Increased endothelin-1 receptor density in ischemic dilated cardiomyopathy.

Endothelin-1 (ET-1) stimulates hypertrophy of cardiomyocytes and proliferation of cardiac fibroblasts by binding to two different receptor subtype (ETA and ETB receptors). In the final stage of human heart failure no differences between ischemic and idiopatic dilated cardiomyopathy was observed so that ET receptor downregulation was excluded. However, ET-1 receptors were reported to up-regulate in experimental myocardial ischemia. We investigated whether the cardiac ET-1 receptor expression in ischemic cardiomyopathy reduces with progressive impairment of cardiac function in hearts from 14 patients with IDC and 15 patients with ISC. The two groups were comparable for age, sex, left ventricular dimensions (X to X), function (CI from X to X) and NYHA class (III-IV). Left ventricular membranes were obtained by centrifugation of homogenated tissue. Left ventricular cardiomyocytes were isolated by enzymatical dissociation and fractional centrifugation. ET-1 cell surface receptors were investigated using 125I-ET-1 (2,000 Ci/mmol) as radiolabeled ligand and BMS-182874, BQ123 and BQ788 (0-1 mM) to characterize receptor subtypes. mRNA expression for ETA and ETB receptor subtype was assessed “ex vivo” by Northern blot in homogenated tissue and by hybridyzation studies in section of left ventricular free wall using specific cDNA probes (ATCC 105194 and ATCC 1250426 for ETA and ETB receptors respectively). Cell membranes: In IDC patients 125I-ET-1 specific receptor binding (Bmax) was 179±32 fmol/mg protein with a receptor affinity (Kd) of 0.36±0.08 nM. Competition curves performed with BMS-182874 showed a biphasic pattern (n Hill=0.24) indicating the presence of both ETA and ETB endothelin receptor subtypes with a mean proportion of 64±3% and 36±2% respectively. In ISC patients the number of ET-1 receptor sites was significantly higher than in IDC patients (Bmax of 305±68 fmol/mg protein, p<0.001) with no differences in receptor affinity (Kd=0.36±0.08 nM) and in ETA/ETB mean proportion (64±3% and 36±2% respectively). Isolated cardiomyocytes: In IDC patients Bmax was 43±6 fmol/mg protein with a Kd of 0.37±0.08 nM and BMS-182874 displaced more than 90% of 125I-ET-1 specific binding with an Hill coefficient of 0.85. In ISC patients ET-1 receptor density was again significantly higher than in IDC patients (62±4 fmol/mg protein) without differences in Kd (0.35±0.04) and ETA/ETB proportion (91±4% and 6±3% reapectively) In conclusion 1) ETA receptor subtype is predominant on isolated cardiomyocytes from failing heart 2) both on cardiac membranes and isolated cardiomyocytes endothelin receptor density is significantly higher in ISC than in IDC with no differences in receptor affinity and subtype population.