Interaction between “mental stress” and baroreceptor reflexes concerning effects on heart rate, mean arterial pressure and renal sympathetic activity in conscious spontaneously hypertensive rats

Abstract

Spontaneously hypertensive rats (SHR) and normotensive Wistar Kyoto rats (WKY) were compared concerning the interactions between cortico-hypothalamic alerting responses and baroreflex influences on neurogenic cardiovascular control. Mean arterial pressure (MAP), heart rate (HR) and renal sympathetic nerve activity (RSNA) were continuously recorded during nighttime in conscious, otherwise undisturbed rats. Baroreceptor sensitivity was assessed as percentage HR and RSNA reductions per mm Hg MAP elevation when a standard phenylephrine infusion was performed. A state of acute mental stress could be induced by a likewise standardized sudden blowing of air. These 2 opposing influences on neurogenic cardiovascular control were also experimentally superimposed in various ways and the effects on MAP, HR and RSNA followed. During rest, RSNA was higher in SHR than in WKY and it also increased more during mental stress. The baroreflex sensitivity was clearly reduced in SHR and WKY concerning HR reduction (0.44 .+-. 0.06 vs. 0.78 .+-. 0.08%/mm Hg; P < 0.01) but not so concerning RSNA, which was similar in SHR and WKY (2.6 .+-. 0.2 vs. 2.9 .+-. 0.4%/mm Hg). Single fiber recordings on anesthetized rats showed the same principle difference between SHR and WKY. Addition of mental stress during phenylephrine baroreflex activation clearly increased both HR (24 .+-. 7%) and RSNA (114 .+-. 21%) in SHR, while almost no change then occurred in WKY (HR + 1 .+-. 3%; P < 0.025 vs. SHR and RSNA + 11% .+-. 10, P < 0.01 vs. SHR). A modestly accentuated cortico-hypothalamic activity evidently ordinarily prevails in SHR, explaining the suppressed baroreflex control of heart rate and the augmented sympathetic activity to, e.g., renal and splanchnic areas. Environmental alerting stimuli induce in SHR more powerful defence reactions which, unlike the situation in WKY, readily overcome baroreflex inhibitory influences on sympathetic activity.