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 10-6m, and is progressively inhibited to 10% of the maximal values by increasing calcium to 10-4m or higher (K0.5 = 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 × 10-6m and decreases to 30% of maximal values at calcium concentrations of 2 × 10-7m or lower (K0.5 = 10-6M). Unlike frog membranes, transverse t