Modulation of Cardiac Myocyte Function by Reactive Oxygen Species
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Previous investigations have demonstrated that reactive oxygen species such as hydrogen peroxide (H2O2) have the ability to alter electrophysiological and mechanical properties of rat ventricular cardiac myocytes. However, despite the breadth of the literature, there is little definitive consensus on the cellular mechanisms. The purpose of this study, therefore, was to study the cellular mechanism of action of H2O2 and test whether H2O2-mediated affects were partially a result of reverse-mode Na+/Ca2+ exchanger (NCX) activity. Unloaded cell shortening, intracellular Ca2+ transients, caffeine-induced Ca2+ transients, L-type Ca2+ channel recordings, and action potential waveforms were recorded in the presence of combinations of different compounds including Cd2+, H2O2, and KB-R7943. H2O2 was found to cause significant positive inotropy by an increase in contractility of 80 ± 20 % (n=6) and an increased amplitude of Ca2+ transients by 24 ± 14 % (n=8), relative to pre-treatment values. Interestingly, H2O2 caused an increase in contractility even in the presence of Cd2+ block from 4 ± 1 % (n=9) to 15 ± 3 % (n=5) of resting cell length. Using caffeine pulse experiments to induce unloading of the sarcoplasmic reticulum (SR), we found that 100µM H2O2 did not significantly alter SR Ca2+ load. Under control conditions, H2O2 significantly increased L-type Ca2+ currents while this H2O2-induced increase was not observed in myocytes pretreated with Cd2+. Positive inotropy in the presence of H2O2 was blocked using 10µM KB-R7943, a selective reverse-mode inhibitor of the NCX. However, it was found that 10µM KB-R7943 alone altered action potential profile and suppressed normal contraction. Altogether, the major finding of this study is that H2O2 has the ability to enhance myocardial contractility, even under conditions of L-type Ca2+ channel inhibition, through a mechanism that likely involves reverse-mode of the NCX.