Age-related alterations in cardiac parasympathetic responsiveness: A preliminary report

Baroreceptor control of the heart rate is reduced by ageing in animals and man. This has been ascribed to an age-related reduction in β-adrenergic receptor density and cardiac responsiveness to sympathetic modulation. However, the baroreceptor-heart rate reflex largely depends on the vagus and the age-related changes in cardiac parasympathetic responsiveness have never been tested directly. We examined the heart rate responses to acetylcholine in six young (3-5 months) and six old (22-24 months) ketamine-anaesthetized, bilaterally vagotomized Sprague-Dawley rats instrumented with arterial and venous catheters. The acetylcholine was given as 2, 4 and 8μg/kg intravenous bolus injections. Linear regressions between each dose of acetylcholine and the ensuing bradycardia were calculated. The acetylcholine- induced bradycardia was strikingly larger in old than in young rats, amounting to 20.9 ± 4.4 and 8.6 ± 1.5 beats/min per μg per kg, respectively (P <0.05). Thus cardiac muscarinic receptor responsiveness is increased rather than reduced by ageing. Therefore not all functions involved In cardiovascular reg-ulation show an age-related impairment, and some may even be enhanced as age progresses. It is also clear that mech-anisms other than attenuation of cardiac responses to auto-nomic stimuli (central and/or afferent) account for the age-re-lated impairment in the baroreceptor-heart rate reflex.