Bombesin Receptors Inhibit G Protein-Coupled Inwardly Rectifying K⁺ Channels Expressed in Xenopus Oocytes through a Protein Kinase C-Dependent Pathway

Abstract

Although activation of G protein-coupled inward rectifying K⁺ (GIRK) channels by G_i/G_o-coupled receptors has been shown to be important in postsynaptic inhibition in the central nervous system, there is also evidence to suggest that inhibition of GIRK channels by G_q-coupled receptors is involved in postsynaptic excitation. In the present study we addressed whether the G_q-coupled receptors of the bombesin family can couple to GIRK channels and examined the mechanism by which this process occurs. Different combinations of GIRK channel subunits (Kir3.1, Kir3.2, and Kir3.4) and bombesin receptors (BB₁and BB₂) were expressed in Xenopus oocytes. In all combinations tested GIRK currents were reversibly inhibited upon application of the bombesin-related peptides, neuromedin B or gastrin-releasing peptide in a concentration-dependent manner. Incubation of oocytes in the phospholipase C inhibitor U73122 or the protein kinase C (PKC) inhibitors chelerythrine and staurosporine significantly reduced the inhibition of GIRK currents by neuromedin B, whereas the Ca²⁺ chelator, BAPTA-AM had no effect. The involvement of PKC was further demonstrated by direct inhibition of GIRK currents by the phorbol esters, phorbol-12,13-dibutyrate and phorbol-12-myristate-13-acetate. In contrast, the inactive phorbol ester 4α-phorbol and protein kinase A activators, forskolin and 8-bromo cAMP did not inhibit GIRK currents. At the single-channel level, direct activation of PKC using phorbol ester phorbol-12,13-dibutyrate caused a dramatic reduction in open probability of GIRK channels due to an increase in duration of the interburst interval.