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

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 (V-m). However, experimental evidence for the mechanism by which alternans of calcium transients produces alternation of V-m (V-m-ALT) is lacking. OBJECTIVE To provide experimental evidence that Ca-to-V-m coupling during alternans is determined by the balanced influence of 2 Ca-sensitive electrogenic sarcolemmal ionic currents: I-NCX and I-Ca. METHODS AND RESULTS V-m-ALT and Ca-ALT were measured simultaneously from isolated guinea pig myocytes (n = 41) by using perforated patch and Indo-1(AM) fluorescence, respectively. There were 3 study groups: (1) control, (2) I-NCX predominance created by adenoviral-induced NCX overexpression, and (3) I-Ca predominance created by I-NCX inhibition (SEA-0400) or enhanced I-Ca (As2O3). During alternans, 14 of 14 control myocytes demonstrated positive Ca-to-V-m coupling, consistent with I-NCX, but not I-Ca, as the major electrogenic current in modulating action potential duration. Positive Ca-to-V-m coupling was maintained during I-NCX predominance in 8 of 8 experiments with concurrent increase in Ca-to-V-m gain (P <.05), reaffirming the role of increased forward-mode electrogenic I-NCX. Conversely, I-Ca predominance produced negative Ca-to-V-m coupling in 14 of 19 myocytes (P < .05) and decreased Ca-to-V-m gain compared with control (P <.05). Furthermore, computer simulation demonstrated that Ca-to-V-m coupling changes from negative to positive because of a shift from I-Ca to I-NCX predominance with increasing pacing rate. CONCLUSIONS These data provide the first direct experimental evidence that coupling in phase and magnitude of Ca-ALT to V-m-ALT is strongly determined by the relative balance of the prominence of I-NCX vs I-Ca currents.