Calcium regulation in the human myocardium affected by dilated cardiomyopathy: A structural basis for impaired Ca2+-sensitivity

Calcium regulation in the human heart is impaired during idiopathic dilated cardiomyopathy (IDC). Here, we analyze the structural basis for impairment in the regulatory mechanism. Regulation of contractility was monitored by MgATPase and Ca2+-binding assays as a function of calcium. Myofibrillar pro teolysis and expression of troponin T isoforms were established by gel elec trophoresis and by Western blots. Myofibrillar ATPase assays in low salt ho wever, revealed a drastic lowering of calcium sensitivity in IDC myofibrils as indicated by reductions in both activation by high calcium and in EGTA- mediated inhibition of MgATPase. Structural changes in myofilament proteins were found in most IDC hearts, specifically proteolysis of myosin light ch ain 2 (LC2), troponin T and I (TnT and TnI), and sometimes large isoform sh ift in TnT. IDC did not induce mutations in LC2 and troponin C (TnC), as es tablished by cDNA sequence data from IDC cases, thus, calcium binding to ID C myofibrils was unaffected. Reassociation of IDC myofibrils with native LC 2 raised MgATPase activation at high Ca2+ to control levels, while repletio n with intact, canine TnI/TnT restored inhibition at low Ca2+. A model, ide ntifying possible steps in the steric blocking mechanism of regulation, is proposed to explain IDC-induced changes in Ca2+-regulation. Moreover, shift s in TnT isoforms may imply either a genetic or a compensatory factor in th e development and pathogenesis of some forms of IDC.