Circulatory responses to heat after celiac ganglionectomy or adrenal demedullation.

Chloraloseanesthetized rats were implanted with Doppler flow probes on the mesenteric, renal, and external caudal arteries and were exposed to an ambient temperature of 40 degrees C. Heart rate, core (Tc) and tail-skin temperatures, and mean arterial blood pressure (MAP) were also monitored. Before heating, the celiac ganglion was removed (ganglionectomy) from one group of animals (n = 11) and a bilateral adrenal demedullation was performed in a second group (n = 14). As Tc progressively increased from 37 degrees C to 43 degrees C, MAP rose to a plateau then fell precipitously as Tc exceeded 41 degrees C. Ganglionectomy eliminated the rise in mesenteric resistance (P less than 0.05) and attenuated the rise in MAP compared with an intact control group (n = 11). Ganglionectomy also increased the heating rate (P less than 0.05) and reduced heat tolerance time (P less than 0.05). Demedullation attenuated the rise in both mesenteric resistance and MAP (P less than 0.05) and increased the rate of heating (P less than 0.05) compared with controls (n = 10). Renal and caudal resistance changes were similar in all groups. These data show the importance of intact adrenal medullas and sympathetic innervation to the splanchnic region in contributing to thermal tolerance in the rat. However, neither factor alone can explain splanchnic vasoconstriction during severe heat stress.