Activation of Small Conductance Ca2+-Activated K+ Channels Suppresses Electrical and Calcium Alternans in Atrial Myocytes

Figure 1. Effect of SK channel inhibition and activation on atrial APD. (A): (a) Overlay of APs recorded from the same atrial myocyte in control and after application of SK channel blocker apamin (100 nM) at 1 Hz stimulation. (b) Mean ± SD and individual APD70 and APD90 values in control (Ctrl) and in apamin (n/N = 15/10; paired t-test). (B): (a) Overlay of APs recorded from the same cell in control and after application of SK channel activator NS309 (2 µM). (b) Mean ± SD and individual APD70 and APD90 measurements in control and after application of NS309 (n/N = 6/2; paired t-test).

Figure 2. Effect of SK channel activation on CaT alternans. (A): CaTs recorded from the same atrial myocyte in control, in the presence of SK channel activator NS309 (2 µM) and after washout of NS309 (stimulated at 1.6 Hz). CaT alternans were induced by increasing field stimulation frequency. (B): Mean ± SD and individual cell CaT AR values in control during NS309 exposure (n/N = 17/6) and after washout in a subset of cells (n/N = 9/3). Statistical analysis was performed using Tukey’s mixed-effects groups comparison test.

Figure 3. Activation of SK channels abolishes APD alternans. (A): APs recorded from the same current-clamped atrial myocyte stimulated at 2 Hz in control and in the presence of SK channel activator NS309 (2 µM). (B): Mean ± SD and individual APD70 and APD90 ratios of pairs of alternating APs in control and in the presence of NS309 (n/N = 6/4). A dashed line marks a ratio of 1, indicating the absence of alternans. Statistical analysis was performed by paired t-test.

Figure 4. The inhibition of SK channels abolishes NS309’s effects on APD and CaT alternans. (A): (a) Overlay of APs recorded at 1 Hz from the same atrial myocyte in control, in apamin (100 nM), and in apamin together with NS309 (2 µM). (b) Changes in APD70 and APD90 compared to control (dashed lines) in the presence of NS309 alone (n/N = 15/10) and in the presence of NS309 + apamin (n/N = 9/5). Mean ± SD and individual cell measurements are shown. Cells were pretreated with apamin for 3 min before measurements. Statistical analysis was performed using the Welsh t-test. (B): (a) CaTs recorded from the same field stimulated (2.8 Hz) atrial myocytes in control, in the presence of apamin, during simultaneous application of NS309 and apamin and in NS309 alone. (b) Mean ± SD and individual CaT ARs recorded using the experimental protocol from panel Ba (n/N = 9/5); Tukey’s multiple group comparison test.

Figure 5. Effect of SK channel activation on CaT amplitude. (A): Overlay of CaTs recorded from the same field stimulated (0.5 Hz; no alternans) atrial myocyte in control (black), in NS309 (red), and after washout of NS309 (grey). (B): Mean ± SD and individual values of CaT amplitudes (ΔF/F0) recorded in control, in the presence of NS309 and during subsequent application of NS309 together with two different SK channel blockers: (a) apamin (n/N = 11/3) and (b) UCL1684 (1 µM; n/N = 9/3). Statistical analysis was performed using Tukey’s multiple-group comparison test. (C): Mean ± SD and individual values of CaT amplitudes observed in control and in the presence of 100 nM of apamin (n/N = 6/2), demonstrating that blocking SK channels had no effect on CaT amplitude. (D): (a) Caffeine (10 mM) induced CaTs recorded in the same atrial cell (stimulated at 0.5 Hz) before and after NS309 application, shown at different time scales. The top trace shows the decrease in CaT amplitude over time in the presence of NS309. (b) Mean ± SD and individual values of caffeine-induced CaT amplitudes (ΔF/F0) recorded in control and in the presence of NS309 (n/N = 8/3; paired t-test).

Figure 6. Activation of SK channels reduces the risk for CaT alternans in atrial myocytes with simulated LQT syndrome type-1. (A): (a) Overlay of APs recorded at 1 Hz from the same atrial myocyte in control and after application of Kv7.1 potassium channel blocker HMR1556 (1 µM) simulating LQT syndrome type-1 conditions. (b) Mean ± SD and individual APD70 and APD90 values recorded in control and in the presence of HMR1556 (n/N = 8/5; paired t-test). (B): (a) CaTs recorded from the same field stimulated (1.8 Hz) atrial myocyte in control, in HMR1556, and during simultaneous application of HMR1556 and NS309. (b) Mean ± SD and individual CaT AR values recorded in control, in the presence of HMR1556 (n/N = 14/8), followed by simultaneous application of HMR1556 and NS309 (n/N = 13/7). Statistical analysis with Tukey’s mixed-effects group comparison test.