Evidence for coupling of resynthesis to hydrolysis in the phosphoinositide cycle

Previous data suggest that agonist-induced hydrolysis of phosphatidylinositol bisphosphate is accompanied by resynthesis through phosphatidylinositol such that these metabolic events function in a cyclic manner. However, it is not known whether resynthesis depends on the presence of agonist or is a direct result of agonist-induced breakdown. In the present study we demonstrate that: (1) increasing the intracellular free inositol concentration will not stimulate phosphatidylinositol synthesis, as measured by assessing the amount of [32P]Pi incorporation; (2) regeneration of free inositol is required for resynthesis; however, addition of exogenous inositol can sustain resynthesis under conditions which inhibit the regeneration of endogenous inositol; (3) resynthesis can take place in the absence of agonist provided that cells have been previously incubated under conditions which prevent resynthesis; and (4) the presence of agonist does not increase the rate of resynthesis. Thus the resynthetic phase of the phosphoinositide cycle is a compensatory event triggered either by the decrease in the level of phosphatidylinositol or by an increase in precursor substrates. The agonist itself appears to have no direct effect on the resynthesis process.