CHARACTERIZATION OF CARDIAC ANGIOTENSIN AT1 RECEPTORS BY [H-3] SR-47436

[H-3]SR 47436, a selective and potent novel non-peptide antagonist of angiotensin receptors, was used to characterize the cardiac angiotensi n AT1 receptors. In neonatal rat heart cells, Scatchard analysis showe d a single class of high affinity binding sites (K(d) = 0.24 nM, B(max ) = 28 fmol/mg protein). The binding was saturable, reversible and pre vented by angiotensin II and the AT1 subtype receptor antagonist DuP 7 53 whilst unaffected by the AT2 receptor antagonist PD123177. In the s ame cells, angiotensin II induced a twofold increase in the intracellu lar free Ca2+ concentration ([Ca2+]i), with a half-maximal effect (EC5 0) at 14 nM. This increase was prevented by SR 47436 (IC50 = 1.03 nM) and by the AT1 receptor antagonist DuP 753, but at higher concentratio ns (IC50 = 15.6 nM) and was unaffected by PD123177. These data directl y demonstrate the presence of cardiac AT1 receptors in rat neonatal ca rdiomyocytes and confirm the involvement of AT1 receptors in cardiac C a2+ homeostasis.