Guanine nucleotides mediate stimulatory and inhibitory effects on cerebral-cortical membrane phospholipase C activity

In cerebral-cortical membranes, hydrolysis-resistant guanine nucleotides exert a dual regulatory effect on phospholipase C activity. Nanomolar concentrations of guanosine 5'-[beta gamma-imido]triphosphate (p[NH]ppG) or guanosine 5'-[gamma-thio]triphosphate (GTP[S]) inhibited basal phospholipase C activity, with a maximum inhibition of 30% at 10 nM. Increasing the concentration of p[NH]ppG or GTP[S] to over 10 nM resulted in a reversal of the inhibitory effect and onset of stimulation of phospholipase C activity. These inhibitory effects were blocked by 100 microM-guanosine 5'-[beta-thio]diphosphate. GTP was relatively ineffective in producing either stimulation or inhibition of phospholipase C activity. Similarly, ATP, adenosine 5'-[beta gamma-imido]triphosphate and GDP were also ineffective. Expression of the dual effects of guanine nucleotides was affected by the Mg2+ concentration. At 0.3 mM-Mg2+, both the inhibitory and the stimulatory components of p[NH]ppG action were evident. At 2.5 mM-Mg2+, only p[NH]ppG stimulation was observed. Pertussis-toxin treatment blocked the p[NH]ppG-mediated inhibition of phospholipase C activity. These results demonstrate that non-hydrolysable guanine nucleotides exert both a stimulatory and an inhibitory effect on membrane phospholipase C activity. These effects may be mediated through distinct GTP-binding proteins.