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Oxytocin Receptors in Guinea Pig Myometrium Near Term and During Labor¹

Vladimir Plika^b

^a Department of Obstetrics & Gynaecology, Research Centre for Reproductive Medicine, University of Auckland, Auckland, New Zealand ^b Department of Animal Science, Swiss Federal Institute of Technology (ETH), CH-8092 Zürich, Switzerland ^c Institute of Clinical Chemistry, Faculty of Medicine, University Hospital, CH-8091 Zürich, Switzerland

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.

First decision: 6 August 1999.

¹ This research was supported by the New Zealand Health Research Council.

² Correspondence: J-C. Schellenberg, Department of Obstetrics & Gynaecology, Hôpital Cantonal Universitaire, 32 Boulevard de la Cluse CH-1211 Geneva 14, Switzerland. FAX: +41 22 382 4030;jean-claude.schellenberg{at}hcuge.ch