title: 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 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.
authors.affiliation: Departmento de Fisiologia y Biofisica, Facultad de Medicina, Universidad de Chile.
authors.forename: M A
authors.initials: MA
authors.lastname: Carrasco
authors.forename: K
authors.initials: K
authors.lastname: Magendzo
authors.forename: E
authors.initials: E
authors.lastname: Jaimovich
authors.forename: C
authors.initials: C
authors.lastname: Hidalgo
mesh_terms: D019870:1-Phosphatidylinositol 4-Kinase; D000818:Animals; D017301:Ca(2+) Mg(2+)-ATPase / Q000378:metabolism; D002118:Calcium / Q000378:metabolism*; D007700:Kinetics; D009119:Muscle Contraction; D009132:Muscles / Q000201:enzymology*; D010770:Phosphotransferases / Q000378:metabolism*; D011817:Rabbits; D011898:Ranidae
chemical_list: D010770:Phosphotransferases; D019870:1-Phosphatidylinositol 4-Kinase; D017301:Ca(2+) Mg(2+)-ATPase; D002118:Calcium