Remodeling of atrial ATP-sensitive K⁺ channels in a model of salt-induced elevated blood pressure

Abstract

Hypertension is associated with the development of atrial fibrillation; however, the electrophysiological consequences of this condition remain poorly understood. ATP-sensitive K(+) (K(ATP)) channels, which contribute to ventricular arrhythmias, are also expressed in the atria. We hypothesized that salt-induced elevated blood pressure (BP) leads to atrial K(ATP) channel activation and increased arrhythmia inducibility. Elevated BP was induced in mice with a high-salt diet (HS) for 4 wk. High-resolution optical mapping was used to measure atrial arrhythmia inducibility, effective refractory period (ERP), and action potential duration at 90% repolarization (APD(90)). Excised patch clamping was performed to quantify K(ATP) channel properties and density. K(ATP) channel protein expression was also evaluated. Atrial arrhythmia inducibility was 22% higher in HS hearts compared with control hearts. ERP and APD(90) were significantly shorter in the right atrial appendage and left atrial appendage of HS hearts compared with control hearts. Perfusion with 1 μM glibenclamide or 300 μM tolbutamide significantly decreased arrhythmia inducibility and prolonged APD(90) in HS hearts compared with untreated HS hearts. K(ATP) channel density was 156% higher in myocytes isolated from HS animals compared with control animals. Sulfonylurea receptor 1 protein expression was increased in the left atrial appendage and right atrial appendage of HS animals (415% and 372% of NS animals, respectively). In conclusion, K(ATP) channel activation provides a mechanistic link between salt-induced elevated BP and increased atrial arrhythmia inducibility. The findings of this study have important implications for the treatment and prevention of atrial arrhythmias in the setting of hypertensive heart disease and may lead to new therapeutic approaches.

Fig. 1.

Increased left atrial (LA) fibrosis in high-salt diet-fed (HS) animals. A and B: trichrome-stained sections of hearts from normal-salt diet-fed (NS; A) and HS (B) animals. Bar = 1 mm. RA, right atrium; LV, left ventricle; RV, right ventricle. C and D: high-magnification images of the boxed regions indicated in A and B, respectively. Bar = 0.1 mm. E: average fibrosis values for LA and RA free walls and appendages (LAA and RAA, respectively). Numbers in bars indicate numbers of animals in each group. *P < 0.05.

Fig. 2.

Altered electrophysiological characteristics in vivo. A and B: representative surface (ECG), intra-atrial (A), and intraventricular (V) electrograms obtained from NS (A) and HS (B) animals. The shaded bar indicates when the programmed stimulation was delivered. C: average LA and RA effective refractory period (ERP) values. Numbers in bars indicate numbers of animals in each group. *P < 0.05.

Fig. 3.

Increased atrial arrhythmia dominant frequencies (DFs). A and B: voltage maps showing RAA activity recorded during a sustained atrial arrhythmia in NS (A) and HS (B) hearts. Arrows indicate the locations of the pivot points for reentrant activation. Bar = 1 mm. C: power spectra of the atrial arrhythmias shown in A and B. D and E: DF maps for the arrhythmias shown in A and B, respectively. Bar = 2.5 mm. F: average atrial DFs in NS and HS hearts. Numbers in bars indicate numbers of animals in each group. *P < 0.05.

Fig. 4.

Decreased atrial ERP. A and B: representative activation maps from the LAA (A) and RAA (B) of a NS heart. C and D: representative activation maps from the LAA (C) and RAA (D) of a HS heart. E and F: average conduction velocity (CV; E) and ERP (F) values. Numbers in bars indicate numbers of animals in each group. Bar = 1 mm. *P < 0.05.

Fig. 5.

Decreased atrial action potential (AP) duration (APD). A and B: representative LAA (A) and RAA (B) single-pixel optical AP traces obtained at an S2 coupling interval of 100 ms. C and D: representative LAA (C) and RAA (D) single-pixel optical AP traces obtained at an S2 coupling interval of 25 ms. E and F: average LAA (E) and RAA (F) APD at 90% repolarization (APD₉₀) values obtained at an S2 coupling interval of 100 ms. G and H: average LAA (G) and RAA (H) APD₉₀ values obtained at an S2 coupling interval of 25 ms. Numbers in bars indicate numbers of animals in each group. *P < 0.05.

Fig. 6.

Glibenclamide and tolbutamide increase APD. A and B: average LAA (A) and RAA (B) APD₉₀ values obtained at an S2 coupling interval of 100 ms. C and D: average LAA (C) and RAA (D) APD₉₀ values obtained at an S2 coupling interval of 25 ms, respectively. HS-GLY, HS group perfused with 1 μM glibenclamide; HS-TOLB, HS group perfused with 300 μM tolbutamide. The dashed lines indicate untreated NS average APD₉₀ values. Numbers in bars indicate numbers of animals in each group. *P < 0.05.

Fig. 7.

Increased atrial ATP-sensitive K⁺ (K_ATP) channel density. A: representative single channel currents from NS and HS atrial myocytes recorded during a slow ramp (−100 to 100 mV). Inset, representative single channel recording and activity histogram. B: average current (I)-voltage (V) curve. C: representative responses of excised patches to decreasing concentrations of ATP. D: average responses to varying concentrations of ATP. E: average numbers of K_ATP channels per patch. Numbers in bars indicate numbers of animals in each group. *P < 0.05.

Fig. 8.

Elevated sulfonylurea receptor 1 (SUR1) protein levels. A and B: representative immunoblot (A) and average SUR1 expression levels (B). N-cadherin (N-Cad) was used as a loading control. Protein expression levels were normalized to the NS group for each chamber. C and D: representative immunoblot (C) and average Kir6.2 expression levels (D). Numbers in bars indicate numbers of animals in each group. *P < 0.05.