Inhibition of calmodulin and protein kinase C by amiodarone and other class III antiarrhythmic agents

Publisher Website
Google Scholar
Abstract
Class III antiarrhythmic agents may prolong refractoriness via modulation of ion channels, which may be sensitive to Ca2+ regulatory proteins or enzymes. Accordingly, the purpose of this study was to quantitate the effects of several structurally diverse class III antiarrhythmic agents on calmodulin-regulated enzymes and protein kinase C activity, and to evaluate the ability of these agents and known calmodulin antagonists to prolong cardiac refractoriness in vivo. The rank order of potency (IC50;μM) of selected class III antiarrhythmic agents and reference calmodulin antagonists as inhibitors of calmodulin-regulated phosphodiesterase activity were: calmidazolium (0.12 μM)>amiodarone (0.62 μM)>desethylamiodarone (1.5 μM)>trifluoperazine (4.3 μM), bepridil (5 μM)>W-7 (7.5 μM), clofilium (13 μM). Similar concentration-related inhibition was evident in a second calmodulin-regulated system, inhibition of myosin light-chain phosphorylation and superprecipitation light-chain phosphorylation and superprecipitation of arterial actomyosin. Sotalol and tetraethylammonium were inactive at 100 μM. Protein kinase C activity was also inhibited by some of these agents; desethylamiodarone (IC50=11 μM) was more potent than the reference agent, H-7 (IC50=79 μM), or amiodarone (38% inhibition at 100 μM) and clofilium (32% inhibition at 100 μM). In vivo, the minimally effective doses required to increase ventricular effective refractory doses required to increase ventricular effective refractory periods in paced guinea pigs were (in mg/kg) bepridil, sotalol [1]>clofilium [3]>amiodarone [10]>W-7, desethylamiodarone [20]. No changes in refractory period were noted with maximum testable doses of calmidazolium or trifluoperazine. These studies show that some, but not all, class III antiarrhythmic agents are effective and potent calmodulin antagonists or protein kinase C inhibitors. Moreover, some calmodulin antagonists are effective at prolonging refractoriness in vivo. However, a lack of correlation between these agents suggests that these mechanisms are not solely responsible for the prolongation of refractoriness of all class III agents.