Increasing evidence suggests that angiotensin II (AngHI) acts as a modulator for ventricular remodeling after myocardial infarction. Using competitive reverse-transcriptase polymerase chain reaction, nuclear runoff, and binding assays, we examined the regulation of AngHI type la and lb (ATla-R and ATlb-R) and type 2 receptor (AT2-R) expression in the infarcted rat heart as well as the effects of AngiH receptor antagonists. ATla-R mRNA levels were increased in the infarcted (4.2-fold) and noninfarcted portions (2.2-fold) of the myocardium 7 d after myocardial infarction as compared with those in sham-operated controls, whereas ATlb-R mRNA levels were unchanged. The amount of detectable AT2-R mRNA increased in infarcted (3.1-fold) and noninfarcted (1.9-fold) portions relative to that in the control. The transcription rates for ATla-R and AT2-R genes, determined by means of a nuclear runoff assay, were significantly increased in the infarcted heart. The Angil receptor numbers were elevated (from 12 to 35 fmol/mg protein) in the infarcted myocardium in which the increases in AT1-R and AT2-R were 3.2-and 2.3-fold, respectively, while the receptor affinity was unchanged. Therapy with AT1-R antagonist for 7 d reduced the increase in AT1-R and AT2-R expressions in the infarcted heart together with a decrease in blood pressure, whereas therapy with an AT2-R antagonist did not affect mRNA levels and blood pressure. Neither ATi-R nor AT2-R antagonists affected the infarct sizes. These results demonstrated that myocardial infarction causes an increase in the gene transcription and protein expression of cardiac ATla-R and AT2-R, whereas the ATlb-R gene is unaffected, and that therapy with an AT1-R antagonist, but not with an AT2-R antagonist, is effective in reducing the increased expression of AngII receptor subtypes induced by myocardial infarction. (J. Clin. Invest. 1995.95:46-54.)
These findings demonstrate that AT2-R is re-expressed by cardiac fibroblasts present in fibrous regions in failing CM hearts and that the increased AT2-R exerts an anti-AT1-R action on the progression of interstitial fibrosis during cardiac remodeling by inhibiting both fibrillar collagen metabolism and growth of cardiac fibroblasts.
Angiotensin (Ang) II has two major receptor isoforms, AT1 and AT2. Currently, AT1 antagonists are undergoing clinical trials in patients with cardiovascular diseases. Treatment with AT1 antagonists causes elevation of plasma Ang II which selectively binds to AT2 and exerts as yet undefined effects. Cardiac AT2 level is low in adult hearts, whereas its distribution ratio is increased during cardiac remodeling and its action is enhanced by application of AT1 antagonists. Although in AT2 knock-out mice sensitivity to the pressor action of Ang II was increased, underlying mechanisms remain undefined. Here, we report the unexpected finding that cardiac-specific overexpression of the AT2 gene using alpha-myosin heavy chain promoter resulted in decreased sensitivity to AT1-mediated pressor and chronotropic actions. AT2 protein undetectable in the hearts of wild-type mice was overexpressed in atria and ventricles of the AT2 transgenic (TG) mice and the proportions of AT2 relative to AT1 were 41% in atria and 45% in ventricles. No obvious morphological change was observed in the myocardium and there was no significant difference in cardiac development or heart to body weight ratio between wild-type and TG mice. Infusion of Ang II to AT2 TG mice caused a significantly attenuated increase in blood pressure response and the change was completely blocked by pretreatment with AT2 antagonist. This decreased sensitivity to Ang II-induced pressor action was mainly due to the AT2-mediated strong negative chronotropic effect and exerted by circulating Ang II in a physiological range that did not stimulate catecholamine release. Isolated hearts of AT2 transgenic mice perfused using a Langendorff apparatus also showed decreased chronotropic responses to Ang II with no effects on left ventricular dp/dt max values, and Ang II-induced activity of mitogen-activated protein kinase was inhibited in left ventricles in the transgenic mice. Although transient outward K+ current recorded in cardiomyocytes from AT2 TG mice was not influenced by AT2 activation, this study suggested that overexpression of AT2 decreases the sensitivity of pacemaker cells to Ang II. Our results demonstrate that stimulation of cardia AT2 exerts a novel antipressor action by inhibiting AT1-mediated chronotropic effects, and that application of AT1 antagonists to patients with cardiovascular diseases has beneficial pharmacotherapeutic effects of stimulating cardiac AT2.
Abstract-The expression pattern of angiotensin (Ang) II type 2 receptor (AT 2 -R) in the remodeling process of human left ventricles (LVs) remains poorly defined. We analyzed its expression at protein, mRNA, and cellular levels using autopsy, biopsy, or operation LV samples from patients with failing hearts caused by acute (AMI) or old (OMI) myocardial infarction and idiopathic dilated cardiomyopathy (DCM) and also examined functional biochemical responses of failing hearts to Ang II. In autopsy samples from the nonfailing heart group, the ratio of AT 1 -R and AT 2 -R was 59% and 41%, respectively. The expression of AT 2 -R was markedly increased in DCM hearts at protein (3.5-fold) and mRNA (3.1-fold) levels compared with AMI or OMI. AT 1 -R protein and mRNA levels in AMI hearts showed 1.5-and 2.1-fold increases, respectively, whereas in OMI and DCM hearts, AT 1 -R expression was significantly downregulated. AT 1 -R-mediated response in inositol phosphate production was significantly attenuated in LV homogenate from failing hearts compared with nonfailing hearts. AT 2 -R sites were highly localized in the interstitial region in either nonfailing or failing heart, whereas AT 1 -R was evenly distributed over myocardium at lower densities. Mitogen-activated protein kinase (MAPK) activation by Ang II was significantly decreased in fibroblast compartment from the failing hearts, and pretreatment with AT 2 -R antagonist caused an additional significant increase in Ang II-induced MAPK activity (36%). Cardiac hypertrophy suggested by atrial and brain natriuretic peptide levels was comparably increased in OMI and DCM, whereas accumulation of matrix proteins such as collagen type 1 and fibronectin was much more prominent in DCM than in OMI. These findings demonstrate that (1) AT 2 -R expression is upregulated in failing hearts, and fibroblasts present in the interstitial regions are the major cell type responsible for its expression, (2) AT 2 -R present in the fibroblasts exerts an inhibitory effect on Ang II-induced mitogen signals, and (3) AT 1 -R in atrial and LV tissues was downregulated during chronic heart failure, and AT 1 -R-mediated functional biochemical responsiveness was decreased in the failing hearts. Thus, the expression level of AT 2 -R is likely determined by the extent of interstitial fibrosis associated with heart failure, and the expression and function of AT 1 -R and AT 2 -R are differentially regulated in failing human hearts. (Circ Res. 1998;83:1035-1046.)Key Words: angiotensin II type 2 receptor Ⅲ AT 2 receptor Ⅲ angiotensin II type 1 receptor Ⅲ AT 1 receptor, angiotensin II T he presence of 2 isoforms of angiotensin (Ang) II receptor was originally proposed on the basis of differences in sensitivity of receptor-ligand binding to dithiothreitol. Ang type 2 receptor (AT 2 -R), which is insensitive to dithiothreitol and has a high affinity for PD123319 and CGP42112A, was isolated, and this receptor was shown to have the same seventransmembrane domain of AT 1 -R but only minimal homology (see Review i...
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