The human myometrial CRH receptor: G proteins and second messengers

The initiation of human parturition remains an enigma but is thought to involve a number of hormonal signals such as oxytocin and prostaglandins. One other possible signal is plaeentally derived corticotropin-releasing hormone (CRH). We have recently reported that the human myometrium expresses a specific receptor for CRH which changes to a high affinity state prior to term. In view of this we sought to determine whether this receptor is functionally linked to some of the known modulators of myometrial function. Myometrial membranes were prepared by differential centrifugation from either pregnant (caesarian section) or non-pregnant (hysterectomy) myometrium. For binding studies the membranes were incubated with radiolabelled oCRH at 22°C for 2 h. For second messenger studies they were incubated at 37°C for 10 or 30 min with either 0.5 mM ATP and 10 mM theophylline (cAMP) or 0.05 mM arachidonic acid or 0.5 mM linoleic acid (PGE₂). When increasing concentrations of membranes were incubated with radiolabelled oCRH an interesting phenomenon was observed. In non-pregnant membranes the binding reached a plateau, whereas in membranes prepared from pregnant myometrium, the binding decreased at concentrations above 130 μg/ml. Possible explanations for this phenomenon include an inhibitor which prevents ligand-receptor binding or an enzyme which destroys the receptor binding region of the ligand. Incubation of both types of membranes with GTP or its analogue, GppNHp, resulted in a dose-dependent inhibition of specific binding suggesting that the myometrial CRH receptor is linked to a G regulatory protein. Human CRH caused a dose-dependent stimulation of cAMP and PGE₂ production in the pregnant myometrial membranes suggesting that, in the pregnant state, the myometrial CRH receptor becomes linked to adenylate cyclase and cyclo-oxygenase enzymes which can be rapidly activated following CRH binding.