Oxytocin receptors in myometrium of women, rats, and rabbits rise markedly before the onset of labor, suggesting a role in the initiation of labor. In guinea pigs, a previous study reported no such rise by one-point determination of oxytocin binding. The purpose of this study was to use a more rigorous method to determine whether the binding characteristics of myometrial oxytocin receptors change in relation to labor in guinea pigs. Competitive binding studies were carried out in microsomes from inner and outer myometrium between 42 days of gestation and labor. Binding to analogs was also tested. Data were analyzed with affinity spectra and LIGAND. Oxytocin bound to one site with a dissociation constant of 6.3 +/- 0.65 x 10(-9) M. Binding capacity was 1.0 +/- 0.1 x 10(-12) mol/mg protein. The Hill coefficient was near unity. No significant changes occurred with gestation or labor in dissociation constant, binding capacity, or Hill coefficient (all P >/= 0.2, nested ANOVA). Binding capacity was higher in the outer than in the inner layer (1.2 +/- 0.2 vs. 0.8 +/- 0.1 x 10(-12) mol/mg protein, P = 0.02), but the dissociation constants were similar. Differences existed in the dissociation constants of the analogs tested. The main conclusion is that oxytocin receptors are unlikely to have a regulatory role in the initiation of labor in guinea pigs.
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