Dopamine and carotid body function in the newborn lamb.

The effect of dopamine on the acute ventilatory response to hypoxia was investigated in four newborn lambs studied on the 1st day of postnatal life and eight lambs studied between 7 and 12 days of age. Studies were accomplished during nonrapid-eye-movement sleep in unanesthetized tracheotomized animals. Changes in minute ventilation (VE/kg), tidal volume, and frequency induced by intravenous bolus injection of dopamine (10 micrograms/kg) or saline control were assessed while animals were breathing room air or N2, before and after carotid body denervation (CBD). Dopamine depressed resting ventilation at both postnatal ages. This effect was greater in the older animals. Dopamine blunted the hypoxia-induced increase in VE/kg at both ages. The magnitude of this depression did not vary with postnatal age. Dopamine induced cessation of respiratory effort at end expiration (apnea) during room air and N2 breathing significantly more often in the older animals. The effect of dopamine was mediated at the carotid body. CBD decreased ventilation by an increase in breath-to-breath interval in older animals, suggesting carotid sinus nerve afferent activity is more important during eucapnic respiration in older animals than in the immediate newborn period.