Angiotensin II receptors are not downregulated on ventricular myocytes isolated from human failing hearts

Recent studies demonstrated that cardiac angiotensin II (Ang II) type 1 (AT1) receptors are downregulated in human failing ventricles from both ischemic (ICM) and idiopathic (DCM) dilated cardiomyopathy but binding characteristics of Ang II to human myocytes are to be defined. In the present study, Ang II receptors were investigated on ventricular myocytes and homogenated hearts from 5 potential transplant donors (NF) and 17 patients who underwent cardiac transplantation due to ICM (n=9) or DCM (n=8). Purity of cell suspension was assessed using specific monoclonal antibodies (anti-myosin, vimentin and von Willebrand factor). Ang II receptor subtypes were characterized by binding assay both on homogenated hearts and isolated cardiomyocytes using 125I-Ang II (2,000 Ci/mmol) as radioligand and Ang II or AT1 selective antagonist (Valsartan) as cold displacers. mRNA for AT1 receptors was quantified with RT-PCR using specific primers, with GAPDH gene expression as internal control. All data are given as mean±SD. 125I-Ang II showed a slow pattern of association reaching equilibrium after about 90 minutes with similar characteristics in homogenated hearts and isolated cardiomyocytes. 125I-Ang II bound to isolated cardiomyocytes from NF hearts with high affinity (Kd=0.14±0.04 nM) showing a receptor density of 0.57±0.21 fmol/mg protein (744±278 binding sites per myocyte). Competition experiments with Valsartan showed an AT1:AT2 ratio of 78:22. Cardiomyocytes from both ICM and DCM hearts showed no differences in Ang II receptor density and AT1:AT2 ratio vs NF hearts. The expression of mRNA for AT1 subtype was detected both in nonfailing and failing ventricular myocytes with no differences in AT1 mRNA expression levels among groups. Conversely both 125I-Ang-II binding and RT-PCR studies revealed a significant downregulation of total Ang II receptors and AT1 subtype in both ICM and DCM hearts. In conclusion human ventricular myocytes express high affinity receptors for Ang II which at variance with homogenated hearts are not downregulated in cardiac failure.