Agonist Binding to Mammalin Beta 1 and Beta 2 Adrenoceptors: Modulation by Guanine Nucleotides and Magnesium

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Abstract
Agonist interaction with beta1 and beta2 adrenoceptors in rat rabbit lung has been examined using ligand binding techniques and the results are discussed in relation to current models of beta-adrenoceptor-adenylate cyclase coupling. Agonist binding has been assessed by examining the ability of isoprenaline or salbutamol to displace the labelled antagonist 3H-dihydroalprenolol (3H-DHA) from specific receptor sites. beta1 and beta2 adrenoceptors, even within the same tissue, exhibited different ion and temperature requirements for guanine nucleotide modulation of agonist binding. Thus, in contrast to the situation at beta2 sites, agonist binding to beta1 adrenoceptors was only sensitive to GTP if incubations were performed at physiological temperatures in the presence of Mg++ ions. These findings suggest that there may be different receptor-effector coupling relationships between the two beta-adrenoceptor subtypes.