Time course of changes in baroreceptor reflex control of heart rate in conscious SHR and WKY: contribution of the cardiac vagus and sympathetic nerves.

1. The aim of this study was to assess the vagal and sympathetic nerve contribution to the relationship between mean arterial pressure (MAP) and heart rate (HR) at 6, 9, 14 and 20 weeks of age in conscious Wistar-Kyoto (WKY) and spontaneously hypertensive rats (SHR) with methoxamine- and nitroprusside-induced steady-state changes in blood pressure. 2. MAP increased with age in both strains but was 17-23% higher in SHR. 3. By contrast baroreflex parameters (HR range: difference between upper and lower HR plateaus, and gain: average slope between inflection points of the logistic MAP-HR relationship) decreased with age in SHR but increased in WKY. 4. After methylatropine, no differences in the cardiac sympathetic baroreflex range or gain parameters were observed between strains or ages. 5. It was concluded that older SHR have normal sympathetic but reduced vagal capacity to control HR in response to changes in MAP, but that this deficit was not dependent on the absolute level of blood pressure. 6. Because the differences were confined to one effector, SHR may have different central rather than arterial baroreceptor afferent mechanisms.