Intracellular Calcium and the Mechanism of Anodal Supernormal Excitability in Langendorff Perfused Rabbit Ventricles

Abstract

Background: Anodal stimulation hyperpolarizes the cell membrane and increases the intracellular Ca²⁺ (Ca_i) transient. This study tested the hypothesis that the maximum slope of the Ca_i decline (-(dCa_i/dt)_max) corresponds to the timing of anodal dip on the strength-interval curve and the initiation of repetitive responses and ventricular fibrillation (VF) after a premature stimulus (S₂). Methods and Results: We simultaneously mapped the membrane potential (V_m) and Ca_i in 23 rabbit ventricles. A dip in the anodal strength-interval curve was observed. During the anodal dip, ventricles were captured by anodal break excitation directly under the S₂ electrode. The Ca_i following anodal stimuli is larger than that following cathodal stimuli. The S₁-S₂ intervals of the anodal dip (203±10ms) coincided with the -(dCa_i/dt)_max (199±10ms, P=NS). BAPTA-AM (n=3), inhibition of the electrogenic Na⁺-Ca²⁺ exchanger current (I_NCX) by low extracellular Na⁺ (n=3), and combined ryanodine and thapsigargin infusion (n=2) eliminated the anodal supernormality. Strong S₂ during the relative refractory period (n=5) induced 29 repetitive responses and 10 VF episodes. The interval between S₂ and the first non-driven beat was coincidental with the time of -(dCa_i/dt)_max. Conclusions: Larger Ca_i transient and I_NCX activation induced by anodal stimulation produces anodal supernormality. The time of maximum I_NCX activation is coincidental to the induction of non-driven beats from the Ca_i sinkhole after a strong premature stimulation. (Circ J 2011; 75: 834-843)