William G. Schrage

Ageing reduces nitric-oxide- and prostaglandin-mediated vasodilatation in exercising humans

In older humans, infusions of endothelial agonists suggest endothelial dysfunction, due in part to less nitric oxide (NO)‐ and prostaglandin (PG)‐mediated vasodilatation, and a shift toward PG‐mediated vasoconstriction. Ageing can also be associated with lower exercise blood flow (exercise hyperaemia), but the vascular mechanisms mediating this remain unknown. Notably, in young adults, inhibition of NO and PGs during exercise decreases exercise hyperaemia by ∼20 and ∼12%, respectively. We tested our first hypothesis that in older humans inhibition of NO would decrease hyperaemia, but that inhibition of PGs would increase hyperaemia by blocking vasoconstrictor PGs. Fifteen older subjects (65 ± 3 years) performed dynamic forearm exercise for 20 min (20 contractions min−1). Forearm blood flow (FBF) was measured beat‐to‐beat with Doppler ultrasound, while saline or drugs were infused sequentially via brachial artery catheter in the exercising forearm. After achieving steady‐state exercise, l‐NAME (25 mg) was infused over 5 min to inhibit NO synthase. After a further 2 min of exercise (saline), ketorolac (6 mg) was infused over 5 min to inhibit PGs, followed by a futher 3 min of exercise with saline. Drug order was reversed in seven subjects. l‐NAME reduced steady‐state exercise hyperaemia by 12 ± 3% in older subjects (P < 0.01), whereas ketorolac had no net effect on blood flow (3 ± 6%, P > 0.4). The effects of l‐NAME and ketorolac were independent of drug order. By comparing these results with our previous results in young adults, we tested our second hypothesis that in older humans inhibition of NO or PGs would have less impact on exercise hyperaemia due to less vasodilatation from these signals. Our results suggest that, compared with young adults, in older humans the relative contribution of NO to exercise hyperaemia is reduced ∼45% (22 ± 4 versus 12 ± 3%), but the role of PG in mediating vasodilatation is lost in ageing human skeletal muscle. Lower exercise hyperaemia in older humans may be mediated in part by less NO‐ and PG‐mediated vasodilatation during exercise.

Endothelium-dependent vasodilatation and exercise hyperaemia in ageing humans: impact of acute ascorbic acid administration

Age‐related increases in oxidative stress impair endothelium‐dependent vasodilatation in humans, leading to the speculation that endothelial dysfunction contributes to impaired muscle blood flow and vascular control during exercise in older adults. We directly tested this hypothesis in 14 young (22 ± 1 years) and 14 healthy older men and women (65 ± 2 years). We measured forearm blood flow (FBF; Doppler ultrasound) and calculated vascular conductance (FVC) responses to single muscle contractions at 10, 20 and 40% maximum voluntary contraction (MVC) before and during ascorbic acid (AA) infusion, and we also determined the effects of AA on muscle blood flow during mild (10% MVC) continuous rhythmic handgrip exercise. For single contractions, the peak rapid hyperaemic responses to all contraction intensities were impaired ∼45% in the older adults (all P < 0.05), and AA infusion did not impact the responses in either age group. For the rhythmic exercise trial, FBF (∼28%) and FVC (∼31%) were lower (P= 0.06 and 0.05) in older versus young adults after 5 min of steady‐state exercise with saline. Subsequently, AA was infused via brachial artery catheter for 10 min during continued exercise. AA administration did not significantly influence FBF or FVC in young adults (1–3%; P= 0.24–0.59), whereas FBF increased 34 ± 7% in older adults at end‐exercise, and this was due to an increase in FVC (32 ± 7%; both P < 0.05). This increase in FBF and FVC during exercise in older adults was associated with improvements in vasodilator responses to acetylcholine (ACh; endothelium dependent) but not sodium nitroprusside (SNP; endothelium independent). AA had no effect on ACh or SNP responses in the young. We conclude that acute AA administration does not impact the observed age‐related impairment in the rapid hyperaemic response to brief muscle contractions in humans; however, it does significantly increase muscle blood flow during continuous dynamic exercise in older adults, and this is probably due (in part) to an improvement in endothelium‐dependent vasodilatation.

α-Adrenergic control of blood flow during exercise: effect of sex and menstrual phase

Sex differences exist in autonomic control of the cardiovascular system. This study was designed to directly test sex or female menstrual phase-related differences in α-adrenergic control of blood flow during exercise. We hypothesized that women would exhibit reduced α-adrenergic vasoconstriction compared with men during exercise; in addition, women would constrict less during the early luteal than the early follicular phase of the female menses. Young men (n = 10) were studied once and women (n = 9) studied twice, once during the early follicular phase and once during the early luteal phase of female menses. We measured forearm blood flow (FBF; Doppler ultrasound of the brachial artery) during rest and steady-state dynamic exercise (15 and 30% of maximal voluntary contraction, 20 contractions/min). A brachial artery catheter was inserted for the local administration of α-adrenergic agonists [phenylephrine (PE; α(1)) or clonidine (CL; α(2))]. Blood flow responses to exercise [forearm vascular conductance (FVC)] were similar between all groups. At rest, infusion of PE or CL decreased FVC in all groups (40-60% reduction). Vasoconstriction to PE was abolished in all groups at 15 and 30% exercise intensity. Vasoconstriction to CL was reduced at 15% and abolished at 30% intensity in all groups; women had less CL-induced constriction during the early luteal than early follicular phase (P < 0.017, 15% intensity). These results indicate that vasodilator responses to forearm exercise are comparable between men and women and are achieved through similar paths of α-adrenergic vascular control at moderate intensities; this control may differ at low intensities specific to the female menstrual phase.

Roles of nitric oxide synthase and cyclooxygenase in leg vasodilation and oxygen consumption during prolonged low-intensity exercise in untrained humans

The vasodilator signals regulating muscle blood flow during exercise are unclear. We tested the hypothesis that in young adults leg muscle vasodilation during steady-state exercise would be reduced independently by sequential pharmacological inhibition of nitric oxide synthase (NOS) and cyclooxygenase (COX) with NG-nitro-L-arginine methyl ester (L-NAME) and ketorolac, respectively. We tested a second hypothesis that NOS and COX inhibition would increase leg oxygen consumption (VO2) based on the reported inhibition of mitochondrial respiration by nitric oxide. In 13 young adults, we measured heart rate (ECG), blood pressure (femoral venous and arterial catheters), blood gases, and venous oxygen saturation (indwelling femoral venous oximeter) during prolonged (25 min) steady-state dynamic knee extension exercise (60 kick/min, 19 W). Leg blood flow (LBF) was determined by Doppler ultrasound of the femoral artery. Whole body VO2 was measured, and leg VO2 was calculated from blood gases and LBF. Resting intra-arterial infusions of acetylcholine (ACh) and nitroprusside (NTP) tested inhibitor efficacy. Leg vascular conductance (LVC) to ACh was reduced up to 53±4% by L-NAME+ketorolac infusion, and the LVC responses to NTP were unaltered. Exercise increased LVC from 4±1 to 33.1±2 ml.min(-1).mmHg(-1) and tended to decrease after L-NAME infusion (31±2 ml.min(-1).mmHg(-1), P=0.09). With subsequent administration of ketorolac LVC decreased to 29.6±2 ml.min(-1).mmHg(-1) (P=0.02; n=9). While exercise continued, LVC returned to control values (33±2 ml.min(-1).mmHg(-1)) within 3 min, suggesting involvement of additional vasodilator mechanisms. In four additional subjects, LVC tended to decrease with L-NAME infusion alone (P=0.08) but did not demonstrate the transient recovery. Whole body and leg VO2 increased with exercise but were not altered by L-NAME or L-NAME+ketorolac. These data indicate a modest role for NOS- and COX-mediated vasodilation in the leg of exercising humans during prolonged steady-state exercise, which can be restored acutely. Furthermore, NOS and COX do not appear to influence muscle VO2 in untrained healthy young adults.

Dietary sodium restriction and β2‐adrenergic receptor polymorphism modulate cardiovascular function in humans

Dietary Na+ intake influences β2‐adrenergic receptor (β2AR) responsiveness. While receiving a normal Na+ diet (150 mmol day−1), subjects homozygous for glycine at amino acid 16 (Gly16) have greater forearm β2AR‐mediated vasodilatation than subjects homozygous for arginine (Arg16), an effect that is mediated by endothelial NO. We tested the hypothesis that dietary Na+ restriction eliminates genotype differences in forearm and systemic β2AR‐mediated dilatation in these groups. We measured heart rate, mean arterial pressure and cardiac output (CO, acetylene breathing) responses to administration of intravenous terbutaline (TRB) before and after 5 days of low dietary Na+ intake (10 mmol day−1) in healthy Gly16 (n= 17; age, 31 ± 7 year) and Arg16 homozygotes (n= 15; age, 29 ± 8 year). After the low‐Na+ diet, a catheter was placed in the brachial artery to measure forearm blood flow (FBF, plethysmography) responses to administration of isoprenaline (isoproterenol) before and after NO inhibition with NG‐mono‐methyl‐l‐arginine (l‐NMMA). In the Gly16 group, the low‐Na+ diet decreased baseline CO from 6.4 ± 1.4 to 5.5 ± 1.2 l min−1 (P= 0.003, paired t test), tended to decrease stroke volume from 97.0 ± 20.6 to 86.9 ± 21.7 ml (P= 0.06) and increased peripheral resistance from 1106 ± 246 to 1246 ± 222 dynes s cm−5 (P= 0.02); significant effects of the low‐Na+ diet were not observed in Arg16 subjects. In a repeated measures ANOVA, the responses of all cardiovascular measures to systemic administration of TRB were not influenced by genotype or diet. Additionally, the FBF response to incremenetal doses of isoprenaline did not differ between genotype groups before or after administration of l‐NMMA. We conclude that dietary Na+ restriction blunted the increased forearm NO‐mediated β2AR responsiveness in Gly16 homozygotes observed in a previous study after normal dietary Na+ intake, while baseline CO decreased and peripheral resistance increased in this group. This study provides evidence that dietary Na+ modulates effects of the Arg16Gly polymorphism on cardiovascular function.

Microvascular function in younger adults with obesity and metabolic syndrome: role of oxidative stress

Older adults with cardiovascular disease exhibit microvascular dysfunction and increased levels of reactive oxygen species (ROS). We hypothesized that microvascular impairments begin early in the disease process and can be improved by scavenging ROS. Forearm blood flow (Doppler ultrasound) was measured in 45 young (32 ± 2 yr old) adults (n = 15/group) classified as lean, obese, and metabolic syndrome (MetSyn). Vasodilation in response to endothelial (ACh) and vascular smooth muscle [nitroprusside (NTP) and epoprostenol (Epo)] agonists was tested before and after intra-arterial infusion of ascorbic acid to scavenge ROS. Vasodilation was assessed as a rise in relative vascular conductance (ml·min(-1)·dl(-1)·100 mmHg(-1)). ACh and NTP responses were preserved (P = 0.825 and P = 0.924, respectively), whereas Epo responses were lower in obese and MetSyn adults (P < 0.05) than in lean controls. Scavenging of ROS via infusion of ascorbic acid resulted in an increase in ACh-mediated (P < 0.001) and NTP-mediated (P < 0.001) relative vascular conductance across all groups, suggesting that oxidative stress influences vascular responsiveness in adults with and without overt cardiovascular disease risk. Ascorbic acid had no effect on Epo-mediated vasodilation (P = 0.267). These results suggest that obese and MetSyn adults exhibit preserved endothelium-dependent vasodilation with reduced dependence on prostacyclin and are consistent with an upregulation of compensatory vascular control mechanisms.

Muscle blood flow responses to dynamic exercise in young obese humans

Exercise is a common nonpharmacological way to combat obesity; however, no studies have systematically tested whether obese humans exhibit reduced skeletal muscle blood flow during dynamic exercise. We hypothesized that exercise-induced blood flow to skeletal muscle would be lower in young healthy obese subjects (body mass index of >30 kg/m(2)) compared with lean subjects (body mass index of <25 kg/m(2)). We measured blood flow (Doppler Ultrasound of the brachial and femoral arteries), blood pressure (auscultation, Finapress), and heart rate (ECG) during rest and two forms of single-limb, steady-state dynamic exercise: forearm exercise (20 contractions/min at 4, 8, and 12 kg) and leg exercise (40 kicks/min at 7 and 14 W). Forearm exercise increased forearm blood flow (FBF) similarly in both groups (P > 0.05; obese subjects n = 9, lean subjects n = 9). When FBF was normalized for perfusion pressure, forearm vascular conductance was not different between groups at increasing workloads (P > 0.05). Leg exercise increased leg blood flow (LBF) similarly in both groups (P > 0.05; obese subjects n = 10, lean subjects n = 12). When LBF was normalized for perfusion pressure, leg vascular conductance was not different between groups at increasing workloads (P > 0.05). These results were confirmed when relative blood flow was expressed at average relative workloads. In conclusion, our results show that obese subjects exhibited preserved FBF and LBF during dynamic exercise.

Rapid onset vasodilatation is blunted in obese humans.

Aim:  Conduit artery function in obese humans is frequently assessed at rest, but very little is known about resistance artery function in response to muscle contraction. We tested the hypothesis that obese adults will exhibit reduced contraction‐induced rapid onset vasodilatation. Single and brief forearm contractions were used to isolate the local effects of muscle contraction on the forearm vasodilatory response, independent of systemic haemodynamic and sympathetic neural influence.

Cerebrovascular regulation in men and women: stimulus-specific role of cyclooxygenase.

Greater cerebral artery vasodilation mediated by cyclooxygenase (COX) in female animals is unexplored in humans. We hypothesized that young, healthy women would exhibit greater basal cerebral blood flow (CBF) and greater vasodilation during hypoxia or hypercapnia compared to men, mediated by a larger contribution of COX. We measured middle cerebral artery velocity (MCAv, transcranial Doppler ultrasound) in 42 adults (24 women, 18 men; 24 ± 1 years) during two visits, in a double‐blind, placebo‐controlled design (COX inhibition, 100 mg oral indomethacin, Indo). Women were studied early in the follicular phase of the menstrual cycle (days 1–5). Two levels of isocapnic hypoxia (SPO2 = 90% and 80%) were induced for 5‐min each. Separately, hypercapnia was induced by increasing end‐tidal carbon dioxide (PETCO2) 10 mmHg above baseline. A positive change in MCAv (ΔMCAv) reflected vasodilation. Basal MCAv was greater in women compared to men (P < 0.01) across all conditions. Indo decreased baseline MCAv (P < 0.01) similarly between sexes. Hypoxia increased MCAv (P < 0.01), but ΔMCAv was not different between sexes. Indo did not alter hypoxic vasodilation in either sex. Hypercapnia increased MCAv (P < 0.01), but ΔMCAv was not different between sexes. Indo elicited a large decrease in hypercapnic vasodilation (P < 0.01) that was similar between sexes. During the early follicular phase, women exhibit greater basal CBF than men, but similar vasodilatory responses to hypoxia and hypercapnia. Moreover, COX is not obligatory for hypoxic vasodilation, but plays a vital and similar role in the regulation of basal CBF (~30%) and hypercapnic response (~55%) between sexes.

Altered neurovascular control of the resting circulation in human metabolic syndrome

•  Young healthy adults exhibit a balance between muscle sympathetic nerve activity (MSNA) and α‐adrenergic‐mediated vasoconstriction such that those with higher MSNA exhibit lower vascular‐adrenergic responsiveness. •  In contrast to healthy adults, the balance between MSNA and α1‐adrenergic‐mediated vasoconstriction is lost in adults with metabolic syndrome. In addition, adults with metabolic syndrome exhibit increased α2‐adrenergic responsiveness. •  This study uncovered some of the earliest sympathetic–haemodynamic changes in the progression from metabolic syndrome to cardiovascular disease and diabetes. •  Considering metabolic syndrome subjects were relatively young and free of overt cardiovascular disease, it is reasonable to speculate as the disease progresses the observed uncoupling between MSNA and α‐adrenergic responsiveness may lead to reduced whole‐limb blood flow, altered blood flow distribution, reduced glucose delivery and/or increased hypertension severity.