Specific binding of progesterone to the cell surface and its role in the meiotic divisions in Rana oocytes.

Progesterone is believed to act at the cell surface to induce the resumption of the meiotic divisions in amphibian oocytes. Analysis of [3H]- and [14C] progesterone uptake and exchange by the plasma-vitelline membrane complex, nucleus and cytoplasm of the isolated Rana oocyte indicates that progesterone uptake by the plasma membrane is saturable, specific and temperature-dependent, and has a slow off-rate. Estradiol (a noninducer) did not compete with progesterone, whereas testosterone (an inducer) blocked progesterone uptake by the membrane complex. Scatchard-type plots indicate an apparent Kd of 5.1.10-7 M over the [progesterone]0 range of 0.01-1.0 microM with maximum binding at about 70 fmol per oocyte. Membrane uptake at higher [progesterone]0 (2-40 microM) indicates apparent cooperative binding, with saturation up to 10 pmol per oocyte. Cytoplasmic uptake was apparently nonspecific and less temperature-dependent than membrane uptake and steroid concentrations (progesterone and pregnanediones) exceeded water solubility by 30-60 min. Nuclear uptake was saturable and specific but uptake was independent of temperature. A comparison of membrane binding and a physiological response (nuclear breakdown) indicated only about 10% of the membrane sites need be filled to initiate a 50% response.