Recurrent hypoxic exposure and reflex responses during development in the piglet.

The effects of recurrent hypoxia on cardiorespiratory reflexes were characterized in anesthetized piglets at 2-10 d (n=15), 2-3 weeks (n=11) and 8-10 weeks (n=8). Responses of phrenic and hypoglossal electroneurograms (ENG(phr) and ENG (hyp)) to hypoxia (8% 0(2), bal N(2), 5 min), hypercapnia (7% CO(2) bal O(2), 5 min) and intravenous capsaicin were tested before and after recurrent exposure to 11 episodes of hypoxia (8% O(2) bal N(2), 5 min). In piglets 2-10 d, ENG(phr) response to hypoxia declined in proportion to the number of hypoxic exposures; however, ENG (hyp) response to hypoxia was unchanged. In piglets at 2-10 d, intracisternal injection of bicuculline (GABA(A) receptor antagonist) reversed effects of recurrent hypoxia on ENG(phr) hypoxic response, eliminated apnea during hypoxia, as well as the delay in appearance of ENG(phr) after hypoxia. The ENG(phr) response to 7% CO(2) inhalation also decreased after recurrent hypoxia; however, the ENG(phr) response to C-fiber stimulation by capsaicin was unaltered. Piglets at 2-3 and 8-10 weeks were resistant to the depressive effects of recurrent hypoxia on respiratory reflex responses. We conclude that the response of the anesthetized newborn piglet to recurrent hypoxia is dominated by increasing inhibition of phrenic neuroelectrical output during successive hypoxic exposures. Central GABAergic inhibition may contribute significantly to the cumulative effects of repeated hypoxia in the newborn piglet experimental model.