Cytosolic calcium changes induced by angiotensin II in neonatal rat atrial and ventricular cardiomyocytes are mediated via angiotensin II subtype 1 receptors

We determined the effects of angiotensin II (Ang II) on cytosolic free calcium concentrations ([Ca2+](i)) in the absence and presence of the selective angiotensin subtype 1 (AT(1)) receptor antagonist losartan or the selective AT(2) antagonist PD 123319 in cultured neonatal rat atrial and ventricular cardiomyocytes. We also assessed Ang II receptor density, affinity, and mRNA expression. [Ca2+](i) was measured in single cells microphotometrically and by fluorescent digital imaging with fura 2 methodology. Receptor parameters were assessed by competitive binding studies with I-125-[Sar(1),Ile(8)]Ang II in the presence of increasing concentrations of [Sar(1),Ile(8)]Ang II, losartan; and PD 123319. AT(1) receptor (types AT(1A) and AT(1B)) mRNA abundance was measured by reverse transcription-polymerase chain reaction. Ang II produced concentration-dependent increases in [Ca2+](i). Basal [Ca2+](i) values in atrial and ventricular cells were similar but Ang II (10(-9) mol/L)-induced [Ca2+](i) changes were significantly greater in atrial compared with ventricular cells. Ang II responses were blocked by losartan (10(-7) mol/L) but not PD 123319 (10(-7) mol/L). Binding studies demonstrated a single class of high-affinity Ang II binding sites on cardiomyocyte membranes (K-d=0.71+/-0.11 mu mol/L). I-125-[Sar(1),Ile(8)]Ang II was displaced by losartan but not by PD 123319. AT, receptor mRNA was detected by reverse transcription-polymerase chain reaction in cells from atria and ventricles. In atrial cardiomyocytes, both AT(1A) and AT(1B) receptor genes were expressed, whereas in ventricular cardiomyocytes, only the AT(1A) receptor gene was expressed. These data demonstrate that neonatal cardiomyocytes possess Ang II receptors of the AT(1) receptor subtype that are linked to [Ca2+](i) signaling pathways. The different Ang II-induced [Ca2+](i) responses between atrial and ventricular cells may be related to differences in the distribution of AT(1) receptor subtype subvariants.