MYOSIN LIGHT-CHAIN PHOSPHORYLATION IN DIABETIC CARDIOMYOPATHY IN RATS

The regulatory myosin light chain (MLC) is phosphorylated in cardiac m uscle by Ca2+/calmodulin-dependent MLC kinase (MLCK) and is considered to play a modulatory role in the activation of myofibrillar adenosine triphosphatase (ATPase) and the process of force generation. Since th e depression in cardiac contractile function in chronic diabetes is as sociated with a decrease in myofibrillar ATPase activity, we investiga ted changes in MLC phosphorylation in diabetic heart. Rats were made d iabetic by injecting streptozotocin (65 mg/kg intravenously), and the hearts were removed 8 weeks later; some 6-week diabetic animals were i njected with insulin (3 U/d) for 2 weeks. Changes in the relative MLC and MLCK protein contents were measured by electrophoresis and immunob lot assay, whereas phosphorylated and unphosphorylated MLCs were separ ated on 10% acrylamide/urea gel and identified by Western blot. MLC an d MLCK contents were decreased markedly (40% to 45%) and MLC phosphory lation was decreased significantly (30% to 45%) in the diabetic rat he art homogenate in comparison to control values. The changes in MLC and MLCK content in diabetic heart were partially reversible, whereas cha nges in MLC phosphorylation were normalized upon treatment with insuli n. These results suggest that decreased protein contents of MLC and ML CK and phosphorylation of MLC may contribute to the depression of card iac myofibrillar ATPase activity and heart dysfunction in diabetic car diomyopathy. Copyright (C) 1997 by W.B. Saunders Company