Adrenergic Receptors in Human and Experimental Pheochromocytoma

Abstract

Radio1 igand binding to adrenergic receptors has become an important technique for identification and quantitation of these receptors. In this article we review our recent findings using sybtype-selective radioligand for studying alphal (e.g., [³H]prazosin) and alpha (e.g. [³H]yohimbine) receptors in human platelets and rat rena? cortical membranes, and we report new results on changes in adrenergic receptors of patients and animals with pheochromocytoma. Five patients with pheochromocytoma were tested prior to surgery. The number of a1 pha₂-adrenergic recep- tors on plateley from these patients and the affinity of these receptors for [H]yohimbine were similar t o values for control subjects. In rats with a transplantable pheochromocytoma we found a 50-fold increase in plasma norepinephrine levels. The number of renal cortical beta-adrenergic receptors (identified using [I]iodohydroxybenzyl pindolol) and alpha-adrenergic receptors were decreased while the number of alpha-aenergic receptors did n o t change. Receptor affinitis for radigl igands were unaltered in animals with pheochromocytma. These findings indicate that pheo- chromocytoma is associated with selective decreases in the number of renal beta- and alpha-adrenergic receptors without changi nga1 pha2-adrenergic receptorl number or affinity. Furthermore, the number and affinity of alpha adrenergic receptors are not altered in platelets of patients with-pheochromocytoma. We conclude that down regulation (an agonist-mediated decrease in receptor number) occurs in vivo for some, but not all, types of adrenergic receptors.