Temperature related effects on the binding characteristics of beta-adrenergic receptor agonists and antagonists by rabbit lung

The effects of incubation temperature on binding characteristics of beta-adrenergic receptor agonists and antagonists were examined in rabbit whole lung membrane fragments. [3H] Dihydroalprenolol ([3H] DHA) exhibited single component, non-cooperative binding at 37 degrees C and 25 degrees C with characteristics representative of beta-adrenergic receptors. Inhibition of [3H] DHA binding by l-propranolol showed single component binding at either incubation temperature, but with slightly higher affinity at 25 degrees C, when analyzed by Hofstee plots. The binding of l-isoproterenol, l-epinephrine and l-norepinephrine were changed qualitatively, as well as quantitatively, by incubation temperature. Hofstee plot analysis of l-isoproterenol inhibition curves showed a linear plot at 37 degrees C and a curvilinear plot at 25 degrees C, whereas l-epinephrine and l-norepinephrine showed curvilinear plots at both temperatures, but with different relative distributions and affinities of the components. The binding characteristics of several selective beta 1- and beta 2-agonists and antagonists were also influenced by incubation temperature. Practolol and prenalterol, beta 1-selective agents, exhibited multiple component binding at 25 degrees C, but only single component binding at 37 degrees C. In contrast, salbutamol, a beta 2-selective agonist, showed single component binding at 25 degrees C and apparent multiple component binding at 37 degrees C. Such temperature-related changes in binding characteristics were not consistent with receptor subtype interconversion and were not restricted to either agonists or antagonists. These studies suggest that the choice of incubation temperature may result in different binding characteristics of various beta-adrenergic receptor ligands. These effects are especially important when binding studies are designed to determine the classification and density of beta-adrenergic receptor subtypes in individual tissues.