ARGINASE EXPRESSION IN THE MOUSE HEART POST-MYOCARDIAL INFARCTION.

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Background Following acute myocardial infarction (MI), there are a number of changes in gene expression that take place in all areas of the heart. It has recently been shown that mRNA levels for arginase (specifically the type I isozyme) are significantly elevated following acute MI in a murine model. The expression of this enzyme could have a significant impact on arginine and polyamine metabolism in the post-MI myocardium and vasculature since it would compete with enzymes such as nitric oxide synthase for access to its substrate, arginine. The goal of this project was to confirm that the protein itself was also overexpressed in the post-MI heart, as well as to define that expression temporally and spatially in the ischemic mouse heart. Study Design and Methods Using primary antibodies directed against arginase I, we evaluated the expression of this enzyme in naive liver, brain, kidney, and heart tissue, as well as tissue from experimentally infarcted (LAD ligature) mice at 4, 24, and 48 hours post-MI. Expression was analyzed via Western blot and immunofluorescence microscopy. Results Western blot data indicated that in naive mouse tissue, arginase expression is highest in the liver, with fairly high levels also noted in the kidney. Naive heart showed very low levels of constitutive arginase expression, whereas brain tissue showed no detectable expression. Experiments on infarcted tissue showed very little induction of protein expression at 4 hours post-MI but notable elevations at 24 hours and even higher levels at 48 hours. These data were further confirmed by immunofluorescence microscopy, which indicated very little evident antibody binding in naive heart and increasing levels of fluorescent activity from 4 to 48 hours post-MI. The 48-hour time point clearly showed the highest levels of arginase I. The fluorescence was consistently localized to that area of the left ventricle (LV) corresponding to the infarct area, with minimal expression noted in the right ventricular (RV) free wall and other remote areas. Additionally, the fluorescence was most intense at the epicardial surface of the heart, although the specific cell type expressing the enzyme could not be definitively defined. Possibilities include subepicardial cardiac myocytes, fibroblasts in the epicardial connective tissue, epithelial cells forming the visceral pericardium (epicardium), or peripheral infiltrating neutrophils. Conclusion These results show that arginase I is preferentially expressed in the epicardial infarct region of the post-MI mouse heart. These results may hold promise for further elucidating the changes in arginine and nitric oxide biochemistry that occur following ischemic insult to the heart.