Hyperthermia: a hyperadrenergic state.

E vaporation of 1 liter of sweat can remove 580 kcal from the body.This has enabled human subjects to withstand oven temperatures as high as 205° C while meat in the same oven became well-cooked.1 Our well-developed sweating mechanism provides us with a unique capacity to withstand high ambient temperature.Without it, we are more susceptible than other mammals.This editorial was stimulated by a report in this journal entitled "Arterial baroreceptor reflex modulation of sympathetic-cardiovascular adjustments to heat stress" by Kregel, Johnson, Tipton, and Seals. 2 They exposed rats, a nonsweating mammal, to an environmental temperature of 42° C before and after denervation of carotid sinus and aortic baroreceptors.Before denervation, body temperature (T c ) rose to 41° C in approximately 1.5 hours while mean arterial pressure, heart rate, estimated mesenteric and renal vascular resistance and plasma norepinephrine concentration all rose significantly.Denervation of arterial baroreceptors markedly exaggerated each of these responses, so that T c rose to 41° C in approximately one half the time and during that time the other variables all rose faster and to much higher values.Their work indicates that hyperthermia by itself is such a powerful stimulus to sympathoadrenal activity in the rat that the cardiovascular effects of this activity must be opposed by arterial baroreceptor reflexes to prevent severe hypertension and elevated cardiac output, which in turn would lead to more rapid heat gain.Kregel and colleagues 2 postulate that the responses of rats and those of humans may become