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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 microliters of high-Ca2+ solution (Ca2+, 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 (delta HR) and changes in mean arterial pressure (delta MAP) produced by bolus injections of phenylephrine or sodium nitroprusside (delta HR/delta MAP; 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 Ca2+ 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 Ca2+ (n = 6) or artificial cerebrospinal fluid (aCSF; n = 5). Administration of high Ca2+ reduced the bradycardic plateau and increased HR range without changes in average gain. Our results suggest a modulatory role for central Ca2+ in the baroreceptor reflex control of HR in SHR.