Biochemical studies of beta-adrenoceptor heterogeneity
The properties of the specific 3H-labelled (-)-dihydroalprenolol (3H-DHA) binding sites on membrane preparations from a number of mammalian tissues have been examined. Specific 3H-DHA binding to all membrane sites was rapid, reversible, saturable, stereoselective and displaceable by beta-adrenoceptor agents with affinities which were similar to those found using intact tissue preparations. These findings indicated that 3H-DHA binding was to sites that had all the characteristics of beta-adrenoceptors. The pharmacological characteristics of these sites have been examined using highly selective beta-adrenoceptor agents. The results indicated that membranes of rat and rabbit lung, rat cerebral cortex, and rat myocardium possessed heterogeneous sites which were pharmacologically equivalent to beta1 and beta2 -adrenoceptors. The single cell types rat erythrocytes and reticulocytes possessed a homogeneous population of beta2-adrenoceptors. The pharmacological properties of the beta-adrenoceptor subtypes were conserved between tissue and species and they were independent of the proportion of the subtypes in the preparation. This finding suggested that there were only two mammalian beta-adrenoceptors. The rat erythrocyte and reticulocyte beta-adrenoceptor systems have been compared. The results indicated that these cell types possessed identical beta-adrenoceptor binding sites but markedly different receptor-adenylate cyclase coupling relationships. Rat reticulocytes contained approximately four-fold greater density of beta-adrenoceptors, and approximately twenty-fold greater maximum isoprenaline stimulated adenylate cyclase activity. The intrinsic activities of the partial agonists salbutamol and procaterol were higher for rat reticulocytes than erythrocyte membranes. Guanine nucleotides modulated agonist binding to beta-adrenoceptors of rat reticulocytes but not to rat erythrocytes. These results suggested that maturation of the rat reticulocyte was associated with functional uncoupling of the receptor-effector system. The beta-adrenoceptors of non-mammalian tissues have also been characterised. Chick and frog erythrocytes were shown to possess beta-adrenoceptor binding sites which did not correspond exactly to mammalian beta1 and beta2 -adrenoceptors. The atypical nature of these non-mammalian beta-adrenoceptors is discussed in relation to their previous extensive use as model beta-adrenoceptor systems. beta1 and beta2-adrenoceptors have been solubilised from rabbit and rat lung using the detergent digitonin. A charcoal separation technique has been developed for the assay of soluble beta-adrenoceptors and its application indicated that the major pharmacological characteristies of the beta-adrenoceptors were conserved upon solubilisation. The potency of several catecholamine agonists and the steepness of the displacement curves were greater in the solubilised preparations. The co-presence of beta1 and beta2-adrenoceptors has been demonstrated in solubilised preparations of rabbit lung. Soluble beta1- adrenoceptors appeared to be more labile than soluble beta2-adrenoceptors, which provided support for the concept that the beta-adrenoceptor subtypes were separate entities. The hydrodynamic properties of soluble rat and rabbit lung beta-adrenoceptors have been examined using gel exclusion chromatography. Agonist interaction with membrane preparations of beta1 and beta2 -adrenoceptors have been examined, and the results are discussed in relation to current models of beta-adrenoceptor effector coupling. beta1 and beta2-adrenoceptor preparations exhibited differential ion and temperature requirements in order for guanine nucleotide modulation of agonist binding to be expressed. Agonist binding to beta1-adrenoceptors was characterised by thermodynamic parameters which differed from those of beta2-adrenoceptors. The partial agonist salbutamol activated the adenylate cyclase coupled to beta2-adrenoceptors but not that coupled to beta1-adrenoceptors. These findings suggested that there may be different receptor-effector coupling relationships between the two beta-adrenoceptor subtypes. Evidence is also presented which suggested that the in vivo selectivity of salbutamol was related not to selective affinity, but to selective efficacy at beta2-adrenoceptors.