Regulation and Role of Myocardial Collagen Matrix Remodeling in Hypertensive Heart Disease

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Abstract
In hypertensive heart disease, reactive myocardial fibrosis represents as an excessive accumulation of fibrillar collagen within the normal connective tissue structures of the myocardium. The fact, that the myocardium of both ventricles is involved, irrespective of ventricular loading conditions, suggests that circulating factors, and not the hemodynamic load are primary responsible for this adverse response of the myocardial fibrous tissue. In various experimental in vivo models, it has been shown that myocardial fibrosis is always associated with activation of circulating or local renin-angiotensin-aldosterone systems (RAAS). Cardiac collagen metabolism is regulated by cardiac fibroblasts which express mRNAs for types I and III collagens, the major fibrillar collagens in the heart, and for interstitial collagenase or matrix metalloproteinase (MMP) 1 which is the key enzyme for interstitial collagen degradation. In order to elucidate the role of the RAAS effector hormones, angiotensin II (AngII) and aldosterone (ALDO), in the regulation of collagen synthesis or inhibition of MMP 1 production, adult human cardiac fibroblasts were cultured. Collagen synthesis was determined by 3H-proline incorporation, and MMP 1 activity by degradation of 14C-collagen measured under serum-free conditions in confluent fibroblasts after 24 hour-incubation with either AngII or ALDO over a wide range of concentrations (10-11–10-6M). In addition, the effects of the mineralocorticoid, deoxycorticosterone (DOC), and prostaglandin E2 (PGE2) on cardiac fibroblast function were determined. Compared with untreated control fibroblasts, collagen synthesis, normalized per total protein synthesis, showed a significant and dose-dependent increase after incubation with either mineralocorticoid hormone, ALDO or DOC, or after incubation with AngII. In contrast, collagen synthesis of cardiac fibroblasts was significantly decreased by PGE2 treatment. Angli type 1 or mineralocorticoid receptor antagonists, respectively, were able to completely inhibit the AngII- or mineralocorticoid-mediated increase of collagen synthesis. Furthermore, AngII significantly decreased MMP 1 activity while ALDO or DOC had no effect on cardiac fibroblast-mediated collagen degradation. In contrast, PGE2 significantly increased MMP 1 activity. Thus cardiac fibroblast function is modulated by either effector hormone of the RAAS, AngII and ALDO, via specific receptors that lead to progressive myocardial fibrosis in disease states where circulating or local RAAS is activated, i.e., in hypertensive heart disease. In contrast, PGE2, which would be elevated in myocardial tissue after angiotensin-converting enzyme inhibition, counteracts the fibrotic effects of the RAAS on myocardial tissue.