The Ca Channel b3 Subunit Differentially Modulates G-Protein Sensitivity of a1A and a1B Ca 21 Channels

We have shown previously that the Ca channel b3 subunit is capable of modulating tonic G-protein inhibition of a1A and a1B Ca channels expressed in oocytes. Here we determine the modulatory effect of the Ca channel b3 subunit on M2 muscarinic receptor-activated G-protein inhibition and whether the b3 subunit modulates the G-protein sensitivity of a1A and a1B currents equivalently. To compare the relative inhibition by muscarinic activation, we have used successive ACh applications to remove the large tonic inhibition of these channels. We show that the resulting rebound potentiation results entirely from the loss of tonic G-protein inhibition; although the currents are temporarily relieved of tonic inhibition, they are still capable of undergoing inhibition through the muscarinic pathway. Using this rebound protocol, we demonstrate that the inhibition of peak current amplitude produced by M2 receptor activation is similar for a1A and a1B calcium currents. However, the contribution of the voltage-dependent component of inhibition, characterized by reduced inhibition at very depolarized voltage steps and the relief of inhibition by depolarizing prepulses, was slightly greater for the a1B current than for the a1A current. After co-expression of the b3 subunit, the sensitivity to M2 receptorinduced G-protein inhibition was reduced for both a1A and a1B currents; however, the reduction was significantly greater for a1A currents. Additionally, the difference in the voltage dependence of inhibition of a1A and a1B currents was heightened after co-expression of the Ca channel b3 subunit. Such differential modulation of sensitivity to G-protein modulation may be important for fine tuning release in neurons that contain both of these Ca channels.