Physiological Evidence for Multiple Calcium Sites in Smooth Muscle

Inherent smooth muscle tone and acetylcholine-induced contractions of the isolated longitudinal muscle from guinea pig ileum are inhibited by 1.2 M ethanol. The inhibitions are antagonized by high concentrations of calcium ion in the external medium. Previous work has indicated that an acetylcholine-induced increase in potassium efflux from the ileal muscle is also inhibited by ethanol and reactivated by high concentrations of calcium ion. It was found in this study that, in addition to ethanol, a drastic reduction in the calcium ion concentration of the bathing medium appeared to produce a depression of this drug-induced increase in potassium efflux. Preincubating the muscle in a reduced calcium ion concentration also inhibited the increase in potassium efflux initiated by a high potassium medium. Conversely, the exposure of the muscle to 1.2 M ethanol did not depress the potassium-induced increase in potassium efflux. Increases in smooth muscle tone produced by a high potassium medium have been reported to be inhibited both by ethanol and by a depletion of external calcium. These data suggest that the calcium ions which activate or enhance a potassium-induced increase in potassium efflux are not bound to the same loci in the muscle fiber as those involved in an acetylcholine-induced increase in potassium efflux or those involved in a potassium-induced increase in smooth muscle tone.