Oestrogen receptors in the genital tract of the Australian marsupial Trichosurus vulpecula

Macromolecules with high binding affinity for oestrogens (K_a = 10⁹–10¹⁰M^?1 at 0°) are detected in cytosols of vaginal and uterine endothelium of the marsupial brush-tailed possum Trichosurus vulpecula. There are two distinct classes, of high (8 S) and low (4 S) molecular weight, the proportions of which vary in relation to the reproductive state or hormone treatment. In the endometrium, the 8-S form predominates during all phases of the breeding cycle; but there is a relative increase in the 4-S form, up to 40% of the total, during lactation. In the vaginal endothelium, an 8-S form is predominant in cytosols from follicular and lactating animals, with little or no 4-S binding, but the reverse occurs in luteal or pregnant animals. Only the 4-S form is found in cytosols of endometrium or vaginal endothelium of 16-day castrate females. After treatment of castrates with oestradiol, testosterone, or progesterone, however, only the 8-S form is detected in the endometrium. In the vaginal endothelium, the high-affinity binding remains in the 4-S region in castrates treated with oestradiol or testosterone, but is in the 8-S form after treatment with progesterone. The concentration of high-affinity binding sites per milligram of protein is significantly increased in both endometrial and vaginal endothelial cytosols during the follicular phase of the cycle and in castrates treated with oestradiol. This increase is associated with a marked increase in size of the two organs. Treatment with progesterone or testosterone has no effect on the concentration of binding sites in castrates. These findings are discussed in relation to the unique method of reproduction in marsupials.