Prolactinotrophic effect of endogenous and exogenous heat loads in human male adults.

Factors associated with heat-induced increase in blood prolactin (PRL) were investigated. Ten male volunteers (23.7 +/- 2.2 yr) were exposed to exogenous heating (head-out immersion) in 41 degrees C water (control 37 degrees C) for 30 min with and without face fanning and cooling. In seven of the subjects, endogenous heating was produced by a 45-min exercise in a warm environment (41 degrees C; control 10 degrees C) with and without selective face fanning. Venous blood was collected before and after each trial; blood hormones were analyzed by radioimmunologic techniques. Heat loading, whether exogenous or endogenous in origin, induced significant increases in blood PRL, beta-endorphin, and vasoactive intestinal peptide (VIP) levels. Blood thyrotropin (TSH) level decreased significantly during water immersion and more significantly with face cooling. From measurement in peripheral blood, the differential beta-endorphin, VIP, and TSH responses to selective face ventilation during exogenous and endogenous heat exposures suggest that blood PRL released in heat derives from secretory stimuli that are independent of these prolactinotropic factors.