Intrinsic Versus Extrinsic Intra‐Subject Variability in Sweat Rate of Individual Eccrine Glands

The purpose of the current study was twofold. First, it was to determine the intra‐subject variability of sweat rate per gland for a given skin location during exercise in the heat. Second, it was to determine the relative importance of intrinsic vs. extrinsic factors as the source of the intra‐subject variability in sweat rate per gland. Sweat rate of individual eccrine glands on the forearm was measured following pilocarpine iontophoresis and during exercise in the heat. In five subjects during exercise in the heat the measured sweat rate for individual forearm eccrine glands (n= 500) ranged from 0.5 nL/gland/min to 16 nL/gland/min, or over a 30‐fold difference. The mean ± SD intra‐subject coefficient of variation in sweat rate per gland was 36±5% and 49±10% (p = 0.008) following pilocarpine iontophoresis and during exercise in the heat, respectively. Such results suggest that intrinsic factors (i.e., sweat gland size and cholinergic sensitivity) contribute approximately three times more than extrinsic factors (i.e., sweat gland nerve fiber density and threshold amplitude) towards explaining the large intra‐subject variability in sweat rate per gland seen during exercise in the heat.