HCN4 is equally stabilized by the open pore blocker Ivabradine and by Cesium. Relative fluorescence intensity (FI) for GFP-HCN4 protein samples heated for 10 min at 20°C (control: gray, 100%) and the same protein heated at 55°C in the absence (black, 16.2 ± 1%) or presence of 0.5 mM Ivabradine (green, 25.5 ± 0.8%) or 50 mM CsCl (blue, 23.1 ± 0.4%). Normalized FI values are mean of n ¼3 SEM. Statistical analysis performed with one-way ANOVA, followed by Fisher’s test (**P < 0.001).