Ventilatory response to hypoxia in rats: gender differences

The ventilatory response to hypoxia of adult conscious rats, measured during sleep as change in ventilation-oxygen consumption ratio (VE/VO2) while breathing 10% O2 for 15-30 min, was found to be approximately 23% larger in females (female) than in males (male, p < 0.001). Also arterial PCO2 during hypoxia decreased more in female. The gender difference occurred at all metabolic levels, and was not related to normoxic VE/VO2 or the hypometabolic response to hypoxia; it was solely due to greater hyperpnea in female. With hypoxia, changes in blood pressure, heart rate and body temperature were similar between male and female; hence, gender differences in baro- or thermal-stimuli were not a contributing factor. Hematocrit, hemoglobin, lung, heart and diaphragm mass/body weight were also similar, whereas respiratory system compliance was higher, and resistance lower, in female. Ovariectomy did not change the female response. In prepubertal rats the VE/VO2 hypoxic response was also larger in female (approximately 12%, p < 0.05). After prolonged (approximately 4 months) hypoxia, the gender difference in the response to acute hypoxia was no longer statistically significant. In conclusion, conscious adult female rats have a higher ventilatory response to hypoxia than male. The reason for the difference remains elusive, but is probably not due to ovarian hormones.