Limited baroreflex control of heart rate in young stroke-prone spontaneously hypertensive rats

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Abstract
Controversy regarding possible differences of baroreflex gain in spontaneously hypertensive rats (SHR) and their relationship to the rise in blood pressure may be due in part to variations in the methods used to assess baroreflex function. In this study, we have compared the baroreflex control of heart rate in normotensive (Wistar-Kyoto, WKY) and stroke-prone spontaneously hypertensive (SHRSP) rats at 1 and at 7 months of age. Mean arterial pressure and heart rate were monitored in conscious rats following implantation of arterial and venous catheters. Phenylephrine and nitroprusside were given intravenously and the peak responses of mean arterial pressure and heart rate were recorded. In the young rats, these recordings were repeated under anaesthesia. Individual slopes for responses to phenylephrine or nitroprusside were obtained by linear regression. A single relationship covering both sets of responses was also obtained by fitting the data to a sigmoidal curve. The latter approach enabled the baroreflex to be represented as a single function which has a single determinant of gain, operates within defined limits and can be readily related to resting mean and arterial pressure and heart rate. This approach demonstrated that: (1) in adult SHRSP, the baroreflex had reset to operate at higher resting levels of mean arterial pressure; (2) the range of heart rate control was smaller in both young and adult SHRSP compared with WKY; (3) average gain was slightly, but not significantly lower in adult SHRSP; (4) anaesthesia reduced heart rate range and average gain in both strains of rat. The results indicate that in SHRSP, even before hypertension develops, there is a deficiency in the baroreflex mechanism which may compromise its ability to counteract pressor influences.