OUABAIN RECEPTOR-BINDING OF HYDROXYPROGESTERONE DERIVATIVES

A specific and sensitive radioreceptor assay has been devised which is based on high affinity, saturable binding of 9 nm [3H]‐ouabain to the total particulate fraction isolated from dog heart. Ouabain and other cardiac glycosides, including the aglycones, were about equipotent in their ability to displace [3H]‐ouabain from its receptor, the IC50s ranging from 10 to 30 nm. The only other substances found to compete significantly in the assay were derivatives of hydroxy‐progesterone having a 17α‐acetate substituent: chlormadinone acetate, megestrol acetate, cyproterone acetate and medroxyprogesterone acetate, with IC50s of 2, 7.4, 9 and 21 μm, respectively. Prednisolone‐3,20‐bisguanyl‐hydrazone, reported to have inotropic activity, gave an IC50 of 6.4 μm. Cyproterone‐17α‐OH was less active (IC50 90 μm) than cyproterone‐17α‐acetate. A large number of peptide and protein hormones, steroid hormones and their metabolites, amines, and drugs were inactive. 1979 British Pharmacological Society