with similar angiogenic potential with old age (25, 57), would increase the number of arteries perforating skeletal muscle in the old group. Contrary to our hypothesis, there were no increases in the number of feed arteries in the old group (Fig. 5), whereas there was an increase in the young group. However, the old animals demonstrated an increase in luminal CSA after exercise training in the Gast_Red and Gast_White, whereas the young group exhibited a trend (P = 0.08) for a greater CSA in the Gast_Red with no change in the Gast_White (Fig. 6).

It is unclear why there are differential training-induced adaptations with age, but it may be related to differences in relative exercise intensity. According to Musch et al. (44), the heavier body mass of the old group would have resulted in these animals exercising at ∼10–20% higher relative intensity than the young group. With this higher relative intensity of exercise and fewer perforating arteries, the blood flow rate through the arteries would be greater in the old group, which is a likely contributing mechanism to the enhanced CSA observed with age and exercise training.

Skeletal muscle blood flow pattern with aging and exercise training.

In the current study, young and old groups demonstrated an elevated flow to the Gast_Red and a lower flow to the Gast_White during exercise after endurance training (Fig. 4), suggesting a training effect in both muscles. Although citrate synthase activity in the Gast_White from young and old groups was enhanced after training (Table 1), we did not measure citrate synthase activity of the Gast_Red. However, a similar training intensity enhances citrate synthase activity in the Gast_Red from young and aged animals (45), and the Gast_Red shows a robust hyperemia during exercise (Fig. 2A), which provides further evidence of recruitment and training in the Gast_Red of young and aged animals with this training protocol.

A paradox exists, in that the Gast_Red and Gast_White demonstrated significant increases in CSA after training with old age (Fig. 6, A and C), whereas the Gast_Red demonstrated an enhanced hyperemia and the Gast_White demonstrated a reduced hyperemia. On the basis of Poiseuille's law, a small change in vessel radius results in a large alteration in flow through a vessel or tube, if it is assumed that pressure gradient, vessel length, and blood viscosity are unaltered. Therefore, in the Gast_Red, even if there were no functional alterations in vasomotor responsiveness, an increase in feed artery radius of ∼7.5% via greater vasodilation or a structural expansion would result in an ∼35% increase in blood flow through a given vessel. Given that exercising blood flow in the Gast_Red from the old group after training was ∼35% greater, with an average radius increase of ∼10% in the feed arteries, the changes in vascular morphology could fully account for the increased flow. However, with aging in the Gast_White, a similar increase in feed artery radius was observed after training but was associated with a reduction in exercising blood flow vs. the sedentary condition. Therefore, changes in feed artery CSA after exercise training with age can be a poor predictor of skeletal muscle blood flow at these low intensities of exercise. Indeed, Laughlin and Roseguini (36) stated that training-induced increases in blood flow capacity are not mediated solely by structural adaptations, a conclusion that is supported by the findings from the current study. Previous studies demonstrated an old age-related endothelial dysfunction in skeletal muscle feed arteries (70, 71) and arterioles (13, 41) that would expectedly affect total muscle blood flow and intramuscular blood flow distribution, respectively (69). Therefore, the enhanced endothelial function in these vascular branches with aging after exercise training (63, 64, 66) likely works in concert with structural alterations to improve muscle perfusion during exercise. Whether similar alterations in large, conduit artery structure and function with aging and exercise training (15, 16, 23) affect downstream muscle perfusion during exercise remains to be determined.

Ramifications of altered muscle blood flow with age after exercise training.

Within skeletal muscle, there must be a coordinated balance in the rates of O₂ delivery to O₂ consumption, as an imbalance between these two variables will promote fatigue (50, 51). In skeletal muscle, with advancing age, there are significant impairments in capillary hemodynamics (e.g., decreased lineal density of capillaries supporting red blood cell flow) at rest (59) and during exercise (10). These altered capillary hemodynamics with old age contribute to an O₂ delivery-O₂ consumption mismatch at rest (6, 38), in response to an increased metabolic demand (6, 11, 21), as well as during the recovery from exercise (29). This O₂ delivery-O₂ consumption mismatch with old age, due, in part, to a blunted rate of vasodilation of upstream resistance arteries (5, 30), would reduce the functional capacity to perform daily activities (e.g., climbing stairs) in older adults. After exercise training with old age, there was a significant increase in blood flow to the Gast_Red, which, considering that this fiber type demonstrates the highest oxidative capacity of the hindlimb skeletal muscles (14), indicates a better matching of O₂ delivery to the metabolic characteristics of the muscle at these running speeds. Indeed, it has been demonstrated that there is a more precise matching of O₂ delivery to O₂ requirements after exercise training with old age (38, 43), which is likely a key mechanism for the increased exercise capacity in the aged individual after training.

Conclusions.

The current study examined the effects of aging and exercising training on resting and exercising blood flow to muscle sections composed of high-oxidative (Gast_Red), mixed (Gast_Mixed), and low-oxidative glycolytic (Gast_White) fiber types. Similar to the findings of Musch et al. (44) in untrained animals, we observed a reduced flow to the Gast_Red and an increased flow to the Gast_White in the old vs. young Sed group during exercise. This pattern of muscle perfusion was reversed after endurance training; i.e., there was a greater exercise hyperemia to the Gast_Red and a lower flow to the Gast_White of old rats. Unlike young animals, in old animals, exercise training did not result in an increase in the number of feed arteries to the muscle. Rather, there was an increase in feed artery CSA after training in the old group that did not occur in young rats. These findings suggest that training-induced increases in resistance artery CSA in old rats may contribute to the elevation of vascular conductance and blood flow capacity in the high-oxidative muscles, whereas the decrease in vascular conductance and exercise hyperemia in the low-oxidative muscles is not the result of structural vascular adaptations but is likely due to changes in the functional vasomotor properties of the resistance vasculature [enhanced vasodilatory responsiveness (63, 64) and myogenic autoregulation (unpublished observations)] and altered fiber type recruitment patterns. Collectively, structural and functional changes in the aging resistance vasculature that allow for a more precise matching of O₂ delivery to muscle oxidative capacity appear to be a key mechanism for the increased exercise capacity in aged individuals after exercise training.

GRANTS

This study was supported, in part, by National Aeronautics and Space Administration Grants NAG2-1340 and NCC2-1166 (M. D. Delp), National Institutes of Health Grants AG-31317 (B. J. Behnke) and R01 HL-077224 (J. M. Muller-Delp), Florida Biomedical Research Program Grant 1BN-02 (B. J. Behnke), and the Jane Adams Edmonds Endowed Doctoral Fellowship (J. M. Dominguez 2nd) and LaPradd Fellowship (D. J. McCullough and R. T. Davis 3rd) from the Department of Applied Physiology and Kinesiology at the University of Florida.

DISCLOSURES

No conflicts of interest, financial or otherwise, are declared by the authors.

AUTHOR CONTRIBUTIONS

B.J.B., J.M.M.-D., and M.D.D. are responsible for conception and design of the research; B.J.B., M.W.R., J.N.S., J.M.D., R.T.D., and D.J.M. performed the experiments; B.J.B. analyzed the data; B.J.B., M.W.R., J.N.S., and M.D.D. interpreted the results of the experiments; B.J.B. prepared the figures; B.J.B. and M.D.D. drafted the manuscript; B.J.B., M.W.R., J.N.S., J.M.D., R.T.D., D.J.M., J.M.M.-D., and M.D.D. edited and revised the manuscript; B.J.B., J.N.S., J.M.D., R.T.D., D.J.M., J.M.M.-D., and M.D.D. approved the final version of the manuscript.