Grant H. Simmons

Vascular Effects of Exercise: Endothelial Adaptations Beyond Active Muscle Beds

Endothelial adaptations to exercise training are not exclusively conferred within the active muscle beds. Herein, we summarize key studies that have evaluated the impact of chronic exercise on the endothelium of vasculatures perfusing nonworking skeletal muscle, brain, viscera, and skin, concluding with discussion of potential mechanisms driving these endothelial adaptations.

Increased muscle sympathetic nerve activity acutely alters conduit artery shear rate patterns

Escalating evidence indicates that disturbed flow patterns, characterized by the presence of retrograde and oscillatory shear stress, induce a proatherogenic endothelial cell phenotype; however, the mechanisms underlying oscillatory shear profiles in peripheral conduit arteries are not fully understood. We tested the hypothesis that acute elevations in muscle sympathetic nerve activity (MSNA) are accompanied by increases in conduit artery retrograde and oscillatory shear. Fourteen healthy men (25 +/- 1 yr) performed three sympathoexcitatory maneuvers: graded lower body negative pressure (LBNP) from 0 to -40 Torr, cold pressor test (CPT), and 35% maximal voluntary contraction handgrip followed by postexercise ischemia (PEI). MSNA (microneurography; peroneal nerve), arterial blood pressure (finger photoplethysmography), and brachial artery velocity and diameter (duplex Doppler ultrasound) in the contralateral arm were recorded continuously. All maneuvers elicited significant increases in MSNA total activity from baseline (P < 0.05). Retrograde shear (-3.96 +/- 1.2 baseline vs. -8.15 +/- 1.8 s(-1), -40 LBNP, P < 0.05) and oscillatory shear index (0.09 +/- 0.02 baseline vs. 0.20 +/- 0.02 arbitrary units, -40 LBNP, P < 0.05) were progressively augmented during graded LBNP. In contrast, during CPT and PEI, in which MSNA and blood pressure were concomitantly increased (P < 0.05), minimal or no changes in retrograde and oscillatory shear were noted. These data suggest that acute elevations in MSNA are associated with an increase in conduit artery retrograde and oscillatory shear, an effect that may be influenced by concurrent increases in arterial blood pressure. Future studies should examine the complex interaction between MSNA, arterial blood pressure, and other potential modulatory factors of shear rate patterns.

Increased brachial artery retrograde shear rate at exercise onset is abolished during prolonged cycling: role of thermoregulatory vasodilation

Acute leg exercise increases brachial artery retrograde shear rate (SR), while chronic exercise improves vasomotor function. These combined observations are perplexing given the proatherogenic impacts of retrograde shear stress on the vascular endothelium and may be the result of brief protocols used to study acute exercise responses. Therefore, we hypothesized that brachial artery retrograde SR increases initially but subsequently decreases in magnitude during prolonged leg cycling. Additionally, we tested the role of cutaneous vasodilation in the elimination of increased retrograde SR during prolonged exercise. Brachial artery diameter and velocity profiles and forearm skin blood flow and temperature were measured at rest and during 50 min of steady-state, semirecumbent leg cycling (120 W) in 14 males. Exercise decreased forearm vascular conductance (FVC) and increased retrograde SR at 5 min (both P < 0.05), but subsequently forearm and cutaneous vascular conductance (CVC) rose while retrograde SR returned toward baseline values. The elimination of increased retrograde SR was related to the increase in FVC (r(2) = 0.58; P < 0.05) and CVC (r(2) = 0.32; P < 0.05). Moreover, when the forearm was cooled via a water-perfused suit between minutes 30 and 40 to blunt cutaneous vasodilation attending exercise, FVC was reduced and the magnitude of retrograde SR was increased from -49.7 ± 13.6 to -78.4 ± 16.5 s(-1) (P < 0.05). Importantly, these responses resolved with removal of cooling during the final 10 min of exercise (retrograde SR: -46.9 ± 12.5 s(-1)). We conclude that increased brachial artery retrograde SR at the onset of leg cycling subsequently returns toward baseline values due in part to thermoregulatory cutaneous vasodilation during prolonged exercise.

Changes in the control of skin blood flow with exercise training: where do cutaneous vascular adaptations fit in?

Heat is the most abundant byproduct of cellular metabolism. As such, dynamic exercise in which a significant percentage of muscle mass is engaged generates thermoregulatory demands that are met in part by increases in skin blood flow. Increased skin blood flow during exercise adds to the demands on cardiac output and confers additional circulatory strain beyond that associated with perfusion of active muscle alone. Endurance exercise training results in a number of physiological adaptations which ultimately reduce circulatory strain and shift thermoregulatory control of skin blood flow to higher levels of blood flow for a given core temperature. In addition, exercise training induces peripheral vascular adaptations within the cutaneous microvasculature indicative of enhanced endothelium‐dependent vasomotor function. However, it is not currently clear how (or if) these local vascular adaptations contribute to the beneficial changes in thermoregulatory control of skin blood flow following exercise training. The purpose of this Hot Topic Review is to synthesize the literature pertaining to exercise training‐mediated changes in cutaneous microvascular reactivity and thermoregulatory control of skin blood flow. In addition, we address mechanisms driving changes in cutaneous microvascular reactivity and thermoregulatory control of skin blood flow, and pose the question: what (if any) is the functional role of increased cutaneous microvascular reactivity following exercise training?

Brachial artery vasodilatation during prolonged lower limb exercise: role of shear rate

We recently observed a marked increase in brachial artery (BA) diameter during prolonged leg cycling exercise. The purpose of the present study was to test the hypothesis that this increase in BA diameter during lower limb exercise is shear stress mediated. Accordingly, we determined whether recapitulation of cycling‐induced BA shear rate with forearm heating, a known stimulus evoking shear‐induced conduit artery dilatation, would elicit comparable profiles and magnitudes of BA vasodilatation to those observed during cycling. In 12 healthy men, BA diameter and blood velocity were measured simultaneously using Doppler ultrasonography at baseline and every 5 min during 60 min of either steady‐state semi‐recumbent leg cycling (120 W) or forearm heating. At the onset of cycling, the BA diameter was reduced (−3.9 ± 1.2% at 5 min; P < 0.05), but it subsequently increased throughout the remainder of the exercise bout (+15.1 ± 1.6% at 60 min; P < 0.05). The increase in BA diameter during exercise was accompanied by an approximately 2.5‐fold rise in BA mean shear rate (P < 0.05). Similar increases in BA mean shear with forearm heating elicited an equivalent magnitude of BA vasodilatation to that observed during cycling (P > 0.05). Herein, we found that in the absence of exercise the extent of the BA vasodilator response was reproduced when the BA was exposed to comparable magnitudes of shear rate via forearm heating. These results are consistent with the hypothesis that shear stress plays a key role in signalling brachial artery vasodilatation during dynamic leg exercise.

Impact of Aging on Conduit Artery Retrograde and Oscillatory Shear at Rest and During Exercise: Role of Nitric Oxide

Aging has been recently associated with increased retrograde and oscillatory shear in peripheral conduit arteries, a hemodynamic environment that favors a proatherogenic endothelial cell phenotype. We evaluated whether nitric oxide (NO) bioavailability in resistance vessels contributes to age-related differences in shear rate patterns in upstream conduit arteries at rest and during rhythmic muscle contraction. Younger (n=11, age 26±2 years) and older (n=11, age 61±2 years) healthy subjects received intra-arterial saline (control) and the NO synthase inhibitor NG-Monomethyl-l-arginine. Brachial artery diameter and velocities were measured via Doppler ultrasound at rest and during a 5-minute bout of rhythmic forearm exercise. At rest, older subjects exhibited greater brachial artery retrograde and oscillatory shear (−13.2±3.0 s−1 and 0.11±.0.02 arbitrary units, respectively) compared with young subjects (−4.8±2.3 s−1 and 0.04±0.02 arbitrary units, respectively; both P<0.05). NO synthase inhibition in the forearm circulation of young, but not of older, subjects increased retrograde and oscillatory shear (both P<0.05), such that differences between young and old at rest were abolished (both P>0.05). From rest to steady-state exercise, older subjects decreased retrograde and oscillatory shear (both P<0.05) to the extent that no exercise-related differences were found between groups (both P>0.05). Inhibition of NO synthase in the forearm circulation did not affect retrograde and oscillatory shear during exercise in either group (all P>0.05). These data demonstrate for the first time that reduced NO bioavailability in the resistance vessels contributes, in part, to age-related discrepancies in resting shear patterns, thus identifying a potential mechanism for increased risk of atherosclerotic disease in conduit arteries.

Abnormal digital neurovascular response to local heating in systemic sclerosis

OBJECTIVES To investigate neurovascular dysfunction using the axon reflex-dependent hyperaemia (initial peak of skin local heating response) in fingers of patients with SSc or primary RP. METHODS Ten healthy subjects were initially enrolled to compare axon reflex-dependent thermal hyperaemia between the finger and forearm cutaneous circulations. Then, 10 patients with primary RP and 16 patients with SSc participated in a similar protocol focusing on the finger circulation only. Lidocaine/prilocaine cream was applied for 1 h to produce local blockade of cutaneous sensory nerves. After lidocaine/prilocaine pre-treatment, laser Doppler probes were heated from skin temperature to 42 degrees C for 30 min, and 44 degrees C for 5 min to achieve maximal skin blood flow. Data were expressed as a percentage of maximal cutaneous vascular conductance. RESULTS In healthy volunteers, we observed a significantly higher initial peak on the finger compared with the forearm, with both responses blunted following topical anaesthesia. In primary RP patients, we observed a decreased initial peak following lidocaine/prilocaine pre-treatment in the finger circulation [96.7% (33.4) vs 75.9% (29.5) with anaesthesia, P = 0.02]. In contrast, pre-treatment did not alter the initial peak in patients with SSc. A minute-by-minute analysis showed no delay of the initial peak. CONCLUSIONS We show an abnormal digital neurovascular response to local heating in SSc. Thermal hyperaemia could be monitored as a clinical test for neurovascular function in SSc. Further studies are required to test whether the abnormal digital neurovascular response correlates to the degree of peripheral vascular involvement.

Endothelial nitric oxide synthase mediates the nitric oxide component of reflex cutaneous vasodilatation during dynamic exercise in humans

Increases in skin blood flow and sweating also occur during exercise; however, it is not known if the mechanisms controlling these responses are the same during passive heat stress and exercise. The prevailing thought has been that mechanisms of cutaneous vasodilatation during passive heat stress and sustained dynamic exercise are the same, or very similar. Nitric oxide (NO) has been shown to be important for increasing skin blood flow during passive heat stress but it is unknown if this molecule is also involved during sustained dynamic exercise. The findings from our study suggest NO is involved in increasing skin blood flow during sustained dynamic exercise in humans but the NO is produced from a different enzyme compared to passive heat stress. These findings may help us better understand and aid individuals who have difficulty regulating their body temperature during sustained dynamic exercise (e.g. ageing).

Long-term exercise training does not alter brachial and femoral artery vasomotor function and endothelial phenotype in healthy pigs

Although the beneficial effects of exercise training on conduit artery endothelial function are well-established in animals and humans with compromised basal function, whether exercise exerts favorable effects on a healthy endothelium is inconclusive. We sought to determine whether long-term exercise training enhances endothelial function in peripheral conduit arteries of healthy pigs. Using a retrospective analysis of data collected in our laboratory (n = 127), we compared in vitro brachial and femoral artery endothelium-dependent and -independent relaxation between a group of pigs that exercise-trained for 16-20 wk and a group that remained sedentary. No differences in vasomotor function were found between the 2 groups (P > 0.05). Additionally, in a subset of pigs (n = 16), expression levels of 18 proteins that are typically associated with the atherosclerotic process were measured by immunoblot analysis of endothelial cell scrapes obtained from the brachial and femoral arteries. We found no differences (P > 0.05) in endothelial gene expression between these exercise-trained and sedentary healthy pigs. These results indicate that pigs exhibiting the classic training-induced adaptations do not demonstrate enhanced endothelium-dependent dilation nor reveal a more atheroprotected endothelial cell phenotype in their brachial and femoral arteries than their sedentary but otherwise healthy counterparts.

Heterogeneity of endothelial cell phenotype within and amongst conduit vessels of the swine vasculature

The purpose of this study was to investigate the extent of endothelial cell phenotypic heterogeneity throughout the swine vasculature, with a focus on the conduit vessels of the arterial and venous circulations. We tested the hypothesis that atheroprone arteries exhibit higher expression of markers of inflammation and oxidative stress than do veins and atheroresistant arteries. The study sample included tissues from 79 castrated, male swine. Immediately after the animals were killed, endothelial cells were mechanically scraped from isolated segments of the thoracic and abdominal aorta, carotid, brachial, femoral and renal arteries, and the vein regionally associated with each of these vessels, as well as the internal mammary and right coronary arteries. Cells were also taken from two regions of the aortic arch contrasted by atheroprone versus atherosusceptible haemodynamics. Endothelial cell phenotype was assessed by either immunoblotting or quantitative real‐time PCR for a host of both pro‐ and anti‐atherogenic markers (e.g. endothelial nitric oxide synthase, p67phox, cyclo‐oxygenase‐1 and superoxide dismutase 1). Marked heterogeneity across the vasculature was observed in the expression of both pro‐ and anti‐atherogenic markers, at both the protein and transcriptional levels. In particular, the coronary vascular endothelium expressed higher levels of the oxidative stress marker p67phox (P < 0.05 versus other arteries). In addition, differential expression of endothelial nitric oxide synthase and KLF4 was evident between atheroprone and atherosusceptible regions of the aorta, while expression of endothelial nitric oxide synthase, KLF2, KLF4 and cyclo‐oxygenase‐1 was lower in both areas of the aortic arch compared with the internal mammary artery. Conduit arteries typically expressed higher levels of both pro‐ and anti‐atherogenic markers relative to their associated veins. We show, for the first time, that endothelial cell phenotype is variable within vessels, across six major vascular territories, and between the arterial and venous circulations. Importantly, even straight vessel segments from systemic conduit arteries (e.g. brachial and carotid arteries) exhibited regional phenotypic heterogeneity; a finding not expected on the basis of local haemodynamic forces alone.