Changes in IGFs in cardiac tissue following myocardial infarction

Abstract

We have studied changes in the IGF axis in an ovine model of myocardial infarction (MI), in order to determine the relationship between time-based changes in post-infarct myocardium and IGF levels. IGF localization was studied by immunocytochemistry, production by in situ hybridization, and specific binding by radioligand studies. In surviving tissue, IGF-I peptide localized to cardiomyocytes, with strongest immunostaining at 1 and 2 days post-infarct in the immediate border area adjoining the infarct, where IGF-I mRNA also increased, reaching a maximum at 2 days. Binding of radiolabelled IGF-I in surviving tissue was initially lower than that seen in cardiomyocytes in control myocardium, subsequently increasing to become significantly greater by 6 days post-infarct. In necrotic tissue, IGF-I peptide was still detectable in cardiomyocytes at 0.5 days post-infarct, but had cleared from this area by 1 day, becoming detectable again at 6 days post-infarct in macrophages and fibroblasts infiltrating the repair zone. IGF-I mRNA was not detected in necrotic tissue until 6 days, when probe hybridized to macrophages and fibroblasts. Within the necrotic zone, high levels of radiolabelled IGF-I binding to a combination of receptors and binding proteins were observed in cardiomyocytes in islands of viable tissue located close to the border. Weak immunostaining for IGF-II was observed in cardiomyocytes of the surviving tissue. IGF-II mRNA was not detected in either surviving or necrotic areas. Binding of radiolabelled IGF-II was predominantly to macrophages in both surviving and infarct areas, although as with IGF-I, high levels of binding of radiolabelled IGF-II to a combination of receptors and binding proteins were observed in islands of viable tissue close to the border within the necrotic area. We conclude that, following MI, surviving cardiomyocytes at the infarct border show marked changes in IGF-I localization, production, and specific binding, indicating that the IGF axis is directly involved in post-infarct events, possibly in the maintenance of cardiac function by the induction of hypertrophy and in cell survival by decreasing apoptotic cell death, which has been demonstrated in other cell types.