Using adenohypophyses from normal female rats, we demonstrate that estradiol binds pituitary membranes to one homogeneous population of sites with high affinity [dissociation constant (Kd) = 0.041 +/- 0.014 nM; n = 6] and low capacity [maximum binding (Bmax) = 13.6 +/- 5.6 fmol/mg protein]. The binding is thermolabile. Association experiments show that the best experimental conditions are an overnight incubation at 0 C. When the amount of proteins is increased more than 0.3 mg/ml of membrane suspension, binding is rapidly nonlinear. The presence of 0.5 M leupeptin does not improve the binding. Extensive washing of the membranes does not decrease the amount of sites, indicating that the binding is not loosely attached to the membranes. Parenthetically, it should be noted that the membrane fraction was devoid of the cytosolic enzyme marker, lactate dehydrogenase. Binding is specific for estrogenic compounds. When 100% specific binding was determined in the presence of 10(-6) M diethylstilbestrol, 17 beta-estradiol, estrone, and estriol displaced total binding by 110, 80, and 75%, respectively. Neither 4-OH-tamoxifen nor dihydrotestosterone, progesterone, or cortisol displaced the binding. Taken together, these data argue in favor of the presence of specific membrane recognition sites for estradiol in the rat pituitary.

Dopaminergic receptors were observed on the membranes of 26 human PRL-secreting adenomas. Two binding sites were found for [3H]domperidone, a selective dopamine antagonist, with a Kd1 of 0.29 +/- 0.06 nM and a Kd2 of 4.19 +/- 0.7 nM (n = 5). Bmax1 and Bmax2 (maximum numbers of binding sites, first and second sites, respectively) varied from one adenoma to another. When considering Bmax1, two different categories of PRL-secreting adenomas were distinguished, one with a concentration of receptor over 250 fmol/mg protein and others with lower concentrations of receptor (< 250 fmol/mg protein). In the latter, when considering the volume of the tumor, a higher plasma PRL level was found. Two binding sites for [3H]domperidone also were found in the normal human hypophysis with Kd values similar to those of the adenomas (0.18 +/- 0.04 and 3.95 +/- 0.35 nM). The same number of binding sites was observed in normal pituitaries and in PRL-secreting adenomas. However, when considering the density of PRL-secreting cells in the two different tissues (prolactinomas contain 2 or 3 times more PRL-secreting cells than human pituitary tissue), one may suspect a defect in the dopaminergic inhibiting control in PRL-secreting adenomas.

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