Centrally administered calcium increases the maximum vagal activation of baroreceptor reflex control of heart rate in spontaneously hypertensive rats

Baroreceptor reflex control of heart rate (HR) was examined before and 15 min after intracerebroventricular infusion (i.c.v.) of 10 μl of high-Ca 2+ solution (Ca 2+ , 16.3 mM) in conscious spontaneously hypertensive (SHR) and Wistar-Kyoto rats (WKY). The slope of the individual regression line of the relation between reflex HR changes (ΔHR) and changes in mean arterial pressure (AMAP) produced by bolus injections of phenylephrine or sodium nitroprusside (ΔHR/ΔMAP; beats/min/ mm Hg) for bradycardia was significantly less in SHR (-0.60 ± 0.30; n = 10) than in WKY (-1.78 ± 0.27; n = 10; p < 0.01) at baseline. The slope increased in SHR during administration of high Ca 2+ to -1.39 ± 0.17 (p < 0.01) but not in WKY. In contrast, no significant changes were observed for the reflex tachycardia both in SHR (n = 7) and WKY (n = 10). Further, we analyzed sigmoidal MAP-HR reflex curves in SHR with i.c.v. of either high Ca 2+ (n = 6) or artificial cerebrospinal fluid (aCSF; n = 5). Administration of high Ca 2+ reduced the bradycardic plateau and increased HR range without changes in average gain. Our results suggest a modulatory role for central Ca 2+ in the baroreceptor reflex control of HR in SHR.