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 a trial and ventricular cardiomyocytes. We also assessed Ang II receptor density, affinity, and mRNA expression. [Ca2+](i) was measured in sin gle 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 in creasing concentrations of [Sar(1),Ile(8)]Ang II, losartan; and PD 123 319. AT(1) receptor (types AT(1A) and AT(1B)) mRNA abundance was measu red by reverse transcription-polymerase chain reaction. Ang II produce d concentration-dependent increases in [Ca2+](i). Basal [Ca2+](i) valu es in atrial and ventricular cells were similar but Ang II (10(-9) mol /L)-induced [Ca2+](i) changes were significantly greater in atrial com pared with ventricular cells. Ang II responses were blocked by losarta n (10(-7) mol/L) but not PD 123319 (10(-7) mol/L). Binding studies dem onstrated a single class of high-affinity Ang II binding sites on card iomyocyte membranes (K-d=0.71+/-0.11 mu mol/L). I-125-[Sar(1),Ile(8)]A ng II was displaced by losartan but not by PD 123319. AT, receptor mRN A was detected by reverse transcription-polymerase chain reaction in c ells from atria and ventricles. In atrial cardiomyocytes, both AT(1A) and AT(1B) receptor genes were expressed, whereas in ventricular cardi omyocytes, only the AT(1A) receptor gene was expressed. These data dem onstrate 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.