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The guanine nucleotides guanosine 5'beta, gamma-imidotriphosphate (GppNHp), guanosine 5'-gamma-thio-triphosphate (GTP gamma S), GMP, GDP and GTP stimulated the hydrolysis of inositol phospholipids by a phosphodiesterase in rat cerebral cortical membranes. Addition of 100 microM-GppNHp to prelabelled membranes caused a rapid accumulation of 3H )inositol phosphates (less than 30 s) for up to 2 min. GTP gamma S and Gpp [NHp caused a concentration-dependent stimulation of phosphoinositide phosphodiesterase with a maximal stimulation of 2.5-3-fold over control at concentrations of 100 microM. GMP was as effective as the nonhydrolysable analogues, but much less potent (EC50 380 microM). GTP and GDP caused a 50% stimulation of the phospholipase C at 100 microM and at higher concentrations were inhibitory. The adenine nucleotides AppNHp and ATP also caused small stimulatory effects (64% and 29%). The guanine nucleotide stimulation of inositide hydrolysis in cortical membranes was selective for inositol phospholipids over choline-containing phospholipids. GppNHp stimulated the production of inositol trisphosphate and inositol bisphosphate as well as inositol monophosphate, indicating that phosphoinositides are substrates for the phosphodiesterase. EGTA (33 microM) did not prevent the guanine nucleotide stimulation of inositide hydrolysis. Calcium addition by itself caused inositide phosphodiesterase activation from 3 to 100 microM which was additive with the GppNHp stimulation. These data suggest that guanine nucleotides may play a regulatory role in the modulation of the activity of phosphoinositide phosphodiesterase in rat cortical membranes.
Gonzales et al. (Sun,) studied this question.