Oxytocin signalling in human myometrium

A physiological role for oxytocin in stimulating uterine contractions during labour is well accepted, but has not yet been well defined. Oxytocin activates phospholipase C to produce inositol 1,4,5-trisphosphate, which releases Ca2+ from intracellular stores. There is considerable evidence that G-proteins are involved in this signalling pathway. The objectives of the present study were to determine the mechanisms of action of oxytocin in human myometrium. We have measured the effect of oxytocin on the formation of inositol phosphates (InsPs) in cultured human myometrial cells labelled with [3H] inositol and on changes in intracellular free Ca2+ concentration ([Ca2+i]) in single cells using a dynamic calcium imaging system. Pertussis toxin was used to obtain information on the G-proteins involved. Oxytocin induced InsPs formation and [Ca2+i] mobilisation in a concentration-dependent manner in human myometrial cells. Our data suggest that two distinct types of G-proteins are involved in the oxytocin response: one most probably a member of the Gq family (pertussis toxin-resistant) and another of the Gi family (pertussis toxin-sensitive). Using Western blotting, we have found that the pertussis toxin-resistant G-proteins alpha(q), alpha(11) and alpha(2), and pertussis toxin-sensitive alpha(i1), alpha(i2), and alpha(i3) are expressed in these cells. We have also detected the phospholipase C isoforms beta(1), beta(2) and beta(3) which are regulated by G-proteins, and phospholipase C isoforms gamma(1) and gamma(2), regulated by receptor tyrosine kinase pathways. However, oxytocin does not stimulate tyrosine phosphorylation in myometrial cells. Extracellular Ca2+ does not play a direct role in the activation of phospholipase C by oxytocin. Protein kinase C causes a strong inhibitory feedback on the oxytocin pathway: protein kinase C activators abolish the response to oxytocin while inhibitors potentiate it. Oxytocin responsiveness is upregulated by incubating the cells in the presence of oestradiol. This effect is reversed by the anti-oestrogen tamoxifen. Oestrogens exert their effects on the oxytocin pathway at a postreceptor level, possibly by affecting the expression of G-proteins and/or phospholipase C isoforms.