Endothelin-1 and angiotensin II receptors in cells from rat hypertrophied heart - Receptor regulation and intracellular Ca2+ modulation

This study investigates the cellular localization and regulation of endothelin-l (ET-1) and angiotensin II (Ang II) receptors and the effects of ET-1 and Ang II on [Ca2+](i) in cardiac hypertrophy due to volume overload in the rat. Radioligand binding assays and [Ca2+](i) measurements by fura 2 methodology were performed on isolated ventricular cardiomyocytes and fibroblasts from the heart of rats with a 4-week aortocaval shunt. In the hypertrophied myocardium, ET-1 and Ang II concentrations were unchanged in ventricles. Ventricular ET-1 receptors had a cell-specific distribution: >90% of ET receptors in cardiomyocytes are of the ET(A) subtype, whereas fibroblasts had a nearly equal proportion of the ET(A) and ET(B) subtypes. ET-1 receptor densities, affinities, and ET-1-induced [Ca2+](i) were not significantly different from control in both Ventricular cell types from hypertrophied myocardium. Ang II specific binding was very low on isolated ventricular cardiomyocytes, suggesting few receptors in control conditions. However, [Ca2+](i) responses induced by Ang II at concentrations >10(-8) mol/L were detectable and were significantly higher in hypertrophied cardiomyocytes. Ang II receptor density (exclusively AT(1)) on fibroblasts was significantly reduced (42 970+/-3330 versus 73 870+/-7940 sites per cell for control cells, P<.01), but AT(1) receptor affinity was unchanged after volume overload. The maximum increase in [Ca2+](i) evoked by 10(-6) to 10(-4) mol/L Ang II was significantly lower in fibroblasts from overloaded hearts. In conclusion, ET-1 receptor proportion is cell specific, with cardiomyocytes possessing predominantly the ET(A) subtype and fibroblasts possessing both ET(A) and ET(B) receptors. Plasma and cardiac ET-1 concentrations and ET-1 receptor regulation on both ventricular cell types are not altered in cardiac volume overload, suggesting that cardiac ET-1 may not play a significant role in this model. Cardiac hypertrophy induced a significant down regulation of AT(1) receptors on fibroblasts, whereas total binding and [Ca2+](i) sensitivity to Ang II were significantly enhanced in hypertrophied cardiomyocytes. This suggests that cardiac Ang II may be involved in the pathophysiology of the cardiac hypertrophy of volume overload.