This study investigated whether changing sympathetic activity, acting via -receptors, might induce the progressive ventilatory changes observed in response to prolonged hypoxia. The responses of 10 human subjects to four 8-h protocols were compared: 1) isocapnic hypoxia (end-tidal PO₂ = 50 Torr) plus 80-mg doses of oral propranolol; 2) isocapnic hypoxia, as in protocol 1, with oral placebo; 3) air breathing with propranolol; and 4) air breathing with placebo. Exposures were conducted in a chamber designed to maintain end-tidal gases constant by computer control. Ventilation (E) was measured at regular intervals throughout. Additionally, the subjects' ventilatory hypoxic sensitivity and their residual E during hyperoxia (5 min) were assessed at 0, 4, and 8 h by using a dynamic end-tidal forcing technique. -Blockade did not significantly alter either the rise in E seen during 8 h of isocapnic hypoxia or the changes observed in the acute hypoxic ventilatory response and residual E in hyperoxia over that period. The results do not provide evidence that changes in sympathetic activity acting via -receptors play a role in the mediation of ventilatory changes observed during 8 h of isocapnic hypoxia.

ventilation; hypoxic sensitivity; high-altitude acclimatization

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