Role of calcium released from the sarcoplasmic reticulum of smooth muscle cells as induced by inositol phosphatides.

Abstract

Phosphatidylinositol 4,5-bisphosphate (PI-P2), distributed at the sarcolemma in smooth muscles, as a phospholipid, is hydrolyzed into inositol 1,4,5-trisphosphate (below Ins-1,4,5-P3) and diacylglycerol (DG) by activations of each receptor through actions of GTP binding protein (Gprotein) and by those of phospholipase C. The latter is also synthesized from phosphatidyl inositide monophosphate (Berridge, 1984; Berridge and Irvine, 1984; Abdel-Latif, 1986). It is now clear that Ins-1,4,5-P3 releases Ca from the sarcoplasmic reticulum (SR) of smooth muscle cells and increases free Ca concentration in the cytosol, thereby initiating contraction (Streb et al., 1983; Suematsu et al., 1984; Somlyo et al., 1985; Yamamoto and van Breemen, 1985; Hashimoto et al., 1986; Sasaguri et al., 1985). DG with phosphatidylserine (PS) activates protein kinase C and phosphorylates the proteins required to regulate physiological functions in the cells (Nishizuka, 1984; 1986). In smooth muscles, 12-0-tetradecanoylphorbol-13-acetate (TPA), a phorbol ester and a substitute of DG, together with PS increases the Ca sensitivity of contractile proteins and enhances the mechanical response evoked by high K (below 59 mM) in intact smooth muscles and also by Ca (below 0.5 μM) in skinned muscle tissues (Itoh et al. 1987). TPA also inhibits the hydrolysis of PI-P2 triggered by agonists and reduces the synthesis of Ins-1,4,5-P3 (Itoh et al., 1988).