Role of endogenous adenosine in hypoxic ventilatory response in humans: a study with dipyridamole

To examine the role of endogenous adenosine in hypoxic ventilatory response, we measured, in nine normal young adults, ventilatory responses to isocapnic progressive hypoxia and subsequent sustained hypoxia [arterial O2 saturation (SaO2); 80%, 20 min] with and without pretreatment with dipyridamole in a double-blind crossover fashion. Dipyridamole, an adenosine uptake blocker, was expected to enhance the effect of endogenous adenosine. Pretreatment with dipyridamole (0.5 mg/kg) significantly augmented the slope of the ventilatory response to isocapnic progressive hypoxia from 0.35 +/- 0.13 (SE) to 0.70 +/- 0.25 l.min-1.%fall of SaO2(-1) (P < 0.01), although there were no significant changes in resting ventilation. On the other hand, minute ventilation, when expressed as a percentage of peak ventilation, declined to 68.4 +/- 4.3% with dipyridamole at the 9-11th min of sustained hypoxia, which was significantly lower than the 90.2 +/- 8.3% with a placebo (P < 0.05), and finally reached 56.1 +/- 7.2% with dipyridamole and 78.7 +/- 9.2% with the placebo (P < 0.1) at the 18-20th min of sustained hypoxia. In an attempt to more specifically examine the role of adenosine, aminophylline (5 mg/kg), an adenosine receptor antagonist, was injected before pretreatment with dipyridamole in four subjects. Aminophylline infusion abolished or at least attenuated the effect of dipyridamole in all four subjects. These data suggest that endogenous adenosine has a modulatory role in hypoxic ventilatory response in adult humans.