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]phosphatidylinositol (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 degraded, 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.