Oxidative Stress–Induced Afterdepolarizations and Calmodulin Kinase II Signaling

Abstract

In the heart, oxidative stress caused by exogenous H 2 O 2 has been shown to induce early afterdepolarizations (EADs) and triggered activity by impairing Na current (INa) inactivation. Because H 2 O 2 activates Ca 2+/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 (Ca i) imaging in Fluo-4 AM–loaded rabbit ventricular myocytes, we found that exposure to H 2 O 2(0.2 to 1 mmol/L) for 5 to 15 minutes consistently induced EADs that were suppressed by the INa blocker tetrodotoxin (10 μmol/L), as well as the ICa,L blocker nifedipine. H 2 O 2 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, H 2 O 2-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 μmol/L; n=4), but not its inactive analog KN-92 (1 μmol/L, n=5), prevented H 2 O 2-induced EADs and DADs, and the selective CaMKII peptide inhibitor AIP (autocamtide-2–related inhibitory peptide) (2 μmol/L) significantly delayed their onset. In conclusion, H 2 O 2-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.