Polyphosphoinositide formation in isolated cardiac plasma membranes

Abstract

Phosphatidylinositol (PtdIns) and phosphatidylinositol 4-phosphate (PtdIns4P) kinase activities in plasma membranes isolated from canine left ventricle were partially characterized, and their sensitivity to a number of intracellular variables was established. PtdIns and PtdIns4P kinase activities were estimated by the formation of [³²P]PtdIns4P and [³²P]phosphatidylinositol 4,5-bisphosphate ([³²P]PtdIns(4,5)P₂), respectively, when membranes were incubated with [γ-³²P]ATP and 0.1% Triton X-100. Unlike [³²P]-PtdIns4P formation, [³²P]PtdIns(4,5)P₂ formation required exogenous (PtdIns4P) substrate. [³²P]-PtdIns4P and [³²P]PtdIns(4,5)P₂ formation were insensitive to Ca²⁺ at concentrations ranging from 0.1–30 μM. The hydrolysis of [³²P]PtdIns4P was less than 15% under standard assay conditions for measuring its formation, and was unaffected by any of the variables tested. The apparent Km of the PtdIns kinase for ATP was 53±13 (S.E.M.) μM (N=3). ADP inhibited [³²P]PtdIns4P formation competitively with respect to ATP, the Ki being 0.4 mM. The data indicate that ADP is a poor competitive inhibitor of PtdIns kinase at the concentrations which are believed to be present intracellularly normally or which may be attained during mild hypoxia provided ATP levels are maintained in the millimolar range. Hence, any response of the myocardium to α-adrenergic hormones during mild hypoxia would be largely unimpaired by effects of Ca²⁺ on PtdIns and PtdIns(4,5)P₂, or of ADP on PtdIns kinase activity.