Demonstration of alpha sub(2)-adrenergic receptors in rat pancreatic islets using radioligand binding (41498)

The type of the alpha -adrenergic receptors on rat pancreatic islet cells was characterized directly using specific radioligands and displacement agonists and antagonists. Scatchard plots for binding of ( super(3)H)clonidine ( alpha sub(2)-agonist) revealed a dissociation constant, K sub(d) of 0.552 plus or minus 0.1 nM and density of binding sites (B sub(max)) of 50.4 plus or minus 3.6 fmole/mg protein. Similar values were obtained with ( super(3)H)dihydroergocryptine (antagonist). The various agonists displaced ( super(3)H)clonidine with the following order of potency: clonidine > epinephrine approximately equals norepinephrine > isoproterenol. Yohimbine, the alpha sub(2)-antagonist, was very effective in displacing ( super(3)H)clonidine, whereas the alpha sub(1)-antagonist, prazosin, was much less effective. The data indicate that the alpha -adrenergic receptors on rat pancreatic islets are of the alpha sub(2) subtype.