Receptor binding of oxytocin and vasopressin antagonists and inhibitory effects on isolated myometrium from preterm and term pregnant women
Authors: Åkerlund Mats , Bossmar Thomas , Brouard Rémi , Kostrzewska Anna , Laudanski Tadeusz , Lemancewicz Adam , Gal Claudine Secradeil‐Le , Steinwall Margareta 
Published: August 2005

Objective
To test binding affinities for, and inhibitory effects on, myometrium of some oxytocin and vasopressin antagonists with respect to their therapeutic potential.

Design
Receptor binding studies on transfected cell lines. In vitro contractility studies of human myometrium.

Setting
The Research Laboratory of Sanofi Recherche, Centre de Toulouse, France and the Departments of Obstetrics and Gynecology, Lund University Hospital, Sweden and Bialystok University Hospital, Poland.

Participants
Nine women delivered by caesarean section preterm and 37 delivered at term for routine obstetric indications.

Interventions
The binding affinities of oxytocin, arginine vasopressin, atosiban (1‐deamino‐2‐D‐Tyr(OEt)‐4‐Thr‐8‐Orn‐oxytocin), SR 49059 and SR 121463 for the human oxytocin and different subtypes of vasopressin receptors were determined. Concentration–response curves with oxytocin and arginine vasopressin were recorded on myometrium from preterm– and term–delivered women in control experiments and in the presence of 2.5 and 10 nmol/L of SR 49059. Furthermore, using term myometrium, the influence of SR 49059 and SR 121463 in concentrations of 3, 10, 30 and 100 nmol/L on responses to the EC50 concentrations of oxytocin and vasopressin were compared.

Main outcome measures
Receptor binding affinities. In vitro contractile effects and their inhibitions.

Results
Oxytocin had a high affinity for the oxytocin receptor (Ki in mean = 6.8 nmol/L) and bound, to some extent, to the vasopressin V1a receptor (Ki= 34.9 nmol/L). Vasopressin displayed higher affinities for vasopressin V1a, V1b, and V2 receptors (Ki= 1.4,0.8 and 4.2 nmol/L, respectively) than for the oxytocin receptor (Ki= 48 nmol/L). Atosiban and SR 49059 both had a high affinity for the vasopressin V1a, receptor (Ki= 4.7 and 7.2 nmol/L, respectively, and a moderate one for the oxytocin receptor (Ki= 397 and 340 nmol/L, respectively). SR 121463 exerted a predominant binding to the V2 receptor (Ki= 3.0 nmol/L). In the concentration–response experiments levels of up to 10 nmol/L of SR 49059 had no influence on the effect of oxytocin on myometrium from women preterm and at term pregnancy. However, a concentration–dependent inhibition of the responses of both these type of tissues to vasopressin was seen. The effects of EC50 concentrations of oxytocin and vasopressin on term pregnant myometrium were markedly inhibited by 10 nmol/L and higher concentrations of SR 49059, the inhibition of the response to vasopressin being more pronounced than that of the oxytocin response. SR 121463 at maximal concentration only caused slight inhibitions of the oxytocin and vasopressin responses.

Conclusions
Atosiban and SR 49059 both have moderate binding affinities for the human oxytocin receptor and high binding affinities for the vasopressin V1a one. We demonstrated that SR 49059 inhibits the response of term myometrium to oxytocin and that of both preterm and term myometrium to vasopressin. These observations suggest a therapeutic potential of SR 49059 in preterm labour. The vasopressin V2 receptor is apparently not involved to any significant degree in the activation of the pregnant human uterus.