No evidence for temperature-dependent changes in the pharmacological specificity of beta 1- and beta 2-adrenoceptors in rabbit lung membranes.

In rabbit lung membranes known to contain both beta 1- and beta 2-adrenoceptors it was studied whether changes in incubation temperature may affect binding characteristics and/or selectivity of beta 1- and beta 2-adrenoceptor drugs. For this purpose inhibition of binding of the highly specific beta-adrenoceptor radioligand (+/-)-125iodocyanopindolol (ICYP) by beta 1-and beta 2-selective as well as non-selective drugs was determined at incubation temperatures of 37 degrees C and 18 degrees C and analyzed by Hofstee-plots. 1. The density of beta-adrenoceptors in rabbit lung membranes was identical independently of the incubation temperature. 2. At both incubation temperatures beta 1- and beta 2-selective drugs showed biphasic displacement curves and non-linear Hofstee-plots, while inhibition of binding by the non-selective drugs resulted in monophasic displacement curves with linear Hofstee-plots. With decreasing temperature affinity of antagonists to beta-adrenoceptors increased only slightly, while affinity of agonists increased markedly. 3. For all beta 1- and beta 2-selective drugs the same ratio beta 1/beta 2-adrenoceptors was calculated from the Hofstee-plots independently of the incubation temperature: It amounted to about 80% beta 1- and 20% beta 2-adrenoceptors in rabbit lung. 4. At an incubation temperature of 37 degrees C the displacement curve of the agonist isoprenaline was biphasic in the absence of GTP with a non-linear Hofstee-plot indicating that at 37 degrees C isoprenaline binds to high and low affinity states of the beta-adrenoceptors in rabbit lung. At 18 degrees C, however, beta-adrenoceptors do not form the high affinity GTP-sensitive complex with agonists, since GTP had no influence on isoprenaline displacement curves. 5. It is concluded that a decrease in the incubation temperature of ICYP binding assay from 37 degrees C to 18 degrees C does neither alter the relative amount of beta 1- and beta 2-adrenoceptors in rabbit lung membranes nor the selectivity of beta 1- and beta 2-selective adrenoceptor drugs.