AJP - Regulatory, Integrative and Comparative Physiology, Vol 241, Issue 1 77-R86, Copyright © 1981 by American Physiological Society
Effect of methacholine on Na+ pump activity and ion content of dispersed avian salt gland cells
S. R. Hootman and S. A. Ernst
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.
