Acute hypoosmolality attenuates the suppression of cutaneous vasodilation with increased exercise intensity

We examined the hypothesis that elevation of the body core temperature threshold for forearm skin vasodilation (TH_FVC) with increased exercise intensity is partially caused by concomitantly increased plasma osmolality (P_osmol). 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 (V̇o_{2 peak}) under isoosmotic conditions (C), and at 65% V̇o_{2 peak} under isoosmotic (H_EXI_OS) and hypoosmotic (H_EXL_Os) conditions. In H_EXL _OS, hypoosmolality was attained by hypotonic saline infusion with DDAVP, a V₂ agonist, before exercise. P_osmol (mosmol/kgH₂O) increased after the start of exercise in both HEX trials (P < 0.01) but not in C. The average P_osmol at 5 and 10 min in H_EXI_OS was higher than in C (P < 0.01), whereas that in H_EXL_OS was lower than in H_EXI _OS (P < 0.01). The change in TH_FVC was proportional to that in P_osmol in every subject for three trials. The change in TH_FVC per unit change in P_osmol (ΔTH _FVC/ΔP_osmol, °C · mosmol^-1 · kgH₂O^-1) was 0.064 ± 0.012 when exercise intensity increased from C to H_EXI_OS, similar to 0.086 ± 0.020 when P_osmol decreased from H_EXI _OS to H_EXL_OS (P > 0.1). Moreover, there were no significant differences in plasma volume, heart rate, mean arterial pressure, and plasma lactate concentration around TH_FVC between H_EXI_OS and H_EXI_OS (P > 0.1). Thus the increase in TH_FVC due to increased exercise intensity was at least partially explained by the concomitantly increased P_osmol.