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Presence of ectopic beta-adrenergic receptors on human adrenocortical cortisol-producing adenomas
Abstract:
A direct binding study of radioligand [3H]dihydroalprenolol (DHA), a potent beta-adrenergic antagonist, was performed on the particulate fractions of four adrenocortical adenomas (three cortisol-producing adenomas and one aldosterone-producing adenoma) and normal adrenal tissues. The effect of epinephrine on cortisol production was also evaluated in vitro from the cultured tumor cells from one cortisol-producing adenoma. Saturable binding of [3H]DHA to the tumor membranes was observed in two of three cortisol-producing adenomas, but not in the aldosterone-producing adenoma or in normal adrenal tissues. Scatchard analysis of equilibrium binding of [3H]DHA revealed a single class of binding sites on the tumor membranes; the apparent dissociation constant (Kd) was 1 nM in each, and the numbers of binding sites were 108 and 45 fmol/mg protein, respectively. Competition by adrenergic agents with [3H]DHA for binding sites on the membranes from one cortisol-producing adenoma revealed that (+/-)propranolol, a beta-adrenergic antagonist, was about 350-fold more potent than phentolamine, an alpha-adrenergic antagonist, suggesting the beta-adrenergic nature of receptor sites. In addition, stereospecificity was demonstrated by about 1000-fold greater affinity of (-)alprenolol than to (+)alprenolol, both of which are stereoisomers of the beta-adrenergic antagonist. Furthermore, production of cortisol from the cultured tumor cells prepared from the same adenoma was significantly stimulated by epinephrine in addition to ACTH. These data indicate that ectopic beta-adrenergic receptor sites are present in some human adrenocortical tumors which may be functionally related to the activation of adenylate cyclase by catecholamines other than ACTH in those tumors, as previously demonstrated. The mechanism by which such altered cellular membrane characteristics occur in association with neoplastic alteration of the endocrine tissues remains unanswered.