Sex‐specific repolarization heterogeneity in mouse left ventricle: Optical mapping combined with mathematical modeling predict the contribution of specific ionic currents

Figures

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  FIGURE 1. Optical mapping allows for sex‐specific comparison of the left ventricular transmural mouse action potential. (a) Schematic illustration of the areas of interest analyzed in the left ventricle (LV). An appropriate angle was chosen to exclude anterior and inferior heart regions. Whole LV free wall, epicardium (one third of the LV free wall), and the transmural gradient of action potential duration (APD) were investigated parameters. Papillary muscles were explicitly excluded from LV wall analysis. (b) Optical mapping of APD at 90% repolarization (APD90) in a representative male (left panel) and female (right panel) LV short‐axis heart slice, respectively. (c) Bar graphs showing APD90 and its standard deviations (SD), calculated from whole LV free wall (LV) and epicardium (epi) of slices from male (M) and female (F) mice (M: n = 6, F: n = 9; **p = 0.0061; #MSLRT = 5.1, p = 0.023). (d) Quantification of transmural gradient vectors of APD90 (M: n = 6, F: n = 9; ***p = 0.000018; ###MSLRT = 18.4, p = 1.7 × 10−5) (e) Overlay of male and female epicardial APs (regions defined in b by dashed lines), respectively, and quantification of the APD at indicated % repolarization (n = 6 slices…

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  FIGURE 2. Mathematical modeling of the mouse action potential upon alteration of ion channel conductivities contributing to the repolarization reserve. (a) Schematic diagram of a model for a single‐cell AP in left ventricular mouse cardiomyocytes comprising indicated ionic currents and calcium fluxes (upper panel). Relative contribution of indicated K+ conductivities implemented in the model (lower panel). (b) Simulation of APs under the gradual increase and decrease (5% per trace) in indicated K+ conductivities from control to ±50% (left) and corresponding changes in APD30 and APD80 (right). (c) State diagram of a Markov model for the Nav channel (left). CNa1, CNa2, and CNa3 resemble closed states; ONa resembles an open state; IFNa resembles a fast‐inactivated state; I1Na and I2Na resemble intermediate inactivated states; and ICNa2 and ICNa3 resemble inactivated closed states. The effect of modifying the rate constant k for channel re‐opening from IFNa is simulated during AP generation and APDs are quantified by plotting their relative change at indicated % of late sodium current.

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  FIGURE 3. Sex‐specific increase in action potential duration upon L‐type calcium channel activation. (a) Optical mapping of APD80 in a representative region of the left ventricle (LV) before and after the application of the pharmacological calcium channel activator FPL 64176 (250 nM) in male (upper) and female (lower) heart slice preparations. Two representative APs averaged from indicated areas before and after application of FPL 64176 are overlaid (right). (b) Overlay of male and female LV APs (before and after FPL application) and quantification of APD at indicated % of repolarization (n = 5 slices of N = 5 male mice, n = 5 slices of N = 5 female mice; APD50: p = 0.00077, APD80: p = 0.00623, APD90: p = 0.00899). Color codes for male (black), female (red), and after FPL 64176 application (blue), respectively.

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  FIGURE 4. Simulation of sex‐specific contribution of ionic currents on action potential duration upon L‐type calcium channel activation. (a) Simulation of APs upon a gradual shift in voltage‐dependence of ICaL activation: voltage of half‐maximum activation (V1/2) was shifted to negative potentials by up to 15 mV at four different conditions: control (male sex); IKto (−23%): 23% reduction in gKto; IKur (−21%): 21% reduction in gKur; INaL (+52%), 52% increase in rate constant (k, Figure 2c). Note the striking delay and heterogeneity in late repolarization, particularly under increasing INaL. (b) Calculations of APD80 upon the indicated shift in voltage‐dependence of activation of ICaL under corresponding sex‐specific current modifications. (c) APD80 alteration under indicated sex‐specific current modification with (gray) and without (white) a 15 mV shift in V1/2 of ICaL to negative potentials.