Methylcholine-activated eccrine sweat gland density and output as a function of age

The purpose of this investigation was to examine eccrine sweat gland responsiveness to intradermal injections of methylcholine (MCh) across three age groups of men [young (Y) = 22-24; middle (M) = 33-40; older (O) = 58-67 yr old, n = 5 per group]. Subjects were matched with respect to maximum O₂ consumption, body size, and body composition, and were thoroughly heat acclimated before participation. Randomly ordered concentrations of acetyl-β-methylcholine chloride ranging from 0% (saline) to 0.1% (5 x 10^-3 M) were injected into the skin of the dorsal thigh in a thermoneutral environment, and activated sweat glands were photographed at 30-s intervals for the next 8 min. Density of MCh-activated glands was independent of both age and ]MCh] (e.g., 2 min after injection of 5 x 10^-3 M [MCh]: Y = 45 ± 7, M = 46 ± 12, O = 42 ± 5 glands/cm²). However, sweat gland output (SGO) per active gland was significantly lower for the O group and failed to increase with increasing [MCh] above 5 x 10^-4 M. When MCh (5 x 10^-3 M) was injected after 1 h of exercise in the heat, higher SGO's were elicited in each group; however, the SGO of the O group was again significantly lower than that of the Y group (91 ± 11 vs. 39 ± 4 ng/gland, P < 0.02) with the M group intermediate (69 ± 11 nl/gland; 2 min postinjection data). It was concluded that eccrine glands of aged skin exhibit a decreased sweat output in response to a cholinergic stimulus. Hence the locus of the thermoregulatory sweating decrement often reported in older individuals may be the sweat gland itself. Furthermore this attenuated response characteristic appears to be unrelated to acclimation state, aerobic fitness level, or body composition.