Telemetry augments the validity of the rat as a model for heat acclimation

Repeated exposure to heat stress results in physiological adaptations which increase tolerance to heat stress. Core temperature (Tc) and heart rate (HR), two indices of successful heat acclimation, are both increased by the handling and restraint usually required to measure these variables in the rat. This study compares measurement of these variables in telemetry- and nontelemetry-equipped heat-acclimated rats. Four groups of male Sprague-Dawley rats were used: CAC (24, control for acclimated), HAC (24, heat acclimated), TCAC (3, telemetry implanted control for acclimation), and THAC (3, telemetry implanted, heat acclimated). The CAC and TCAC rats were maintained at 26 degrees C for 2 weeks; HAC and THAC groups were maintained at 32-33 degrees C for 2 weeks. The telemetry rats were surgically implanted with transmitters which measured activity, temperature, blood pressure (BP), and ECG. Following acclimation, the rats were subjected to a heat-stress test as follows: animals were placed in a chamber maintained at 41.5 degrees C until a Tc of 41.5 degrees C was reached, when the animals were removed to a 26 degrees C chamber to recover. The Tc of the CAC and HAC rats was manually measured with a rectal probe every 15 min. The TCAC and THAC rats had Tc, HR, and BP monitored electronically every 2.5 min from 30 min prior to heat stress through 30 min after heat. The THAC rats maintained lower Tc, HR, and BP than the TCAC rats throughout the heat stress. The CAC and HAC rats had higher Tc and slower rates of cooling after heat than either of the telemetry groups. Simultaneous measurement of HR, BP, and Tc has not previously been reported in unrestrained heat-acclimated rats. Measurement of these variables without the confounding effects of restraint or handling has increased the validity of the rat as a model for human heat acclimation.