Effect of methacholine on Na+ pump activity and ion content of dispersed avian salt gland cells.

The effects of the cholinergic agonist methacholine chloride (MCh) on cellular ion content and Na+ pump activity of dissociated duck salt gland cells were studied. Dispersed salt gland cells regulate intracellular ion levels in a ouabain-sensitive manner. MCh (0.5 mM) caused no detectable change in cell Na+ levels over the first 10 min of exposure of cells to the agonist but elicited decreases of 23 and 13%, respectively, in intracellular Cl- and K+ content. The rate of turnover of salt gland cell plasmalemmal Na+ pumps, as measured by [3H]ouabain binding to the dissociated cells, was markedly stimulated by 0.5 mM MCh, although the total number of binding sites at equilibrium remained unchanged. Replacement of medium Na+ with choline abolished the MCh-stimulated increase in ouabain binding but had no effect on the rate of glycoside binding in the absence of the agonist. Substitution of Cl- in the medium by NO3-, SO42-, or benzene sulfonate- reduced the stimulated component of Na+ pump turnover by 85-90%. Addition of 1 mM furosemide to the medium abolished the increase in ouabain binding and ouabain-sensitive oxygen consumption observed after exposure of salt gland cells to MCh. These data are consistent with the hypothesis that cholinergic stimulation of salt gland cells triggers a Cl--dependent uptake of Na+, which elicits a compensatory increase in Na+ pump turnover. In addition, the decrease in cellular Cl- content caused by MCh suggests that the agonist either directly or indirectly mediates an efflux of Cl- from the cells.