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Molecular Pharmacology, Vol 19, 194-204, Copyright © 1981 by the American Society for Pharmacology and Experimental Therapeutics

Homogeneity of Beta2-Adrenoceptors on Rat Erythrocytes and Reticulocytes

A Comparison with Heterogeneous Rat Lung Beta-Adrenoceptors

K. DICKINSON 1, A. RICHARDSON 1, and S. R. NAHORSKI 1

1 Department of Pharmacology and Therapeutics, University of Leicester, Leicester, United Kingdom

The characteristics of specific 3H-labeled (-)-dihydroalprenolol ([3H]DHA) binding sites on rat erythrocytes and reticulocytes have been examined in an attempt to determine whether different beta-adrenoceptor subtypes could coexist in the same population of cells. [3H]DHA binds specifically to erythrocyte membranes with a dissociation equilibrium constant (KD) of 0.24 nM and a maximal number of binding sites (Bmax) of 195 fmoles/mg of protein. Reticulocytes induced by phenylhydrazine treatment possessed a considerably higher density of binding sites (Bmax 760 fmoles/mg of protein) but identical affinity for [3H]DHA (KD 0.25 nM).

A detailed pharmacological characterization using highly selective beta1- and beta2- adrenoceptor agents revealed that both cell populations possessed only homogeneous beta2-adrenoceptor binding sites that were pharmacologically identical with the beta2 component observed in rat lung which possesses both beta1- and beta2-adrenoceptors. Further evidence for the heterogeneous nature of beta-adrenoceptor binding sites in rat lung was provided by the demonstration that selective occupation of the beta1 sites with atenolol results in a loss of receptor heterogeneity, and suggests that the receptor subtypes are probably separate noninteracting entities. Methods to analyze heterogeneous binding data (Hofstee and Hill plots of transformed data and computer-assisted iterative curve-fitting of the raw data) were compared and indicate that, with highly selective competing drugs, linear transformations provide data in excellent agreement with those obtained with curve-fitting.

Note:
ACKNOWLEDGMENT We would like to thank Jenny Bell for preparing the manuscript.

Submitted on May 28, 1980
Accepted on October 6, 1980




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