OXYTOCIN RECEPTOR COUPLES TO THE 80-KDA G(H-ALPHA) FAMILY PROTEIN IN HUMAN MYOMETRIUM

One of the primary functions of the oxytocin receptor is to modulate i ntracellular calcium levels in myometrium. The oxytocin receptor has b een purified and cloned. Although it has been suggested that oxytocin receptor couples with a GTP-binding regulatory protein (G-protein), th e identity of this G-protein remains unclear. To elucidate the mechani sm of oxytocin receptor signalling, we used the oxytocin-receptor-G-pr otein ternary complex preparation from human myometrium, and evaluated oxytocin-mediated activation of [S-35]guanosine 5'-[gamma-thio]tripho sphate ([S-35]GTP[S]) binding and [alpha-P-32]GTP photoaffinity labell ing to a G-protein. Binding of[S-35]GTP[S] and the intensity of the [a lpha-P-32]GTP photoaffinity labelled protein resulting from activation of the oxytocin receptor were significantly attenuated by the selecti ve oxytocin antagonist, desGlyNH(2)d(CH2)(5)[Tyr(Me)(2),Thr(4)]OVT. Fu rthermore, the molecular mass of the specific GTP-binding protein was similar to 80 kDa; homologous with the G(h alpha) family, the new clas s of GTP-binding proteins first identified in rat liver that couples t o the alpha(1B)-adrenoceptor. Consistent with these observations, in c oimmunoprecipitation and co-immunoadsorption of the oxytocin receptor in the ternary complex preparation by anti-G(h7 alpha) antibody, the G (h alpha) family protein tightly coupled to the oxytocin receptor. The se findings demonstrate that oxytocin receptor couples with similar to 80 kDa G(h alpha) in signal mediation.