Putative β 4 ‐adrenoceptors in rat ventricle mediate increases in contractile force and cell Ca 2+ : comparison with atrial receptors and relationship to (−)‐[ 3 H]‐CGP 12177 binding

We identified putative β 4 ‐adrenoceptors by radioligand binding, measured increases in ventricular contractile force by (−)‐CGP 12177 and (±)‐cyanopindolol and demonstrated increased Ca 2+ transients by (−)‐CGP 12177 in rat cardiomyocytes. (−)‐[ 3 H]‐CGP 12177 labelled 13–22 fmol mg −1 protein ventricular β 1 , β 2 ‐adrenoceptors (pK D ∼9.0) and 50–90 fmol mg −1 protein putative β 4 ‐adrenoceptors (pK D ∼7.3). The affinity values (pK i ) for (β 1 ,β 2 ‐) and putative β 4 ‐adrenoceptors, estimated from binding inhibition, were (−)‐propranolol 8.4, 5.7; (−)‐bupranolol 9.7, 5.8; (±)‐cyanopindolol 10.0,7.4. In left ventricular papillary muscle, in the presence of 30 μ M 3‐isobutyl‐1‐methylxanthine, (−)‐CGP 12177 and (±)‐cyanopindolol caused positive inotropic effects, (pEC 50 , (−)‐CGP 12177, 7.6; (±)‐cyanopindolol, 7.0) which were antagonized by (−)‐bupranolol (pK B 6.7–7.0) and (−)‐CGP 20712A (pK B 6.3–6.6). The cardiostimulant effects of (−)‐CGP 12177 in papillary muscle, left and right atrium were antagonized by (±)‐cyanopindolol (pK P 7.0–7.4). (−)‐CGP 12177 (1 μ M ) in the presence of 200 n M (−)‐propranolol increased Ca 2+ transient amplitude by 56% in atrial myocytes, but only caused a marginal increase in ventricular myocytes. In the presence of 1 μ M 3‐isobutyl‐1‐methylxanthine and 200 n M (−)‐propranolol, 1 μ M (−)‐CGP 12177 caused a 73% increase in Ca 2+ transient amplitude in ventricular myocytes. (−)‐CGP 12177 elicited arrhythmic transients in some atrial and ventricular myocytes. Probably by preventing cyclic AMP hydrolysis, 3‐isobutyl‐1‐methylxanthine facilitates the inotropic function of ventricular putative β 4 ‐adrenoceptors, suggesting coupling to G s protein‐adenylyl cyclase. The receptor‐mediated increases in contractile force are related to increases of Ca 2+ in atrial and ventricular myocytes. The agreement of binding affinities of agonists with cardiostimulant potencies is consistent with mediation through putative β 4 ‐adrenoceptors labelled with (−)‐[ 3 H]‐CGP 12177.British Journal of Pharmacology (1999) 128 , 1445–1460; doi: 10.1038/sj.bjp.0702936