Autonomic regulation of reflex bradycardia in rats declines with age.

Heart rate responses, elicited reflexly by elevating blood pressure with phenylephrine or lowering it with sodium nitroprusside, were compared in groups of rats aged 4 and 14 months. All tests were done while the rats were awake to avoid artefacts due to anesthesia. Parasympathetic and sympathetic contributions were assessed by repeating baroreflex tests after cholinergic blockade with atropine or beta-adrenergic blockade with propranolol. Magnitude of reflex bradycardia and tachycardia was initially smaller in the 14-month-old rats than in the younger rats thereby indicating that baroreflex sensitivity had diminished with age. After beta-adrenergic or cholinergic blockade, adjusted means (obtained by covariance analysis) for reflex tachycardia did not differ significantly between rat groups, but those for reflex bradycardia were significantly smaller in the 14-month-old rats than in the younger rats. Selective attenuation of reflex bradycardia after either cholinergic or beta-adrenergic blockade indicates that concurrent parasympathetic activation and sympathetic withdrawal during reflex bradycardia were also reduced in 14-month-old rats. These results suggest that as autonomic mediation of reflex bradycardia diminishes with age, old rats may no longer be able to slow the heart as easily whenever blood pressure rises, but they can still accelerate it whenever blood pressure falls.