Small conductance calcium activated K+ channel inhibitor decreases stretch induced vulnerability to atrial fibrillation

Abstract

Background: Atrial dilation is an important risk factor for atrial fibrillation (AF) and animal studies have found that acute atrial dilation shortens the atrial effective refractory period (AERP) and increases the risk of AF. Stretch activated ion channels (SACs) and calcium channels play a role in this. The expression profile and calcium dependent activation makes the small conductance calcium activated K⁺ channel (K_Ca2.x) a candidate for coupling stretch induced increases in intracellular calcium through K⁺-efflux and thereby shortening of atrial refractoriness.

Objectives: We hypothesized that K_Ca2.x channel inhibitors can prevent the stretch induced shortening of AERP and protect the heart from AF.

Methods: The effect of K_Ca2 channel inhibitor (N-(pyridin-2-yl)-4-(pyridin-2-yl)thiazol-2-amine (ICA) 1 µM) was investigated using the isolated perfused rabbit heart preparation. To stretch the left atrium (LA) a balloon was inserted and inflated. AERP and action potential duration (APD) were recorded before and after atrial stretch. AF was induced by burst pacing the LA at different degrees of atrial stretch.

Results: Stretching of the LA by increasing the balloon pressure from 0 to 20 mmHg shortened the AERP by 8.6 ± 1 ms. In comparison, the K_Ca2 inhibitor ICA significantly attenuated the stretch induced shortening of AERP to 2.5 ± 1.1 ms. Total AF duration increased linearly with atrial balloon pressure. This relationship was not found in the presence of ICA. ICA lowered the incidence of AF induction and total AF duration.

Conclusion: The K_Ca2 channel inhibitor ICA attenuates the acute stretch induced shortening of AERP and decreases stretch induced vulnerability to AF.

Keywords: Atrial fibrillation; Ion channel; KCa2; Pharmacology; SK channel; Stretch.

Fig. 1

Graphical representation of the protocols used for the two studies, and picture of the preparation. mmHg refers to LA balloon pressure. Please note that the atrial stretch was held for 5 min before AERP or burst pacing commenced. For study 1n = 22; study 2n = 13.

Fig. 2

Effect of atrial stretch on atrial refractoriness. Time plot of the changes in AERP during the study before and after stretch in the vehicle control group (A; black) and treatment group (ICA 1 µM) (B; red). Data are presented as mean ± SEM, n = 11. Multiple comparisons tests were performed. For simplicity only the comparisons: baseline to 30 min; baseline to 0 mmHg, 0 mmHg to 20 mmHg; 20 mmHg to 0 mmHg are shown. mmHg refers to LA balloon pressure. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 3

Effect of atrial stretch and ICA on action potential durations. (A,B) Effects of intervention on atrial APD₉₀ (A,B), n = 11; and ventricular APD₉₀ recorded at pacing basic cycle length of 200 ms (C,D) n = 9 and 10. Data are presented as mean ± SEM. Vehicle control group at top (black) and treatment group (ICA 1 µM) at bottom (red). Multiple comparisons tests were performed. For simplicity only the comparisons: baseline to 30 min; 0 mmHg to 20 mmHg; 20 mmHg to 0 mmHg are shown. mmHg refers to LA balloon pressure. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 4

Number of sustained AF episodes. Accumulative number of sustained AF (AF > 2 min) episodes during the 5 atrial stretch levels before (black) and after (red) perfusion with ICA 1 µM. n = 13. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 5

Individual AF durations. Individual AF durations (>2 s) induced by atrial burst pacing before (A, black) and after perfusion with ICA 1 µM (B, red) at 5 different stretch levels. Top panel shows example of atrial action potential recordings before, during and after burst. Data are presented as box and whiskers with median and 25–75 percentile, n = 13. Note that the Y-axis is logarithmic. mmHg refers to LA balloon pressure. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 6

Incidence of AF. Incidence (%) of bursts resulting in AF > 2 s. Dots represents the incidence for each animal at each level of balloon pressure. A) before (black) and B) after perfusion with ICA 1 µM (red). Data are presented as box and whiskers with median and 25–75 percentile, n = 13. mmHg refers to LA balloon pressure. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 7

Frequency distribution of AF episode durations. Relative frequencies of the AF (>2 s) episodes before (A, black) and after ICA 1 µM (B, red). X-axis is divided in 2 s bins, except for the last bin which includes all AF episodes longer than 10 s. n = 13. mmHg refers to LA balloon pressure. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 8

AF duration increases with stretch. Total AF duration recorded at the different stretch levels before (black) and after ICA 1 µM (red). A positive linear correlation was found for the effect of stretching the left atrium and the AF duration (insert formula) at baseline. The two linear regressions (stretch and AF duration (baseline) vs stretch and AF duration (ICA) are significantly different. n = 13. mmHg refers to LA balloon pressure. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)