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The goal of this study was to determine whether endothelium-dependent responses of the microcirculation are altered during cardiomyopathy. We examined in vivo responses of cheek pouch arterioles to an endothelium-dependent agonist (acetylcholine) and an endothelium-independent agonist (nitroglycerin) in normal and in cardiomyopathic hamsters. In normal hamsters, acetylcholine produced dose-related dilatation of arterioles. In contrast, acetylcholine produced constriction of arterioles in cardiomyopathic hamsters. Nitroglycerin produced similar dose-related dilatation in normal and cardiomyopathic hamsters. We also examined whether impaired responses to acetylcholine in cardiomyopathic hamsters were related to an alteration in the L-arginine/nitric oxide pathway. We found that L-arginine (100 μM) restored endotheliumdependent vasodilatation to acetylcholine in cardiomyopathic hamsters. Thus …