The Change of Expression of Troponin T and I Isoforms in the Failing Myocardium of Rat after Myocardial Infarction

BACKGROUND: Contractile dysfunction of rat myocardium in postinfarction remodeling is characterized by decreased response of myofilament both to calcium(Ca++) and adrenergic stimuli. This study tested the hypothesis that above alterations may be linked to molecular switches among the isoforms of troponin T and troponin I, the major regulators of myocardial responsiveness. METHODS: Using Sprague-Dawley rat as a model, myocardiums of fetal heart, 1 day postnatal heart, normal adult heart(n=4), and non-infarcted area of postinfarction heart(n=4, 3 months after left coronary artery ligation, mean LVEDP=21.4mmHg) were studied. For the detection on molecular switches, western blotting and semiquantitative RT-PCR were employed. RESULTS: Immunoblotting of rat myocardium showed normal developmental isoform switch of troponin protein. There was distinct isoform patterns specific for postinfarction rat heart. The postinfarction myocardium developed a marked increase in the fetal isoform and marked decrease in the adult isoform of troponin T, resulting in the reversed ratio of fetal/adult cardiac troponin T isoform(normal adult rat=0.60+/-0.1 vs postinfarction rat=1.79+/-0.15, p<0.01). Also, the postinfarction heart showed approximately 20% decrease in the amount of adult troponin I isoform. In RT-PCR experiments, the postinfarction hearts were characterized by increased amplification of fetal troponin R isoform cDNA(fetal troponin R/GAPDH ; normal adult rat=0.22, postinfarction rat=0.84). However, there was no siginificant difference in the amplification of troponin I cDNA between normal and postinfarction heart. CONCLUSION: These experimental findings indicate that there are molecular switches operative among the regulatory protein troponin T and I in the rat myocardium with development of postianfarction heart failure.

A Study on the Cardiac Myofibrillar ATPase Activity in Diabetic Rats

Diabetes mellitus is known to be associated with a specific cardiomyopathy. This is evident from the clinical-pathological work and the epidemiologic data. An investigation was made in this study to determine whether diabetic cardiomyopathy in rats is associated with an alteration of biochemical characteristics of cardiac contractile proteins. Rats were made diabetic with intravenous injection of streptozotocin and hearts removed 8 weeks later for the isolation of myofibrils. The basal ATPase activity of myofibrils from diabetic hearts was significantly lower than that of the controls, suggesting the presence of some subtle structural and conformational changes in diabetic myofibrils. The activating effect of Mg ions on the myofibrillar actomyosin system of rat heart muscle was also demonstrated. Sodium dodecylsulfate gel electrophoresis showed the presence of myosin heavy chain, light chain 1 and 2, actin and troponin but failed to reveal differences in the patterns of these contractile proteins of light subunits between diabetics and controls. The deficiency in utilization of energy rich phosphates by the myofibrillar protein may be one of of the main mechanisms of cardiodepression observed in diabetic hearts. The cardiac myofibrillar ATPase activity may be one of useful measurements in evaluating pathophysiological states of cardiac contractile proteins.