Gender difference in baroreflex-mediated bradycardia in young rats: role of cardiac sympathetic and parasympathetic components

In a previous clinical study we have demonstrated a significantly lower baroreflex-mediated bradycardic response in young women compared with men. The present study determined whether sexual dimorphism in baroreflex sensitivity in young rats also covers the reflex tachycardic response. The study was then extended to test the hypothesis that an attenuated cardiac cholinergic component of the baroreflex heart rate response in females may account for the gender difference. Baroreflex sensitivity (BRS) was expressed as the regression coefficient of the reciprocal relationship between evoked changes in blood pressure and heart rate. BRS measured in conscious rats with phenylephrine (BRSPE) and nitroprusside (BRSNP) represented the reflex bradycardic and tachycardic responses, respectively. Female rats exhibited significantly lower BRSPE compared with male rats (-1.53 +/- 0.1 vs. -2.36 +/- 0.13 beats.min(-1).mmHg(-1); p < 0.05) but similar BRSNP (-2.60 +/- 0.20 vs. -2.29 +/- 0.17 beats.min(-1).mmHg(-1)). Blockade of cardiac muscarinic receptors with atropine methyl bromide elicited greater attenuation of BRSPE in male than in female rats (72 +/- 4.6 vs. 53 +/- 6.7% inhibition; p < 0.01) and abolished the gender difference. In male rats cardiac muscarinic blockade attenuated BRSPE significantly more than did cardiac beta-adrenergic receptor blockade with propranolol (72 +/- 4.6 vs. 43 +/- 2.7; p < 0.01), which suggests greater dependence of BRSPE on the parasympathetic component. In females, muscarinic and beta-adrenergic blockade elicited similar attenuation of BRSPE. The findings suggest that (i) BRS is differentially influenced by gender; female rats exhibit substantially lower BRSPE but similar BRSNP compared with age-matched male rats and (ii) the sexual dimorphism in BRSPE results, at least partly, from a smaller increase in vagal outflow to the heart in response to baroreceptor activation.