INHIBITION OF THE INTERACTION OF G-PROTEIN G(O) WITH CALCIUM CHANNELS BY THE CALCIUM-CHANNEL BETA-SUBUNIT IN RAT NEURONS

1. The beta-subunit has marked effects on the biophysical and pharmacological properties of voltage-dependent calcium channels. In the present study we examined the ability of the GABA(B) agonist (-)-baclofen to inhibit calcium channel currents in cultured rat dorsal root ganglion neurones following depletion of beta-subunit immunoreactivity, 108-116 h after microinjection of a beta-subunit antisense oligonucleotide. 2. We observed that, although the calcium channel current was markedly reduced in amplitude following beta-subunit depletion, the residual current (comprising both N- and L-type calcium channel currents) showed an enhanced response to application of (-)-baclofen. Therefore, it is possible that there is normally competition between activated G protein G(o) and the calcium channel beta-subunit for binding to the calcium channel alpha(1)-subunit; and this competition shifts in favour of the binding of activated G(o) following depletion of the beta-subunit, resulting in increased inhibition. 3. This hypothesis is supported by evidence that an antibody against the calcium channel beta-subunit completely abolishes stimulation of the GTPase activity of G(o) by the dihydropyridine agonist S-(-)-Bay K 8644 in brain membranes. This stimulation of GTPase is thought to result from an interaction of G(o) alpha-subunit (G alpha(o)) with its calcium channel effector Which may operate as a GTPase-activating protein. 4. These data suggest that the calcium channel beta-subunit when complexed with the alpha(1)-subunit normally inhibits its association with activated G(o). It may function as a GTPase-activating protein to reduce the ability of activated G(o) to associate with the calcium channel, and thus limit the efficacy of agonists such as (-)-baclofen.