ABC | Volume 114, Nº4, April 2020

Brief Communication Miranda et al. Ranolazine and type 3 long QT syndrome Arq Bras Cardiol. 2020; 114(4):732-735 present larger I NaL than EPI cells. However, it is also important to note that 30 µM RANO could also block L-type calcium current in cardiomyocytes. 9 To better understand the mechanism underlying sarcomere shortening induced by RANO, subsequent experiments were performed at 35 o C. Cardiomyocytes were loaded with Fura 2-AM to monitor calcium oscillation during cell contraction, and cells were exposed to ATX-II to increase I NaL and induce an LQT3 phenotype3 (Figure 2). ENDO (Figures 2A, B and C) and EPI (Figures 2D, E and F) cells exposed to ATX-II showed clear calcium disturbances and simultaneous mechanical arrhythmias. RANO (30 µM) strongly attenuated the arrhythmic phenotype induced by ATX-II in both cell groups to a similar extent. To confirm that the arrhythmic phenotype observed in our experiments was truly attributed to I NaL , cells were exposed to 6 nM ATX-II [Figure 2A (iv) and Figure 2D (iv)], following exposure to 10 µM TTX and 6 nM ATX-II [Figure 2A (v) and Fig 2D (v)]. The results confirmed that the observed arrhythmic phenotype occurred due to I NaL augmentation. Despite the fact that rat ENDO cells present larger sodium currents than EPI cells, 7,10 the arrhythmic phenotype induced by ATX-II and the extent of antiarrhythmic effects of RANO were similar in both cell groups. Interestingly, the therapeutic concentration range of RANO is 1–10 µM. 11 The apparent discrepancy in RANO potency may be explained by the fact that ATX-II at doses of 1–10 nM induces larger I NaL in cardiomyocytes than that observed in cardiovascular disease. 3,6 Conclusion RANO exerted a minor impact on sarcomere shortening of healthy cardiomyocytes and abrogated arrhythmias induced by I NaL to a similar extent in ENDO and EPI cells. Author contributions Conception and design of the research and Writing of the manuscript: Campos DR; Acquisition of data: Miranda VM, Beserra SS; Analysis and interpretation of the data and Statistical analysis: Miranda VM, Campos DR. Figure 1 – Inotropic effect of ranolazine (RANO) on sarcomere shortening of ENDO and EPI cardiomyocytes. Representative sarcomere shortening recordings before (black (25ºC) and blue (35ºC)) and after (light gray (25ºC) and red (35ºC)) exposure of ENDO (left) and EPI (right) cardiomyocyte to RANO ((A) 10 and (B) 30 µM). Inotropic effect of 0.1, 1, and 10 µM RANO (C) and 30 µM (D) on sarcomere shortening (upper bars); Normalized time to 50% sarcomere contraction (T50C) (middle bars)) and; normalized time to 50% of sarcomere relaxation (T50R) (bottom bars) Hatched bars represent EPI cells (n = 3–6 cells/concentration). *p < 0.05 comparing before and after RANO exposure. 733