Endogenous opioids and ventilatory adaptation to prolonged hypoxia in goats

Abstract

To investigate whether endogenous opioid peptides mediate time-dependent changes in ventilatory control during prolonged hypoxia, we studied four adult goats at rest during 14 days at simulated high altitude in a hypobaric chamber (P B∼450 Torr). Arterial PCO 2 fell during the first several hours of hypoxia, remained stable over the next 7 days, and then rose slightly (but without statistical significance) by day 14. Ventilatory responsiveness to CO 2 increased during the first week of hypoxia. By day 14, while still greater than control, the ventilatory response to CO 2 was less than that observed on day 7. Immunoactive β-endorphin levels in plasma and CSF did not change during the 14-day period. Administration of naloxone on day 14 did not restore the ventilatory response to CO 2 to the level observed during the first week of acclimatization. We conclude that in adult goats, time-dependent changes in ventilatory response to CO 2 during acclimatization to prolonged hypoxia are not primarily attributable to alterations in endogenous opioid peptide activity.