EFFECT OF COLD-EXPOSURE ON ELECTROPHYSIOLOGICAL PROPERTIES OF RAT-HEART

Male rats exposed to the cold (4 degrees C) for five or ten days exhib ited modifications in their thyroid state, as documented by increases in serum thyroid hormone levels, to which differently graded modificat ions of heart weight/body weight ratio, heart rate, and resting metabo lic rate were associated. The values of the above mentioned thyroid st ate indicators returned to those of the control when the animals, kept at cold for ten days, were re-exposed to room temperature (24 degrees C) for an additional 10 days. The configuration of action potentials, recorded in vitro at 26 degrees C from fibres of anterior papillary m uscles, was different in control rats of different age and was affecte d by prolonged cold exposure. In fact, the action potential duration ( APD) increased after ten days of cold exposure. In the re-exposed grou p the APD was not different from that of the controls. Such a pattern was not significantly modified when the stimulation frequency increase d from 1 Hz to 5 Hz. The above results suggest that in cold exposure, as in experimental hyperthyroidism, thyroid hormone might exert a card iac chronotropic effect by modifying heart electrophysiological proper ties. Thus thyroid hormone should play a basic role during the exposur e to cold environment, stimulating the body metabolism and increasing heart rate as a response to the requirement for greater tissue perfusi on.