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Previous autoradiographic studies have suggested that the regulation of oxytocin receptors differs between endometrial cell types during the ovine oestrous cycle, and that those present on luminal epithelial cells are of particular importance to the regulation of prostaglandin F₂ release during luteal regression. The present autoradiographic study compares the distribution of the endometrial oxytocin receptor in day-15 non-pregnant and pregnant ewes. The distribution of the endometrial oxytocin receptor in day-15 non-pregnant ewes infused with systemic or intrauterine oxytocin has also been investigated.

Continuous, s.c. infusion of oxytocin (150 mmol/24 h) into ewes (n = 6) between days 10 and 15 of the oestrous cycle significantly increased plasma oxytocin concentrations (to approximately 100 pmol/l). There was no similar increase in systemic oxytocin concentrations in ewes receiving intrauterine (i.u.) oxytocin infusions (10 nmol/24 h) between days 10 and 15 of the oestrous cycle (n = 6). Luteolysis was inhibited in all six animals infused with oxytocin (s.c.) and endometrial oxytocin receptor concentrations were significantly lower on day 15 in these animals (12·8 ± 6·5 (S.E.M.) fmol/mg protein; P<0·001) and in pregnant ewes (18·4 ± 15·4 fmol/mg protein; P <0·001; n = 8) than in ewes infused with saline (248·6±67·1 fmol/mg protein; n = 6). While the ¹²⁵I-labelled oxytocin receptor antagonist, [1-(β-mercapto-β,β-cyclopentamethylene propionic acid), 2-(ortho-methyl)-Tyr², Thr⁴, Orn⁸, Tyr⁹-NH₂]-vasotocin (¹²⁵I-labelled OTA) clearly labelled glandular epithelia, luminal epithelium and caruncular stromal cells specifically on day 15 in saline (s.c.)-infused ewes, such specific labelling appeared to be reduced or absent from pregnant animals and those infused with oxytocin (s.c.). A significant reduction in the density of labelling of caruncular stroma (P < 0·05) and luminal epithelium (P < 0·001) was confirmed using quantitative densitometric analysis. The reduction in the labelling of endometrium in oxytocin-infused ewes was not caused by the binding of exogenous oxytocin to endometrial binding sites. Oxytocin infusion (i.u.) did not inhibit luteolysis, nor was there any significant difference in the endometrial oxytocin receptor concentration in this group of ewes on day 15 compared with those infused with saline (i.u.). There was also clear specific labelling of luminal epithelial cells with ¹²⁵I-labelled OTA in ewes receiving oxytocin infused i.u. and quantification of autoradiograms failed to differentiate between these animals and those infused with saline (i.u.).

It was concluded that systemic oxytocin infusion and the early establishment of pregnancy led to a clear reduction in the endometrial oxytocin receptor concentration on luminal epithelial cells, glandular epithelial cells and caruncular stromal cells, but that i.u. oxytocin infusions did not affect any of these receptor populations and notably not the luminal epithelial oxytocin receptor. The results support the contention that the luminal epithelial oxytocin receptor is involved in the luteolytic process.

Journal of Endocrinology (1993) 137, 423–431

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