Healing processes affect ventricular remodeling, an important prognostic factor in acute myocardial infarction (AMI). Type IV collagen, a major component of the basement membrane, has been demonstrated to appear in the infarct zone. Six type IV alpha chains, alpha 1 to alpha 6 [alpha 1(IV) to alpha 6(IV)], have been identified. We examined the expression of the alpha 1(IV), alpha 2(IV), alpha 3(IV) and alpha 5(IV) in experimental rat AMI induced by permanent ligation of the left coronary artery in male Sprague-Dawley rats. Neonatal cardiac fibroblasts cultured to reach confluency were also used to examine the hypoxic induction of the expression of these chains. Northern blotting, in situ hybridization, and immunohistochemical staining were employed to examine the alpha chain expression. With the in situ hybridization, alpha 1(IV) and alpha 2(IV) mRNA signals were observed in spindle-shaped mesenchymal cells in the intercellular spaces, in the vascular endothelial cells and in some myocytes in the normal myocardium. Signals for both alpha 1(IV) and alpha 2(IV) mRNA appeared in the spindle-shaped mesenchymal cells located between the surviving myocytes in the infarct peripheral zone at 24 h after the ligation. These signals extended into the central zone and reached the center point of the infarct 5 days after the ligation. Neither alpha 3(IV) nor alpha 5(IV) mRNA signals were observed in the infarct zone. Northern blotting demonstrated increased expressions of alpha 1(IV) and alpha 2(IV) mRNA in the infarct zone compared to the sham-operated hearts. The expression of alpha 5(IV) mRNA was weak in the sham-operated hearts but was not detected in the infarct zone. alpha 3(IV) mRNA was not expressed in the infarct zones or sham-operated hearts. In the cultured cardiac fibroblasts, a hypoxic induction of the expression of both alpha 1(IV) and alpha 2(IV) mRNA was demonstrated by Northern hybridization. Immunohistochemistry revealed alpha 1(IV) and alpha 2(IV) staining in the infarct lesion. The findings demonstrate that among the type IV collagen alpha chains examined, alpha 1(IV) and alpha 2(IV) produced by mesenchymal cells appeared in the infarct zone, suggesting that type IV collagen composed by alpha 1 and alpha 2 chains contributes to the pathological course of myocardial infarction.