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Structural changes in the myocardium following inhibition of nitric oxide synthesis were studied quantitatively within two different periods. Four groups of 10 rats were studied: control and L-NAME (NG-nitro-methyl-ester-L-arginine) groups for 25 and for 40 days. L-NAME was administered at 50 mg/kg per day in the drinking water. On the 26th and 41st days, the hearts were examined. Volume densities of myocytes (Vv[m]), cardiac interstitium (Vv[int], numerical density of myocytes (Nv[m]) and mean cross-sectional area of the myocytes (A[m]) were determined. Comparing the L-NAME animals with their respective controls showed the arterial pressure (AP) and the heart weight (HW) to be increased in the L-NAME animals. At 25 days, and more obviously at 40 days, the myocytes were hypertrophied with increase of myofibrils (A[m], greater in L-NAME rats). There were some areas with ischaemic lesions, inflammatory infiltrates and perivascular and interstitial fibrosis. The intramyocardial arteries had a thick tunica media and tunica intima. At 25 days the myocardium showed no stereological difference between L-NAME and controls, but by 40 days there was decreased Vv[m] and Nv[m] and increased Vv[int] in the exposed group. Inhibition of NO synthesis provoked a time progressive myocardial change, quantified by stereology.

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