Progestin receptors in adult rat ovary during estrous cycle.

Using a synthetic progestin (P)(i.e. R5020), characteristics of P receptors were determined in the ovarian tissue cytosols from adult estradiol benzoate-treated or 4-day cycling rats. In the estradiol benzoate-treated rats a specific 3H-R5020 binding in the cytosol was found with a number of binding sites, V max = 110 fmol/mg protein and the equilibrium dissociation constant, Kd = 14 nM. In the 4-day cycling rats, specific binding was found in the 6-7 S region with the Vmax which fluctuated during the estrous cycle: the sequence of Vmax (fmol/mg protein) was 395 (proestrus) greater than 122 (diestrus) greater than 96 (late estrus) greater than 62 (metestrus) greater than 40 (early estrus). The Kd value varied during the cycle, the highest (22 nM) in proestrus and the lowest (5 nM) in early estrus. In addition, 3H-R5020 binders were thermolabile and of protein in nature. These results suggest that the rat ovary contains P receptors in its cytoplasm with high affinity and low capacity of P binding, the level of which fluctuates during the estrous cycle.