Phosducin and phosducin-like protein attenuate G-protein-coupled receptor-mediated inhibition of voltage-gated calcium channels in rat sympathetic neurons.

Phosducin (PDC) has been shown in structural and biochemical experiments to bind the Gbetagamma subunit of heterotrimeric G-proteins. A proposed function of PDC and phosducin-like protein (PDCL) is the sequestration of "free" Gbetagamma from the plasma membrane, thereby terminating signaling by Gbetagamma. The functional impact of heterologously expressed PDC and PDCL on N-type calcium channel (CaV2.2) modulation was examined in sympathetic neurons, isolated from rat superior cervical ganglia, using whole-cell voltage clamp. Expression of PDC and PDCL attenuated voltage-dependent inhibition of N-type calcium channels, a Gbetagamma-dependent process, in a time-dependent fashion. Calcium current inhibition after short-term exposure to norepinephrine was minimally altered by PDC or PDCL expression. However, in the continued presence of norepinephrine, PDC or PDCL relieved calcium channel inhibition compared with control neurons. We observed similar results after activation of heterologously expressed metabotropic glutamate receptors with 100 microM L-glutamate. Neurons expressing PDC or PDCL maintained suppression of inhibition after re-exposure to agonist. Unlike other Gbetagamma sequestering proteins that abolish the short-term inhibition of Ca2+ channels, PDC and PDCL require prolonged agonist exposure before effects on modulation are realized.