A Gq-type G protein couples muscarinic receptors to inositol phosphate and calcium signaling in exocrine cells from the avian salt gland.

Muscarinic acetylcholine receptor (mAChR) activation in isolated cells from the nasal salt gland of the domestic duck (Anas platyrhynchos) results in a rapid increase in the rate of phosphatidylinositol hydrolysis and pronounced intracellular calcium signals. Both responses can be elicited by treating these cells with fluoroaluminate (AlF4-) indicating the involvement of a heterotrimeric G protein in the transmembrane signaling process. To characterize this G protein, electrophoretically separated membrane proteins were blotted onto nitrocellulose filters and probed with peptide-antibodies raised against portions of different alpha-subunits of mammalian G proteins. We could demonstrate the presence of at least four different G proteins in salt gland cell membranes. Two of these proteins (40 and 41 kD) were ADP-ribosylated by pertussis toxin and were recognized by an antiserum against a common sequence in all G protein alpha-subunits. One protein (46 kD) was a cholera toxin-substrate and was recognized by a Gs-specific antiserum; the other (42 kD) was recognized by Gq-specific antisera and was resistant to ADP-ribosylation. Since the initial inositol phosphate production upon receptor activation with carbachol and the resulting calcium signals were not affected by pertussis toxin-pretreatment of salt gland cells, we conclude that muscarinic receptors are coupled to phospholipase C by a Gq-type G protein.