Putative β4‐adrenoceptors in rat ventricle mediate increases in contractile force and cell Ca2+: comparison with atrial receptors and relationship to (−)‐[3H]‐CGP 12177 binding

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Abstract
We identified putative β4‐adrenoceptors by radioligand binding, measured increases in ventricular contractile force by (−)‐CGP 12177 and (±)‐cyanopindolol and demonstrated increased Ca2+ transients by (−)‐CGP 12177 in rat cardiomyocytes. (−)‐[3H]‐CGP 12177 labelled 13–22 fmol mg−1 protein ventricular β1, β2‐adrenoceptors (pKD ∼9.0) and 50–90 fmol mg−1 protein putative β4‐adrenoceptors (pKD ∼7.3). The affinity values (pKi) for (β1,β2‐) and putative β4‐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 μM 3‐isobutyl‐1‐methylxanthine, (−)‐CGP 12177 and (±)‐cyanopindolol caused positive inotropic effects, (pEC50, (−)‐CGP 12177, 7.6; (±)‐cyanopindolol, 7.0) which were antagonized by (−)‐bupranolol (pKB 6.7–7.0) and (−)‐CGP 20712A (pKB 6.3–6.6). The cardiostimulant effects of (−)‐CGP 12177 in papillary muscle, left and right atrium were antagonized by (±)‐cyanopindolol (pKP 7.0–7.4). (−)‐CGP 12177 (1 μM) in the presence of 200 nM (−)‐propranolol increased Ca2+ transient amplitude by 56% in atrial myocytes, but only caused a marginal increase in ventricular myocytes. In the presence of 1 μM 3‐isobutyl‐1‐methylxanthine and 200 nM (−)‐propranolol, 1 μM (−)‐CGP 12177 caused a 73% increase in Ca2+ 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 β4‐adrenoceptors, suggesting coupling to Gs 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 agonists with cardiostimulant potencies is consistent with mediation through putative β4‐adrenoceptors labelled with (−)‐[3H]‐CGP 12177. British Journal of Pharmacology (1999) 128, 1445–1460; doi:10.1038/sj.bjp.0702936