Early Maturation of the Hypoxic Ventilatory Response in Hyperoxic Rat Pups: Evidence for Trait‐Specific Heterokairy

Abstract

Newborn mammals exhibit a biphasic hypoxic ventilatory response (HVR) in which an initial increase in ventilation is followed by a decline back toward baseline levels. The magnitude of the secondary decline diminishes with age, but rat pups reared in moderate hyperoxia make this transition earlier (Bavis et al., J. Appl. Physiol. 109(3): 796–803, 2010). This is consistent with heterokairy, a form of developmental plasticity in which environmental factors alter the rate and/or timing of developmental events. The present study investigated whether this effect is specific to the HVR or if hyperoxia instead accelerates overall development. Rat pups reared in 60% O2 (Hyperoxia) exhibited a less biphasic HVR at both P4 and P10‐11 compared to pups reared in 21% O2 (Control). However, growth curves and the average ages at which pups attained key developmental milestones (i.e., fur development at P5, incisor eruption at P9, and eye opening at P15) were similar between treatment groups. We also studied the relationship between metabolic rate and ambient temperature as another indicator of physiological maturity. The lower critical temperature (LCT) decreased with age, but the LCT was the same for Control and Hyperoxia rats at both P4 (~32°C) and P10‐11 (~30°C). The ability to increase metabolic rate at low ambient temperatures also increased with age, but this thermogenic capacity tended to be reduced in Hyperoxia rats at both P4 and P10‐11 (i.e., lower rates of metabolic CO2 production at temperatures below the LCT). We conclude that hyperoxia accelerates the rate at which the HVR matures without affecting the maturation of other morphological and physiological traits.