MOLECULAR DIVERSITY AND DOUBLE REGULATORY MECHANISM OF ACTIVATION OF PHOSPHOLIPASE-C IN RAT-BRAIN

Whereas evidence for a G protein-dependent stimulation of phospholipas e C (PLC) is abundant, reports on the inhibition of PLC through a G pr otein-mediated pathway have only recently begun to appear. In the pres ent study, cerebral cortex membranes were chosen since they have a rea dily measurable Gpp[NH]p and Ca2+-stimulated PLC activity. Nanomolar c oncentrations of Gpp[NH]p, a hydrolysis-resistant GTP analogue, inhibi ted basal inositol 1,4,5-trisphosphate (IP3) production, with a maximu m inhibition of 25% at 10nM. Increasing the concentrations of Gpp[NH]p to over 10nM resulted in a reversal of the inhibitory effect and onse t of stimulation of IP3 production. GDP beta S as a G protein inhibito r and U-73122 as a putative PLC-beta inhibitor had little effect on ba sal IP3 production at 100 mu M and 1 mu M, respectively. However, GDP beta S and U-73122 completely antagonized both the inhibition and the stimulation of IP3 production produced by lower and higher concentrati ons, respectively, of Gpp[NK]p. Rat cortical membranes expressed a gre ater amount of PLC-beta(1). These data suggest that PLC-beta(1) isozym es may be regulated by both inhibitory and stimulatory G protein-media ted mechanisms.