Oxidative Stress–Induced Afterdepolarizations and Calmodulin Kinase II Signaling

In the heart, oxidative stress caused by exogenous H2O2 has been shown to induce early afterdepolarizations (EADs) and triggered activity by impairing Na current (INa) inactivation. Because H2O2 activates Ca2+/calmodulin kinase (CaMK)II, which also impairs INa inactivation and promotes EADs, we hypothesized that CaMKII activation may be an important factor in EADs caused by oxidative stress. Using the patch-clamp and intracellular Ca (Cai) imaging in Fluo-4 AM–loaded rabbit ventricular myocytes, we found that exposure to H2O2 (0.2 to 1 mmol/L) for 5 to 15 minutes consistently induced EADs that were suppressed by the INa blocker tetrodotoxin (10 &mgr;mol/L), as well as the ICa,L blocker nifedipine. H2O2 enhanced both peak and late ICa,L, consistent with CaMKII-mediated facilitation. By prolonging the action potential plateau and increasing Ca influx via ICa,L, H2O2-induced EADs were also frequently followed by DADs in response to spontaneous (ie, non–ICa,L-gated) sarcoplasmic reticulum Ca release after repolarization. The CaMKII inhibitor KN-93 (1 &mgr;mol/L; n=4), but not its inactive analog KN-92 (1 &mgr;mol/L, n=5), prevented H2O2-induced EADs and DADs, and the selective CaMKII peptide inhibitor AIP (autocamtide-2–related inhibitory peptide) (2 &mgr;mol/L) significantly delayed their onset. In conclusion, H2O2-induced afterdepolarizations depend on both impaired INa inactivation to reduce repolarization reserve and enhancement of ICa,L to reverse repolarization, which are both facilitated by CaMKII activation. Our observations support a link between increased oxidative stress, CaMKII activation, and afterdepolarizations as triggers of lethal ventricular arrhythmias in diseased hearts.