To investigate the mechanism of expression of atrial natriuretic polypeptide (ANP) in human ventricles, we conducted an immunohistochemical study of ANP in biventricular endomyocardial biopsy specimens obtained from a total of 49 patients with cardiac dilatation due to dilated cardiomyopathy (21 patients), postmyocarditis (18 patients), or volume overload (five patients) and subjects with no dilatation as controls (five patients). Four-micron thick sections were stained by an indirect immunoperoxidase method using monoclonal antibody to ahuman ANP as the primary antibody. The frequency of ANP-present myocytes was calculated in each specimen and compared with clinical, echocardiographic, hemodynamic, angiographic, and histologic parameters. ANP-present myocytes were noted in all of the 21 patients with dilated cardiomyopathy, in 11 of the 18 patients with postmyocarditis, in four of the five patients with volume overload, and in zero of the five controls. The mean percentage of ANP-present myocytes was significantly greater in the left-side specimens (35±37%) than in the right-side ones (2±4%o). The percentage of ANP-present myocytes in the left-side specimens significantly correlated with peak systolic or end-diastolic wall stress (r=0.67 and 0.58), left ventricular end-systolic or end-diastolic volume index (r=0.75 and 0.69), or left ventricular end-diastolic pressure (r=0.42) and inversely correlated with ejection fraction (r=-0.73), systolic left ventricular wall thickness (r= -0.58), or cardiac index (r= -0.30). Expression of ANP was rarely seen in the cases with normal wall stresses, normal ejection fraction, normal volume, or normal myocyte size. However, it was seen frequently even in hearts with normal levels of left ventricular end-diastolic pressure and cardiac index (compensated hearts). The percent of ANP-present myocytes in both sides significantly correlated with size of myocytes (r=0.48 at right and r=0.57 at left side) or degree of fibrosis (r=0.45 at right and r=0.48 at left side). These results suggest that ANP expression is augmented in the dilated ventricles regardless of the causes of dilatation and that the augmentation is a compensatory mechanism as prevention against decompensation responding to reduced contractility, excess of wall stresses, or both, concomitantly occurring with cardiac dilatation and myocardial hypertrophy. (Circulation

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