EVIDENCE FOR INVOLVEMENT OF THE VOLTAGE-DEPENDENT NA-INDUCED ACTIVATION OF G-PROTEINS( CHANNEL GATING IN DEPOLARIZATION)

Evidence for activation of pertussis-toxin-sensitive G-proteins by mem brane depolarization in rat brain-stem synaptoneurosomes was recently reported (Coben-Armon, M., and Sokolovsky, M. (1991) J. Biol. Chem. 26 6, 2595-2605; (1991) Neurosci. Lett. 126, 87-90) and is further suppor ted in this study by the observation that the depolarization-induced e ffect is inhibited when G-proteins are stabilized in the non-activated state with guanosine 5'-O-(2-thiodiphosphate) (GDPbetaS), which was i ntroduced into synaptoneurosomes during the process of permeabilizatio n and resealing. In the present study, agents that either keep the vol tage-dependent Na+ channel in persistently activated state (while Nacurrents are blocked) or prevent it from activation were used in an at tempt to determine whether the voltage-dependent Na+ channels are invo lved in the depolarization-induced activation of pertussis-toxin-sensi tive G-proteins. The main probe employed was the cardiotonic and antia rrhythmic agent DPI, which is a racemic mixture of two enantiomers, on e of which (the R enantiomer) reportedly prevents depolarization-induc ed activation of the Na+ channel while the other (the S enantiomer) in hibits Na+ channel inactivation. The results suggest that while inacti vation of the voltage-dependent Na+ channel does not interfere with th e putative depolarization-induced activation of G-proteins, membrane d epolarization affects G-proteins and the coupled muscarinic receptors only if the voltage-dependent Na+ channels are capable of being activa ted. Thus, inhibition of the depolarization-induced activation of Nachannels was accompanied by inhibition of the depolarization-induced a ctivation of pertussis-toxin-sensitive G-proteins and by modifications of both the coupling of G-proteins to muscarinic receptors and the AD P-ribosylation of G(o)-proteins. These effects could be counteracted b y persistent activation of the voltage-dependent Na+ channels (while N a+ current was blocked). Our observations may suggest that the voltage -dependent Na+ channel gating is involved in the depolarization-induce d activation of pertussis toxin-sensitive G-proteins and may provide e vidence for a possible mechanism of membrane depolarization signal tra nsduction in excitable cells.