Abstract
1 The apparent dissociation constants (Kd) of four competitive antagonists of oxytocin were estimated from their ability to compete with [3H]-oxytocin for binding sites in particulate fractions from rat uterine homogenates. 2 These apparent Kd values were not significantly different from the Kd values calculated from the published potency of each compound as an antagonist of oxytocin-induced uterine contractions. 3 These results support the conclusion that the binding sites for oxytocin are part of the receptor complex. Furthermore, 'spare receptors' for oxytocin do not appear to be present in significant quantities, and the relative potency of each antagonist appears to depend upon its affinity for the receptor site rather than its intrinsic activity. 4 The antagonists used in these studies were [N-acetyl, 2-O-methyltyrosine]oxytocin, [1-(beta-mercapto-beta,beta-diethylpropionic acid)]oxytocin, [1-(beta-mercapto-beta,beta-pentamethylenepropionic acid)]oxytocin, and [1-(deaminopenicillamine), 4-threonine]oxytocin.