Mediation of reflex tachycardia becomes exclusively beta-adrenergic in old Fischer 344 rats.

To study how baroreflex regulation changes with age we compared reflex heart rate responses elicited in awake Fischer 344 rats of different ages during intravenous infusions of phenylephrine or sodium nitroprusside. Underlying neural mechanisms were assessed by repeating baroreflex tests following cholinergic blockade with methylatropine or beta-adrenergic blockade with propranolol. Basal heart rates always tended to be lower in old than in young rats, but the differences became statistically significant only after beta-adrenergic or combined cholinergic and beta-adrenergic blockade. Most reflex heart rate responses (i.e. except reflex tachycardia in males during depressor responses to sodium nitroprusside) were initially weaker in old than in younger rats. Reflex bradycardia and tachycardia were reduced equally in both age groups after cholinergic blockade, but were reduced more in old than in young rats after beta-adrenergic blockade. beta-adrenergic blockade alone reduced reflex tachycardia as much as did combined blockade thereby suggesting that the predominant neural mechanism was beta-adrenergic. Taken altogether these results are compatible with the interpretation that efferent pathways for mediating heart rate reflexes in Fischer 344 rats are altered with age such that as parasympathetic mediation diminishes, residual mediation particularly of reflex tachycardia, becomes almost exclusively beta-adrenergic or sympathetic.