Calcium modulation of phosphoinositide kinases in transverse tubule vesicles from frog skeletal muscle

Abstract

Highly purified transverse tubule membranes isolated from frog skeletal muscle phosphorylate phosphatidylinositol to phosphatidylinositol 4-phosphate and phosphatidylinositol (4,5)-bisphosphate. The two phosphorylation reactions have different calcium requirements. Phosphorylation of phosphatidylinositol to phosphatidylinositol 4-phosphate, which takes place in both isolated transverse tubules and sarcoplasmic reticulum membranes, is independent of calcium in a range of concentrations from 10 −9 to 10 −6 m, and is progressively inhibited to 10% of the maximal values by increasing calcium to 10 −4 m or higher ( K 0.5 = 5 × 10 −6M). In contrast, phosphorylation of phosphatidylinositol 4-phosphate to phosphatidylinositol (4,5)-bisphosphate, a reaction exclusively present in transverse tubule membranes, is maximal at calcium concentrations higher than 2 × 10 −6 m and decreases to 30% of maximal values at calcium concentrations of 2 × 10 −7 m or lower ( K 0.5 = 10 −6M). Unlike frog membranes, transverse tubules from rabbit muscle need exogenous phosphatidylinositol 4-phosphate in order to produce the bisphosphate derivative in the same range of calcium concentrations. Inositol (1,4,5)-trisphosphate has been proposed recently as a chemical messenger in excitation-contraction coupling in skeletal muscle. Calcium regulation of the synthesis of phosphatidylinositol (4,5)-bisphosphate, the membrane-bound precursor of inositol (1,4,5)-trisphosphate, might have physiological implications regarding modulation of excitation-contraction coupling by intracellular calcium levels.