Isoproterenol sensitivity in heat tolerant and relatively heat intolerant men.

Recent research has demonstrated that pharmacologic blockade of beta-adrenoceptors predisposes to hyperthermia during prolonged exercise. To investigate the hypothesis that beta-adrenoceptor sensitivity to catecholamines may be an important determinant of exertional heat tolerance, we performed a cross-sectional study comparing the heart rate responses to graded doses of isoproterenol in 6 heat tolerant and 6 relatively heat intolerant men. We observed no significant difference (p greater than 0.1) between the heat tolerant (0.9 +/- 0.68 microgram) and heat intolerant (1.19 +/- 0.61 microgram) subjects in the dose of isoproterenol that produced a 25 beat.min-1 increment in heart rate. Although the possibility of a relationship between beta-adrenoceptor sensitivity and the ability to tolerate exercise in heat cannot be entirely excluded on the basis of these data, our study clearly demonstrates the lack of a correlation between cardiac pacemaker sensitivity to isoproterenol and exertional heat tolerance.