Riley, Paul Richard; (1996) The ovine endometrial oxytocin receptor. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

The ovine endometrial oxytocin receptor (OTR) plays a pivotal role in both luteolysis and maternal recognition of pregnancy. The purpose of this PhD thesis was to investigate structure, function, localisation and regulation of OTR in light of this role. An ovine endometrial OTR cDNA was sequenced. The encoded receptor was a 391 amino acid polypeptide, 94% homologous to a previously reported human myometrial OTR, and was assigned to the G-protein-coupled seven transmembrane domain receptor family. The ovine OTR contained two extra amino acids compared to the human OTR, located in the third intracytoplasmic loop; the region associated with G-protein coupling. Northern blot and primer extension analysis revealed multiple OTR transcript sizes in oestrous endometrium of 1.65, 2.5, 3.9 and 6.2 kb and a possible transcription start site 90 base pairs upstream of the ATG translation initiation codon. The OTR cDNA was transiently expressed in Cos-7 cells and shown to be functional. Binding parameters for the expressed receptor were consistent with native OTR. Accumulation of inositol phosphates in OTR cDNA transfected Cos-7 cells was measured in response to treatment with ligand, non-specific G-protein analogues (A1F4-, GTP?S), pertussis toxin (PTX) and a phospholipase C (PLC) inhibitor to characterise oxytocin-induced stimulation of PLC activity. OTR mRNA was localised, using in situ hybridisation, primarily to the epithelial cells. It first appeared in the luminal epithelium (LE) on day 14 of the cycle and reached a peak at oestrus. OTR mRNA was absent during early pregnancy, appearing only on day 21 which coincides with the time at which the inhibition of OTR by trophoblast interferon (IFN?) ceases. In situ hybridisation on uterine sections from ovariectomized steroid-treated ewes demonstrated that OTR mRNA was under transcriptional control such that progesterone loses its inhibitory influence in the LE on day 14, whereas development in the myometrium at oestrus is oestrogen-dependent. Oxytocin had no effect on its own receptor at the level of gene transcription, suggesting its reported inhibitory effect occurs at the level of translation or down-regulation of functional OTR.

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