Intracellular ion concentrations and cell volume during cholinergic stimulation of eccrine secretory coil cells

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Abstract
Methacholine (MCh)-induced changes in intracellular concentrations of Na, K, and Cl ([Na]i, [K]i, and [Cl]i, respectively) and in cellular dry mass (a measure of cell shrinkage) were examined in isolated monkey eccrine sweat secretory coils by electron probe X-ray microanalysis using the peripheral standard method. To further confirm the occurrence of cell shrinkage during MCh stimulation, the change in cell volume of dissociated clear and dark cells were directly determined under a light microscope equipped with differential interference contrast (DIC) optics. X-ray microanalysis revealed a biphasic increase in cellular dry mass in clear cells during continuous MCh stimulation; an initial increase of dry mass to 158% (of control) followed by a plateau at 140%, which correspond to the decrease in cell volume of 37 and 29%, respectively. The latter agrees with the MCh-induced cell shrinkage of 29% in dissociated clear cells. The MCh-induced increase in dry mass in myoepithelial cells was less than half that of clear cells. During the steady state of MCh stimulation, both [K]i and [Cl]i of clear cells decreased by about 45%, whereas [Na]i increased in such a way as to maintain the sum of [Na]i+[K]i constant. There was a small (12–15mm) increse in [Na]i and a decrease in [K]i in myoepithelial cells during stimulation with MCh. Dissociated dark cells failed to significantly shrink during MCh stimulation. The decrease in [Cl]i in the face of constant [Na]i+[K]i suggests the accumulation of unknown anion(s) inside the clear cell during MCh stimulation. While the decrease in [K]i and [Cl]i may be instrumental in facilitating influx of ions via Na−K−2Cl cotransporters, the functional significance of MCh-induced cell shrinkage remains unknown.