Predictability of foot shock differentially affects the phasic blood pressure of SHR, BHR, and WKY rats

Because animals prefer predictable over unpredictable shock, it was hypothesized that rats receiving unpredictable foot shock would show greater phasic blood pressure responses. Furthermore, because some studies utilizing human subjects suggest that blood pressure responses to stress are greater in those with a positive family history of hypertension, the current study examined the phasic blood pressure response to alterations in foot shock predictability in rats with either zero, one, or two hypertensive parents. Predictability was manipulated by altering the percentage of trials during which foot shock was associated with a tone, while at the same time keeping the total number of foot shocks equal in all groups. During the first few trials, the most unpredictable foot shock was associated with higher blood pressure, a trend that was significant for the borderline hypertensive rat (BHR) and marginal for the spontaneously hypertensive rat (SHR). It was also noted that, over the 28 foot-shock trials comprising the stress session, habituation of the blood pressure response was obtained for the most unpredictable shock group. Again, this occurred in those rats with a genetic history of hypertension (SHR and BHR). The results are discussed in relation to studies of stress and blood pressure reactivity in humans with a positive family history of hypertension.