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 bar oreflex-mediated bradycardic response in young women compared with men. The present study determined whether sexual dimorphism in baroreflex sensitivi ty in young rats also covers the reflex tachycardic response. The study was then extended to test the hypothesis that an attenuated cardiac cholinergi c component of the baroreflex heart rate response in females may account fo r the gender difference. Baroreflex sensitivity (BRS) was expressed as the regression coefficient of the reciprocal relationship between evoked change s in blood pressure and heart rate. BRS measured in conscious rats with phe nylephrine (BRSPE) and nitroprusside (BRSNP) represented the reflex bradyca rdic and tachycardic responses, respectively. Female rats exhibited signifi cantly lower BRSPE compared with male rats (-1.53 +/- 0.1 vs. -2.36 +/- 0.1 3 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 recep tors 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 blocka de attenuated BRSPE significantly more than did cardiac beta-adrenergic rec eptor blockade with propranolol (72 +/- 4.6 vs. 43 +/- 2.7; p < 0.01), whic h suggests greater dependence of BRSPE on the parasympathetic component. In females, muscarinic and beta-adrenergic blockade elicited similar attenuat ion of BRSPE. The findings suggest that (i) BRS is differentially influence d by gender; female rats exhibit substantially lower BRSPE but similar BRSN P compared with age-matched male rats and (ii) the sexual dimorphism in BRS PE results, at least partly, from a smaller increase in vagal outflow to th e heart in response to baroreceptor activation.