Effect of progesterone on respiratory response to moderate hypoxia and apnea frequency in developing rats.

We used whole-body plethysmography and pulse oximetry to assess the effects of acute administration of progesterone (4 mg/kg, i.p.) on normoxic ventilation, hypoxic ventilatory response (HVR: FiO(2)=12% over 20 min), metabolism, and apnea frequency in rats on postnatal (P) days P1, P4, P7, and P12. Arterial oxygen saturation was continuously measured, and apneas were discriminated based on the degree of associated desaturation, at least 5 units less than the value before the desaturation. In normoxia, progesterone did not alter ventilation, metabolism or the coefficient of variation of minute ventilation at any age studied when compared with the control group (saline). However, it decreased apnea frequency and apnea associated with desaturation only in P1 rats. In hypoxia: progesterone increased the peak HVR in P4 and P7 rats, increased the steady-state HVR (mean at 15-20 min of exposure) in P1, P4 and P7 without affecting the rats' metabolic rate, decreased the coefficient of variation of minute ventilation in P4 and P7 rats, and finally, decreased apnea frequency only in the P1 rats with no effect on apnea associated with desaturation at any age. We conclude that acute administration of progesterone has no effect on baseline ventilation, but it increases HVR in rats younger than 7 days, and decreased the frequency of apnea only in P1 rats.