Demonstration of α 2 -Adrenergic Receptors in Rat Pancreatic Islets Using Radioligand Binding 1

Abstract

The type of the α-adrenergic receptors on rat pancreatic islet cells was characterized directly using specific radioligands and displacement agonists and antagonists. Scatchard plots for binding of [³H]clonidine (α₂-agonist) revealed a dissociation constant, K _d of 0.552 ± 0.1 nM and density of binding sites (B_max) of 50.4 ± 3.6 fmole/mg protein. Similar values were obtained with [³H]dihydroergocryptine (antagonist). The various agonists displaced [³H]clonidine with the following order of potency: clonidine > epinephrine > norepinephrine > isoproterenol. Yohimbine, the α₂-antagonist, was very effective in displacing [³H]clonidine, whereas the α₁-antagonist, prazosin, was much less effective. The data indicate that the α-adrenergic receptors on rat pancreatic islets are of the α₂ subtype.