Effects of papaverine on the relative distribution of 45Ca ions in subcellular fractions of hog biliary muscles

Abstract

The mechanisms by which papaverine (Pap) relaxes smooth muscles of hog bile ducts were investigated with respect to the effects of Pap on the distribution of Ca ions in intracellular organelles. Muscles loaded with 45Ca ions in 139 mM KCl-Tyrode's solution were incubated with Pap, [ 3H]Pap or theophylline (ThP) in Tyrode's solution. The Pap and Ca contents of various subcellular fractions were measured. Pap (40 μM) had no effect on Ca influx into the muscle in normal and 139 mM KCl-Tyrode's solutions. Pap was distributed to a greater extent in the cytosol fraction (F 4) than in microsome and mitochondria-enriched fractions (F 3) and in the nuclei-enriched fraction (F 2). Pap selectively accumulated in the plasma membrane-enriched fraction (PM-F) and was negligible in the sarcoplasmic reticulum-enriched fraction (SR-F) and the mitochondria-enriched fraction (Mit-F). The Ca release of PM-F was greater and more rapid than the release from SR-F or Mit-F. The increase in the content of Ca in PM-F induced by Pap was enhanced by oxalate, suggesting that the increase is based on the increase in Ca uptake, not on the reduced efflux. Pap increased dose-dependently the Ca content of PM-F and appeared to decrease the Ca contents of SR-F and Mit-F. ThP and cyclic AMP (4 μM) had no effect on the Ca uptake of PM-F and no influence on the effects of Pap. These results suggest that Pap relaxation of hog biliary smooth muscle results from the interaction of the drug with the plasma membrane causing cyclic AMP-independent sequestration of Ca.