Exogenous heating induces a more pronounced rise in prolactin concentration than exertional heating in young men, despite a closely matched body core temperature increase.

This study aimed to compare blood prolactin concentrations and thermal perceptions between exertional heating (Exer-H) and exogenous heating (Exo-H) eliciting a closely matched elevation of rectal temperature (Trec) but different changes in skin temperature (Tsk) in young active males. Fourteen participants were randomly exposed to Exer-H (ergocycle at 60 % VO2max at 30 °C) and Exo-H (hot-water immersion up to the waist, 43-44 °C). The heating phase ended when Trec (Peak) reached 39 °C, or was <39 °C but reached the volunteers' limit point of tolerance. The post-heating phase (passive sitting) ended when Trec decreased to 37.5 °C. Blood prolactin concentration and Tsk were assessed during the heating and post-heating phases, and thermal perceptions were determined during the heating phase. Peak Trec was slightly higher in Exo-H than Exer-H (39.00 ± 0.00 °C vs. 38.82 ± 0.27 °C; P = 0.028). The Δ changes in prolactin concentration and Tsk (especially Calf and Thigh) were larger in Exo-H during the heating and post-heating phases (P < 0.05). Sweating/shivering, thermal comfort, and thermal sensation scores did not differ between Exer-H and Exo-H during heating. Positive relationships were found between Δ changes in prolactin concentration and Δ changes in Tsk, with the best associations observed for Tsk Calf and Tsk Thigh in both phases. Thermal comfort/sensation correlated positively with Δ changes in prolactin concentration, while sweating/shivering correlated negatively with body mass loss (%). In conclusion, prolactin concentrations show a more pronounced increase during Exo-H compared to Exer-H, despite a closely matched elevation of Trec. This result may be explained by the large differences in Tsk of the lower limbs between the two heating conditions. Moreover, thermal perceptions are not differently affected during Exer-H and Exo-H. Our findings suggest that sweating sensation could partially reflect the hydration status, while thermal comfort and sensation may be partly connected to changes in prolactin concentrations.