New experimental evidence for mechanism of arrhythmogenic membrane potential alternans based on balance of electrogenic INCX/ICa currents

Abstract

Background Computer simulations have predicted that the balance of various electrogenic sarcolemmal ion currents may control the amplitude and phase of beat-to-beat alternans of membrane potential (Vm). However, experimental evidence for the mechanism by which alternans of calcium transients produces alternation of Vm (Vm-ALT) is lacking. Objective We sought to provide experimental evidence that Ca-to-Vm coupling during alternans is determined by the balanced influence of two Ca-sensitive electrogenic sarcolemmal ionic currents, INCX and ICa. Methods and Results Vm-ALT and Ca-ALT were measured simultaneously from isolated guinea pig myocytes (n=41) using perforated patch and Indo-1AM fluorescence, respectively. There were three study groups: 1) Control, 2) INCX predominance created by adenoviral-induced NCX overexpression, and 3) ICa predominance created by INCX inhibition (SEA-0400) or enhanced ICa (As2O3). During alternans, 14 of 14 control myocytes demonstrated positive Ca-to-Vm coupling, consistent with INCX, but not ICa as the major electrogenic current in modulating action potential duration. Positive Ca-to-Vm coupling was maintained during INCX predominance in 8 of 8 experiments with concurrent increase in Ca-to-Vm gain (p<0.05), reaffirming the role of increased forward mode electrogenic INCX. Conversely, ICa predominance produced negative Ca-to-Vm coupling in 14 of 19 myocytes (p<0.05) and decreased Ca-to-Vm gain compared to control (p<0.05). Furthermore, computer simulation demonstrated that Ca-to-Vm coupling changes from negative to positive was due to a shift from ICa to INCX predominance with increasing pacing rate. Conclusion These data provide the first direct experimental evidence that coupling in phase and magnitude of Ca-ALT to Vm-ALT is strongly determined by relative balance of prominence of INCX versus ICa currents.

In Silico Investigation into Cellular Mechanisms of Cardiac Alternans in Myocardial Ischemia

“…Previous study also shows that I NCX is the key factor that translates alternans from Ca to APD [12]. More precisely, the balance of I NCX and I Ca determines coupling in phase of Ca i alternans to APD alternans [27]. Alternans presented by Wan et al can arise from the shifted balance of I NCX and I Ca at higher pacing rates.…”

“…Alternans presented by Wan et al can arise from the shifted balance of I NCX and I Ca at higher pacing rates. In our simulation, the extent of unbalance between these currents shifted by increasing I NCX at cycle length of 400 ms could be too small to produce stable alternans [27]. I Kr and I Ks contribute to the occurrence of APD alternans in ischemia [15].…”

Nonlinear and stochastic dynamics in the heart

“…When γ is negative, a larger Ca 2+ amplitude causes a shorter APD, which is called negative Ca 2+ -to-APD coupling (equivalent to discordant electromechanical coupling). Direct experimental evidence of positive and negative Ca 2+ -to-APD coupling by altering I Ca,L and I NCX was demonstrated recently [294]. …”

Alternans in atria: Mechanisms and clinical relevance

“…Contrary to computational results, numerous experimental studies could not confirm these theoretical findings and actually show a poor relationship between experimentally determined APD restitution kinetics and inducibility of alternans [34–38]. While it was shown that in ventricular myocytes AP kinetics affect sarcoplasmic reticulum (SR) Ca 2+ release [39,40], activity of the electrogenic Na + /Ca 2+ exchanger (NCX) [41–43] and L-type Ca 2+ channels (LCC) [42,44–46], experimental evidence for the intricate details of how V m → [Ca 2+ ] i coupling modulates the occurrence of alternans is still lacking. Such discrepancy in theoretical and experimental findings can be explained, at least to some extent, by the fact that because of the slow recovery of ion channels and gradual change in intracellular ionic concentrations, cardiac myocytes exhibit a “memory” of the preceding stimulation conditions and thus APD is not determined solely by the preceding diastolic interval, and therefore constitute more complex APD dynamics than are simulated by most computational models (discussed in detail in [47,48]).…”