We examined the hypothesis that elevation of the body core temperature threshold for forearm skin vasodilation (THFVC) with increased exercise intensity is partially caused by concomitantly increased plasma osmolality (Posmol). Eight young male subjects, wearing a body suit perfused with warm water to maintain the mean skin temperature at 34 ± 1°C (ranges), performed 20-min cycle-ergometer exercise at 30% peak aerobic power (V̇o2 peak) under isoosmotic conditions (C), and at 65% V̇o2 peak under isoosmotic (HEXIOS) and hypoosmotic (HEXLOs) conditions. In HEXL OS, hypoosmolality was attained by hypotonic saline infusion with DDAVP, a V2 agonist, before exercise. Posmol (mosmol/kgH2O) increased after the start of exercise in both HEX trials (P < 0.01) but not in C. The average Posmol at 5 and 10 min in HEXIOS was higher than in C (P < 0.01), whereas that in HEXLOS was lower than in HEXI OS (P < 0.01). The change in THFVC was proportional to that in Posmol in every subject for three trials. The change in THFVC per unit change in Posmol (ΔTH FVC/ΔPosmol, °C · mosmol-1 · kgH2O-1) was 0.064 ± 0.012 when exercise intensity increased from C to HEXIOS, similar to 0.086 ± 0.020 when Posmol decreased from HEXI OS to HEXLOS (P > 0.1). Moreover, there were no significant differences in plasma volume, heart rate, mean arterial pressure, and plasma lactate concentration around THFVC between HEXIOS and HEXIOS (P > 0.1). Thus the increase in THFVC due to increased exercise intensity was at least partially explained by the concomitantly increased Posmol.
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