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 (T_rec) but different changes in skin temperature (T_sk) in young active males. Fourteen participants were randomly exposed to Exer-H (ergocycle at 60 % VO_2max at 30 °C) and Exo-H (hot-water immersion up to the waist, 43–44 °C). The heating phase ended when T_rec (Peak) reached 39 °C, or was <39 °C but reached the volunteers’ limit point of tolerance. The post-heating phase (passive sitting) ended when T_rec decreased to 37.5 °C. Blood prolactin concentration and T_sk were assessed during the heating and post-heating phases, and thermal perceptions were determined during the heating phase. Peak T_rec 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 T_sk (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 T_sk, with the best associations observed for T_sk Calf and T_sk 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 T_rec. This result may be explained by the large differences in T_sk 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.