It is well known that hyperosmolality suppresses thermoregulatory responses and that plasma osmolality (Posmol) increases with exercise intensity. We examined whether the decreased esophageal temperature thresholds for cutaneous vasodilation (THFVC) and sweating (THSR) after 10-day endurance training (ET) are caused by either attenuated increase in Posmol 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 (V̇O2 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% V̇O2 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, V̇O2 peak and blood volume (BV) both increased by ∼4% (P < 0.05), followed by a decrease in THFVC (P < 0.05) but not by that in THSR. Although there was no significant decrease in Posmol at the thresholds after ET, the sensitivity of increase in THFVC at a given increase in Posmol [ΔTHFVC/ ΔPosmol, °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 ΔTHFVC/ ΔPosmol after ET were highly correlated with their increases in BV around THFVC (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 THFVC after ET was partially explained by the blunted sensitivity to hyperosmolality, which occurred in proportion to the increase in BV.
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