Characteristics of IKAch in rat atrioventricular nodal myocytes.

Nicholas Gourd, Kathryn Yuill. University of Plymouth, Plymouth, UK

The atrioventricular node (AVN), is the primary conduction pathway from the atria to the ventricles, and is essential for normal cardiac function. The region displays unique properties, co-ordinating conduction between atria and ventricles, therefore timing ventricular contraction. The AVN also displays intrinsic spontaneous electrical activity, acting as a secondary pacemaker during sinoatrial node dysfunction. The AVN region is richly innervated, and changes in AVN dromotropy are mediated by vagal stimulation and release of acetylcholine. This muscarinic activation modifies AVN electrical activity via activation of an inwardly rectifying potassium conductance (IKAch), causing membrane hyperpolarisation (Hancox et al, 1993; Nishimura et al, 1988). Characterisation of such channels opens up opportunities for possible pharmacological intervention, and IKAch has been postulated as an atrial selective drug target for treatment of atrial fibrillation (Erlich & Nattel 2009). At the present time, activation of IKAch has only been observed in rabbit AVN preparations (Choisy et al 2012; Nishimura 1988). However, as rat tissue is used widely to assess changes in cardiac ion channel pharmacology, the intention of this study was to investigate IKAch in rat AVN myocytes.

Individual AVN myocytes were isolated from the hearts of male Wistar rats (<200g), killed via cervical dislocation in accordance with Home Office guidelines and local ethical approval. Using a modified Llangendorff apparatus, hearts were retrogradely perfused with an enzymatic solution, and the AVN region was excised and cells isolated using a combination of mechanical and enzymatic dispersion (Yuill et al 2010). All recordings were carried out at 37°C, using the whole cell patch mode of the voltage clamp technique, using standard physiological recording solutions.

A voltage ramp protocol was used routinely to elicit IKAch, and a dose dependent activation of IKAch was observed. Upon application of 100nM Ach (-60mV, 10.96 ± 2.72 pA/pF) an outward potassium conductance was activated, which was significantly greater than control (-60mV, 1.78 ± 0.37 pA/pF) at all membrane voltages tested (n=7; ≤0.019). The current activated by 100nM Ach, was inhibited in the presence of tertiapin-Q (100 nM). During maintained Ach application, the current amplitude declined over time, indicating a desensitisation of the observed response.

This study confirms that in rat AVN myocytes, application of Ach (mimicking parasympathetic nerve stimulation) has a negative dromotropic action, via activation of a Tertiapin sensitive potassium current IKAch, which also exhibited properties of desensitisation. These findings are consistent with those observed in rabbit AVN (Choisy et al 2012). Further characterisation of IKACh is required in native cardiac tissue, if this is to be a future therapeutic drug target.

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