Protection by 6-aminonicotinamide against oxidative stress in cardiac cells

Oxidative stress at the time of reperfusion is a major aspect of ischemia-reperfusion injury in heart as well as in other organs. There is a continuing interest in development of pharmacological approaches to alleviate this injury. 6-Aminonicotinamide (6AN) has been shown to diminish myocardial necrosis following global ischemia in an isolated rat heart, apparently by limiting the oxidative injury component. We therefore explored the antioxidative potential of 6AN in a model using H9C2(2-1) rat cardiac myoblasts exposed to H2O2 stress. Dependent on the specific protocol, 6AN pretreatment for 6-23 h resulted in a strongly increased cell survival: from 11% to 16% in untreated cells to 56-75% following 6AN treatment. This 6AN-mediated protection was associated with a modest increase (up to 55%) of the cytosolic free Ca2+, and was blocked by ryanodine, but not by verapamil or nifedipine. The protective effect of 6AN was associated with a decrease in total cell content of the reduced glutathione (GSH) by 15-44%, indicative of an oxidative shift in the GSH/GSSG system redox potential. We propose that this redox shift caused an increased Ca2+ leak through ryanodine receptors, reflecting their known sensitivity to redox modulation. In turn, this Ca2+ redistribution appeared to trigger a state of an enhanced antioxidative resistance, somewhat analogous to the phenomenon of Ca2+ preconditioning. Similar to some of the cases of Ca2+ preconditioning, this protected state involved the activity of Ca2+ -independent, but not of Ca2+ -dependent, isoform(s) of protein kinase C.