The apparent in vivo affinity of estradiol binding sites in the uterus varies during the estrous cycle of the rat

An estradiol-exchange assay system was used to determine the amounts of different estradiol receptors in the uterus of rats exhibiting a regular, four day estrous cycle. The total number of all receptors increased from a low at estrus and metestrus to a high at proestrus, before falling sharply between proestrus and estrus. These variations have been explained by alterations in the amounts of estradiol and progesterone in the serum. Similar variations were observed in the classes of free and occupied receptors; the latter varied more dramatically than the former. The changes in the amounts of free and occupied receptors could be explained by variations both in the estradiol levels in the serum and in the total number of receptors in the uterus. However, the amount of occupied receptors did not increase as much from diestrus to proestrus as expected from the increase in the estradiol levels in the serum, indicating that the apparent in vivo affinity of the receptors for estradiol was greater at diestrus than at proestrus. This difference could not be explained in terms of (i) receptor stability during the exchange assay, or (ii) of estradiol retention capacity of the serum, or (iii) of limited uptake of estradiol into the uterine cell. It is concluded that some differences must exist in the function or structure of the receptors at diestrus and proestrus.