Wilkins, Brad W.

We previously reported that augmented skeletal muscle vasodilation during mild hypoxic forearm exercise includes beta‐adrenergic mechanisms. Furthermore, hypoxic vasodilation at rest is known to include beta‐adrenergic mediated nitric oxide (NO) production. Therefore, we tested the hypothesis that NO contributes to augmented vasodilation with hypoxic exercise. Twelve subjects (6M/6F; 26 ± 2 years) breathed hypoxic gas to titrate arterial O 2 saturation to 80% while remaining normocapnic via a rebreath system. Subjects performed incremental forearm exercise (10% and 20% of maximum) during saline infusion (control) and NO synthase inhibition ( N G ‐monomethyl‐L‐arginine; L‐NMMA) under normoxic and hypoxic conditions. Forearm vascular conductance (FVC; ml/min/100mmHg) was calculated from blood flow (ml/min) and blood pressure (mmHg). During the control condition, the rise in FVC (Δfrom normoxic baseline) due to hypoxic exercise was 211 ± 23 and 367 ± 34 (10% and 20% respectively). Infusion of L‐NMMA during hypoxic exercise attenuated ΔFVC at 10% (174 ± 20; P < 0.001) and 20% (317 ± 32; P < 0.01) compared to control condition. Our data indicate that NO contributes to the augmented vasodilation observed during hypoxic exercise. Supported by NIH HL‐46493 (MJJ) and AR‐55819 (DPC) and by CTSA RR‐024150.

The key vascular signals regulating muscle blood flow during exercise are unclear. We tested the hypothesis that in young adults, leg blood flow (LBF) during exercise would be reduced independently by pharmacological inhibition of nitric oxide synthase (NOS) and cyclooxygenase (COX). In 13 young adults, we measured heart rate (ECG), blood pressure (femoral artery and vein), blood gases, and venous oxygen saturation (indwelling oximeter) during steady‐state dynamic (60/min) knee extension exercise. LBF was determined by Doppler ultrasound of the femoral artery. Exercise (20W) increased leg vascular conductance (LVC) from 4± 1 to 33± 3 ml/min/100mmHg. The NOS inhibitor L‐NAME reduced LVC by ∼10% to 30± 1. Infusion of the COX inhibitor, ketoralac (n=9), increased LVC from 30± 1 to 34± 2. L‐NAME infusion reduced venous oxygen saturation from 48 to 38%, suggesting greater extraction to maintain oxygen consumption. Control intra‐arterial infusions of acetylcholine (ACH) and nitroprusside (NTP) tested inhibitor efficacy. The change in LVC to two doses of ACh was blunted 53 and 15%, by L‐NAME+ketorolac infusion. In contrast, the LVC responses to NTP were unaltered. These data implicate a modest role for NO‐mediated leg exercise hyperemia in adults and a potentially modest LBF restraint by vasoconstrictor prostaglandins. Funded by the NIH HL‐46493 and the Minnesota Obesity Center.

The augmented skeletal muscle vasodilation during hypoxic exercise is due, in part, to an intensity dependent contribution of beta‐receptor activation. We tested the hypothesis that an adenosine mediated vasodilation increases with hypoxic exercise intensity. Subjects (N=10) breathed hypoxic gas to titrate arterial O 2 saturation to 80% while remaining normocapnic (re‐breath system). Consecutive bouts of forearm exercise (10% and 20% of maximum) were performed under normoxia and hypoxia during phentolamine (alpha‐receptor antagonist) alone and the combination of phentolamine / aminophyline (adenosine receptor antagonist). Exogenous adenosine administered before and after aminophyline established effective receptor blockade. Forearm vascular conductance (FVC; ml/100mmHg/min) was calculated from blood flow (ml/min) and blood pressure (mmHg). During hypoxic exercise, the rise in FVC (Δ from baseline) during phentolamine was 314±30 and 470±37 (10% and 20% respectively). ΔFVC was similar at both 10% (349±45; P=0.3) and 20% (536±47; P=0.2) exercise during hypoxia with combined phentolamine / aminophyline. This occurred despite significant reduction in ΔFVC to exogenous adenosine following phentolamine / aminophyline (P<0.03). Our data suggests that in the absence of overlying vasoconstriction, adenosine does not contribute to the augmented hypoxic exercise hyperemia in humans.

The ganglionic blocker trimethaphan (TMP) is no longer produced; alternatives are needed to inhibit baroreflex control of the circulation during systemic hemodynamic studies. We tested the hypothesis that combination anti‐cholinergic glycopyrrolate (Gly) and α2 receptor agonist dexmedetomidine (Dex) would inhibit baroreflex buffering in response to vasodilation to a similar degree as ganglionic blockade with TMP. In 2 separate trials, 5 males received an I.V. bolus and infusion of Gly‐Dex, and 11 males received TMP infusion. To offset the fall in blood pressure with TMP, phenylephrine (PE) was used to raise pressure to pre‐TMP baseline. Terbutaline was then infused for 15 minutes at 2 doses (33 and 67 ng/kg/min). Heart rate (HR), blood pressure (BP, oscillometric cuff for Gly‐Dex, brachial arterial line for TMP+PE), and cardiac output (CO, acetylene breathing) were recorded. Resting HR was higher with Gly‐Dex (mean ± SEM: 93±7mmHg) than TMP+PE (74±3mmHg, p<0.001) but resting BP, CO, stroke volume (SV), and total peripheral resistance (TPR) were similar between Gly‐Dex and TMP+PE. The cardiovascular responses to terbutaline were similar between Gly‐Dex and TMP+PE, except for the increase in CO which was significantly greater in the TMP + PE group. We conclude that Gly‐Dex is a viable pharmacologic alternative to TMP in physiologic measures of BP and TPR during vasodilator infusions.

Decreased vasoconstrictor responsiveness during hypoxic exercise does not explain the augmented vasodilation in exercising muscle. We tested the hypothesis that β‐mediated vasodilation contributes to a portion of the enhanced muscle blood flow during hypoxic exercise. Subjects breathed hypoxic gas to titrate arterial O 2 saturation to 80% while remaining normocapnic via a re‐breath system. Two consecutive bouts of forearm handgrip exercise (10 and 20% of maximum) were performed under normoxia and hypoxia during saline infusion, during phentolamine (α‐antagoinst) alone and during the combination of phentolamine and propranolol (β‐antagonist). Forearm blood flow (FBF) and blood pressure (MAP) were assessed and forearm vascular conductance (FVC) was calculated (FBF/MAP). Hypoxia alone increased heart rate (62±3 vs. 82±3 bpm) with no change in MAP (86±2 vs. 89±2 mmHg for normoxia and hypoxia respectively). During hypoxic exercise the rise (Δ from baseline) in FVC with phentolamine alone was 166±26 and 448±31 ml/min/100mmHg (10% and 20 % respectively). This rise in blood flow was blunted (143±36 and 279±73 ml/min/100mmHg) with combined phentolamine and propranolol infusion. Thus, preliminary data suggests that in the absence of overlying vasoconstriction, β‐mediated vasodilation may account for a modest portion of the enhanced blood flow during hypoxic exercise.

Hypoxic vasodilation in skeletal muscle at rest is known to include β-adrenergic receptor-stimulated nitric oxide (NO) release. We previously reported that the augmented skeletal muscle vasodilation during mild hypoxic forearm exercise includes β-adrenergic mechanisms. However, it is unclear whether a β-adrenergic receptor-stimulated NO component exists during hypoxic exercise. We hypothesized that NO-mediated vasodilation becomes independent of β-adrenergic receptor activation with increased exercise intensity during hypoxic exercise. Ten subjects (7 men, 3 women; 23 ± 1 yr) breathed hypoxic gas to titrate arterial O2 saturation to 80% while remaining normocapnic. Subjects performed two consecutive bouts of incremental rhythmic forearm exercise (10% and 20% of maximum) with local administration (via a brachial artery catheter) of propranolol (β-adrenergic receptor inhibition) alone and with the combination of propranolol and nitric oxide synthase inhibition [ NG-monomethyl-l-arginine (l-NMMA)] under normoxic and hypoxic conditions. Forearm blood flow (FBF, ml/min; Doppler ultrasound) and blood pressure [mean arterial pressure (MAP), mmHg; brachial artery catheter] were assessed, and forearm vascular conductance (FVC, ml·min−1·100 mmHg−1) was calculated (FBF/MAP). During propranolol alone, the rise in FVC (Δ from normoxic baseline) due to hypoxic exercise was 217 ± 29 and 415 ± 41 ml·min−1·100 mmHg−1 (10% and 20% of maximum, respectively). Combined propranolol-l-NMMA infusion during hypoxic exercise attenuated ΔFVC at 20% (352 ± 44 ml·min−1·100 mmHg−1; P < 0.001) but not at 10% (202 ± 28 ml·min−1·100 mmHg−1; P = 0.08) of maximum compared with propranolol alone. These data, when integrated with earlier findings, demonstrate that NO contributes to the compensatory vasodilation during mild and moderate hypoxic exercise; a β-adrenergic receptor-stimulated NO component exists during low-intensity hypoxic exercise. However, the source of the NO becomes less dependent on β-adrenergic mechanisms as exercise intensity increases.

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 (V̇o2) 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 V̇o2 was measured, and leg V̇o2 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 V̇o2 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 V̇o2 in untrained healthy young adults.

We tested the hypothesis that adenosine contributes to augmented skeletal muscle vasodilation during hypoxic exercise. In separate protocols, subjects performed incremental rhythmic forearm exercise (10% and 20% of maximum) during normoxia and normocapnic hypoxia (80% arterial O2saturation). In protocol 1 ( n = 8), subjects received an intra-arterial administration of saline (control) and aminophylline (adenosine receptor antagonist). In protocol 2 ( n = 10), subjects received intra-arterial phentolamine (α-adrenoceptor antagonist) and combined phentolamine and aminophylline administration. Forearm vascular conductance (FVC; in ml·min−1·100 mmHg−1) was calculated from forearm blood flow (in ml/min) and blood pressure (in mmHg). In protocol 1, the change in FVC (ΔFVC; change from normoxic baseline) during hypoxic exercise with saline was 172 ± 29 and 314 ± 34 ml·min−1·100 mmHg−1(10% and 20%, respectively). Aminophylline administration did not affect ΔFVC during hypoxic exercise at 10% (190 ± 29 ml·min−1·100 mmHg−1, P = 0.4) or 20% (287 ± 48 ml·min−1·100 mmHg−1, P = 0.3). In protocol 2, ΔFVC due to hypoxic exercise with phentolamine infusion was 313 ± 30 and 453 ± 41 ml·min−1·100 mmHg−1(10% and 20% respectively). ΔFVC was similar at 10% (352 ± 39 ml·min−1·100 mmHg−1, P = 0.8) and 20% (528 ± 45 ml·min−1·100 mmHg−1, P = 0.2) hypoxic exercise with combined phentolamine and aminophylline. In contrast, ΔFVC to exogenous adenosine was reduced by aminophylline administration in both protocols ( P < 0.05 for both). These observations suggest that adenosine receptor activation is not obligatory for the augmented hyperemia during hypoxic exercise in humans.