Effect of heat stress on cutaneous vascular responses to the initiation of exercise.

To explore further the competition between vasoconstrictor and vasodilator reflexes in the regulation of skin blood flow, responses in forearm blood flow (FBF) to the initiation of supine leg exercise were measured by plethysmography against a background of rising internal temperature. In 17 studies involving six men, skin temperature (Tsk) was controlled with water-perfused suits first at normothermic levels, followed by a 40- to 50-min period during which Tsk was held at 38-38.5 degrees C. Supine leg exercise at a moderate intensity (100-150 W) was performed for 5-6 min of each 15 min throughout, yielding one period of exercise performed during normothermic conditions and three periods of exercise performed during the period of elevated Tsk. On the average, FBF fell significantly with the beginning of each period of exercise (P less than 0.05). Furthermore, the amount by which FBF fell tended to increase with increasing levels of preexercise FBF. Thus the average fall in FBF associated with the onset of the last period of exercise, 2.45 ml X 100 ml-1 X min-1, significantly exceeded the 1.12 ml X 100 ml-1 X min-1 fall in FBF seen with onset of work in normothermic conditions. These responses were not due to changes in internal temperature as reflected by esophageal temperatures. However, individual studies occasionally revealed a reduction or abolition of the vasoconstrictor response with the last period of exercise. These findings are in agreement with earlier studies showing a cutaneous participation in the vasoconstrictor responses to exercise but also indicate that sufficient hyperthermia can attenuate or even abolish this response.