Differential age-dependent attenuation of reflex tachycardia by verapamil in rats

To determine if verapamil alters baroreflex function differently depending on age, reflex heart rate responses to intravenous infusions of phenylephrine or sodium nitroprusside were compared in conscious 5- and 14-month-old rats before and after daily oral administration of verapamil (100 mg/kg) for 6 days. The effects of verapamil on parasympathetic and sympathetic mediation of heart rate were also assessed by repeating baroreflex tests after treatment with either propranolol or atropine. All reflex heart rate responses were initially smaller in 14- than in 5-month-old rats. Regardless of age, magnitude of reflex bradycardia or the effects on it of either cholinergic or beta-adrenergic blockade, were unaffected by verapamil. By contrast, reflex tachycardia which was attenuated in both age groups, was decreased further by subsequent cholinergic or beta-adrenergic blockade in 5-month-old rats, but only by cholinergic blockade in 14-month-old rats. These findings suggest that while verapamil did not affect autonomic mediation of reflex bradycardia, it reduced that of reflex tachycardia differently depending on age. Whereas it attenuated both sympathetic and parasympathetic mediation of reflex tachycardia in 5-month-old rats, it attenuated only sympathetic mediation in 14-month-old rats.