Figure 2. Effects of I _KAS blockade on APD during isoproterenol infusion in female rabbit ventricles.

A, representative V _m traces, APD₂₅, APD₈₀ and APD₈₀ − APD₂₅ maps at baseline, during isoproterenol, after apamin and after washout (Protocol II). At baseline, the AP exhibited a prominent phase 2 plateau. Isoproterenol markedly shortened APD and more prominently APD₂₅ than APD₈₀, leading to a short and triangular AP. Representative V _m traces were obtained at the LV base. B, C and D, apamin significantly prolonged both APD₂₅ and APD₈₀, but more prominently APD₂₅ (D), consequently reversing the AP triangulation and restoring the AP plateau. After washout, the APD was further prolonged to a level similar to the baseline. E, apamin significantly increased the ratio of APD₂₅/APD₈₀, resulting in AP squaring and plateau restoration. F, apamin significantly abbreviated APD₈₀ − APD₂₅, indicating an acceleration of phase 3 repolarization. Student's paired t tests were performed in C–F. G, the prolongation of the total repolarization by apamin (represented by ΔAPD₈₀) corresponded to the shortening of phase 3 repolarization (Δ(APD₈₀ − APD₂₅)) in 7 out of 8 females (circles in quadrant IV). Therefore, the apamin predominantly lengthened APD₂₅. H, representative TMP recording in ventricular cardiomyocytes from a female rabbit. ISO, isoproterenol. [Color figure can be viewed at http://wileyonlinelibrary.com]