Proceedings of The Physiological Society

University of Oxford (2011) Proc Physiol Soc 23, PC34

Poster Communications

Slow desensitization of muscarinic K+ channel is associated with chronic vulnerability to atrial fibrillation

Z. Shui1,2, Y. Song1, H. Xiao1, M. Boyett1, J. Liu2, A. Luo2, K. Liu2, X. Yang2, H. Wang2, Y. Du2

1. Cardiovascular Research Group, School of Biomedicine, University of Manchester, Manchester, United Kingdom. 2. Departments of Cardiovascular Surgery and Cardiology, Union Hospital, Huazhong University of Science and Technology, Wuhan, China.

Background — Cholinergic stimulation (CS) and pulmonary venoatrial junctions (PVJ) play a key role in atrial fibrillation (AF) that is correlated with ageing. In contrast, cardiac vagal control decreases with age, and effects of continuous CS fade with time. Yet, their relationship and mechanism are unclear. Methods and Results — Extracellular potentials were record in the PVJ region and atria in rabbit heart perfused physiologically in vitro to study AF induced by ACh and/or electrical pacing. Whole cell current clamp and voltage clamp were employed to measure action potential (AP) and ion currents in single myocytes isolated from left atrium (LA) and PVJ. Single cell quantitative PCR was used to estimate the abundance of mRNAs for receptor and ion channels that were visualized and located by immunofluorescence. During 30 s application of 10 μM ACh, episodes of fibrillation wavelet (FW) were longer and peak FW was higher at the centre of PVJ (CPV) than in LA (CPV 23.5±5.1 vs. LA 8.9±2.7 s, n=9). The dominant frequency of ectopic activity (DFEA) was lower in LA and faded away with FW, whereas DFEA hid in noise at CPV after FW fadeaway. After 2 h ACh application and in the absence of ACh, long lasting FW was induced by burst pacing, and DFEA was higher at CPV than in LA (16.1±2.4 vs. 9.7±2.6 Hz, n=9). During 30 s ACh application, the effects of ACh on RP hyperpolarization and APD shortening and muscarinic K+ current (IK,ACh) in LA and PVJ myocytes declined (RP: LA 20.7±3.1 vs. PVJ 18.4±2.8%, n=15/9; APD: LA 15.7±2.1 vs. PVJ 11.4±1.2%, n=15/9; IK,ACh: LA 40.8±2.3 vs. PVJ 29.3±3.2%, n=38/76) as a result of fast desensitization to ACh, which were reversible following wash-off of ACh. After 2 h exposure to ACh, the effects of ACh on RP and APD and IK,ACh faded further (RP: LA 57.9±3.7 vs. PVJ 83.2±4.6%, n=11/7; APD: LA 31.3±5.1 vs. PVJ 46.8±3.4%, n=11/7; IK,ACh: LA 58.4±6.4 vs. PVJ 42.6±7.1%, n=22/33) as a result of slow desensitization, which were largely irreversible following ACh wash-off. The mean intensity of M2 receptor (M2R) labelling in the cell membrane was reduced or internalised (M2R: LA 82.8±6.7 vs. PVJ 71.3±5.1%, n=37/24). As compared with LA myocytes, in PVJ myocytes, IK1 and IK,ACh and the abundances of Kir2.1, M2R and Kir3.1/3.4 mRNAs were lower, and decrease in the effects of ACh on RP and APD and IK,ACh was smaller during the fast desensitization, but generally larger during the slow desensitization. Conclusions — The fast IK,ACh desensitization may reduce the effects of CS and prevent the prolongation of cholinergic AF, whereas the slow IK,ACh desensitization causes irreversible APD shortening and thus increase chronic vulnerability to AF even in the absence of CS. The PVJ region is more susceptible to CS.

Where applicable, experiments conform with Society ethical requirements