Changes in progesterone and oestrogen receptor mRNA and protein and oxytocin receptors in endometrium of ewes after intrauterine injection of ovine trophoblast interferon

This study determined whether twice-daily intrauterine injections of ovine conceptus secretory proteins (oCSP) containing type-I trophoblast interferon (25 μg/uterine horn) from day 11 to day 15 post-oestrus (oestrus = day 0) could alter the binding capacities of endometrial receptors for oxytocin, progesterone and oestrogen in cyclic ewes when compared with control ewes receiving serum protein (SP) injections. Injections of oCSP on days 11-15 post-oestrus decreased concentrations of oestrogen receptors (P < 0.06), oestrogen receptor mRNA (P < 0.05) and progesterone receptors (P < 0.08) in endometrium on day 16 when compared with SP-infused control ewes, which were undergoing corpus luteum regression on days 14-16. Injection of oCSP also decreasedd the number (P < 0.10) and affinity (P < 0.06) of oxytocin receptors. Inositol phosphate formation induced in the endometrium on day 16 by 100 nM oxytocin in vitro was highly correlated with the concentration (r ≥ 0.93, P < 0.001) and K(d) (r = -0.91, P < 0.01) of oxytocin receptors in SP-infused ewes, but was not as highly correlated with concentration (r ≤ 0.83, P < 0.06) and K(d) (r ≤ -0.40, P > 0.40) of oxytocin receptors in oCSP-infused ewes. This indicates that oCSP disrupted the relationship between oxytocin receptor binding and oxytocin-induced activation of its second messenger system. These results indicate that antiluteolytic type-I trophoblast interferon may prevent oxytocin-induced luteolytic pulsatile secretion of prostaglandin F(2α) during maternal recognition of pregnancy in sheep, by reducing the synthesis and affinity of endometrial oxytocin receptors. Inhibition of other components of the oxytocin receptor-phospholipase C system by ovine trophoblast interferon may also be involved in reducing endometrial responsiveness to oxytocin. Ovine trophoblast interferon may inhibit the synthesis of endometrial oestrogen receptors to inhibit responsiveness ot oxytocin during early pregnancy in ewes.