Chlorpromazine increases the turnover of metabolically active phosphoinositides and elevates the steady-state level of phosphatidylinositol-4-phosphate in human platelets

Abstract

Non-permeabilizing concentrations (<40 μM) of chlorpromazine (CPZ) increase the radioactivity of phosphatidylinositol-4-phosphate (PIP) in platelets pre-labelled with [ 32P]P i, but the biochemical mechanisms underlying this increase are poorly understood. Incubation of [ 32P]P i-labelled, gelfiltered platelets with 25 μM CPZ for 10 min increased: (1) the mass of PIP from 315 to 476 nmol 10 11 platelets but not the total inositol phospholipid mass, (2) the specific phosphodiester radioactivities in phosphatidylinositol (PI), PIP and phosphatidylinositol-4,5-bisphosphate (PIP 2) by 34, 63 and 37%, respectively, and (3) the specific phosphomonoester radioactivities in PIP and PIP 2 by 53 and 10%, respectively. In control platelets (no CPZ) the specific radioactivity of the phosphodiester was the same in PI, PIP and PIP 2, and the specific radioactivity in the phosphomonoester in PIP and PIP 2 was 55% of that of the γ-phosphoryl in ATP, measured as metabolically active, actin-bound ADP. These results suggest that 55% of each of PI, PIP and PIP 2 constitutes a metabolic pool which is labelled by 32P in the platelets, while the remainder is in a metabolically inactive pool and not labelled. CPZ has two major effects: (1) CPZ interferes with the kinase and phosphohydrolase reactions that maintain the steady-state level of PIP in the metabolic phosphoinositide pool, resulting in a 92% increase in the PIP level of this pool, and (2) CPZ causes synthesis (45% in 10 min) of new phosphodiester in the metabolically active phosphoinositides by tentative stimulation of the turnover of the phosphoinositide cycle, de novo phosphoinositide synthesis and/or diacylglycerol formation through phospholipases C and D. The marked alteration by CPZ of phosphoinositide metabolism may be part of the mechanism by which this drug effects its psychotropic action.