Maturation of the respiratory response to acute hypoxia in the newborn rat.

1. We used a non-invasive method to measure the breathing of conscious newborn rat pups continuously and studied the maturation of the respiratory response to reductions in inspired O2 fraction (FI,O2) from 0.21 to 0.15, 0.12 or 0.08 for 6 min. Newborn rats, like many other species, showed a "biphasic" respiratory response, ventilation (VE) increasing during the first 1-2 min of hypoxia (phase 1) and then falling towards control, or below it, over the next 4 min (phase 2). 2. The secondary fall in VE decreased with age up to 14 days. The presence of a significant fall in VE in phase 2 depended on the FI,O2 to which the pups were exposed acutely; the lower the FI,O2 the greater the age at which a biphasic VE response was observed. 3. Our results are discussed in terms of two opposing components which determine VE during acute hypoxia in the neonate: (a) a fast arterial chemoreceptor-mediated stimulation of VE and (b) a slower central nervous component which inhibits VE, possibly a persistence of a suprapontine mechanism which inhibits fetal breathing movements during hypoxia in utero. Both of these components undergo maturation after birth.