Functional angiotensin II receptors have been documented in cardiac fibroblasts as well as an intracardiac aldosterone system which responds to short- and long-term physiological stimuli. In vitro, angiotensin II increased cardiac fibroblasts-mediated collagen synthesis and mRNA levels of collagen type I, type III, pro- α1 (I) collagen, pro- α1 (III) collagen and fibronectin, and inhibited matrix metalloproteinase I activity. The angiotensin II- stimulated secretion and expression of collagen was completely abolished by AT1 receptor antagonism, but not affected by AT2 receptor antagonism.
In vivo, chronic infusion of angiotensin II increased the collagen volume fraction in the ventricles. Angiotensin-converting enzyme (ACE) inhibition and AT1 receptor antagonism, but not AT2 receptor antagonism, reduced collagen deposition in the myocardium in spontaneously hypertensive rats and in rat myocardium following myocardial infarction.
During chronic aldosterone infusion in uninephrectromized rats on a high-salt diet, a marked accumulation of interstitial and to a lesser extent perivascular collagen occurs in the heart in both ventricles. This cardiac fibrosis in this aldosterone model is prevented by spironolactone. During the continuous infusion of aldosterone in the rat the appearance of fibrosis was delayed and starts 4 weeks after the beginning of the infusion which argues against a direct effect of aldosterone. The mechanism of aldosterone-salt induced cardiac fibrosis possibly involves angiotensin II acting through upregulated AT1 receptors and the cardiac AT1 receptor is the target for aldosterone. An accumulation of collagen in the heart has also been found in patients with adrenal adenomas and during chronic activation of the renin-angiotensin-aldosterone system such as in surgically induced unilateral renal ischemia, unilateral renal artery banding or renovascular hypertension. Spironolactone prevents aortic collagen accumulation in spontaneously hypertensive rats. In patients with stable chronic heart failure spironolactone treatment in addition to diuretics and angiotensin-converting enzyme (ACE) inhibition reduced circulating levels of procollagen type III N-terminal aminopeptide. Also, in the Randomized Aldactone Evaluation Study spironolactone coadministered with conventional therapy of ACE inhibitors, loop diuretics and digitalis in patients with symptomatic heart failure defined as NYHA classes III-IV reduces total mortality by 30 %.
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