Exhaled nitric oxide during normoxic and hypoxic exercise in endurance athletes

Abstract

Endogenous nitric oxide (NO) through its relaxing effect on smooth muscle cells may be involved in pulmonary gas exchange as well as in the modulation of the hypoxic pulmonary vasoconstriction. As athletes with exercise-induced hypoxaemia (EIH) present pulmonary gas exchange abnormalities in normoxia that could be even greater in hypoxia, we hypothesized that pulmonary NO may be lower in such athletes with EIH. Eleven athletes with EIH [decrease in arterial oxygen blood partial pressure (PaO2) > 12 mmHg] and 9 without EIH (NEIH) exercised at 40%, 60% (10 min) and 90% (5 min) of normoxic maximal power output (Pmax) in normoxia, and at 40% and 60% (10 min) of Pmax in hypoxia (FiO2 = 15%). Exhaled NO concentration during a constant flow exhalation (FENO(0.170)) and arterialized blood gases were measured at every power output. FENO(0.170) decreased from rest to exercise both in normoxia (-27.8 +/- 22.8% at 90% Pmax, P < 0.001) and hypoxia (-23.8 +/- 17.5% at 60% Pmax, P < 0.001). At 90% Pmax in normoxia, EIH athletes showed lower PaO2 (76.7 +/- 5.4 vs. 82.8 +/- 4.4 mmHg, P = 0.013) and greater FENO(0.170) decrement (-37.0 +/- 24.7% vs. -16.6 +/- 14.6%, P = 0.042) than NEIH athletes. During hypoxic exercise, P(a)O(2) and FENO(0.170) decreases were similar in both groups (P > 0.05). The present study shows lower pulmonary NO in athletes with gas exchange abnormalities during intense exercise in normoxia, while EIH and NEIH athletes have similar decreases in blood gases and pulmonary NO during hypoxic exercise. Decreased pulmonary NO in such conditions may contribute to ventilation-perfusion inequality and/or increase pulmonary vascular tone in athletes.