Exercise training and α1-adrenoreceptor-mediated sympathetic vasoconstriction in resting and contracting skeletal muscle

Exercise training and vascular heterogeneity in db/db mice: evidence for regional- and duration-dependent effects

Exercise training (ET) has several health benefits; however, our understanding of regional adaptations to ET is limited. We examined the functional and molecular adaptations to short- and long-term ET in elastic and muscular conduit arteries of db/db mice in relation to changes in cardiovascular risk factors. Diabetic mice and their controls were exercised at moderate intensity for 4 or 8 weeks. The vasodilatory and contractile responses of thoracic aortae and femoral arteries isolated from the same animals were examined. Blood and aortic samples were used to measure hyperglycemia, oxidative stress, inflammation, dyslipidemia, protein expression of SOD isoforms, COX, eNOS, and Akt. Short-term ET improved nitric oxide (NO) mediated vasorelaxation in the aortae and femoral arteries of db/db mice in parallel with increased SOD2 and SOD3 expression, reduced oxidative stress and triglycerides, and independent of weight loss, glycemia, or inflammation. Long-term ET reduced body weight in parallel with reduced systemic inflammation and improved insulin sensitivity along with increased SOD1, Akt, and eNOS expression and improved NO vasorelaxation. Exercise did not restore NOS- and COX-independent vasodilatation in femoral arteries, nor did it mitigate the hypercontractility in the aortae of db/db mice; rather ET transiently increased contractility in association with upregulated COX-2. Long-term ET differentially affected the aortae and femoral arteries contractile responses. ET improved NO-mediated vasodilation in both arteries likely due to collective systemic effects. ET did not mitigate all diabetes-induced vasculopathies. Optimization of the ET regimen can help develop comprehensive management of type 2 diabetes.

Does sympathetic vasoconstriction contribute to metabolism: Perfusion matching in exercising skeletal muscle?

The process of matching skeletal muscle blood flow to metabolism is complex and multi-factorial. In response to exercise, increases in cardiac output, perfusion pressure and local vasodilation facilitate an intensity-dependent increase in muscle blood flow. Concomitantly, sympathetic nerve activity directed to both exercising and non-active muscles increases as a function of exercise intensity. Several studies have reported the presence of tonic sympathetic vasoconstriction in the vasculature of exercising muscle at the onset of exercise that persists through prolonged exercise bouts, though it is blunted in an exercise-intensity dependent manner (functional sympatholysis). The collective evidence has resulted in the current dogma that vasoactive molecules released from skeletal muscle, the vascular endothelium, and possibly red blood cells produce local vasodilation, while sympathetic vasoconstriction restrains vasodilation to direct blood flow to the most metabolically active muscles/fibers. Vascular smooth muscle is assumed to integrate a host of vasoactive signals resulting in a precise matching of muscle blood flow to metabolism. Unfortunately, a critical review of the available literature reveals that published studies have largely focused on bulk blood flow and existing experimental approaches with limited ability to reveal the matching of perfusion with metabolism, particularly between and within muscles. This paper will review our current understanding of the regulation of sympathetic vasoconstriction in contracting skeletal muscle and highlight areas where further investigation is necessary.

Sympathetic vasoconstriction in skeletal muscle: Modulatory effects of aging, exercise training, and sex

The sympathetic nervous system (SNS) is a critically important regulator of the cardiovascular system. The SNS controls cardiac output and its distribution, as well as peripheral vascular resistance and blood pressure at rest and during exercise. Aging is associated with increased blood pressure and decreased skeletal muscle blood flow at rest and in response to exercise. The mechanisms responsible for the blunted skeletal muscle blood flow response to dynamic exercise with aging have not been fully elucidated; however, increased muscle sympathetic nerve activity (MSNA), elevated vascular resistance and a decline in endothelium-dependent vasodilation are commonly reported in older adults. In contrast to aging, exercise training has been shown to reduce blood pressure and enhance skeletal muscle vascular function. Exercise training has been shown to enhance nitric oxide-dependent vascular function and may improve the vasodilatory capacity of the skeletal muscle vasculature; however, surprisingly little is known about the effect of exercise training on the neural control of circulation. The control of blood pressure and skeletal muscle blood flow also differs between males and females. Blood pressure and MSNA appear to be lower in young females compared to males. However, females experience a larger increase in MSNA with aging compared to males. The mechanism(s) for the altered SNS control of vascular function in females remain to be determined. Novelty: • This review will summarize our current understanding of the effects of aging, exercise training and sex on sympathetic vasoconstriction at rest and during exercise. • Areas where additional research is needed are also identified.

Effects of sex and exercise training on β-adrenoreceptor-mediated opposition of evoked sympathetic vasoconstriction in resting and contracting muscle of rats

This study investigated the hypothesis that β-adrenoreceptor-mediated inhibition of sympathetic vasoconstriction would be enhanced in female compared with male rats, and that endurance exercise training would augment β-adrenoreceptor-mediated inhibition of sympathetic vasoconstriction in male and female rats. Sprague-Dawley rats were randomized into sedentary (male: n = 7; female: n = 8) and exercise-trained (male: n = 9; female: n = 9) groups. Following 4 wk of exercise training or being sedentary, rats were anesthetized and surgically instrumented for stimulation of the lumbar sympathetic chain, muscle contraction and measurement of arterial blood pressure and femoral artery blood flow (FBF). Femoral vascular conductance (FVC) was calculated as FBF/mean arterial pressure. The percentage change of FVC in response to sympathetic stimulation delivered at 2 and 5 Hz was measured at rest and during contraction of the triceps surae muscles before and after β-adrenoreceptor blockade (propranolol: 0.075 mg·kg^-1 iv). We found that, at rest, β-adrenoreceptor blockade decreased (main effect of drug, 2 Hz: P < 0.001; 5 Hz: P < 0.001) sympathetic vasoconstriction. During contraction, sympathetic vasoconstrictor responsiveness was lower (main effect of sex, 2 Hz: P = 0.001; 5 Hz: P = 0.023) in female compared with male rats, and sympatholysis was enhanced (main effect of sex, 2 Hz: P = 0.001; 5 Hz: P < 0.001) in female rats. β-adrenoreceptor blockade decreased (main effect of drug, 2 Hz: P = 0.049; 5 Hz: P < 0.001) evoked sympathetic vasoconstriction in contracting muscle. The present study demonstrated that β-adrenoreceptors do not blunt sympathetic vasoconstriction in resting or contracting skeletal muscle of male or female rats. Sympatholysis was enhanced in female rats; however, this was not attributable to β-adrenoreceptor-mediated blunting of sympathetic vasoconstriction.NEW & NOTEWORTHY β-adrenoreceptors do not inhibit sympathetic vasoconstriction in resting or contracting muscle of male or female rats, regardless of training status. Sympatholysis was enhanced in female, compared to male rats; however, β-adrenoreceptors were not responsible for the enhanced sympatholysis. These findings indicate that β-adrenoreceptors do not contribute to the regulation of sympathetic vasoconstriction in resting and contracting skeletal muscle and suggest that β-adrenoreceptors do not underlie sex differences in the neural control of the circulation.

β-Adrenoreceptors do not oppose sympathetic vasoconstriction in resting and contracting skeletal muscle of male rats

Sympathetic nervous system (SNS) vasoconstriction is primarily achieved through the binding of norepinephrine (NE) to α-adrenoreceptors. However, NE may also bind to β-adrenoreceptors and cause vasodilation that may oppose/blunt SNS-mediated vasoconstriction. Therefore, this study investigated the hypothesis that β-adrenoreceptor–mediated vasodilation opposes evoked vasoconstriction in resting and contracting skeletal muscle. Male (n = 9) Sprague–Dawley rats were anesthetized and surgically instrumented for stimulation of the lumbar sympathetic chain and measurement of arterial blood pressure and femoral artery blood flow. The percentage change of femoral vascular conductance in response to sympathetic chain stimulation delivered at 2 and 5 Hz was determined at rest and during triceps surae skeletal muscle contraction before (control) and after β-adrenoreceptor blockade (propranolol; 0.075 mg·kg−1, intravenously). β-Adrenoreceptor blockade did not alter (P > 0.05) baseline hemodynamics or the hyperemic response to exercise. At the 2 Hz stimulation frequency, sympathetic vasoconstriction was similar (P > 0.05) in control and β-blockade conditions in resting (control, −34% ± 6%; β-blockade, −33% ± 8%) and contracting (control, −16% ± 6%; β-blockade, −14% ± 7%) muscle. At the 5 Hz stimulation frequency, sympathetic vasoconstrictor responsiveness was reduced (main effect of drug, P < 0.05) following β-blockade (rest: control, −52% ± 7%; β-blockade, −51% ± 9%; contraction: control, −32% ± 11%; β-blockade, −29% ± 13%).

Novelty These data indicate that β-adrenoreceptor blockade did not augment sympathetic vasoconstriction at rest or during exercise. The study demonstrates that β-adrenoreceptors do not oppose evoked sympathetic vasoconstriction in resting or contracting skeletal muscle or contribute to functional sympatholysis.

Evaluating the NIRS-derived microvascular O2 extraction "reserve" in groups varying in sex and training status using leg blood flow occlusions

It has been demonstrated that the plateau in the near-infrared spectroscopy (NIRS) derived deoxygenated hemoglobin and myoglobin (deoxy[Hb+Mb]) signal (i.e., deoxy[Hb+Mb]PLATEAU) towards the end of a ramp-incremental (RI) test does not represent the upper-limit in O2 extraction of the vastus lateralis (VL) muscle, given that an O2 extraction reserve has been recently observed. This study aimed to investigate whether this O2 extraction reserve was present in various populations and whether it exhibited sex- and/or training- related differences.Sixteen men- 8 untrained (27±5 years; 83±11 kg; 179±9 cm), 8 trained (27±4 years; 82±10 kg; 182±8 cm) and 9 trained women (27±2 years; 66±10 kg; 172±6 cm) performed a RI cycling test to exhaustion. The NIRS-derived deoxy[Hb+Mb] signal was measured continuously on the VL as a proxy for O2 extraction. A leg blood flow occlusion (i.e., ischemia) was performed at rest (LBFOCC 1) and immediately post the RI test (LBFOCC 2).No significant difference was found between the deoxy[Hb+Mb] amplitude during LBFOCC 1 and the deoxy[Hb+Mb]PLATEAU (p>0.05) nor between baseline (bsln) deoxy[Hb+Mb] values. deoxy[Hb+Mb] amplitude during LBFOCC 2 was significantly greater than LBFOCC 1 and at deoxy[Hb+Mb]PLATEAU (p<0.05) with group means ~30-45% higher than the deoxy[Hb+Mb]PLATEAU and LBFOCC 1 (p<0.05). No significant differences were found between groups in O2 extraction reserve, regardless of sex- or training-statusThe results of this study demonstrated the existence of an O2 extraction reserve in different populations, and that neither sex- nor training-related differences affect the amplitude of the reserve.

Vascular characteristics in young women-Effect of extensive endurance training or a sedentary lifestyle

AIM

To explore whether high-level endurance training in early age has an influence on the arterial wall properties in young women.

METHODS

Forty-seven athletes (ATH) and 52 controls (CTR), all 17-25 years of age, were further divided into runners (RUN), whole-body endurance athletes (WBA), sedentary controls (SC) and normally active controls (AC). Two-dimensional ultrasound scanning of the carotid arteries was conducted to determine local common carotid artery (CCA) geometry and wall distensibility. Pulse waves were recorded with a tonometer to determine regional pulse wave velocity (PWV) and pulse pressure waveform.

RESULTS

Carotid-radial PWV was lower in WBA than in RUN (P < .05), indicating higher arterial distensibility along the arm. Mean arterial pressure was lower in ATH than in CTR and in RUN than in WBA (P < .05). Synthesized aortic augmentation index (AI@75) was lower among ATH than among CTR (-12.8 ± 1.6 vs -2.6 ± 1.2%, P < .001) and in WBA than in RUN (-16.4 ± 2.5 vs -10.7 ± 2.0%, P < .05), suggesting a diminished return of reflection waves to the aorta during systole. Carotid-femoral PWV and intima-media thickness (IMT), lumen diameter and radial distensibility of the CCA were similar in ATH and CTR.

CONCLUSION

Elastic artery distensibility and carotid artery IMT are not different in young women with extensive endurance training over several years and in those with sedentary lifestyle. On the other hand, our data suggest that long-term endurance training is associated with potentially favourable peripheral artery adaptation, especially in sports where upper body work is added. This adaptation, if persisting later in life, could contribute to lower cardiovascular risk.

Sex differences in sympathetic vasoconstrictor responsiveness and sympatholysis

Sex differences in the neurovascular control of blood pressure and vascular resistance have been reported. However, the mechanisms underlying the modulatory influence of sex have not been fully elucidated. Nitric oxide (NO) has been shown to inhibit sympathetic vasoconstriction in resting and contracting skeletal muscle, and estrogen modulates NO synthase (NOS) expression and NO bioavailability. Therefore NO-mediated inhibition of sympathetic vasoconstriction may be enhanced in females. Thus the purpose of the present study was to investigate the hypothesis that sympathetic vasoconstrictor responsiveness would be blunted and NO-mediated inhibition of sympathetic vasoconstriction would be enhanced in females compared with males. Male (M; n = 8) and female (F; n = 10) Sprague-Dawley rats were anesthetized and surgically instrumented for measurement of arterial blood pressure and femoral artery blood flow and stimulation of the lumbar sympathetic chain. The percentage change of femoral vascular conductance in response to sympathetic chain stimulation delivered at 2 and 5 Hz was determined at rest and during triceps surae muscle contraction before (control) and after NOS blockade [N^ω-nitro-l-arginine methyl ester (l-NAME), 10 mg/kg iv]. At rest, sympathetic vasoconstrictor responsiveness was augmented (P < 0.05) in female compared with male rats at 2 Hz [F: -33 ± 8% (SD); M: -26 ± 6%] but was not different at 5 Hz (F: -55 ± 7%; M: -47 ± 7%). During muscle contraction, evoked vasoconstriction was similar (P > 0.05) in females and males at 2 Hz (F: -12 ± 5%; M: -13 ± 5%) but was blunted (P < 0.05) in females compared with males at 5 Hz (F: -24 ± 5%; M: -34 ± 8%). l-NAME increased (P < 0.05) sympathetic vasoconstrictor responsiveness in both groups at rest and during contraction. Contraction-mediated inhibition of vasoconstriction (sympatholysis) was enhanced (P < 0.05) in females compared with males; however, sympatholysis was not different (P > 0.05) between males and females in the presence of NOS blockade, indicating that NO-mediated sympatholysis was augmented in female rats. These data suggest that sex modulates sympathetic vascular control in resting and contracting skeletal muscle and that a portion of the enhanced sympatholysis in female rats was NO dependent.NEW & NOTEWORTHY Sex differences in the neurovascular regulation of blood pressure and vascular resistance have been documented. However, our understanding of the underlying mechanisms that mediate these differences is incomplete. The present study demonstrates that female rats have an enhanced capacity to inhibit sympathetic vasoconstriction during exercise (sympatholysis) and that NO mediates a portion of the enhanced sympatholysis.

Exercise training and α1-adrenoreceptor-mediated sympathetic vasoconstriction in resting and contracting skeletal muscle

Exercise training (ET) increases sympathetic vasoconstrictor responsiveness and enhances contraction‐mediated inhibition of sympathetic vasoconstriction (i.e., sympatholysis) through a nitric oxide (NO)‐dependent mechanism. Changes in α₂‐adrenoreceptor vasoconstriction mediate a portion of these training adaptations, however the contribution of other postsynaptic receptors remains to be determined. Therefore, the purpose of this study was to investigate the effect of ET on α₁‐adrenoreceptor‐mediated vasoconstriction in resting and contracting muscle. It was hypothesized that α₁‐adrenoreceptor‐mediated sympatholysis would be enhanced following ET. Male Sprague Dawley rats were randomized to sedentary (S; n = 12) or heavy‐intensity treadmill ET (n = 11) groups. Subsequently, rats were anesthetized and instrumented for lumbar sympathetic chain stimulation and measurement of femoral vascular conductance (FVC) at rest and during muscle contraction. The percentage change in FVC in response to sympathetic stimulation was measured in control, α₁‐adrenoreceptor blockade (Prazosin; 20 μg, IV), and combined α₁ and NO synthase (NOS) blockade (l‐NAME; 5 mg·kg⁻¹IV) conditions. Sympathetic vasoconstrictor responsiveness was increased (P < 0.05) in ET compared to S rats at low, but not high (P > 0.05) stimulation frequencies at rest (S: 2 Hz: −25 ± 4%; 5 Hz: −45 ± 5 %; ET: 2 Hz: −35 ± 7%, 5 Hz: −52 ± 7%), whereas sympathetic vasoconstrictor responsiveness was not different (P > 0.05) between groups during contraction (S: 2 Hz: −11 ± 8%; 5 Hz: −26 ± 11%; ET: 2 Hz: −10 ± 7%, 5 Hz: −27 ± 12%). Prazosin blunted (P < 0.05) vasoconstrictor responsiveness in S and ET rats at rest and during contraction, and abolished group differences in vasoconstrictor responsiveness. Subsequent NOS blockade increased vasoconstrictor responses (P < 0.05) in S at rest and during contraction, whereas in ET vasoconstriction was increased (P < 0.05) in response to sympathetic stimulation at 2 Hz at rest and unchanged (P > 0.05) during contraction. ET enhanced (P < 0.05) sympatholysis, however the training‐mediated improvements in sympatholysis were abolished by α₁‐adrenoreceptor blockade. Subsequent NOS inhibition did not alter (P > 0.05) sympatholysis in S or ET rats. In conclusion, ET augmented α₁‐adrenoreceptor‐mediated vasoconstriction in resting skeletal muscle and enhanced α₁‐adrenoreceptor‐mediated sympatholysis. Furthermore, these data suggest that NO is not required to inhibit α₂‐adrenoreceptor‐ and nonadrenoreceptor‐mediated vasoconstriction during exercise.