Chronic ventricular myocyte-specific overexpression of angiotensin II type 2 receptor results in intrinsic myocyte contractile dysfunction

ANG II type 2 receptor (AT₂) is upregulated in failing hearts, but its effect on myocyte contractile function is not known. We measured fractional cell shortening and intracellular Ca²⁺ concentration transients in left ventricular myocytes derived from transgenic mice in which ventricle-specific expression of AT₂ was driven by the myosin light chain 2v promoter. Confocal microscopy studies confirmed upregulation of AT ₂ in the ventricular myocytes and partial colocalization of AT ₂ with AT₁. Three components of contractile performance were studied. First, baseline measurements (0.5 Hz, 1.5 mmol/l extracellular Ca²⁺ concentration, 25°C) and study of contractile reserve at faster pacing rates (1-5 Hz) revealed Ca²⁺-dependent contractile dysfunction in myocytes from AT₂ transgenic mice. Comparison of two transgenic lines suggested a dose-dependent relationship between magnitude of contractile dysfunction and level of AT₂ expression. Second, activity of the Na⁺/H⁺ exchanger, a dominant transporter that regulates beat-to-beat intracellular pH, was impaired in the transgenic myocytes. Third, the inotropic response to β-adrenergic versus ANG II stimulation differed. Both lines showed impaired contractile response to β-adrenergic stimulation. ANG II elicited an increase in contractility and intracellular Ca²⁺ in wild-type myocytes but caused a negative inotropic effect in myocytes from AT₂ transgenic mice. In contrast with β-adrenergic response, the depressed response to ANG II was related to level of AT₂ overexpression. The depressed response to ANG II was also present in myocytes from young transgenic mice before development of heart failure. Thus chronic overexpression of AT₂ has the potential to cause Ca²⁺- and pH-dependent contractile dysfunction in ventricular myocytes, as well as loss of the inotropic response to ANG II.