Electrochemical driving forces for K+ secretion by rat paw eccrine sweat gland

Abstract

The secretory portion of the rat paw eccrine sweat gland elaborates a secretory fluid high in K+ and low in Na+ concentrations. Whether K+ is secreted against an electrochemical gradient during K+-rich sweat secretion was studied. The transepithelial potential difference (PD) of an isolated secretory coil of the rat paw sweat gland was less than 1 mV, lumen negative, during the prestimulation period and increased to a mean of 14 mV, lumen negative, during stimulation with acetylcholine (ACh). The electrochemical potential for K+ across the secretory coil epithelium, as calculated from the mean transepithelial PD and K+ concentration gradient, was as much as 76 mV against K+ movement. The electrochemical gradient favors Na+ secretion by 55 mV. The electrochemical gradient against Cl- is only 5 mV. The PD across the basal membrane of the secretory cell measured by glass microelectrodes ranged from 60-90 mV, cell interior negative, with a mean of 80 mV during the resting period. Addition of 10-5 M ACh to the bath initially evoked an instantaneous depolarization of approximately 10 mV. The membrane PD depolarized with an increase in K+ concentration in the bath, [K+]b. The magnitude of depolarization was approximately 30 mV/decade change in [K+]b between [K+]b of 5 and 20 mM, whereas it tended to approach 60 mV/decade change in [K+]b at higher [K+]b. The transepithelial PD during ACh stimulation showed a curvilinear decrease with an increase in [K+]b, approaching 0 mV at 50 mM [K+]b, whereas the PD at rest failed to change by increasing [K+]b from 5 to 20 mM. The permeability of K+ across the extracellular pathway as estimated by equivalent circuit analysis was greater than the permeability of Na+. Apparently the lumen negative PD during sweat secretion is caused at least in part by bi-ionic diffusion potential across the extracellular diffusive pathway.