Stimulation of phosphoinositide degradation and phosphatidylinositol-4-phosphate phosphorylation by GTP exclusively in plasma membrane of rat brain

The effect of GTP on the hydrolysis of ³H]phosphatidyinositol (PI), ³H]phosphatidylinositol-4-phosphate (PIP) and ³H]phosphatidylinositol-4,5-bisphosphate (PIP₂) by phospholipase C of rat brain plasma membrane, microsomes and cytosol was determined. Moreover the regulation of PI and PIP phosphorylation by GTP in brain plasma membrane was investigated.In the presence of EGTA PIP₂ was actively degradted, opposite to PI and PIP which require Ca²⁺ for their hydrolysis. Addition of calcium ions in each case caused stimulation of inositide phosphodiesterase(s). GTP independently of calcium ions activates by about 3 times phospholipase C acting on PIP and PIP₂ exclusively in the plasma membrane. PI degradation was unaffected by GTP. In the presence of Ca²⁺ guanine nucleotides have synergistic stimulatory effect on plasma membrane bound phospholipase C acting on PIP₂. PIP kinase of brain plasma membrane was stimulated by GTP by about 20–100% in the presence of exogenous and endogenous substrate respectively. PI kinase was negligible activated by about 20% exclusively in the presence of endogenous substrate. These results indicated that guanine nucleotide modulates the level of second messengers as diacylglycerol and IP₃ through the activation of phospholipase C acting on PIP₂ exclusively in brain plasma membrane. The stimulation of phospholipase C by GTP may occur directly or through the enhancement of substrate level PIP₂ due to stimulation of PIP kinase.