Cardiovascular effects of whole-body heating in spontaneously hypertensive rats

The cardiovascular response to severe whole-body heating was examined in anesthetized spontaneously hypertensive (SH) (mean BP = 140 Torr) and normotensive (N) rats (mean BP = 96 Torr). Elevation of colonic temperature to 44 degrees C resulted in an initial increase in arterial pressure (mean BP: SH = 199 Torr, N = 124 Torr) with a subsequent severe hypotension. There was little evidence to suggest that this collapse in systemic pressure was related to a direct heat impairment of cardiac function. It was more likely the result of a progressive venodilation which produced venous pooling and decreased venous return. The arterial blood pressure response to exogenous catecholamine challenge decreased at elevated temperatures. This suggests that there was a deterioration of the vasoconstrictor response and indicated a possible effect of heat at the receptor or effector level. There were significant differences between the responses of normotensive and hypertensive rats to whole-body heating suggesting an enhanced cardiovascular sensitivity in hypertensive rats that may partially explain earlier reports of inferior thermoregulation and heat intolerance in these animals.