Multiple pertussis toxin-sensitive G-proteins can couple receptors to GIRKchannels in rat sympathetic neurons when expressed heterologously, but only native G(i)-proteins do so in situ

Although many G-protein-coupled neurotransmitter receptors are potentially capable of modulating both voltage-dependent Ca2+ channels (I-Ca) and G-pro tein-gated K+ channels (I-GIRK), there is a substantial degree of selectivi ty in the coupling to one or other of these channels in neurons. Thus, in r at superior cervical ganglion (SCG) neurons, M-2 muscarinic acetylcholine r eceptors (mAChRs) selectively activate I-GIRK whereas M-4 mAChRs selectivel y inhibit I-Ca. One source of selectivity might be that the two receptors c ouple preferentially to different G-proteins. Using antisense depletion met hods, we found that M-2 mAchR-induced activation of I-GIRK is mediated by G (i) whereas M-4 mAChR-induced inhibition of I-Ca is mediated by G(oA). Expe riments with the betay-sequestering peptides alpha -transducin and beta ARK 1(C-ter) indicate that, although both effects are mediated by G-protein bet ay subunits, the endogenous subunits involved in I-GIRK inhibition differ f rom those involved in I-Ca inhibition. However, this pathway divergence doe s not result from any fundamental selectivity in receptor-G-protein-channel coupling because both I-GIRK and I-Ca modulation can be rescued by heterol ogously expressed G(i) or G(o) proteins after the endogenously coupled alph a -subunits have been inactivated with Pertussis toxin (PTX). We suggest in stead that the divergence in the pathways activated by the endogenous mAChR s results from a differential topographical arrangement of receptor, G-prot ein and ion channel.