Putative beta(4)-adrenoceptors in rat ventricle mediate increases in contractile force and cell Ca2+: comparison with atrial receptors and relationship to (-)-[H-3]-CGP 12177 binding

1 We identified putative beta(4)-adrenoceptors by radioligand binding, meas ured increases in ventricular contractile force by (-)-CGP 12177 and (+/-)- cyanopindolol and demonstrated increased Ca2+ transients by (-)-CGP 12177 i n rat cardiomyocytes. 2 (-)-[H-3]-CGP 12177 labelled 13-22 fmol mg(-1) protein ventricular beta(1 ), beta(2)-adrenoceptors (pK(D) similar to 9.0) and 50-90 fmol mg(-1) prote in putative beta(4)-adrenoceptors (pK(D) similar to 7.3). The affinity valu es (PKi) for (beta(1),beta(2)-) and putative beta(4)-adrenoceptors, estimat ed from binding inhibition, were (-)-propranolol 8.4, 5.7; (-)-bupranolol 9 .7, 5.8; (+/-)-cyanopindolol 10.0,7.4. 3 In left ventricular papillary muscle, in the presence of 30 mu M 3-isobut yl-1-methylxanthine, (-)CGP 12177 and (+/-)-cyanopindolol caused positive i notropic effects, (pEC(50) (-)-CGP 12177, 7.6; (+/-)-cyanopindolol, 7.0) wh ich 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 mu scle, left and right atrium were antagonized by (+/-)-cyanopindolol (pK(i), 7.0-7.4). 4 (-)-CGP 12177 (1 mu M) in the presence of 200 nM (-)-propranolol increase d Ca2+ transient amplitude by 56% in atrial myocytes, but only caused a mar ginal increase in ventricular myocytes. In the presence of 1 mu M 3-isobuty l-1-methylxanthine and 200 nM (-)-propranolol, 1 mu M (-)-CGP 12177 caused a 73% increase in Ca2+ transient amplitude in ventricular myocytes. (-)-CGP 12177 elicited arrhythmic transients in some atrial and ventricular myocyt es. 5 Probably by preventing cyclic AMP hydrolysis, 3-isobutyl-1-methylxanthine facilitates the inotropic function of ventricular putative beta(4)-adrenoc eptors. suggesting coupling to G(s) protein-adenylyl cyclase. The receptor- mediated increases in contractile force are related to increases of Ca2+ in atrial and ventricular myocytes. The agreement of binding affinities of ag onists with cardiostimulant potencies is consistent with mediation through putative beta(4)-adrenoceptors labelled with (-)-[H-3]-CGP 12177.