A G(Q)-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) r esults in a rapid increase in the rate of phosphatidylinositol hydroly sis and pronounced intracellular calcium signals. Both responses can b e elicited by treating these cells with fluoroaluminate (AlF4-) indica ting the involvement of a heterotrimeric G protein in the transmembran e signaling process. To characterize this G protein, electrophoretical ly separated membrane proteins were blotted onto nitrocellulose filter s and probed with peptide-antibodies raised against portions of differ ent alpha-subunits of mammalian G proteins. We could demonstrate the p resence of at least four different G proteins in salt gland cell membr anes. Two of these proteins (40 and 41 kD) were ADP-ribosylated by per tussis toxin and were recognized by an antiserum against a common sequ ence in all G protein alpha-subunits. One protein (46 kD) was a choler a toxin-substrate and was recognized by a G(s)-specific antiserum; the other (42 kD) was recognized by G(q)-specific antisera and was resist ant to ADP-ribosylation. Since the initial inositol phosphate producti on upon receptor activation with carbachol and the resulting calcium s ignals were not affected by pertussis toxin-pretreatment of salt gland cells, we conclude that muscarinic receptors are coupled to phospholi pase C by a G(q)-type G protein.