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Abstract

The specific binding of [³H]oxytoxin to uterine membrane preparations derived from different species at late pregnancy was examined. The highest receptor density (b_max value) was found in membranes derived from the myometria of guinea pigs between day 60 post-conception (b_m = 3.6 ± 0.1 pmol/mg) and day 65 (bmax = 4.4 ± 0.1 pmol/mg). The similarity of Kd values for oxytocin binding (K_d = 2.6 ± 0.2 nM) and for vasopressin binding (K_d = 2.1 ± 0.4 nM) to the same membranes derived from a guinea pig myometrium indicate a homogeneous population of high-affinity binding sites which do not discriminate between these two hormones. Competitive binding experiments with specific oxytocin agonists containing either sarcosine or N-methylalanine in the place of Pro⁷ demonstrated that these myometrial receptors have the pharmacological properties of oxytocin receptors. The analogue of 1-deamino-[8-lysine]vasopressin containing a photoreactive azidophenylamidino group at the sidechain of Lys⁸ retained roughly the same receptor affinity as oxytocin. In photoaffinity labelling experiments with the tritium-labelled analogue a membrane protein from guinea pig myometrium with an apparent relative molecular mass M_r of 78,000 ± 5000 (n = 13) was preferentially labelled. The labelling of this protein was completely suppressed by a 100-fold molar excess of either oxytocin, or [Sar⁷]oxytocin or [Thr⁴, Sar⁷]oxytocin, but not by other peptide hormones. These results provide evidence that the labelled 78,000-M_r protein is a myometrial oxytocin-receptor protein.