Renin-angiotensin system and myocardial collagen matrix: modulation of cardiac fibroblast function by angiotensin II type 1 receptor antagonism.

BACKGROUND: Left ventricular hypertrophy is an adaptive process to increased loading of the left ventricle. This condition becomes pathologic with impaired myocardial function if the various tissue compartments of the myocardium (myocyte, interstitial and vascular compartments) are inhomogeneously altered, particularly if myocardial fibrosis occurs. In arterial hypertension, myocardial fibrosis is known to occur in association with activated circulating or local renin-angiotensin systems and includes reactive perivascular and interstitial fibrosis in both the pressure-overloaded hypertrophied left ventricle and the normotensive non-hypertrophied right ventricle. Therefore, it appears that hemodynamics are not primarily responsible for the adverse myocardial collagen matrix remodeling in hypertensive heart disease. Accordingly, we studied the interaction between cultured adult rat cardiac fibroblasts, which express messenger (m)RNAs for types I and III collagens, the major fibrillar collagens in the heart, and angiotensin II (Ang II), the effector hormone of the renin-angiotensin system.
OBJECTIVES: Specifically, we sought to determine whether Ang II stimulates total collagen synthesis and the expression of type I collagen mRNA in cultured adult rat cardiac fibroblasts, and to investigate the effects of Ang II on intracellular Ca2+ levels.
MATERIALS AND METHODS: Adult rat cardiac fibroblasts were cultured in Dulbecco's Modified Eagle's Medium + 10% fetal calf serum and incubated for 24 h with Ang II with or without specific Ang II type 1 or type 2 receptor antagonists. Collagen synthesis was measured using a 3H-proline incorporation assay, and type I collagen mRNA was determined using reverse-transcriptase polymerase chain reaction. Intracellular Ca2+ transients were measured by fast fluorescence photometry using the fluorescent dye fura-2-acetoxymethylester.
RESULTS: We found a 76% increase in type I collagen mRNA in cultured cardiac fibroblasts after a 24-h incubation with Ang II, and this was abolished by simultaneous incubation with the Ang II type 1 (AT1)-receptor antagonist candesartan. Likewise, total collagen synthesis was stimulated by Ang II in a dose-dependent manner, and this stimulation was also counteracted by candesartan. Additionally, incubation with Ang II resulted in a significant dose-dependent increase in intracellular Ca2+ transients which was also abolished by treatment with candesartan.
CONCLUSIONS: Ang II stimulates collagen synthesis in cultured adult rat cardiac fibroblasts via AT1 receptors, most likely using Ca2+ as a second messenger. These findings suggest a direct interaction between Ang II and cardiac fibroblasts in mediating myocardial fibrosis in arterial hypertension, leading to pathologic left ventricular hypertrophy with initially impaired diastolic and ultimately reduced systolic function of the left ventricle. The AT1-receptor antagonist candesartan cilexetil, which is the prodrug of the active compound candesartan, may prove valuable in preventing or regressing myocardial fibrosis in hypertensive heart disease.