Background:
In the heart, hydrogen peroxide (H
2
O
2
) has been shown to cause early afterdepolarizations (EADs) and triggered activity by impairing Na current (I
Na
) inactivation. Since H
2
O
2
has been recently shown to activate Ca
2+
/calmodulin kinase II (CaMKII), and since CaMKII activation has also been reported to impair I
Na
inactivation and predispose to EADs, we hypothesized that CaMKII activation by H
2
O
2
may be an important factor in the genesis of EADs induced by oxidative stress.
Methods and Results:
Patch-clamped Fluo-4 AM-loaded rabbit ventricular myocytes were exposed to H
2
O
2
(0.1–1mM), which induced spontaneous EADs after 5–15 min. Both the I
Na
blocker tetrodoxtin (TTX, 10 μM) and the I
Ca,L
blocker nifedipine shortened AP duration (APD) and suppressed EADs. H
2
O
2
increased both peak and steady-state I
Ca,L
under square-pulse voltage clamp, and enhanced I
Ca,L
to a greater extent during the AP plateau than during the AP upstroke under AP clamp conditions. In addition, by prolonging the AP plateau and increasing Ca influx via maintained I
Ca,L
, H
2
O
2
-induced EADs frequently caused DADs delayed afterdepolarizations (DADs) due to spontaneous SR Ca release waves after repolarization. KN-93(1 μM), a CaMKII inhibitor, prevented H
2
O
2
-induced EADs (n=4), whereas the inactive analogue KN-92 did not (n=5).
Conclusion:
These findings indicate that H
2
O
2
-induced EADs depend on both impaired I
Na
inactivation to reduce repolarization reserve and enhanced I
Ca,L
to reverse repolarization. Intact CaMKII signaling is necessary for EAD generation in this setting, presumably via its actions on I
Na
and I
Ca,L
, although direct redox effects on other ion channels/transporters may also be important. Our observations support a link between increased oxidative stress, CaMKII activation and afterdepolarizations as triggers of lethal ventricular arrhythmias in diseased heart.
This research has received full or partial funding support from the American Heart Association, AHA National Center.
Early afterdepolarizations in cardiac myocytes: beyond reduced repolarization reserve
