Comparison of follicular estrogen receptors in rat, hamster, and pig

The hypophysectomized or intact hamster-unlike the rat-does not respond to estrogen by increasing the number of large antral follicles. In the pig, estrogen inhibits progesterone synthesis by granulosa cells from large follicles for 36 h before progesterone accumulation is enhanced. The objective of this study was to determine whether these divergent responses of the three species to estrogen are related to differences in follicular estrogen receptors. For the rat antral follicle, the equilibrium dissociation constants (KaS), expressed as nmol/L, were 0.55 +/- 0.07 in the cytosolic fraction and 0.46 -/+ 0.11 in the nuclear fraction; for the hamster and pig follicles, the respective KaS were 1.08 +/- 0.04 and 0.94 +/- 0.11, and 1.16 0.23 and 1.09 +/- 0.17, respectively. The cytosolic and nuclear KaS for the rat follicle were statistically different from those of the other two species, but this is most likely not of biological consequence. Comparison of the maximal numbers of binding sites (NBSmax) in the cytosolic fractions of the antral follicles, expressed as fmol/mg protein, indicated a relationship of hamster > rat = pig; whereas in the nuclear fraction, with the NBSmax expressed ms fmol/mg DNA, the relationship was hamster > pig > rat. The total number of estrogen receptors (sum of the NBSmax of cytosolic and nuclear fractions) was approximately tenfold higher in the hamster than in the rat. Thus, the failure of hamster large follicles to respond in vivo or in vitro to estrogen cannot be explained on the basis of lack of estrogen receptors. It is possible that the greater number of occupied and unoccupied estrogen receptors in the hamster compared to the rat is related to the difference in biologic responses to estrogen. Alternatively, differences in post-receptor events may be involved.