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; how ever, the associated transduction pathway has not been described fully . Experiments were performed in an attempt to gain insight into the co upling mechanism associated with this fundal 5-HT receptor. 5-HT-stimu lated [ 35 S]GTPgammaS binding to a protein which was recognized by an ti-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 sto mach. 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 t he brain Galphaz protein which is recognized by the anti-Galphaz antib ody is a 41 kDa protein. Mixing experiments revealed that the fundal g uanine nucleotide binding protein does not appear to be a proteolytic product of the 41 kDa Galphaz protein. Activating protein kinase C wit h phorbol-12-myristate, 13-acetate induced a concentration-dependent, noncompetitive inhibition of [S-35]GTPgammaS binding to the fundal pro tein, and of 5-HT-induced contraction of fundal strips. Porbol-12-myri state, 13-acetate did not alter carbachol- or KCl-mediated fundus cont raction. Furthermore, the activation of [S-35]GTPgammaS binding by ser otonergic agonists and its inhibition by pharmacological antagonists c orresponded to the known actions of these agents on contraction of fun dal muscle. The results provide evidence that the 5-HT receptor in the rat stomach fundus is coupled directly or indirectly to a GalphaZ-lik e protein which may mediate 5-HT-induced contraction in this tissue.