Sweat rate vs. forearm blood flow during lower body negative pressure.

Skin blood flow is inhibited when hyperthermia and added hypovolemic stresses are superimposed. We tested the hypothesis that part of this inhibition is a reduced drive for cutaneous active vasodilatation (AVD) with sweat rate (SR) taken as an indirect measure of the efferent drive for cutaneous AVD. We also inquired whether SR itself changes with redistribution of blood volume. Six healthy supine men were subjected to lower body negative pressure (LBNP) after heating in water-perfused suits increased esophageal temperatures (Tes) to a mean of 37.2 degrees C and at least doubled SR and forearm vascular conductance (FVC). Heating continued throughout LBNP and recovery. Sweat rate did not decrease with LBNP onset, although SR-Tes slopes during LBNP were reduced 28% from control. In four subjects the SR-Tes slope did not recover when LBNP was discontinued. These observations suggest that SR is not an effector of the low-pressure baroreflex. In contrast to SR, FVC abruptly fell 22% at the onset of LBNP. Thereafter, FVC-Tes slopes near zero or less occurred. The major effector for FVC inhibition with LBNP appears to be the neural vasoconstrictor system. A minor component due to reduced drive for cutaneous AVD probably occurs as well.