Characterisation of the Na, K pump current in atrial cells from patients with and without chronic atrial fibrillation

Abstract

<b>Objective:</b> To assess the contribution of the Na, K pump current (<i>I</i><sub>p</sub>) to the action potential duration (APD) and effective refractory period (ERP) in human atrial cells, and to investigate whether <i>I</i><sub>p</sub> contributes to the changes in APD and ERP associated with chronic atrial fibrillation (AF). <b>Methods:</b> Action potentials and ion currents were recorded by whole-cell patch clamp in atrial myocytes isolated from consenting patients undergoing cardiac surgery, who were in sinus rhythm (SR) or AF (>3 months). <b>Results:</b> In cells from patients in SR, the <i>I</i><sub>p</sub> blocker, ouabain (10 μM) significantly depolarised the membrane potential, Vm, from -80±2 (mean±S.E.) to -73±2 mV, and lengthened both the APD (174±17 vs. 197±23 ms at 90% repolarisation) and ERP (198±22 vs. 266±14 ms; P<0.05 for each, Student's t-test, <i>n</i>=7 cells, 5 patients). With an elevated pipette [Na<sup>+</sup>] of 30 mM, <i>I</i><sub>p</sub> was measured by increasing extracellular [K<sup>+</sup>] ([K<sup>+</sup>]o) from 0 to 5.4 mM. This produced an outward shift in holding current at -40 mV, abolished by 10 muM ouabain. K± and ouabain-sensitive current densities were similar, at 0.99±0.13 and 1.12±0.11 pA/pF, respectively (P>0.05; <i>n</i>=9 cells), confirming the K±induced current as <i>I</i><sub>p</sub>. <i>I</i><sub>p</sub> increased linearly with increasing Vm between -120 and +60 mV (<i>n</i>=25 cells). Stepwise increments in [K<sup>+</sup>]<sub>o</sub> (between 0 and 10 mM) increased Ip in a concentration-dependent manner (maximum response, <i>E</i><sub>max</sub>=1.19±0.09 pA/pF; EC50=1.71±0.15 mM; n=27 cells, 9 patients). In cells from patients in AF, the sensitivity of Ip to both Vm and [K+]o (<i>E</i><sub>max</sub>=1.02±0.05 pA/pF, EC50=1.54±0.11 mM; <i>n</i>=44 cells, 9 patients) was not significantly different from that in cells from patients in SR. Within the group of patients in AF, long-term digoxin therapy (<i>n</i>=5 patients) was associated with a small, but significant, reduction in <i>E</i><sub>max</sub> (0.92±0.07 pA/pF) and EC<sub>50</sub> (1.35±0.15 mM) compared with non-treatment (<i>E</i><sub>max</sub>=1.13±0.08 pA/pF, EC<sub>50</sub>=1.76±0.14 mM; P<0.05 for each, <i>n</i>=4 patients). In cells from non-digoxin-treated patients in AF, the voltage- and [K<sup>+</sup>]<sub>o</sub>-sensitivity (<i>E</i><sub>max</sub> and EC<sub>50</sub>) were similar to those in cells from patients in SR. <b>Conclusions:</b> The Na, K pump current contributes to the human atrial cell Vm, action potential shape and ERP. However, the similarity in Ip sensitivity to both [K<sup>+</sup>]<sub>o</sub> and <i>V</i><sub>m</sub> between atrial cells from patients with and without chronic AF indicates that <i>I</i><sub>p</sub> is not involved in AF-induced electrophysiological remodelling in patients