Carrasco M A, Magendzo K, Jaimovich E, Hidalgo C
Departmento de Fisiologia y Biofisica, Facultad de Medicina, Universidad de Chile.
Arch Biochem Biophys. 1988 Apr;262(1):360-6. doi: 10.1016/0003-9861(88)90199-3.
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 membrane, 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 (K0.5 = 5 X 10(-6) M). 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 X 10(-6) M and decreases to 30% of maximal values at calcium concentrations of 2 X 10(-7) M or lower (K0.5 = 10(-6) M). 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.
