Effect of Nitric Oxide Synthase Inhibition on Plasma ATP during Hypoxia and Exercise Induced Vasodilation in Humans

Abstract

IntroductionAdenosine triphosphate (ATP) is postulated to be a major player in the local control of skeletal muscle vascular tone during exercise. Exercise during hypoxic conditions significantly augments skeletal muscle blood flow and elevates venous plasma levels of ATP compared to normoxic conditions in young healthy adults. This suggests that the elevated levels of ATP during hypoxic exercise contribute to the augmented vasodilation. Nitric oxide synthase (NOS) inhibition has also been shown to blunt the effects of hypoxia on exercise vasodilation. Therefore, the aim of the current study was to evaluate the effect of NOS inhibition and hypoxia on vasodilator responses and plasma ATP levels during exercise in healthy young adults.Methods10 young volunteers (age = 28 ± 1 years, 6 men/4 women) performed four minutes of forearm handgrip exercise at 20% of their maximal voluntary contraction (MVC). The forearm handgrip exercise was performed under normoxia and hypoxia (SpO2 = ~80%) with and without NOS inhibitor, LNMMA administered via a brachial artery catheter. Beat to beat blood pressure and heart rate were recorded. Forearm blood flow was measured using Doppler ultrasound and normalized for arterial pressure to determine forearm vascular conductance (FVC). Venous blood was obtained from a retrograde intravenous catheter at rest and during the last minute of each exercise bout. Plasma ATP was measured using the luciferin‐luciferase technique.ResultsDuring saline and normoxia FVC was 43.4 ± 8.4 at rest and 307.8 ± 22.6 ml/min/100mmHg during exercise. With hypoxia FVC increased to 59.35 ± 10.5 at rest and 370.9 ± 33.3 ml/min/100mmHg during exercise (both p<0.05 vs. normoxia). During LNMMA and normoxia FVC did not change significantly compared to saline 33.9 ± 4.4 at rest and 283.5 ± 23.2 ml/min/100mmHg during exercise. With hypoxia and LNMMA FVC was 35.04 ± 4.1 at rest (p>0.05 vs. normoxia) and 326.0 ± 23.9 ml/min/100mmHg during exercise (p<0.05 vs. normoxia). When changes from baseline are evaluated, hypoxia augmented the vasodilator responses exercise to exercise during both saline and LNMMA.ATP was higher during 20% MVC exercise with saline during hypoxia as compared to normoxia (65.6 ± 12.1 vs. 108.1 ± 23.2 versus nmol/L, p=0.058). It was also higher during 20% MVC exercise with LNMMA (81.7 ± 9.5 vs. 106.1 ± 14.7 nmol/L, p=0.055). When changes from baseline are evaluated, hypoxia augmented venous ATP concentration with exercise during both saline and LNMMA.ConclusionsOur results suggest that increased endogenous ATP release during hypoxic exercise continued to evoke vasodilation during NOS inhibition. A likely source of this ATP is from red blood cells and this interpretation is consistent with findings demonstrating that NOS inhibition has limited effects on ATP mediated dilation in the human forearm.Support or Funding InformationThis research was supported by National Institutes of Health Research Grants HL‐119337, CTSA UL1 TR000135 and the Caywood Professorship via the Mayo Foundation