Solubilization and Properties of Oxytocin Receptors from Rat Mammary Gland*

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Abstract
Oxytocin (OT)-binding activity was extracted with the Zwitterionic detergent 3-[(3-cholamidopropyl)dimethylammonio]2-hydroxy-1-propanesulfonate (CHAPSO) from rat involuted mammary glands with about 20% yield. The binding in detergent extracts was characterized and shown to be similar or identical to that of OT receptors on intact plasma membranes. Solubilized receptors had a high affinity site (Kd, .apprx. 2 nM) and a lower affinity component, whereas the membrane receptor has only the high affinity site. Several synthetic OT analogs inhibited [3H]OT binding in the same rank order in both solubilized and intact membrane preparations. Both solubilized and membrane-associated receptor required Mn2+ for [3H]OT binding. The concentration of OT-binding sites in solubilized extracts of uterine myometrium from rats in late pregnancy was substantially greater in uteri from rats in labor than in that from rats 2 days before labor, as we have seen previously with receptors on intact membranes. The affinity of the solubilized myometrial receptor (Kd, .apprx. 5 nM) was comparable to that of the membrane-associated receptor. Binding of [3H]OT to solubilized extracts of intestinal smooth muscle, which is not a target for OT, was negligible. Gel filtration analysis on columns of Sepharose 6B indicated that the solubilized [3H]OT-binding component from mammary gland was present in multiple mol wt forms, but the smallest major form eluted with an average apparent mol wt of about 40,000. These studies indicate that CHAPSO-solubilized binding sites for [3H]OT are the same as those in intact membranes and, therefore, are components of the OT receptor.