Fig. 4: The GABAergic system controls electrical conduction within the AVN.

a Photograph of isolated rat AVN tissue and optical mapping of the field of view. The pacing electrode is marked with a blue oval. The black box shows the electrical signal mapping field. The yellow triangle shows the position of the Koch triangle. Anatomical landmarks are annotated. FO fossa ovalis, TV tricuspid valve, TT tendon of Todaro, CS coronary sinus, RA right atrium, IVC inferior vena cava, AO aorta. Scale bar, 2 mm. b Representative activation map showing the electrical activation and conduction in isolated rat AVN preparations after perfusion with the solvent control DMSO, the GABA_AR-specific agonist Afloqualone (320 μM), the GABA reuptake inhibitor Tiagabine (64 μM) or the GAT-1 inhibitor SKF89976A (128 μM). The activation map of zoomed-in images shows the electrical activation and conduction in the compact node of the AVN. Electrical activity in rat AVN preparations was recorded by optical mapping using the fluorescent dye Di-4-ANBDQBS. The activation time, conduction velocity, and vector maps were obtained during continuous electrode pacing located at the crista terminalis (5 Hz, 2 V). c Quantification of electrical conduction time within the AVN in distinct groups (n = 6 samples for control group, n = 7 samples for Afloqualone-treated group, n = 9 samples for Tiagabine-treated group, n = 8 samples for SKF89976A-treated group). Data are shown as means ± SD. P values were calculated by one-way ANOVA with Dunnett’s multiple comparison test. d, g, j Representative ECG recordings of perfused rat hearts treated with different concentrations of Afloqualone (0–640 μM) (d), Tiagabine (0–64 μM) (g) or SKF89976A (0–128 μM) (j) under the right atrial pacing (6 Hz). The arrows point to the typical ECG of second-degree type I AV block (d, g) and second-degree AV block (2:1) (j). Scale bars are 100 ms (horizontal) and 2 mV (vertical). Stim stimulation. e, h, k The dose-response of Afloqualone (e), Tiagabine (h) and SKF89976A (k) on PR intervals in perfused rat hearts. n = 5 hearts for Afloqualone and SKF89976A treatment group, n = 6 hearts for Tiagabine-treated group. Data are shown as means ± SD. P values were calculated using one-way ANOVA with Dunnett’s multiple comparison test. f, i, l Concentration-response curves of PR interval alterations induced by Afloqualone (EC₅₀, 165.70 μM) (f), Tiagabine (IC₅₀, 20.22 μM) (i), and SKF89976A (IC₅₀, 37.57 μM) (l). Concentration-response curves were fitted to the Hill equation using nonlinear regression and normalized to the maximal changed PR intervals. Data are shown as means ± SD. n = 5 hearts for Afloqualone- and SKF89976A-treated group, n = 6 hearts for Tiagabine-treated group.