SEROTONIN-INDUCED MUSCLE-CONTRACTION IN RAT STOMACH FUNDUS IS MEDIATED BY A G-ALPHA-Z-LIKE GUANINE-NUCLEOTIDE-BINDING PROTEIN

Serotonin (5-HT) potently contracts the fundus of the rat stomach; however, the associated transduction pathway has not been described fully. Experiments were performed in an attempt to gain insight into the coupling mechanism associated with this fundal 5-HT receptor. 5-HT-stimulated [ 35 S]GTPgammaS binding to a protein which was recognized by anti-Galphaz antiserum in a Mg++-dependent fashion. 5-HT increased [S-35]GTPgammaS binding in the fundus, but not in the corpus of the rat stomach. 5-HT also enhanced the binding of [alpha-P-32]GTP to the fundal protein and increased the hydrolysis of GTP to GDP in fundal membranes. The fundal protein which binds GTP is 25 to 29 kDa in size whereas the brain Galphaz protein which is recognized by the anti-Galphaz antibody is a 41 kDa protein. Mixing experiments revealed that the fundal guanine nucleotide binding protein does not appear to be a proteolytic product of the 41 kDa Galphaz protein. Activating protein kinase C with phorbol-12-myristate, 13-acetate induced a concentration-dependent, noncompetitive inhibition of [S-35]GTPgammaS binding to the fundal protein, and of 5-HT-induced contraction of fundal strips. Porbol-12-myristate, 13-acetate did not alter carbachol- or KCl-mediated fundus contraction. Furthermore, the activation of [S-35]GTPgammaS binding by serotonergic agonists and its inhibition by pharmacological antagonists corresponded to the known actions of these agents on contraction of fundal muscle. The results provide evidence that the 5-HT receptor in the rat stomach fundus is coupled directly or indirectly to a GalphaZ-like protein which may mediate 5-HT-induced contraction in this tissue.