Andrew C. Rankin

The contribution of ionic currents to changes in refractoriness of human atrial myocytes associated with chronic atrial fibrillation

OBJECTIVE To investigate changes in human atrial single cell functional electrophysiological properties associated with chronic atrial fibrillation (AF), and the contribution to these of accompanying ion current changes. METHODS The whole cell patch clamp technique was used to record action potentials, the effective refractory period (ERP) and ion currents, in the absence and presence of drugs, in enzymatically isolated myocytes from 11 patients with chronic (>6 months) AF and 39 patients in sinus rhythm. RESULTS Stimulation at high rates (up to 600 beats/min) markedly shortened late repolarisation and the ERP in cells from patients in sinus rhythm, and depolarised the maximum diastolic potential (MDP). Chronic AF was associated with a reduction in the ERP at physiological rate (from 203+/-16 to 104+/-15 ms, P<0.05), and marked attenuation in rate effects on the ERP and repolarisation. The abbreviated terminal phase of repolarisation prevented fast rate-induced depolarisation of the MDP in cells from patients with AF. The density of L-type Ca(2+) (I(CaL)) and transient outward K(+) (I(TO)) currents was significantly reduced in cells from patients with AF (by 60-65%), whilst the inward rectifier K(+) current (I(K1)) was increased, and the sustained outward current (I(KSUS)) was unaltered. Superfusion of cells from patients in sinus rhythm with nifedipine (10 micromol/l) moderately shortened repolarisation, but had no effect on the ERP (228+/-12 vs. 225+/-11 ms). 4-Aminopyridine (2 mmol/l) markedly prolonged repolarisation and the ERP (by 35%, P<0.05). However, the combination of these drugs had no effect on late repolarisation or refractoriness. CONCLUSION Chronic AF in humans is associated with attenuation in adaptation of the atrial single cell ERP and MDP to fast rates, which may not be explained fully by accompanying changes in I(CaL) and I(TO).

Cellular bases for human atrial fibrillation.

Atrial fibrillation (AF) causes substantial morbidity and mortality. It may be triggered and sustained by either reentrant or nonreentrant electrical activity. Human atrial cellular refractory period is shortened in chronic AF, likely aiding reentry. The ionic and molecular mechanisms are not fully understood and may include increased inward rectifier K(+) current and altered Ca(2+) handling. Heart failure, a major cause of AF, may involve arrhythmogenic atrial electrical remodeling, but the pattern is unclear in humans. Beta-blocker therapy prolongs atrial cell refractory period; a potentially antiarrhythmic influence, but the ionic and molecular mechanisms are unclear. The search for drugs to suppress AF without causing ventricular arrhythmias has been aided by basic studies of cellular mechanisms of AF. It remains to be seen whether such drugs will improve patient treatment.

Adenosine or adenosine triphosphate forsupraventricular tachycardias? Comparative double-blind randomized study in patients with spontaneous or inducible arrhythmias

The effects of intravenous adenosine and adenosine triphosphate (ATP) were studied in a double-blind randomized study during 68 episodes of supraventricular tachycardia in 39 patients. Adenosine restored sinus rhythm in 20 patients (25 of 27 episodes) and produced atrioventricular block to reveal atrial arrhythmias in nine. ATP restored sinus rhythm in 17 patients (22 of 25 episodes) and revealed atrial tachyarrhythmias in six. In patients receiving both compounds, the effective dosage of adenosine was 3.8 mg and of ATP it was 6.6 mg (p less than 0.05), suggesting molar equipotency. Transient side effects were common, occurring in 81% of episodes with adenosine and in 94% with ATP. Symptom scores (0 to 10) were not significantly different (median score for adenosine was 5, for ATP it was 6). Adenosine and ATP were equally effective for the diagnosis and treatment of supraventricular tachycardias and the incidence and severity of side effects were similar. Adenosine has the advantage of being more stable.

Nitric oxide mediates the anti-adrenergic effect of adenosine on calcium current in isolated rabbit atrioventricular nodal cells

The aim of this study was to determine if adenosine exerts an anti-adrenergic effect on rabbit isolated atrioventricular (AV) nodal cells and, if so, the dependence of this effect on nitric oxide (NO) production. Inward Ca current,ICa, was measured in AV nodal cells, enzymatically isolated from rabbit hearts. Isoprenaline (0.1 μM) increasedICa from 676 ± 59 to 1102 ± 86 pA (n = 25). This isoprenaline-induced increase inICa, (178 ± 15 % of control) was abolished in the presence of 10 μM adenosine (ICa 100 ± 2 % of control,n = 9, P < 0.05). This effect of adenosine was completely blocked by the A1 receptor antagonist CPDPX (8-cyclopentyl 1, 3-dipropylxanthine, 0.1 μM). In cells pre-treated with the NO synthase inhibitor,l-nitro-arginine methyl ester (l-NAME, 1 mM) the isoprenaline-induced increase inICa(208 ± 39 % of control,n = 7) was not reduced by the addition of 10 μM adenosine (195 ± 32% of control). Co-incubation of cells inl-NAME withl--arginine (1 mM, the endogenous substrate of NO synthase) restored the adenosine-induced attenuation ofICa. In these cells, isoprenaline increasedICa (157 ± 7% of control,n = 6), and, following addition of adenosine (10 μM)ICa was reduced to 107 ± 8% (P < 0.05). The NO-releasing agent SIN-1 (3-morpholino-sydnonimine, 100 μM) inhibitedICa augmented by isoprenaline (n = 5). It is concluded that adenosine exerts an anti-adrenergic effect on the AV node via A, receptors to attenuate a catecholamine-stimulated increase inICa and that this action involves the intracellular production of NO.

Atrial cellular electrophysiological changes in patients with ventricular dysfunction may predispose to AF

BACKGROUND Left ventricular systolic dysfunction (LVSD) is a risk factor for atrial fibrillation (AF), but the atrial cellular electrophysiological mechanisms in humans are unclear. OBJECTIVE This study sought to investigate whether LVSD in patients who are in sinus rhythm (SR) is associated with atrial cellular electrophysiological changes that could predispose to AF. METHODS Right atrial myocytes were obtained from 214 consenting patients in SR who were undergoing cardiac surgery. Action potentials or ion currents were measured using the whole-cell-patch clamp technique. RESULTS The presence of moderate or severe LVSD was associated with a shortened atrial cellular effective refractory period (ERP) (209 +/- 8 ms; 52 cells, 18 patients vs 233 +/- 7 ms; 134 cells, 49 patients; P <0.05); confirmed by multiple linear regression analysis. The left ventricular ejection fraction (LVEF) was markedly lower in patients with moderate or severe LVSD (36% +/- 4%, n = 15) than in those without LVSD (62% +/- 2%, n = 31; P <0.05). In cells from patients with LVEF <or= 45%, the ERP and action potential duration at 90% repolarization were shorter than in those from patients with LVEF > 45%, by 24% and 18%, respectively. The LVEF and ERP were positively correlated (r = 0.65, P <0.05). The L-type calcium ion current, inward rectifier potassium ion current, and sustained outward ion current were unaffected by LVSD. The transient outward potassium ion current was decreased by 34%, with a positive shift in its activation voltage, and no change in its decay kinetics. CONCLUSION LVSD in patients in SR is independently associated with a shortening of the atrial cellular ERP, which may be expected to contribute to a predisposition to AF.

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

OBJECTIVE To assess the contribution of the Na, K pump current (I(p)) to the action potential duration (APD) and effective refractory period (ERP) in human atrial cells, and to investigate whether I(p) contributes to the changes in APD and ERP associated with chronic atrial fibrillation (AF). METHODS 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). RESULTS In cells from patients in SR, the I(p) blocker, ouabain (10 microM) significantly depolarised the membrane potential, V(m), 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, Students t-test, n=7 cells, 5 patients). With an elevated pipette [Na(+)] of 30 mM, I(p) was measured by increasing extracellular [K(+)] ([K(+)](o)) from 0 to 5.4 mM. This produced an outward shift in holding current at -40 mV, abolished by 10 microM 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; n=9 cells), confirming the K(+)-induced current as I(p). I(p) increased linearly with increasing V(m) between -120 and +60 mV (n=25 cells). Stepwise increments in [K(+)](o) (between 0 and 10 mM) increased I(p) in a concentration-dependent manner (maximum response, E(max)=1.19+/-0.09 pA/pF; EC(50)=1.71+/-0.15 mM; n=27 cells, 9 patients). In cells from patients in AF, the sensitivity of I(p) to both V(m) and [K(+)](o) (E(max)=1.02+/-0.05 pA/pF, EC(50)=1.54+/-0.11 mM; n=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 (n=5 patients) was associated with a small, but significant, reduction in E(max) (0.92+/-0.07 pA/pF) and EC(50) (1.35+/-0.15 mM) compared with non-treatment (E(max)=1.13+/-0.08 pA/pF, EC(50)=1.76+/-0.14 mM; P<0.05 for each, n=4 patients). In cells from non-digoxin-treated patients in AF, the voltage- and [K(+)](o)-sensitivity (E(max) and EC(50)) were similar to those in cells from patients in SR. CONCLUSIONS The Na, K pump current contributes to the human atrial cell V(m), action potential shape and ERP. However, the similarity in I(p) sensitivity to both [K(+)](o) and V(m) between atrial cells from patients with and without chronic AF indicates that I(p) is not involved in AF-induced electrophysiological remodelling in patients.

Ionic basis of a differential effect of adenosine on refractoriness in rabbit AV nodal and atrial isolated myocytes

OBJECTIVES Firstly, to compare effects of adenosine on membrane potential and refractoriness in AV nodal and atrial cells. Secondly, to assess the contribution of the effects of adenosine on IKAdo and ICaL to its effects on the functional electrophysiological properties in the two cell types. METHODS The whole cell patch clamp technique was used to record action potentials and ion currents in AV nodal and left atrial myocytes isolated enzymatically from rabbit hearts. RESULTS Adenosine (10 microM) caused similar hyperpolarisation and shortening of the action potential duration (APD) in both cell types: maximum diastolic potential was hyperpolarised from -59 +/- 3 to -66 +/- 2 and from -70 +/- 2 to -76 +/- 2 mV (mean +/- SEM) and APD90 was shortened by 31 +/- 4 and 30 +/- 7% in AV nodal (n = 14) and atrial cells (n = 8), respectively. Adenosine shortened the effective refractory period (ERP) in atrial cells, from 124 +/- 15 to 98 +/- 14 ms (n = 8). In contrast, ERP in AV nodal cells was not significantly affected (112 +/- 13 vs. 102 +/- 12 ms, n = 14), and post-repolarization refractoriness was prolonged. By contrast, current injection, to induce an equal degree of hyperpolarisation to that produced by adenosine, shortened APD and ERP in both cell types, suggesting an additional action of adenosine in AV nodal cells. Adenosine (10 microM) did not affect peak ICaL in AV nodal cells, but significantly altered the biexponential time course of recovery of ICaL from inactivation. The proportion of recovery in the fast phase (time constant, tau = 102 +/- 10 ms) was reduced from 71 +/- 3 to 55 +/- 5%, with shift to the slow phase (tau = 858 +/- 168 ms), without altering tau in either phase. A similar effect of adenosine was seen in left atrial cells. CONCLUSION Adenosine caused hyperpolarisation, APD-shortening and slowing of recovery of ICaL from inactivation, in both AV nodal and atrial cells, but prolonged post-repolarisation refractoriness in AV nodal cells only. This differential effect of adenosine on refractoriness in the two cell types could not be explained by effects on IKAdo, but may be due to slowed reactivation of ICaL, which is the predominant inward current in AV nodal but not left atrial cells.

Adenosine increases potassium conductance in isolated rabbit atrioventricular nodal myocytes

OBJECTIVE To study the actions of adenosine on the electrophysiology of spontaneously active, rod-shaped cells enzymatically isolated from rabbit atrioventricular (AV) node. METHODS Calcium-tolerant myocytes were isolated from the region of the AV node by enzymatic and mechanical dispersion. They were rod- or spindle-shaped, with spontaneous activity at 35-37 degrees C, and had higher membrane resistances (776 +/- 283 M omega, n = 13), compared to atrial cells (41 +/- 18.2 M omega, n = 7; P < 0.001). Membrane potential, spontaneous action potentials and transmembrane ionic currents were studied using the whole-cell patch-clamp technique, in current-clamp and voltage-clamp mode. RESULTS Adenosine (0.1-50 microM) slowed or abolished the spontaneous activity, with hyperpolarisation of the membrane potential. Voltage-clamp experiments showed that adenosine induced an inwardly rectifying time-independent current. The adenosine-induced current was shown to be carried by potassium ions by the effect of increasing external potassium, which altered the reversal potential in accordance with the calculated potassium equilibrium potential. The A1 adenosine receptor antagonist, CPDPX (8-cyclopentyl-1,3-dypropylxanthine), reversed the effects of adenosine and an A1 receptor agonist, R-PIA [R(-)N(6)-(2-phenylisopropyl)adenosine] had effects similar to adenosine. Adenosine also caused a small decrease in inward calcium current (ICa) in some AV nodal cells. CONCLUSIONS These results indicate that adenosine acts at A1 adenosine receptors to suppress spontaneous activity, hyperpolarise membrane potential and induce a time-independent potassium current in AV nodal cells. These actions, combined with reduction in inward calcium current in some cells, may underlie the negative chronotropic and dromotropic actions of adenosine on rabbit AV nodal cells.

Post-operative atrial fibrillation is influenced by beta-blocker therapy but not by pre-operative atrial cellular electrophysiology

Introduction: We investigated whether post‐cardiac surgery (CS) new‐onset atrial fibrillation (AF) is predicted by pre‐CS atrial cellular electrophysiology, and whether the antiarrhythmic effect of beta‐blocker therapy may involve pre‐CS pharmacological remodeling.

Electrophysiological effects of 5‐hydroxytryptamine on isolated human atrial myocytes, and the influence of chronic β‐adrenoceptor blockade

5‐Hydroxytryptamine (5‐HT) has been postulated to play a proarrhythmic role in the human atria via stimulation of 5‐HT4 receptors. The aims of this study were to examine the effects of 5‐HT on the L‐type Ca2+ current (ICaL) action potential duration (APD), the effective refractory period (ERP) and arrhythmic activity in human atrial cells, and to assess the effects of prior treatment with β‐adrenoceptor antagonists. Isolated myocytes, from the right atrial appendage of 27 consenting patients undergoing cardiac surgery who were in sinus rhythm, were studied using the whole‐cell perforated patch‐clamp technique at 37°C. 5‐HT (1 nM–10 μM) caused a concentration‐dependent increase in ICaL, which was potentiated in cells from β‐blocked (maximum response to 5‐HT, Emax=299±12% increase above control) compared to non‐β‐blocked patients (Emax=220±6%, P<0.05), but with no change in either the potency (log EC50: −7.09±0.07 vs −7.26±0.06) or Hill coefficient (nH: 1.5±0.6 vs 1.5±0.3) of the 5‐HT concentration–response curve. 5‐HT (10 μM) produced a greater increase in the APD at 50% repolarisation (APD50) in cells from β‐blocked patients (of 37±10 ms, i.e. 589±197%) vs non‐β‐blocked patients (of 10±4 ms, i.e. 157±54%; P<0.05). Both the APD90 and the ERP were unaffected by 5‐HT. Arrhythmic activity was observed in response to 5‐HT in five of 17 cells (29%) studied from β‐blocked, compared to zero of 16 cells from the non‐β‐blocked patients (P<0.05). In summary, the 5‐HT‐induced increase in calcium current was associated with a prolonged early plateau phase of repolarisation, but not late repolarisation or refractoriness, and the enhancement of these effects by chronic β‐adrenoceptor blockade was associated with arrhythmic potential.