The sheep endometrial oxytocin receptor.

The sheep endometrial oxytocin receptor plays a central role in determining the time at which luteolysis occurs during the oestrous cycle, and in the events leading to the establishment of pregnancy (the maternal recognition of pregnancy). Expression of the receptor in the uterus is controlled by ovarian steroid hormones, and by trophoblast interferon (IFN-tau). We report here studies on the second messengers involved in the effect of IFN-tau, and on the structure and expression of the oxytocin receptor. The receptor is expressed in ovine endometrial explants during culture, when the explants are taken during the luteal phase of the cycle; this process is partially blocked by inhibitors of protein kinase C, or by down-regulation of protein kinase C. Therefore it is suggested that protein kinase C, rather than tyrosine kinases, is involved in the effect of IFN-tau on oxytocin receptor expression. Northern blotting shows that in common with uterine oxytocin receptor mRNA in other species, the message is heterogeneous. cDNA sequencing indicates the sheep uterine oxytocin receptor is at least 2 amino acids longer than those of other species, and expression of the receptor in Cos-7 cells induces oxytocin responsiveness in terms of phosphoinositide turnover.