Variation of hypophyseal estradiol receptors during rat estrous cycle

The number of free and all receptors present in homogenates of pituitaries was estimated at different days in the rat's estrous cycle. The procedure consisted of a preincubation at 4°C followed by an exchange incubation at 20°C overnight. The former incubation measured the amount of free receptors present. The latter incubation was used to calculate the amount of all receptors present, a calculation which took into account that only 80% of the receptors exchanged their estradiol under this condition. Increasing the temperature did improve the rate of exchange, but was detrimental to the stability of the receptors. The number of all receptors remained stable from estrous to diestrous and decreased somewhat from diestrous to proestrous. The number of free receptors was equal to the total number of receptors, except at proestrous where it was significantly smaller. Thus, measurable amounts of bound receptors were only present at proestrous. The results are compared with similar measurements performed with the uterus and it was concluded that the pituitary is less sensitive to endogenous estradiol than the uterus.