Binding Properties of Alpha-i Adrenergic Receptors in Rat Cerebral Cortex: Similarity to Smooth Muscle1

The characteristics of alpha-i adrenergic receptors in rat cerebral cortex were examined using the radioiodinated alpha-i adrener-gic receptor antagonist [1I]BE 2254 ([1lJBE). [125l]BE labeled a single class of high-affinity binding sites in a particulate fraction of rat cerebral cortex with mass action kinetics and a K of 57 pM. The binding of [125I]BE was inhibited by various alpha adrenergic receptor antagonists, partial agonists and full ago-nists. The potency of these compounds in competing for the [12511BE binding sites suggested that [125I]BE was labeling alpha-1 adreriergic receptors in rat cerebral cortex. In the absence of a physiological concentration of NaCI in the assay medium there was a small (20%) decrease in the density of [1251]BE binding sites with no effect on the K, value. The absence of NaCI also caused a 4-fold increase in the potency of norepinephnne i