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

BackgroundComputer 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.ObjectiveTo provide experimental evidence that Ca-to-Vm coupling during alternans is determined by the balanced influence of 2 Ca-sensitive electrogenic sarcolemmal ionic currents: INCX and ICa.Methods and ResultsVm-ALT and Ca-ALT were measured simultaneously from isolated guinea pig myocytes (n = 41) by using perforated patch and Indo-1AM fluorescence, respectively. There were 3 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 <.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 < .05) and decreased Ca-to-Vm gain compared with control (P <.05). Furthermore, computer simulation demonstrated that Ca-to-Vm coupling changes from negative to positive because of a shift from ICa to INCX predominance with increasing pacing rate.ConclusionsThese data provide the first direct experimental evidence that coupling in phase and magnitude of Ca-ALT to Vm-ALT is strongly determined by the relative balance of the prominence of INCX vs ICa currents.