Seasonal fluctuations of estrogen-binding activity in the testis of the frog, Rana esculenta.

We have studied the annual cycle of estrogen-binding activity in the frog, Rana esculenta, testis to evidence possible fluctuations. The testicular binding for [3H]estradiol-17 beta shows high affinities (cytosolic Kd = 1.94 +/- 0.43 x 10(-9) M; nuclear Kd = 2.72 +/- 1.20 x 10(-9) M) and low capacity (cytosolic: 7.56 +/- 0.66 fmol/mg protein; nuclear: 9.27 +/- 2.5 fmol/mg protein). Nuclear-binding activity appears in spring concomitantly with the estradiol peak in plasma and testes, while, in the remaining periods, binding capacity was extremely low or absent. Present data strongly support an important role played by estrogens as an intratesticular control factor in vertebrates.