Effect of gender on thermoregulation and survival of hypoxic rats

1. Hypothermia is a documented response to hypoxia but little is known about possible gender differences. Because female rats have a greater hypoxic ventilatory response than males, we hypothesized that females would be more tolerant of hypoxia. We studied 18 female and 18 male Long-Evans rats. 2. Radiotelemetry transmitters for body temperature (Tb) were implanted under general anaesthesia (90 mg/mL ketamine and 10 mg/mL xylazine; 0.1 mL/100 g bodyweight, i.p.). 3. Rats were exposed to 21, 16, 12, 10, 8, 6, 4 and 2% O2 (balance N2) for 30 min each in chambers kept at either 31 degrees C (clamped) or 20 degrees C (hypothermic). Survival was defined as ataxic and unresponsive. 4. Females were more hypoxia tolerant than males, often enduring 2% inspired O2 (13 km altitude). 5. This was correlated with a lower Tb in the hypothermic group, but not in the clamped group. 6. Hypothermia increased 'survival' of rats independent of gender. 7. When Tb was clamped, female rats showed significantly greater survival than males. 8. Thus, separate mechanisms (hypothermia or ventilation) may be acting to increase tolerance of clamped and hypothermic female rats to severe hypoxia.