Displacement of Alpha- and Beta-Radioligands by Specific Adrenergic Agonists in Rat Pancreatic Islets

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Abstract
Secretion of insulin is increased by β-adrenergic agonists and inhibited by α-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 α- and β-receptor binding ligands, surprisingly revealed a preponderance of β-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 α-adrenergic receptor sites and 170 fmol/mg protein of β-receptor sites. The affinity of both α- and β-receptors for epinephrine was similar, as judged by the displacement of either radioligand, thus ruling out a preferential affinity of α-receptor binding as an explanation for the α-inhibition of insulin secretion. The data on radioligand displacement by clonidine indicate that the α-receptor is of the α2-type.