Modifying L-Type Calcium Current Kinetics: Consequences for Cardiac Excitation and Arrhythmia Dynamics

Abstract

The L-type Ca current (I(Ca,L)), essential for normal cardiac function, also regulates dynamic action potential (AP) properties that promote ventricular fibrillation. Blocking I(Ca,L) can prevent ventricular fibrillation, but only at levels suppressing contractility. We speculated that, instead of blocking I(Ca,L), modifying its shape by altering kinetic features could produce equivalent anti-fibrillatory effects without depressing contractility. To test this concept experimentally, we overexpressed a mutant Ca-insensitive calmodulin (CaM(1234)) in rabbit ventricular myocytes to inhibit Ca-dependent I(Ca,L) inactivation, combined with the ATP-sensitive K current agonist pinacidil or I(Ca,L) blocker verapamil to maintain AP duration (APD) near control levels. Cell shortening was enhanced in pinacidil-treated myocytes, but depressed in verapamil-treated myocytes. Both combinations flattened APD restitution slope and prevented APD alternans, similar to I(Ca,L) blockade. To predict the arrhythmogenic consequences, we simulated the cellular effects using a new AP model, which reproduced flattening of APD restitution slope and prevention of APD/Ca(i) transient alternans but maintained a normal Ca(i) transient. In simulated two-dimensional cardiac tissue, these changes prevented the arrhythmogenic spatially discordant APD/Ca(i) transient alternans and spiral wave breakup. These findings provide a proof-of-concept test that I(Ca,L) can be targeted to increase dynamic wave stability without depressing contractility, which may have promise as an antifibrillatory strategy.

Redox Control of Cardiac Excitability

“…In another example, Mahajan et al showed that I CaL can be targeted to increase dynamic wave stability by maintaining action potentials without depressing contractility (172). Mahajan et al showed that in rabbit ventricular myocytes, when I CaL inactivation is inhibited/delayed, and when these myocytes were also treated with verapamil (an I CaL blocker), the intracellular Ca + 2 transients were not affected; the action potential duration restitution slope was flattened; and action potential duration alternans (beat-to-beat alternation in action potential duration) were prevented (172). In fact, this approach has been proposed as the possible therapeutic way to increase the usability/safety of I CaL blockers as antiarrhythmic drugs.…”

Intra- and Interdomain Effects Due to Mutation of Calcium-binding Sites in Calmodulin

“…This latter mutation was used previously to inactivate EF-hand Ca 2ϩ -binding sites in cardiac troponin C (10), skeletal troponin C (11), and myosin regulatory light chain (12). Mutation of position 1 has been used extensively to assess the relative functional contribution of Ca 2ϩ binding to the N-and C-domain of CaM by expression of the mutated proteins in cells (13)(14)(15)(16)(17)(18)(19)(20)(21)(22)(23)(24)(25)(26)(27)(28)(29)(30)(31). Thus, we elected to convert Asp to Ala in the first ligand position in all CaM Ca 2ϩ -binding mutants.…”

A rendezvous with the queen of ion channels: Three decades of ion channel research by David T Yue and his Calcium Signals Laboratory

“…Indeed, myocytes overexpressing CaM 1234 exhibit markedly prolonged action potentials. 148,149 These studies highlighted the importance of CaM in the heart, adding credence to the widespread belief that naturally-occurring mutant CaMs would likely be so deleterious to be incompatible with life. However, it was not until a decade later, that a rare group of diseases resulting from heterozygous missense mutations within one of the 3 genes (CALM1-3) encoding identical CaM proteins was discovered.…”