KN-93, an inhibitor of multifunctional Ca++/calmodulin-dependent protein kinase, decreases early after depolarizations in rabbit heart

The multifunctional Ca++/calmodulin-dependent protein kinase II (CaM kinase ) mediates Ca++-induced augmentation of L-type Ca++ current (I-Ca); therefo re it may act as a proarrhythmic signaling molecule during early afterdepol arizations (EADs) due to I-Ca. To investigate the hypothesis that I-Ca-depe ndent EADs are favored by CaM kinase activation EADs were induced with clof ilium in isolated rabbit hearts. All EADs were rapidly terminated with I-Ca antagonists. Hearts were pretreated with the CaM kinase inhibitor KN-93 or the inactive analog KN-92 (0.5 mu M) for 10 min before clofilium exposure. EADs were significantly suppressed by KN-93 (EADs present in 4/10 hearts) compared to KN-92 (EADs present in 10/11 hearts) (P =.024). There were no s ignificant differences in parameters favoring EADs such as monophasic actio n potential duration or heart rate in KN-93- or KN-92-treated hearts. CaM k inase activity in situ increased 37% in hearts with EADs compared to hearts without EADs (P =.015). This increase in CaM kinase activity was prevented by pretreatment with KN-93, in vitro, KN-93 potently inhibited rabbit myoc ardia[ CaM kinase activity (calculated K-i less than or equal to 2.58 mu M) , but the inactive analog KN-92 did not (K-i > 100 mu M). The actions of KN -93 and KN-92 on I-Ca and other repolarizing K+ currents did not explain pr eferential EAD suppression by KN-93. These data show a novel association be tween CaM kinase activation and EADs and are consistent with the hypothesis that the I-Ca and CaM kinase activation both contribute to EADs in this mo del.