Pharmacological approaches to the identification and classification of myometrial oxytocin receptors

Three binding sites have been recently reported on the rat, calf and sheep myometrial cells, with dissociation constants (Kd) roughly 10(-9), 10(-7) and 10(-5) mol/l. Oxytocin receptor for the uterotonic response in vitro was identified pharmacologically: 1) The analysis of dose-response curves has been based on a partial irreversible inhibition of the receptor on isolated rat uterus by the method of Furchgott and Bursztyn, and by the newly suggested plotting of Kd vs. maximal response for an increasing degree of irreversible inhibition. 2) pA2- values (reflecting Kd) of structural analogues of oxytocin acting as competitive inhibitors of the parent hormone have been analysed according to Free and Wilson. Contribution of side chains in individual positions of the nonapeptide chain were computed, tested on additivity and then used for back-computation of a Kd for oxytocin. Results of all experiments reveal a Kd for oxytocin receptor (rat uterus in vitro) of 2 x 10(-7) mol/l. Possible endocrine functions of the high and low affinity sites have not been clarified as yet.