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 β-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 β-adrenergic or combined cholinergic and β-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 β-adrenergic blockade. β-adrenergic blockade alone reduced reflex tachycardia as much as did combined blockade thereby suggestng that the predominant neural mechanism was β-adrenergic. Taken altogether these reuslts 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 β-adrenergic or sympathetic. © 1990.