Ten-day endurance training attenuates the hyperosmotic suppression of cutaneous vasodilation during exercise but not sweating

It is well known that hyperosmolality suppresses thermoregulatory responses and that plasma osmolality (P_osmol) increases with exercise intensity. We examined whether the decreased esophageal temperature thresholds for cutaneous vasodilation (TH_FVC) and sweating (TH_SR) after 10-day endurance training (ET) are caused by either attenuated increase in P_osmol at a given exercise intensity or blunted sensitivity of hyperosmotic suppression. Nine young male volunteers exercised on a cycle ergometer at 60% peak oxygen consumption rate (V̇O_{2 peak}) for 1 h/day for 10 days at 30°C. Before and after ET, thermoregulatory responses were measured during 20-min exercise at pretraining 70% V̇O_{2 peak} in the same environment as during ET under isoosmotic or hyperosmotic conditions. Hyperosmolality by ∼4% mosmol/kgH₂O was attained by acute hypertonic saline infusion. After ET, V̇O_{2 peak} and blood volume (BV) both increased by ∼4% (P < 0.05), followed by a decrease in TH_FVC (P < 0.05) but not by that in TH_SR. Although there was no significant decrease in P_osmol at the thresholds after ET, the sensitivity of increase in TH_FVC at a given increase in P_osmol [ΔTH_FVC/ ΔP_osmol, °C · (mosmol/kgH₂O)^-1], determined by hypertonic infusion, was reduced to 0.021 ± 0.005 from 0.039 ± 0.004 before ET (P < 0.05). The individual reductions in ΔTH_FVC/ ΔP_osmol after ET were highly correlated with their increases in BV around TH_FVC (r = -0.89, P < 0.005). In contrast, there was no alteration in the sensitivity of the hyperosmotic suppression of sweating after ET. Thus the downward shift of TH_FVC after ET was partially explained by the blunted sensitivity to hyperosmolality, which occurred in proportion to the increase in BV.