Effect of hyperthermia induced by environmental heat stress on vascular interoceptor reflexes

Abstract

The effect of hyperthermia induced by environmental heat stress on the vascular interoceptor reflexes was studied in rabbits under urethane anesthesia (l g/kg). The pressor reflex was elicited by occlusion of the common carotid artery, and the depressor reflex by stimulation of the depressor nerve. The arterial blood pressure and the rectal temperature were recorded continuously. Besides, the firing rate of the depressor nerve was recorded by means of an X-Y recorder through a bioelectric impulse analyzer made in our department. The nerve discharges were amplified by a preamplifier and monitored on an oscilloscope. The arterial blood pressure was maintained at a relatively stable level in most cases until the rectal temperature rose to 41°-42 °C, and then fell significantly above this temperature. The decrease of pressor reflex began at 40°-41 °C, and was more marked at a higher rectal temperature, whereas the decrease of the depressor reflex occurred only when the temperature rose to 42 °C and was not so significant as that of the pressor reflex. The firing rate of the depressor nerve showed no decrease in all the seven cases, however it increased significantly in two. The mechanism for the reduction of the vascular interoceptor reflexes under heat stress-induced hyperthermia was analysed experimentally. The results suggested that the reduction of the vascular interoceptor reflexes was not due to impaired responsiveness of the effectors, i.e. heart and vascular smooth muscle, nor due to that of the arterial baroreceptor, but due to weakening of the regulatory function of the cardiovascular centers.

Keywords

heat stress-induced hyperthermia / vascular interoceptor reflexes / pressor reflex / depressor reflex / firing rate of depressor nerve / cardiovascular centers

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