Gender considerations in ventilatory and metabolic development in rats: Special emphasis on the critical period

• Hypoxic response was weakest during the critical period in both male and female rats.
• Age effect was highly significant in both genders in all parameters and ages tested.
• Gender was never significant as a main effect analyzed statistically from P0 to P21.
• Both genders exhibited comparable developmental trends in normoxia and hypoxia.
• The critical period exists in both genders in rats.

In rats, a critical period exists around postnatal day (P) 12–13, when an imbalance between heightened inhibition and suppressed excitation led to a weakened ventilatory and metabolic response to acute hypoxia. An open question was whether the two genders follow the same or different developmental trends throughout the first 3 postnatal weeks and whether the critical period exists in one or both genders. The present large-scale, in-depth ventilatory and metabolic study was undertaken to address this question. Our data indicated that: (1) the ventilatory and metabolic rates in both normoxia and acute hypoxia were comparable between the two genders from P0 to P21; thus, gender was never significant as a main effect; and (2) the age effect was highly significant in all parameters studies for both genders, and both genders exhibited a significantly weakened response to acute hypoxia during the critical period. Thus, the two genders have comparable developmental trends, and the critical period exists in both genders in rats.