Progesterone derivatives that bind to the digitalis receptor: effects on Na+,K+-ATPase and isolated tissues

The steroid nature of the cardiac glycosides (CG) suggests that an endogenous steroid (or steroids) may be the natural ligand for the specific receptor site, i.e., Na+,K+-ATPase. Derivatives of progesterone (PROG) that compete in a [3H]ouabain radioreceptor binding assay (RRA) are characterized by 17 alpha-acetylation and modifications in the B ring. Chlormadinone acetate (CMA) is the most potent analog identified thus far, having about one-twentieth the RRA potency of ouabain. CMA interacts at the ouabain site on Na+,K+-ATPase, inhibits the enzyme in the same rank order of species sensitivity as do the CG, and inhibits the sodium pump in vitro in guinea pig atrium and perfused heart, cardiac myocytes, rat diaphragm, and red blood cells. CMA does not cross-react with digoxin antiserum, which indicates that CG antibodies are not necessarily directed at molecular determinants of biological activity. By crystallographic analysis, the 20-carbonyl moiety in CMA is seen spatially oriented so as to be equivalent to the lactone 23-oxygen atom in the CG. CMA exerts primarily cardiodepressant effects, in accordance with the often-reported similar action of PROG. Negative inotropy may be mediated other than by Na+,K+-ATPase because PROG itself has no significant CG-like actions. Positive inotropy by CG, cardiodepression by CMA and PROG or low concentrations of CG, occasional transient enhancement of contractility by CMA, and pump stimulation by low concentrations of CMA or CG may reflect different affinities of the compounds for sites that mediate Na+,K+-ATPase/pump inhibition, positive inotropy, and negative inotropy. Thus, PROG derivatives related to CMA appear to be likely candidates for endogenous digitalis-like hormones. Body tissues possess the enzymes for conversion of PROG to derivatives related even more closely than the semisynthetic CMA to the CG configuration.