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Activation of several G-protein-coupled receptors leads to voltage-dependent (VD) inhibition of N- and P/Q-type Ca(2+) channels via G-protein betagamma subunits (Gbetagamma). The purpose of the present study was to determine the ability of different Gbetagamma combinations to produce VD inhibition of N-type Ca(2+) channels in rat superior cervical ganglion neurons. Various Gbetagamma combinations were heterologously overexpressed by intranuclear microinjection of cDNA and tonic VD Ca(2+) channel inhibition evaluated using the whole-cell voltage-clamp technique. Overexpression of Gbeta1-Gbeta5, in combination with several different Ggamma subunits, resulted in tonic VD Ca(2+) channel inhibition. Robust Ca(2+) channel modulation required coexpression of both Gbeta and Ggamma. Expression of either subunit alone produced minimal effects. To substantiate the apparent lack of Gbetagamma specificity, we examined whether heterologously expressed Gbetagamma displaced native Gbetagamma from heterotrimeric complexes. To this end, mutant Gbeta subunits were constructed that differentially modulated N-type Ca(2+) and G-protein-gated inward rectifier K(+) channels. Results from these studies indicated that significant displacement does not occur, and thus the observed Gbetagamma modulation can be attributed directly to the heterologously expressed Gbetagamma combinations.
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