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. Author manuscript; available in PMC: 2014 Apr 9.
Published in final edited form as: Acta Physiol (Oxf). 2012 Sep 14;207(2):280–289. doi: 10.1111/j.1748-1716.2012.02481.x

Potassium channel activators differentially modulate the effect of sodium channel blockade on cardiac conduction

Rengasayee Veeraraghavan 1, Anders Peter Larsen 1, Natalia S Torres 1, Morten Grunnet 2, Steven Poelzing 1
PMCID: PMC3980511  NIHMSID: NIHMS560607  PMID: 22913299

Introduction

Loss of potassium channel function leads to a compromised repolarization reserve and contributes to the arrhythmogenic substrate in several pathophysiological states including the long QT syndromes (LQTS). (1;2) The cause of reduced K+ current can vary between genetic mutations in ion channel proteins (inherited LQTS), pathophysiological remodeling and K+ channel inhibition by drugs (acquired LQTS); however, it consistently leads to prolongation of action potential duration (APD) and increased dispersion of repolarization.(1;2) These in turn provide a substrate for potentially lethal reentrant arrhythmias. (3;4)

Recent studies have suggested augmenting the repolarization reserve using pharmacological K+ channel activators as a potential therapy for LQTS. Evidence to support this notion comes primarily from experimental observations that these drugs mitigate action potential duration (APD) prolongation and dispersion of repolarization. (5-7) However, these drugs have yet to be evaluated for other possible pro-arrhythmic effects such as conduction slowing.

We previously demonstrated that pharmacological modulation of the inward rectifier K+ current (IK1) (8) as well as the rapid delayed rectifier K+ current (IKr) (9) can affect cardiac conduction in a cardiac sodium current (INa)-dependent manner. Further, modulation of IK1 only affected conduction when sodium channel availability was not reduced(8) whereas modulation of IKr preferentially affected conduction when conduction was already compromised. (9) Here we tested the hypothesis that pharmacological activation of the slow delayed rectifier K+ current (IKs), a K+ current carried by voltage-gated K+ channels, (9;10) will affect cardiac conduction and its dependence on sodium channel blockade differently from activation of the ATP-sensitive K+ current (IKATP), a K+ current carried by non-voltage-gated, inwardly rectifying channels(9;10).

We demonstrate that augmentation of both IKATP and IKs slows conduction; however, where IKATP activation only had this effect in the absence of sodium channel blockade, IKs activation slowed conduction regardless of sodium channel blockade. These data suggest that K+ channel activators differently modulate cardiac conduction vis-à-vis its dependence on INa. Further, these differences may stem from whether the K+ current being activated is carried by voltage-gated or non-voltage-gated channels. Therefore, K+ channel activators must be evaluated for potential effects on cardiac conduction in order to ensure their safety in a therapeutic context.

METHODS

The investigation conforms to the Guide for the Care and Use of Laboratory Animals published by the US National Institutes of Health (NIH Publication No. 85-23, revised 1996). Our protocols are in accordance with AVMA guidelines and approved by the University of Utah Animal Welfare Review Board.

Guinea Pig Langendorff Preparations

Adult male guinea pig breeders (800-1000g) were anesthetized (200 mg/kg pentobarbital sodium [Nembutal] IP; a dose sufficient for euthanasia), and their hearts were rapidly excised. Depth of anesthesia was monitored via corneal reflex and pedal withdrawal reflex. Isolated ventricles were perfused (at 40-50 mm Hg) in Langendorff configuration with oxygenated Tyrode's solution (containing, in mMol/l, CaCl2 1.25, NaCl 140, KCl 4.5, dextrose 5.5, MgCl2 0.7, HEPES 10; pH 7.4) at 36.5 ± 0.5 °C as previously described. (8;11) 2,3-butanedione monoxime (7.5 mM; BDM) was added to the perfusate for optical mapping experiments only. Hearts were stimulated via a unipolar silver electrode placed on the anterior epicardial surface at the center of the mapping field at 1.5 times the stimulation threshold with a basic cycle length (BCL) of 300 ms unless otherwise specified.

Electrocardiography (ECG)

A volume-conducted bath ECG was obtained using a silver chloride anode located ~2 cm from lateral wall of the RV and a similar cathode located ~2 cm from the lateral wall of the LV. ECGs were recorded at 1 kHz and filtered to remove 60 Hz noise.

Optical Mapping

Optical voltage mapping was performed using di-4-ANEPPS (15 μM) as a voltage indicator to quantify conduction velocity (θ) and anisotropy (ARθ; defined as the ratio of longitudinal θ (θL) to transverse θ (θT)) as previously described. (8;11) Briefly, the preparation was stained with di-4-ANEPPS by direct coronary perfusion for 10 minutes, then excited by three- 60 LED light sources (RL5-A9018, Superbrightleds, St. Louis, MO) fitted with 510±5 nm filters (Chroma, Rockingham, VT). Fluoresced light was passed through a 610 nm LP filter (Newport, Irvine, CA) before being recorded with a SciMedia MiCam02 HS CCD camera (SciMedia, Irvine CA) in a tandem lens configuration capable of resolving membrane potential changes as small as 2 mV with 1 ms temporal resolution from 44 × 30 sites simultaneously.

Motion was reduced by perfusion of 7.5 mM 2,3-butanedione monoxime (BDM). The anterior epicardium was mechanically pressed against the front wall of the perfusion chamber to further stabilize and flatten it.

Activation time was defined as the time of the maximum first derivative of the action potential as described previously. (12) The inter-pixel resolution was 0.313 mm in the x-direction (44 pixels) and 0.34 mm in the y-direction (30 pixels).

A parabolic surface was fitted to the activation times as previously described. (13) The gradient at each point was assigned a conduction velocity vector. The averaged conduction velocity vectors along the slow and fast axes of propagation (±15°) are reported as they reflect transverse and longitudinal propagation. (14)

Interventions

The class IC sodium channel blocker flecainide was perfused in concentrations ranging from 0 to 1 μM to determine conduction dependence on sodium channel availability. Pinacidil (10 μM; Sigma Aldrich, St. Louis, MO) and R-L3 (10 μM; NeuroSearch A/S, Ballerup, Denmark) were applied to test the effects of increasing IKATP and IKs respectively on θ. In the dose response experiments, flecainide was applied in increasing doses, first by itself and then, in the presence of a constant concentration of K+ channel activator.

Statistical analysis

Statistical analysis of the data was performed using a 2-tailed Student's t-test for paired and unpaired data or a single factor ANOVA. The Bonferroni correction was applied to adjust for multiple comparisons. A p<0.05 was considered statistically significant. All values are reported as mean ± standard error unless otherwise noted.

RESULTS

Controls

Activating outward potassium currents is expected to shorten APD, which may alter cardiac conduction by altering sodium channel availability (aka refractoriness). To test this, we globally assessed cardiac conduction (QRS duration) and an index of refractoriness (QT interval). Representative traces of a volume conducted ECG demonstrates that QRS duration was prolonged during activation of the non-voltage-gated channels that carry the ATP-sensitive potassium current (IKATP) by pinacidil relative to control (figure 1A). Interestingly, activation of the voltage-gated channels carrying the slow component of the delayed rectifier potassium current (IKs) by R-L3 did not prolong the QRS (figure 1A). Summary data in figure 1B (white bars) demonstrates that IKATP activation by pinacidil prolonged QRS duration relative to control (*), while IKs activation by R-L3 did not significantly change QRS duration. These data suggest that pinacidil slows cardiac conduction while R-L3 does not under normal conditions.

Figure 1.

Figure 1

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A) Representative bath electrocardiograms recorded during control (top), pinacidil (middle) and R-L3 (bottom) in the absence of di-4-ANEPPS. B) QRS duration. Pinacidil (n=4) but not R-L3 (n=3) prolonged the QRS (*) relative to control (white bars). These measurements were then repeated in the presence of flecainide (black bars). Flecainide by itself prolonged the QRS relative to control and R-L3. Pinacidil + flecainide did not change the QRS relative to flecainide alone, but R-L3 + flecainide did (black bars). C) QT interval. Both pinacidil and R-L3 abbreviated the QT interval (white bars, *). These measurements were then repeated in the presence of flecainide (black bars). Flecainide by itself prolonged the QT interval relative to control (*) and pinacidil + flecainide (†, p<0.05) but not R-L3 + flecainide shortened the QT interval relative to flecainide alone (black bars). In panels B and C: * – p < 0.05 vs. control, † – p < 0.05 vs. flecainide.

Figure 1A also demonstrates that the QT interval, a global metric of action potential duration (APD) is shortened during perfusion of pinacidil and R-L3 as expected. Overall, both agents shortened the QT interval relative to control (figure 1C, white bars) despite differential effects on conduction.

Flecainide

To elucidate the mechanism of preferential conduction slowing during pinacidil versus R-L3 perfusion, conduction velocity was first reduced by perfusion of 1 μM flecainide to partially block sodium channels. Flecainide alone significantly prolonged the QRS duration as well as the QT interval relative to control (figure 1B, C, black bars, *). Interestingly, flecainide + pinacidil did not significantly alter QRS duration (figure 1B, black bars), while it shortened the QT interval (figure 1C, black bars) relative to flecainide alone. In contrast, flecainide + R-L3 prolonged the QRS duration relative to flecainide alone (figure 1B, black bars, †). However, flecainide + R-L3 did not shorten the QT interval (figure 1C, black bars) relative to flecainide alone.

di-4-ANEPPS

Optical mapping was performed with the voltage sensitive dye, di-4-ANEPPS in order to directly quantify conduction velocity in the presence of pinacidil or R-L3. However, di-4-ANEPPS was previously demonstrated to prolong the QRS and unmask conduction slowing during IKr inhibition. (9) Indeed, over all experiments, di-4-ANEPPS perfusion prolonged the QRS (21 ± 3%, p < 0.05). Representative ECG traces in figure 2A demonstrate QRS prolongation and QT interval shortening during IKATP activation by pinacidil in the presence of di-4-ANEPPS relative to di-4-ANEPPS alone. Summary data demonstrate that pinacidil prolonged the QRS (figure 2B, white bars) and abbreviated the QT interval (figures 2C, white bars) overall, in the presence of di-4-ANEPPS, as it did in the absence of di-4-ANEPPS. Activation of IKs by R-L3 in the presence of di-4-ANEPPS had a similar effect, prolonging the QRS (figures 2A, B, white bars) and shortening the QT interval (figures 2A, C, white bars) relative to di-4-ANEPPS alone. This is in contrast to the absence of di-4-ANEPPS, where R-L3 did not significantly alter QRS duration (figures 1A, B, white bars).

Figure 2.

Figure 2

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A) Representative bath electrocardiograms recorded during control (top), pinacidil (middle) and R-L3 (bottom) recorded in the presence of di-4-ANEPPS. B) QRS duration. Both pinacidil (n=5) and R-L3 (n=4) prolonged the QRS (*) relative to control (white bars). These measurements were then repeated in the presence of flecainide (black bars). Flecainide (black bars) by itself prolonged the QRS relative to control (i.e., di-4-ANEPPS alone). Pinacidil + flecainide did not alter the QRS relative to flecainide alone whereas R-L3 + flecainide prolonged the QRS (†) relative to flecainide alone (black bars). C) QT interval. Both pinacidil + di-4-ANEPPS and R-L3 + di-4-ANEPPS abbreviated the QT interval (*, white bars) relative to control (i.e. di-4-ANEPPS alone). These measurements were then repeated in the presence of flecainide (black bars). Flecainide by itself prolonged the QT interval relative to control (i.e. di-4-ANEPPS alone; *). Pinacidil + flecainide but not R-L3 + flecainide shortened the QT interval (†) relative to flecainide alone (black bars). In panels B and C: * – p < 0.05 vs. control, † – p < 0.05 vs. flecainide.

Flecainide, perfused in the presence of di-4-ANEPPS, again significantly prolonged both the QRS duration and the QT interval relative to di-4-ANEPPS alone (figures 2B, C). As before, flecainide + pinacidil did not significantly alter QRS duration (figure 2B, black bars) but shortened the QT interval (figure 2C, black bars) relative to flecainide alone. Lastly, flecainide + R-L3 prolonged the QRS duration (figure 2B, black bars) but did not alter the QT interval (figure 2C, black bars) relative to flecainide alone.

Action potential duration (APD): di-4-ANEPPS

Action potential duration (APD) was quantified from optical action potentials. Representative action potentials in figure 3A demonstrate similar APD between control and flecainide while APD was shorter in the presence of pinacidil and R-L3. Overall, flecainide did not significantly alter APD relative to control while both pinacidil and R-L3 lowered APD relative to control and did so to a similar extent (figure 3 A, B).

Figure 3.

Figure 3

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A) Representative optical action potentials. Solid vertical line marks activation time. Repolarization time is marked by dashed vertical line for control and flecainide and by the dotted vertical line for pinacidil and R-L3. B) Flecainide (n=9) did not alter action potential duration (APD) relative to control while pinacidil (n=5) and R-L3 (n=4) significantly lowered APD (*, p<0.05 vs. control).

Conduction Velocity: di-4-ANEPPS

Longitudinal (θL) and transverse (θT) conduction velocities were subsequently quantified by optical mapping. Representative action potential (AP) upstrokes recorded during control conditions in figure 4A demonstrate shorter delays between equally spaced sites along the longitudinal axis (top traces) relative to sites along the transverse axis (bottom traces). This produces the elliptical activation pattern characteristic of cardiac tissue seen in figure 4B.

Figure 4.

Figure 4

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A) Action potential upstrokes from equally spaced sites. Representative activation isochrone maps: B) Control, C) Pinacidil, D) R-L3 and E) Flecainide. F) Longitudinal (θL) and transverse (θT) conduction velocity. Overall, pinacidil, R-L3 and flecainide decreased both θL and θT.

IKATP activation by pinacidil decreased the spacing between isochrones (figure 4C), suggesting mild conduction slowing. Indeed, over all experiments, pinacidil decreased θL by 9.1 ± 2.6 % and θT by 10.0 ± 0.6 % respectively (p<0.05; figure 4F). Similarly, IKs activation by R-L3 also decreased isochrone spacing, suggesting slowed conduction (figure 4D). Overall, R-L3 decreased θL by 11.7 ± 2.2 % and θT by 15.0 ± 3.8% respectively (p<0.05; figure 4F). No intervention altered anisotropy of conduction (ARθ; p=ns).

Conduction Velocity: di-4-ANEPPS + Flecainide

As reported above, pinacidil or R-L3 in the presence of flecainide produced diverse conduction changes. Conduction was assessed by optical mapping in the presence of different concentrations of flecainide (0, 0.5, 1 μM) to determine conduction dependence on INa during perfusion of pinacidil or R-L3. By itself, flecainide produced a linear decrease in both θL and θT with increasing dose (figures 4, 5 – dashed lines, R2 ≥ 0.95 for a linear fit). Also, flecainide did not alter ARθ relative to control (n = 9; p = ns).

Figure 5.

Figure 5

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A) Plots of θL and θT versus flecainide concentration in the presence (solid lines) and absence (dashed lines) of pinacidil (n=5). Whereas pinacidil decreased both θL and θT by itself (0 μM flecanide), pinacidil + flecainide did not significantly alter either parameter relative to flecainide alone at any flecainide dose. * – p < 0.05 vs. 0 pinacidil. B) Conduction dependence on sodium channel blockade was quantified as the absolute slope of θ vs. flecainide dose. Pinacidil (grays bars) significantly decreased conduction dependence on sodium channel blockade (*) relative to control (white bars).

IKATP activation by pinacidil significantly decreased conduction in the absence (0 μM) but not in the presence of either 0.5 or 1 μM flecainide (figure 5 – solid lines). Yet, θL and θT decreased linearly with flecainide dose in the presence of pinacidil (figure 5 – solid lines, R2 ≥ 0.95) and ARθ was not different relative to control or flecainide alone (n = 4, p =ns). Consequently, pinacidil blunted conduction dependence on pharmacological INa inhibition quantified as the absolute slope of θ versus flecainide dose (figure 6, p < 0.05) as demonstrated by the crossover of the two lines in figure 5A.

Figure 6.

Figure 6

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A) Plots of θL and θT versus flecainide concentration in the presence (solid lines) and absence (dashed lines) of R-L3 (n=4). R-L3 by itself (0 μM flecanide), decreased θL and θT (*). R-L3 + flecainide decreased both θL and θT (*) relative to flecainide alone for both 0.5 and1 μM doses of flecainide was present. * – p < 0.05 vs. 0 R-L3. B) Conduction dependence on sodium channel blockade was quantified as the absolute slope of θ vs. flecainide dose. R-L3 (black bars) did not significantly alter conduction dependence on sodium channel blockade relative to control (white bars).

In contrast, R-L3 activation of IKs decreased θL and θT by itself and also in the presence of all flecainide doses (figure 6 – solid lines). Similar to pinacidil, θL and θT decreased linearly with flecainide dose in the presence of R-L3 (figure 6 – solid lines, R2 ≥ 0.95) and ARθ was not altered relative to control or flecainide (n = 5, p =ns). Consequently, R-L3 did not alter θL or θT dependence on pharmacological INa inhibition (figure 6, n=5, p = ns).

DISCUSSION

It is well established that membrane excitability, in particular the amplitude of the cardiac sodium current (INa), which in turn depends on sodium channel availability, is a key determinant of velocity (θ). It has long been recognized that decreasing INa slows conduction. (15) We recently demonstrated that outward K+ currents such as the inward rectifier K+ current (IK1) (8) and the rapid delayed rectifier (IKr) (9) can modulate θ. Further, we demonstrated that modulation of IK1 and IKr altered conduction under different conditions. Therefore, we explored the hypothesis that non-voltage-gated and voltage-gated potassium channels heterogeneously affect conduction. Here we demonstrate that activation of non-voltage-gated IKATP(10;16) slowed conduction by itself but flattened the response of conduction velocity to pharmacological INa inhibition. In contrast, voltage-gated IKs(10) activation did not slow conduction by itself, and it did not affect the response of conduction velocity to pharmacological INa activation.

Pinacidil: effects on conduction

By itself, 10 μM pinacidil, an IKATP agonist, significantly broadened QRS duration. This is in contrast to the study by Yang et al. which did not report a change in QRS duration up to 50 μM pinacidil. (17) Important differences between the Yang et al study and our study include the sites of pacing (atria and ventricle respectively), location of ECG electrodes (on the ventricle and bath respectively), and temporal resolution of ECG quantification (Polaroid photos of the oscilloscope and 1 kHz digital data acquisition, respectively). Further, the QRS complex during atrial pacing is narrower relative to ventricular pacing, and as a result changes in QRS duration in the Yang et al. study may have been below the detection limit. Therefore, experimental differences likely explain these discrepant findings. Importantly, the mechanism of pinacidil mediated conduction slowing we observed under normal conditions may be due to pinacidil's effect of hyperpolarizing myocytes (supplementary figure 1) (18) and increasing outward current during early depolarization. Hyperpolarization is a well-established mechanism for conduction slowing(19), where the amount of charge required to reach the activation threshold for INa is increased.

When INa was inhibited by flecainide, a drug demonstrated to block sodium channels in their open state, (20) the QRS broadened. In the presence of pinacidil + flecainide, QRS did not significantly (p=0.52) prolong more than flecainide alone. Importantly, this is consistent with the lack of QRS change during pinacidil + flecainide in the Yang study, with the same caveats mentioned above. This result is also consistent with our previous finding that modulation of another inwardly-rectifying K+ current, IK1, impacted conduction under normal conditions but not when INa was reduced. (8) The question remains as to why pinacidil slowed conduction by itself but not in the presence of flecainide. One explanation is that resolution of the measurements may have been insufficient to detect a change in the pinacidil + flecainide groups. Alternatively, the lower resting potential produced by pinacidil(18) would decrease random openings of sodium channels during diastole, (21) which may mitigate the impact of flecainide, a drug that preferentially binds to sodium channels in the open state. (20)

In order to directly quantify conduction, hearts were optically mapped with the voltage sensitive dye, di-4-ANEPPS. Pinacidil prolonged the QRS and slowed conduction in the presence of di-4-ANEPPS as it did in the absence of di-4-ANEPPS, suggesting that the effects of pinacidil on conduction were not altered by di-4-ANEPPS. Furthermore, ARθ was not significantly different, which we interpret to mean that pinacidil principally affected sarcolemmal currents. (19) Pinacidil + di-4-ANEPPS + flecainide did not change QRS duration, slow conduction or change ARθ relative to flecainide + di-4-ANEPPS. These data suggest that pinacidil has the same effect with and without di-4-ANEPPS.

When flecainide concentration was increased in the presence of pinacidil + di-4-ANEPPS, we observed damping of conduction dependence on pharmacological INa inhibition. Specifically, the slope of the conduction velocity-flecainide relationship was flattened by the addition of pinacidil. In our previous study, inhibiting IK1 (which is also carried by channels open during diastole) can increase the steepness of the conduction velocity-flecainide curve. (8) Importantly, these data are consistent in that opening the non-voltage-gated potassium channels flattens the conduction velocity-flecainide relationship, while inhibiting non-voltage-gated potassium channels steepens the same relationship.

Taken together, the pinacidil data suggest that activation of an inwardly-rectifying potassium channel may lower resting membrane potential and slow conduction but without exacerbating conduction slowing by sodium channel blockers that bind in the open state.

R-L3: effects on conduction

In contrast to pinacidil, R-L3 activates the voltage-activated IKs. In addition to differences in voltage-dependent kinetics between IKATP and IKs, IKs activation by R-L3 has not been associated with changes in resting membrane potential. (6;22) As might be expected, R-L3 perfusion alone did not significantly change QRS (p = 0.95). Thus, it was interesting to note that R-L3 + flecainide significantly broadened QRS more that flecainide alone, whereas we could not measure a difference between pinacidil + flecainide and pinacidil alone. This is consistent with our previous finding that activation of the voltage-activated IKr also prolonged the QRS only when conduction was already impaired. (9) This is different from pinacidil which slowed conduction by itself, but did not cause further slowing in the presence of flecainide. These data suggest that there are mechanistic differences between the modulation of conduction by K+ currents like IKr and IKs which are carried by voltage-gated channels as opposed to K+ currents like IK1 and IKATP which are carried by inwardly-rectifying, non-voltage-gated channels.

It is interesting to note that flecainide prolongs time to dV/dtmax (i.e. increases latency) and slows conduction by delaying the time to maximal INa activation. (20) As a voltage-gated potassium channel agonist, R-L3 significantly increases IKs. Delaying time to peak INa activation may provide sufficient time for IKs to provide a significant current opposing depolarization by INa and thereby, slow conduction. In isolated guinea pig myocytes, R-L3 concentrations from 1 to 10 μM have been demonstrated to increase IKs between 750 and 1500 %,(22) which could represent a significant repolarizing current during depolarization. This hypothesis requires validation by single cell measurements.

In the presence of di-4-ANEPPS, R-L3 prolonged the QRS whereas it did not do so by itself. This difference is likely related to the impairment of conduction by di-4-ANEPPS: it prolonged the QRS in the current study as well as in our previous study.(9) This effect of di-4-ANEPPS may be related to its previously suggested interaction with the sodium potassium ATPase. (23) Indeed, sodium-potassium ATPase inhibition by cardiac glycosides is known to increase intracellular sodium(24) and also broaden the QRS complex. (25;26) Therefore, di-4-ANEPPS's interaction with the sodium potassium ATPase could underlie the dye's effects on conduction; however, the specific mechanism warrants further study.

Further, the results obtained with R-L3 are consistent with our previous findings that activation of another voltage-gated potassium current (IKr) slows conduction only when conduction is already impaired by di-4-ANEPPS or flecainide. (9) In contrast, pinacidil prolonged the QRS even in the absence of di-4-ANEPPS. These results further suggest conduction slowing secondary to voltage-gated potassium channel inhibition may require a delay in peak INa activation in order to provide significant opposing current to depolarization. On the other hand the non-voltage-gated, inwardly-rectifying channels are active through diastole, (10) and affect conduction by decreasing resting membrane potential as well as increasing outward current during early depolarization.

Lastly, when flecainide doses were increased during perfusion of R-L3 + di-4-ANEPPS, conduction was slowed relative to flecainide + di-4-ANEPPS alone. As a result, the slope of the conduction velocity-flecainide relationship was not significantly altered by R-L3. This is in contrast with pinacidil which did not further decrease θ in the presence of flecainide and also decreased conduction dependence on INa blockade. These differences further suggest that R-L3 modulates conduction by a different mechanism from pinacidil.

Conclusions

In summary, we demonstrate here that pharmacological activation of IKATP and IKs both decrease θ; however, they exhibit differential response to θ dependence on INa. Importantly, modulating K+ currents that do not display voltage-dependent kinetics may only affect conduction when Na+ channel availability is not reduced, whereas modulating K+ currents that display voltage-dependent kinetics may impact conduction when Na+ channel availability is reduced.

Clinical Relevance

Conduction slowing is an established proarrhythmic factor. (19) Here we demonstrate conduction slowing secondary to pharmacologic K+ channel activation. Such effects could be deleterious, particularly in the presence of other pathophysiologic structural / electrophysiologic changes. Therefore, pharmacological K+ channel activators should be evaluated for any possible effects on conduction velocity and safety of conduction before being used as antiarrhythmic therapy.

Limitations

The drugs used in the study, while relatively selective, do have off-target effects. For instance, pinacidil and flecainide are known to affect the transient outward current (Ito). (27;28) However, guinea pigs do not functionally express Ito; therefore, these effects are unlikely to affect the results of this study.(29) R-L3 has been reported to block the L-type Ca2+ current (ICa,L) in isolated myocytes as well as IKr in mouse atrial tumor (AT-1) cells. (22) However, the QT abbreviation observed with R-L3 in the present study argues against significant IKr blockade by R-L3 in our experiments. ICa,L is normally active after the action potential upstroke, however possible ICa,L block by R-L3 may contribute to conduction slowing by the drug when INa is reduced. BDM has been shown to cause no significant change in conduction velocity at the dose used here; (30-32) therefore, BDM is unlikely to have significantly affected the principal findings of this study. This study does not address any possible effects of K+ channel activation on regional conduction heterogeneities within the heart secondary to heterogeneous distribution of ion channels. Lastly, the study was conducted in structurally normal hearts with no remodeling; K+ channel activation may affect conduction differently in diseased hearts.

Supplementary Material

01

Acknowledgments

Funding

This work was supported by a Nora Eccles Harrison Treadwell Foundation Grant and National Institutes of Health R01 HL102298-01A1 awarded to Dr. Poelzing.

Footnotes

Conflicts of Interest

The authors report no conflicts of interest.

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