Characterization of beta-adrenoreceptors of rainbow trout (Oncorhynchus mykiss) erythrocytes

Although many studies have characterized these receptors according to pharmacological criteria, this work represents only the second direct characterization of the rainbow trout beta-adrenergic receptors. Radioligand binding assays using (+/−)-4-(3-t-butylamino-2-hydroxy-propoxy)-[5,7-3H]benzimidazol-2- one ([3H]CGP 12177) and 1-[4,6-propyl 3H]dihydroalprenolol ([3H]DHA) were conducted to determine equilibrium binding times, ligand-receptor dissociation constants (KD) and binding capacities (Bmax). Furthermore, we assessed the influence of erythrocyte handling, suspension medium and endogenous catecholamines on Bmax and KD. Maximal binding was obtained when erythrocytes were handled minimally and maintained suspended in plasma rather than physiological saline. Washing and resuspending the erythrocytes, as well as the transfer of the erythrocytes into saline, significantly impaired apparent radioligand affinity and receptor density. Endogenous catecholamines, at levels considered normal for non-stressed animals, did not interfere with the radioligand binding assays, and thus eliminated the need to wash and resuspend erythrocytes. Based on the binding characteristics after intentional lysis of erythrocytes, it is shown that the total receptor population of trout erythrocytes can be estimated by propranolol-displaceable DHA binding, the density of high-affinity surface receptors can best be determined by isoproterenol-displaceable CGP binding, and the number of receptors located within the erythrocytes can be calculated from the difference between the total receptor density and the number of isoproterenol-displacable DHA binding sites. Each of these components must be considered when performing radioreceptor assays using these radioligands and this has significant implications for the interpretation of erythrocyte beta-adrenorecptor localization and mobilization.