Characterization of Ectopicα-Adrenergic Binding Receptors of Adrenocortical Carcinoma Cells*

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Abstract
In the accompanying communication, we provided evidence for the formation of cGMP via the activation of α-adrenergic receptors in isolated adrenocortical carcinoma cells. The investigations of Williams et al. have demonstrated that the compound [3H]dihydroergocryptine (DHE) can be used as a specific ligand for the direct characterization of α-adrenergic receptors. In the present communication, we have used this agent for the direct identification of α-adrenergic receptors on adrenocortical carcinoma cells. The results showed that the binding of [3H]DHE with the adrenocortical carcinoma cell receptors is rapid, reversible, and of high affinity, with an apparent dissociation constant of 0.55 × 10−9m. Binding of [3H]DHE was a saturable process corresponding to 85 fmol of sites/2 × 106 cells. α-Adrenergic agonists and antagonists competed far more effectively for the binding receptors of DHE than the β-adrenergic agents. Acetylcholine, a cholinergic agonist, was devoid of binding activity. No detectable binding of [3H]DHE with normal adrenal cell receptors was observed. These studies thus demonstrate ectopic production of α-adrenergic receptors in the adrenocortical carcinoma cells. Taken together with the evidence provided in the accompanying paper, in which we have shown that catecholamines stimulate the tumor guanylate cyclase via α-adrenergic receptors, these studies present the first complete enzymatic and pharmacological analyses of adrenocortical carcinoma α-adrenergic receptors and their relationship to the guanylate cyclase system. (Endocrinology106: 1594, 1980)

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