Agonist interactions with alpha-adrenergic receptors.
alpha-Adrenergic receptors have been grouped into two major subtypes, termed alpha 1- and alpha 2-receptors. Radioligand binding techniques have been utilized to measure the number of alpha 1- and alpha 2-receptors in a variety of tissues. [3H]Dihydroergocryptine labels the entire alpha-receptor population; the alpha-receptor subtypes may be delineated by constricting competition curves with unlabeled selective antagonists and analyzing the data with computer modeling techniques. Alternatively, alpha 1- and alpha 2-receptors may be directly identified with selective radioligands such as [3H]prazosin and [3H]yohimbine, respectively. For example, rat liver membranes have been shown to contain alpha 1- (80%) and alpha 2- (20%) receptors; the alpha 1-receptors activate glycogen phosphorylase. Radioligands have also been used to probe the mechanism by which alpha 2-receptors may inhibit adenylate cyclase activity. Agonist competition curves with [3H]dihydroergocryptine at eht human platelet's alpha 2-receptor may be resolved into two affinity components, interconvertible by guanine nucleotides. These data suggest the agonist-promoted association of the alpha 2-receptor with an additional membrane component. More direct evidence in favor of this possibility was indicated by the increase in sedimentation velocity of solubilized agonist-labeled receptor on sucrose density gradients.
D000316:Adrenergic alpha-Agonists / Q000494:pharmacology*; D000818:Animals; D001667:Binding, Competitive; D001792:Blood Platelets / Q000378:metabolism; D006150:Guanine Nucleotides / Q000494:pharmacology; D007700:Kinetics; D008024:Ligands; D008099:Liver / Q000378:metabolism; D011224:Prazosin / Q000494:pharmacology; D051381:Rats; D011941:Receptors, Adrenergic / Q000187:drug effects*; D011942:Receptors, Adrenergic, alpha / Q000145:classification / Q000187:drug effects*; D015016:Yohimbine / Q000494:pharmacology
D000316:Adrenergic alpha-Agonists; D006150:Guanine Nucleotides; D008024:Ligands; D011941:Receptors, Adrenergic; D011942:Receptors, Adrenergic, alpha; D015016:Yohimbine; D011224:Prazosin