Bombesin receptors inhibit G protein-coupled inwardly rectifying K+ channels expressed in Xenopus oocytes through a protein kinase C-dependent pathway

Although activation of G protein-coupled inward rectifying K+ (GIRK) channe ls by G(i)/G(o)-coupled receptors has been shown to be important in postsyn aptic inhibition in the central nervous system, there is also evidence to s uggest that inhibition of GIRK channels by G(q)-coupled receptors is involv ed in postsynaptic excitation. In the present study we addressed whether th e G(q)-coupled receptors of the bombesin family can couple to GIRK channels and examined the mechanism by which this process occurs. Different combina tions of GIRK channel subunits (Kir3.1, Kir3.2, and Kir3.4) and bombesin re ceptors (BB1 and BB2) were expressed in Xenopus oocytes. In all combination s tested GIRK currents were reversibly inhibited upon application of the bo mbesin-related peptides, neuromedin B or gastrin-releasing peptide in a con centration-dependent manner. Incubation of oocytes in the phospholipase C i nhibitor U73122 or the protein kinase C (PKC) inhibitors chelerythrine and staurosporine significantly reduced the inhibition of GIRK currents by neur omedin B, whereas the Ca2+ chelator, BAPTA-AM had no effect. The involvemen t of PKC was further demonstrated by direct inhibition of GIRK currents by the phorbol esters, phorbol-12,13-dibutyrate and phorbol-12-myristate-13-ac etate. In contrast, the inactive phorbol ester 4 alpha-phorbol and protein kinase A activators, forskolin and 8-bromo cAMP did not inhibit GIRK curren ts. At the single-channel level, direct activation of PKC using phorbol est er phorbol-12,13-dibutyrate caused a dramatic reduction in open probability of GIRK channels due to an increase in duration of the interburst interval .