Priming of the sweat glands explains reflex sweating in the heat.

The purpose of this study was to determine whether reflex sweating during isometric handgrip exercise (IHG) in the heat was due to a priming effect in the sweat glands or an increase in skin temperature. Ten male subjects completed four trials where they performed IHG for three minutes at 40% of their maximal voluntary contraction (MVC). The four trials included: (1) a control trial in thermoneutral conditions (23±1°C), (2) after sitting in hyperthermic conditions (35±1°C) for 30 min, (3) a local heating trial after having their non-exercising arm wrapped in a heat pad that maintained forearm skin temperature at ~35°C for 30 min, 4) and after pilocarpine iontophoresis to a 5 cm(2) area of the forearm. The sweating rate (SR), as measured by resistance hygrometry, was not significantly different (P>0.05) from baseline during IHG in either the control or local heating trial, but was significantly increased (P<0.05) from baseline during the hyperthermic and pilocarpine trials. Baseline SR values of the hyperthermic and pilocarpine trials (~0.25mg/cm(2)/min) were significantly greater than the control and local heating trials (~0.05 mg/cm(2)/min). These results suggest that reflex sweating in the heat during IHG is primarily due to a priming effect in the sweat glands and not because of an increase in skin temperature.