Role of intracellular Ca2+ overload in inducing changes in cardiac gene expression

Abstract

Normal expression of cardiac genes for Ca 2+ transport and contractile proteins is known to play a critical role in the maintenance of the function of subcellular organelles such as sarcolemma (SL), sarcoplasmic reticulum (SR) and myofibrils (MFs) in cardiomyocytes. Previous studies have revealed varying degrees of depression in gene expression for SL, SR and MF proteins, as well as defects in subcellular organelles during the development of heart failure in patients and experimental animal models (1-6). Accordingly, it has been suggested that cardiac dysfunction in failing hearts is due to subcellular remodelling as a consequence of changes in cardiac gene expression (2,(7)(8)(9). Although the occurrence of intracellular Ca 2+ overload is believed to be intimately involved in the genesis of cardiac dysfunction in heart failure (10-15), its role in inducing defects in cardiac gene expression is not fully understood. It was, therefore, the purpose of the present study to investigate whether alterations in gene expression of SL, SR and MF proteins occur during the development of intracellular Ca 2+ overload in the myocardium.Isolated heart reperfused with high concentrations of Ca 2+ following a brief period of perfusion with Ca 2+ -free medium has been demonstrated to be an excellent model (Ca 2+ paradox [CP]) for investigating the effects of intracellular Ca 2+ overload (16)(17)(18)(19)(20)(21)(22)(23)(24)(25)(26). We have reported that the inability of CP hearts to recover contractile function was associated with a marked increase in intracellular Ca 2+ , as well as the development of cardiac contracture, ultrastructural damage and subcellular defects (11,13,15,16,21,23). In the present study, we examined whether hearts perfused with Ca 2+ -free medium followed by reperfusion with medium containing different concentrations of Ca 2+ exhibit alterations in gene expression for SL Na + -K + ATPase and Na + -Ca 2+ exchanger, SR Ca 2+ -pump ATPase, Ca 2+ -release channel and phospholamban (PLB), as well as MF α-and β-myosin heavy chain proteins. Because subcellular remodelling in the failing heart is dependent on the balance between changes in gene expression and activities of proteolytic enzymes, such as calpain (27-33), messenger RNA (mRNA) levels for both calpain-1 and calpain-2 were measured in CP hearts. It may be noted that subcellular remodelling in failing hearts has also been suggested to be the result of activation of different proteolytic enzymes (27,28). METHODSMale Sprague Dawley rats, each weighing 250 g to 300 g, were used in the present study. All experiments were conducted according to the protocol approved by the Animal Care Committee of the University of Manitoba (Winnipeg, Manitoba) as per guidelines established by the Canadian Council on Animal Care. Isolated hearts were perfused according to the Langendorff technique with Krebs-Henselite medium (gassed with 95% O 2 and 5% CO 2 ) containing 1.25 mM Ca 2+ at 37°C for 20 min. The left ventricular developed pressure and the left ventricular end dias...

Mechanisms of Cardiac Dysfunction in Heart Failure due to Myocardial Infarction

“…The depletion of energy stores can also be produced by increased ATP hydrolysis. Furthermore, intracellular Ca 2+ -overload has been shown to produce subcellular remodeling by increasing the activities of different proteases as well as by depressing the cardiac gene expression [79,84]. Thus, both depletion of energy stores and subcellular remodeling have been suggested to explain cardiac dysfunction due to the occurrence of intracellular Ca 2+ -overload.…”