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

Abstract

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.

Fig. 1.

Experimental timeline. A: after ∼60 min of instrumentation subjects rested quietly in supine position for 30 min before drug infusion [acetylcholine (ACh) or nitroprusside (NTP)]. Subjects were then seated in the leg ergometer for at least 5 min before starting continuous, dynamic 1-leg knee extensor exercise. After exercise, subjects lay supine for repeated ACh and NTP infusions. Catheters were then removed, and subjects recovered for 2 h while catheter sites were monitored for healing. B: data were collected specifically during minutes 4 and 5 of each 5-min portion of the exercise bout. Gray boxes indicate drug infusion during each 5-min period. Horizontal bars indicate times for key data collection, and arrows indicate blood sampling. l-NAME, N^G-nitro-l-arginine methyl ester.

Fig. 2.

Leg hemodynamics with exercise during nitric oxide synthase (NOS) and cyclooxygenase (COX) inhibition. Leg blood flow did not change with either N^G-nitro-l-arginine methyl ester (l-NAME) + ketorolac (A) or l-NAME alone (C). When normalized for blood pressure, leg vascular conductance (LVC) decreased slightly with l-NAME infusion (B, P = 0.09; D, P = 0.08). Addition of ketorolac significantly reduced LVC (B, P = 0.02), which returned to control exercise levels shortly after ketorolac infusion. *Statistically different from control exercise (P = 0.02).

Fig. 3.

Whole body and leg oxygen consumption (V̇o₂). Whole body V̇o₂ increased with exercise (protocol 1) but did not change with drug infusion (A). Leg V̇o₂ increased with exercise but did not change with drug infusion in protocol 1 (B) or protocol 2 (C).

Fig. 4.

Continuous monitoring of femoral venous oxygen saturation during exercise. A: venous saturation from indwelling oximeter decreased from rest to exercise (P = 0.01) but did not significantly change during drug infusion (l-NAME + ketorolac) (P = 0.2). B: a similar pattern was observed during l-NAME-only infusion (P = 0.11). *Statistically different from baseline (nonexercising) values (P < 0.05).

Fig. 5.

Leg vasodilation responses to endothelium-dependent and -independent agonist infusion. Control responses before exercise protocol and responses following exercise protocol after l-NAME + ketorolac infusion are shown. l-NAME + ketorolac reduced vasodilation to ACh (A, P = 0.01) but not to NTP (B, P = 0.6). *Statistically different from control (P < 0.05).