Role of plasma osmolality in the delayed onset of thermal cutaneous vasodilation during exercise in humans

To elucidate the role of increased plasma osmolality (P(osmol)), which occurs during exercise in the regulation of cutaneous vasodilation (CVD) during exercise, we determined the relationship between the change in esophageal temperature (ΔT(es)) required to elicit CVD (ΔT(es) threshold for CVD) and P(osmol) during light and moderate exercise (30 and 55% of peak oxygen consumption, respectively) and passive body heating. Then we compared the relationship with the data obtained in our previous study [A. Takamata, K. Nagashima, H. Nose, and T. Morimoto. Am. J. Physiol. 273 (Regulatory Integrative Comp. Physiol. 42): R197-R204, 1997], in which we determined the relationships during passive body heating following isotonic (0.9% NaCl) or hypertonic (2 or 3% NaCl) saline infusions in the same subjects. P(osmol) values at 5 min after the onset of exercise were 287.5 ± 0.9 mosmol/kgH₂O during light exercise and 293.0 ± 1.2 mosmol/kgH₂O during moderate exercise. P(osmol) just before passive body heating was 289.9 ± 1.4 mosmol/kgH₂O. The ΔT(es) threshold for CVD was 0.09 ± 0.05°C during light exercise, 0.31 ± 0.09°C during moderate exercise, and 0.10 ± 0.05°C during passive body heating. The relationship between the ΔT(es) threshold for CVD and P(osmol) was shown to be on the same regression line both during exercise and during passive body heating with or without infusions [A. Takamata, K. Nagashima, H. Nose, and T. Morimoto. Am. J. Physiol. 273 (Regulatory Integrative Comp. Physiol. 42): R197-R204,1997]. Our data suggest that the elevated body core temperature threshold for CVD during exercise could be the result of increased P(osmol) induced by exercise and is not due to reduced plasma volume or the intensity of the exercise itself.