Displacement of alpha- and beta-radioligands by specific adrenergic agonists in rat pancreatic islets.

Secretion of insulin is increased by beta-adrenergic agonists and inhibited by alpha-adrenergic agonists. However, administration of epinephrine, which acts on both types of receptors, inhibits insulin secretion. A preliminary study using [3H]-dihydroergocryptine and [3H]-dihydroalprenolol as the respective alpha- and beta-receptor binding ligands, surprisingly revealed a preponderance of beta-binding sites in normal rat pancreatic islets. The present study, using displacement by epinephrine, norepinephrine, isoproterenol and clonidine validated the use of these radioligands as appropriate for specific receptor binding in pancreatic islet cells. The islets were found to have 55 fmol/mg protein of alpha-adrenergic receptor sites and 170 fmol/mg protein of beta-receptor sites. The affinity of both alpha- and beta-receptors for epinephrine was similar, as judged by the displacement of either radioligand, thus ruling out a preferential affinity of alpha-receptor binding as an explanation for the alpha-inhibition of insulin secretion. The data on radioligand displacement by clonidine indicate that the alpha-receptor is of the alpha 2-type.