Hearts of surviving MLP-KO mice show transient changes of intracellular calcium handling.

The muscle Lim protein knock-out (MLP-KO) mouse model is extensively used for studying the pathophysiology of dilated cardiomyopathy. However, explanation is lacking for the observed long survival of the diseased mice which develop until adulthood despite the gene defect, which theoretically predestines them to early death due to heart failure. We hypothesized that adaptive changes of cardiac intracellular calcium (Ca(i)(2+)) handling might explain the phenomenon. In order to study the progression of changes in cardiac function and Ca(i)(2+) cycling, myocardial Ca(i)(2+)-transients recorded by Indo-1 surface fluorometry were assessed with concomitant measurement of hemodynamic performance in isolated Langendorff-perfused hearts of 3- and 9-month old MLP-KO animals. Hearts were challenged with beta-agonist isoproterenol and the sarcoplasmic reticular Ca(2+)-ATPase (SERCA2a) inhibitor cyclopiazonic acid (CPA). Cardiac mRNA content and levels of key Ca(2+) handling proteins were also measured. A decline in lusitropic function was observed in 3-month old, but not in 9-month old MLP-KO mice under unchallenged conditions. beta-adrenergic responses to isoproterenol were similar in all the studied groups. The CPA induced an increase in end-diastolic Ca(i)(2+)-level and a decrease in Ca(2+)-sequestration capacity in 3-month old MLP-KO mice compared to age-matched controls. This unfavorable condition was absent at 9 months of age. SERCA2a expression was lower in 3-month old MLP-KO than in the corresponding controls and in 9-month old MLP-KO hearts. Our results show time-related recovery of hemodynamic function and an age-dependent compensatory upregulation of Ca(i)(2+) handling in hearts of MLP-KO mice, which most likely involve the normalization of the expression of SERCA2a in the affected hearts.

Altered calcium handling is an early sign of streptozotocin-induced diabetic cardiomyopathy.

The main objective of the present study was to determine alterations of calcium handling in the diabetic rat heart during the transition from adaptive to maladaptive phase of cardiomyopathy. By inhibiting the nuclear enzyme poly(ADP-ribose) polymerase (PARP), we also investigated the possible role of this enzyme in the sequence of pathological events. Six weeks after induction of type I diabetes by injection of streptozotocin in rats, the hearts were perfused according to Langendorff. Intracellular-free calcium (Ca(2+)(i)) levels were measured by surface fluorometry using Indo-1 AM. Cyclic changes in Ca(2+)(i) concentrations and hemodynamic parameters were measured simultaneously. The hearts were challenged by infusion of isoproterenol. Six weeks of diabetes resulted in reduced inotropy and lusitropy. The diabetic hearts (DM) expressed a significantly elevated end-diastolic Ca(2+)(i) level (control, 111-/+20 vs DM, 221-/+35 nM). The maximal transport capacity of SERCA2a and conductance of RyR2 were reduced. These changes were not accompanied by major alterations in the tissue content of SERCA2a, RyR2, phospholamban and Na(+)/Ca(2+) exchanger. In response to beta-adrenergic activation, SERCA2a transport capacity and RyR2 conductance were stunted in the DM hearts. Inhibition of PARP induced minor changes in the mechanical function and calcium handling of the DM hearts. In conclusion, the observed changes in contractility and in Ca(2+)(i) handling are most likely attributable to functional disturbances of SERCA2a and RyR2 in this transitional phase of diabetes. At this stage of diabetes, PARP does not appear to play a significant pathogenetic role in the alterations in contractile function and calcium handling.

Poly(ADP-ribose) polymerase regulates myocardial calcium handling in doxorubicin-induced heart failure.

Reactive oxygen and nitrogen species are overproduced in the cardiovascular system in response to the exposure to doxorubicin, a cardiotoxic anticancer compound. Oxidant-induced cell injury involves the activation of the nuclear enzyme poly(ADP-ribose) polymerase (PARP) and pharmacological inhibition of PARP has recently been shown to improve myocardial contractility in doxorubicin-induced heart failure models. The current investigation, by utilizing an isolated perfused heart system capable of beat-to-beat intracellular calcium recording, addressed the following questions: (1) is intracellular calcium handling altered in hearts of rats after 6-week doxorubicin treatment, under baseline conditions, and in response to oxidative stress induced by hydrogen peroxide exposure in vitro; and (2) does pharmacological inhibition of PARP with the phenanthridinone-based PARP inhibitor PJ34 affect the changes in myocardial mechanical performance and calcium handling in doxorubicin-treated hearts under normal conditions and in response to oxidative stress. The results showed a marked elevation in intracellular calcium in the doxorubicin-treated hearts which was normalized by pharmacological inhibition of PARP. PARP inhibition also prevented the myocardial contractile disturbances and calcium overload that developed in response to hydrogen peroxide in the doxorubicin-treated hearts. We conclude that PARP activation contributes to the development of the disturbances in cellular calcium handling that develop in the myocardium in response to prolonged doxorubicin exposure.