The effects of acetylcholine and atropine on the 22Na+ permeability of intestinal smooth muscle vesicles.

A technique is described which has enabled us to measure change in 22Na+ efflux from smooth muscle plasma membrane vesicles. The resting 22Na+ efflux from these sealed vesicles showed a concentration-dependent increase in response to acetylcholine and other muscarinic agonists, in similar concentrations to those which increased 42K+ efflux in whole muscle. The kinetics of this efflux were complex and could not be described by less than three exponential processes. The response to agonists has, therefore, been characterized by measurement of the half-life of 22Na+ efflux (t1/2). The acetylcholine effect was inhibited by atropine, but unlike the situation in the whole muscle, this inhibition was noncompetitive. Tubocuraine (a nicotinic antagonist) had no effect on this acetyl-choline response. Atropine has no effect by itself on the resting 22Na+ efflux, neither did tetrodotoxin or ouabain. 22Na+ efflux from erythrocyte ghosts and liposomes, prepared from lipid extracts of the smooth muscle plasma membrane, was not modified by acetylcholine or atropine.