INFLUENCE OF EXERCISE INTENSITY AND PLASMA-VOLUME ON ACTIVE CUTANEOUS VASODILATION IN HUMANS

The influence of dynamic exercise on active cutaneous vasodilation was evaluated in eight male subjects. We measured the increase in internal body temperature (esophageal temperature, T-es) required to elicit active cutaneous vasodilation and the slope of the linear relationship between increases in forearm skin vascular conductance (Delta FVC) and T-es during indirect heating (legs immersed in 44 degrees C water for 30 min), 30 min of light exercise (LEX; 75 +/- 5 W = 30% maximal oxygen uptake, VO2max), and 20 min of moderate exercise (MEX, 149 +/- 7 W = 60% VO2max) Studies were conducted in the supine position at 30 degrees C (RH <30%) and mean skin temperature averaged 35.09 +/- 0.12 degrees C. During indirect heating and LEX, cutaneous vasodilation occurred after a similar increase in T-es, 0.03 +/- 0.02 degrees C and 0.11 +/- 0.02 degrees C, respectively. During MEX, T-es increased 0.42 +/- 0.06 degrees C before the onset of cutaneous vasodilation (P < 0.05, different from rest and LEX). The relationship between the increase in T-es threshold for vasodilation and exercise intensity was nonlinear, indicating that some minimal exercise intensity was required to elicit a delay in active cutaneous vasodilation. That minimal exercise intensity was greater than 30% VO2max (75 +/- 5 W). During MEX the increase in T-es threshold for vasodilation was inversely related to resting plasma volume (ml . kg(-1)) with a larger initial plasma volume associated with a smaller increase in T-es threshold for cutaneous vasodilation (r(2) = 0.67, P = 0.03). In addition, peak cutaneous vasodilation represented as the peak Delta FVC/peak Delta T-es, was directly related to plasma volume (r(2) = 0.64, P = 0.03). We conclude that the influence of exercise on the threshold for active cutaneous vasodilation is dependent on an interaction between exercise intensity and the size of the intravascular compartment. This conclusion is consistent with the hypothesis that blood volume expansion is an important adaptation of exercise training and contributes to improved thermoregulatory control during exercise in the heat.