Modifications of sweating responses to thermal transients following heat acclimation.

The sweating response was studied before and after passive humid heat acclimation in four resting male subjects who were exposed to slow thermal transients increasing air and wall temperatures from 28 degrees C to 45 degrees C. The slopes of the ambient temperature increases were +0.19 degrees C . min-1; +0.16 degrees C . min-1 or +0.14 degrees C . min-1. Dew-point temperature and air velocity were kept constant (17.5 degrees C; 0.3 m . s-1). Continuous measurements were made of oesophageal temperature, mean skin temperature, whole-body sweat loss and of right upper limb sweating responses. The local sweating response was measured from an arm chamber under a local thermal clamp (Tsk,1 = 38 degrees C). The results confirmed the fact that heat acclimation to humid heat induces a shortening in the time lag of sweat onset and increases the local sweating rates while internal temperature changes are reduced. These modifications are interpreted as a non-linearity in the response of the central controller, involving both a change in the central gain and an upward resetting of the "local sweating rate-body temperature" curves, without any shifting of the hypothalamic set-point temperature as it is currently described. However, a modification of local sweat gland activity occurring with heat acclimation cannot be ruled out.