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Determination β‐adrenoceptor subtype on rat isolated ventricular myocytes by use of highly selective β‐antagonists
Author(s) -
Kitagawa Yoshihiro,
AdachiAkahane Satomi,
Nagao Taku
Publication year - 1995
Publication title -
british journal of pharmacology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.432
H-Index - 211
eISSN - 1476-5381
pISSN - 0007-1188
DOI - 10.1111/j.1476-5381.1995.tb16384.x
Subject(s) - iodocyanopindolol , myocyte , biology , membrane , medicine , endocrinology , receptor , biophysics , biochemistry , intrinsic activity , agonist
1 The relative proportions of β 1 and β 2 ‐adrenoceptors were determined by radioligand binding studies in three different rat myocardial preparations: membranes prepared from rat ventricle (ventricular membranes), membranes prepared from rat isolated ventricular myocytes (myocyte membranes), and myocytes isolated from rat ventricle (myocytes). 2 Competition experiments using CGP 20712A or ICI 118,551 with [ 125 I]‐iodocyanopindolol ([ 125 1]‐ ICYP) revealed high‐ and low‐affinity binding sites in ventricular membranes. The concentration at which each β‐antagonist occupied 100% of its high‐affinity binding sites was 300 nM for CGP 20712A (β 1 ‐adrenoceptor) and 50 μ M for ICI 118,551 (β 2 ‐adrenoceptor). 3 The density of high‐affinity (β 1 ‐adrenoceptor) and low‐affinity (β 2 ‐adrenoceptor) binding sites for CGP 20712A was measured by a saturation experiment using [ 125 I]‐ICYP in the presence and absence of 300 nM CGP 20712A. In ventricular membranes, the proportions of high‐affinity and low‐affinity binding sites for CGP 20712A were 73% and 27%, respectively, whereas in myocyte membranes, the corresponding figures were 90% and 10%, respectively. The density of low‐affinity binding sites for CGP 20712A in ventricular membranes, defined as [ 125 I]‐ICYP‐specific binding in the presence of 300 nM CGP 20712A, was decreased by addition of 50 nM ICI 118,551, whereas that in myocyte membranes was not affected. 4 In myocytes, specific binding of [ 125 I]‐ICYP and [ 3 H]‐CGP 12177 was not detected by saturation experiments performed in the presence of 300 nM CGP 20712A. 5 In myocytes, the activation of adenylate cyclase caused by β 2 ‐adrenoceptors was not detected in the presence of 10 nM, 100 nM or 1000 nM CGP 20712A, which selectively antagonized β 1 ‐adrenoceptors. Furthermore, the concentration‐response curve for isoprenaline‐stimulated cyclic AMP accumulation was not shifted by 10 nM or 100 nM ICI 118,551, which selectively antagonized β 2 ‐adrenoceptors, but was shifted to the right by 1000 nM ICI 118,551. 6 These results indicate that β 2 ‐adrenoceptors are not present on rat ventricular myocytes and that β 2 ‐adrenoceptor stimulation does not cause any detectable production of cyclic AMP. We conclude that only β 1 ‐adrenoceptors exist on rat ventricular myocytes.