Intermittent hypoxia increases blood pressure in old female rats

Blood pressure (BP) in male rats exposed to intermittent hypoxia (IH) increases (5.4 ± 1.0 mm Hg), but not in females (1.6 ± 1.0 mm Hg). Female sex hormones appear to protect against the BP increase because ovariectomized females respond to IH with an increase in BP (5.1 ± 1.0 mm Hg) similar to males. This study tested the hypothesis that old female rats with low plasma estradiol would also respond with BP increases during IH. BP was measured by radiotelemetry in 24 month old female rats (n=5). The phases of the experiment were control, IH, and recovery (7days/phase). During IH the rats were exposed to 8 hours of IH while sleeping (8am‐4pm) when oxygen levels cycled between 21% (90 sec) and 10% (90 sec). Animals were not exposed to IH during the control and recovery phases. BP during the control and IH phases were compared, and the increase in BP during IH was 4.7 ± 0.6 mm Hg. The BP response to IH was significantly greater in 24 month old females compared to young females (p<0.05). The elevated BP in old females was sustained throughout the 24 hours, and returned to control levels during the recovery phase. Plasma estradiol levels were low, 3.4±1.0 pg/ml, compared to young rats (52±8 pg/ml). This study provides more evidence that female sex hormones protect against the BP increase during IH. These results also predict that the hypertension associated with sleep apnea is more prevalent in postmenopausal compared to premenopausal women.