Agonist versus antagonist binding to alpha-adrenergic receptors.

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Abstract
The binding properties of 2 .alpha.-adrenergic radioligands, [3H]epinephrine (an agonist) and [3H]dihydroergocryptine (an antagonist), were compared in 2 model systems: membranes derived from human platelets and membranes from rat liver. The platelet contains exclusively .alpha.2 and the liver mostly (=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, but [3H]epinephrine at low concentrations labels predominantly the high-affinity form of the .alpha.2 receptor in both platelet and liver. In the liver, .alpha.-adrenergic effects, i.e., glycogen phosphorylase activation are mediated via .alpha.1 receptors. 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.