Aging impairs heart rate reflexes earlier in female than in male Sprague-Dawley rats

We compared heart rate reflexes in conscious male or female Sprague-Dawley rats at ages of 4, 14, or 24 months to determine whether, with advancing age, baroreflex sensitivity diminishes uniformly in both sexes. Phenylephrine or sodium nitroprusside was infused intravenously to elevate or lower systemic arterial pressure and thereby elicit reflex changes in heart rate. Ensuing blood pressure responses to either drug were smaller at 24 months than at 4 or 14 months in males but did not differ between age groups in females. By contrast, reductions in reflex tachycardia or bradycardia were significant at 14 and 24 months in females but only at 24 months in males. Regression slopes from 4 to 14 months of age, though unaltered in males, fell significantly in females (from 2.35 +/- 0.2 to 1.28 +/- 0.2 for tachycardia, and from -2.17 +/- 0.1 to -1.46 +/- 0.1 for bradycardia). Thus, heart rate reflexes though eventually impaired in both sexes, were impaired earlier in females than in males.