Vascular reactivity and baroreflex function during hyperthermia in conscious rats

The hemodynamic responses to vasoconstrictor agents are blunted during heat ing in anesthetized rats. It is unknown whether reflex neural responses to these agents are also altered during hyperthermia. Therefore, the purpose o f this study was to determine the effect of hyperthermia on the hemodynamic and baroreflex-mediated sympathetic neural responses to vasoactive agents in conscious, unrestrained rats. The splanchnic sympathetic nerve activity (SpNA) and systemic and regional hemodynamic responses to injections of phe nylephrine and sodium nitroprusside were measured during normothermia (37 d egrees C) and hyperthermia (41.5 degrees C). The hemodynamic responses to p henylephrine and sodium nitroprusside were blunted with heating, whereas th e SpNA responses to both agents were augmented or unchanged. At 41.5 degree s C, the baroreflex curves relating heart rate (HR) and SpNA to mean arteri al blood pressure were shifted to the right. The operating range and gain o f the blood pressure (BP)-HR reflex were significantly reduced during heati ng, whereas the operating range of the BP-SpNA reflex was augmented at 41.5 degrees C. These results indicate that heating alters the cardiovascular a nd sympathetic neural responses to vasoactive agents in vivo. Furthermore, the data suggest that heating differentially affects arterial baroreflex co ntrol of HR and SpNA, shifting both curves toward higher BP values but sele ctively attenuating baroreflex control of HR.