Solubilization and Characterization of D2-Dopamine Receptors in an Estrone-Induced, Prolactin-Secreting Rat Pituitary Adenoma

Abstract

D₂‐dopamine (3,4‐dihydroxyphenylethylamine) receptors were successfully solubilized with 3‐[(3‐cholami‐dopropyl)‐dimethylammonio]‐l‐propane sulfonate from an estrone‐induced rat pituitary adenoma. Forty‐five percent of initial protein and 48% of initial [³H]spiroperidol binding sites were solubilized. The high affinity as well as the stereoselectivity of the sites was preserved. The order of potency of dopaminergic agonists was found to be typical of D₂ receptors. Target size analysis by radiation inactivation indicated a molecular weight of 143,000 ± 3,000 and of 106,000 ± 4,000 daltons for membrane‐bound and solubilized receptors, respectively. This suggests the loss of a 37,000‐dalton subunit during solubilization without significant modification of binding characteristics. Sodium do‐decyl sulfate‐polyacrylamide gel electrophoresis of receptor protein preparation photolabeled with N‐(p‐azido‐w[¹²⁵I]‐iodophenethyl)spiroperidol confirmed the existence of a 94,000‐dalton peptide which probably constitutes the li‐gand binding site of the receptor. Thus, our data indicate that chronic estrogen treatment of rats, although inducing a pituitary adenoma, does not modify the pharmacological characteristics of D₂ receptors. These data suggest therefore that these adenoma may represent an ideal source of material for further biochemical characterization of D₂ receptors.