Identification of functional oxytocin receptors in lactating rat mammary gland in vitro.

The oxytocin (OT) receptor of the lactating rat mammary gland was further characterized by radioligand binding and functional assays in vitro and compared to the uterine OT receptor. In equilibrium saturation binding studies, [3H]OT bound apparently to a single site in mammary tissue with an affinity (Kd = 0.98 nM) similar to that found in the uterus (Kd = 0.68 nM). Using a variety of ligands for OT and arginine vasopressin (AVP) receptors, binding studies indicated that the recognition profile of the mammary [3H]OT binding site closely resembled that found for the uterus but was distinct from the known AVP receptor subtypes. In functional studies, OT and the highly selective OT agonist, [Thr4,Gly7]OT, were potent activators of phosphatidylinositol (PI) turnover in both mammary and uterine slices (EC50 3-5 nM). L-365,209, a novel potent and selective OT antagonist, inhibited OT-stimulated PI turnover in both tissues with similar potencies. These data provide evidence that the high-affinity [3H]OT binding site found in rat mammary tissue during lactation is a functional OT receptor coupled to PI turnover.