Osmoreceptors do not exhibit a sex-dependent modulation of forearm skin blood flow and sweating

Studies show that increases in plasma osmolality result in a delayed onset threshold of thermoeffector responses. However, it remains unclear if there are sex-related differences in the osmotically induced changes in both sweating and cutaneous vascular conductance (CVC). Nine young men and nine young women were passively heated (water-perfused suit) to 1.5 degrees C above baseline esophageal temperature while in an isosmotic (0.9% NaCl saline infusion) (ISO) and hyperosmotic (3% NaCl saline infusion) (HYP) state. Forearm sweat rate (ventilated capsule), skin blood flow (laser-Doppler), esophageal temperature and skin temperature were continuously recorded. Sweat gland output (SGO) on the forearm was calculated from the number of heat activated sweat glands (modified iodine-paper technique) at the end of heating. The onset threshold and thermosensitivity of sweating and CVC were determined using the linear portion of each response plotted against mean body temperature and analyzed using segmented regression analysis. We show that the osmotically induced delay in the onset threshold of sweating and CVC is similar between males and females. Although the thermosensitivity of CVC was similar between sexes (P = 0.601), the thermosensitivity of sweating was consistently lower in females compared to males (P = 0.018). The lower thermosensitivity in sudomotor response of females was accompanied by a lower SGO (P = 0.003), albeit similar sweat gland activation to males (P = 0.644). We conclude that sex-related differences in thermoeffector activity are independent of osmoreceptor activation. Therefore, osmoreceptors do not exhibit sex-related differences in the modulation of CVC and sweating responses during heat stress.