Aug 1, 1980

Agonist versus antagonist binding to alpha-adrenergic receptors

Proceedings of the National Academy of Sciences of the United States of America
B B HoffmanJ N Fain

Abstract

The binding properties of two alpha-adrenergic radioligands, [3H]epinephrine (an agonist) and [3H]dihydroergocryptine (an antagonist), were compared in two model systems--membranes derived from human platelets and membranes from rat liver. The platelet contains exclusively alpha 2 and the liver mostly (approximately 80%) alpha 1 receptors. Agonists induce the formation of a guanine nucleotide-sensitive high-affinity state of alpha 2 but not alpha 1 receptors. [3H]Dihydroergocryptine labels all the alpha receptors, whereas [3H]epinephrine at low concentrations labels predominantly the high-affinity form of the alpha 2 receptor in both platelet and liver. However, in the liver, alpha-adrenergic effects such as glycogen phosphorylase activation are shown to be mediated via alpha 1 receptors. Thus, in liver membranes the endogenous "physiological" agonist may not label the physiologically relevant alpha 1 receptors in typical radioligand binding assays using low concentrations of [3H]epinephrine.

Mentioned in this Paper

Guanylyl Imidodiphosphate
Adrenergic alpha-Agonists
Adrenergic alpha-Antagonists
Medihaler-Epi
Norepinephrine Receptors
Phosphorylases
Alpha-adrenergic receptor
Ergot Alkaloids, Hydrogenated
Liver
Blood Platelets

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