Increased expression and regional differences of atrial myosin light chain 1 in human ventricles with old myocardial infarction. Analyses using two monoclonal antibodies.

BACKGROUND: This study was designed to examine the expression of atrial/fetal-type myosin light chain 1 (ALC1) in human ventricles with old myocardial infarction and in control hearts. METHODS AND RESULTS: The expression of immunoreactive (ir) ALC1 was examined in the subendocardial and subepicardial myocardium of the infarcted and the noninfarcted regions in the left ventricles with old myocardial infarction (n = 12) and of the control left ventricles (n = 8). For the analysis, we prepared two monoclonal antibodies, KA1 and KB1, that were specific for only ALC1 and for both ALC1 and ventricular myosin light chain 1 (VLC1), respectively. The ir-ALC1 expression ratio ALC1/(ALC1 + VLC1), % of the subendocardial myocardium, determined densitometrically by Western blotting with KB1, was significantly higher in the infarcted region (11.4 +/- 7.3%) than in the noninfarcted region (4.7 +/- 2.3%, p < 0.001) and the control ventricle (1.0 +/- 1.5%, p < 0.0001). In the infarcted region, the subendocardial myocardium contained a significantly greater percentage of ir-ALC1 than the subepicardial myocardium (5.8 +/- 6.7%, p < 0.005). The ir-ALC1 expression ratio had a significant negative correlation with the value of tissue protein concentration (milligrams protein per gram wet weight). The immunohistochemical study with KA1 revealed that the surviving myocytes included in the infarcted region, especially in the ventricular aneurysm, expressed ir-ALC1 strongly in comparison with those in the noninfarcted or the control ventricles. CONCLUSIONS: These results demonstrate increased expression of ALC1 and the regional differences in the failing left ventricles with old myocardial infarction. We conclude that the reexpression of ALC1 in infarcted ventricles occurs as one of the regional responses to increased load and may be a useful biochemical marker for the appearance of fetal-type myocytes.