Effect of acidosis on transient outward potassium current in isolated rat ventricular myocytes

Sophoridine manifests as a leading compound for anti-arrhythmia with multiple ion-channel blocking effects

BACKGROUND

Sophoridine (SR) has shown the potential to be an antiarrhythmic agent. However, SR's electrophysiological properties and druggability research are relatively inadequate, which limits the development of SR as an antiarrhythmic candidate.

PURPOSE

To facilitate the development process of SR as an antiarrhythmic candidate, we performed integrated studies on the electrophysiological properties of SR in vitro and ex vivo to gain more comprehensive insights into the multi-ion channel blocking effects of SR, which provided the foundation for the further drugability studies in antiarrhythmic and safety studies. Firstly, SR's electrophysiological properties and antiarrhythmic potentials were recorded and assessed at the cell and tissue levels by comprehensively integrating the patch clamp with the Electrical and Optical Mapping systems. Subsequently, the antiarrhythmic effects of SR were validated by aconitine and ouabain-induced arrhythmia in vivo. Finally, the safety of SR as an antiarrhythmic candidate compound was evaluated based on the guidelines of the Comprehensive in Vitro Proarrhythmia Assay (CiPA).

STUDY DESIGN

The antiarrhythmic effect of SR was evaluated at the in vitro, ex vivo, and in vivo levels.

METHODS

Isolated primary cardiomyocytes and stable cell lines were prepared to explore the electrophysiologic properties of being a multiple ion-channel blocker in vitro by whole-cell patch clamp. Using electrical and optical mapping, the negative chronotropic effect of SR was determined in langendorff-perfused rat or guinea-pig hearts.The antiarrhythmic activity of SR was assessed by the ex vivo tachyarrhythmia models induced by left coronary artery ligation (LCAL) and isoproterenol (ISO). Canonical models of aconitine and ouabain-induced arrhythmia were used to verify the antiarrhythmic effects in vivo. Finally, the pro-arrhythmic risk of SR was detected in Human-Induced Pluripotent Stem Cell-Derived Cardiomyocytes (hSCCMs) using a Microelectrode array (MEA).

RESULTS

Single-cell patch assay validated the multiple ion-channel blockers of SR in transient outward current potassium currents (Ito), l-type calcium currents (ICa-l), and rapid activation delayed rectifier potassium currents (IKr). SR ex vivo depressed heart rates (HR) and ventricular conduction velocity (CV) and prolonged Q-T intervals in a concentration-dependent manner. Consistent with the changes in HRs, SR extended the active time of hearts and increased the action potential duration measured at 90% repolarization (APD90). SR could also significantly lengthen the onset time and curtail the duration of spontaneous ventricular tachycardia (VT) in the ex vivo arrhythmic model induced by LCAL. Meanwhile, SR could also significantly upregulate the programmed electrical stimulation (PES) frequency after the ISO challenge in forming electrical alternans and re-entrant excitation. Furthermore, SR exerted antiarrhythmic effects in the tachyarrhythmia models induced by aconitine and ouabain in vivo. Notably, the pro-arrhythmic risk of SR was shallow for a moderate inhibition of the human ether-à-go-go-related gene (hERG) channel. Moreover, SR prolonged field potential duration (FPDc) of hSCCMs in a concentration-dependent manner without early after depolarization (EAD) and arrhythmia occurrence.

CONCLUSION

Our results indicated that SR manifested as a multiple ion-channel blocker in the electrophysiological properties and exerts antiarrhythmic effects ex vivo and in vivo. Meanwhile, due to the low pro-arrhythmic risk in the hERG inhibition assay and the induction of EAD, SR has great potential as a leading candidate in the treatment of ventricular tachyarrhythmia.

Carvedilol blockage of delayed rectifier Kv2.1 channels and its molecular basis

Previous studies indicated that one of the action targets of carvedilol is the voltage-gated potassium (Kv) channel, which has a fundamental role in the control of electrical properties in excitable cells. It is not clear whether this compound exerts any actions specifically on delayed rectifier Kv2.1 channels. The present study is conducted on Kv2.1 currents heterologously expressed in HEK293 cells to characterize the block by carvedilol in detail, identifying molecular determinants and providing biophysical insights of the block. Macroscopic Kv2.1 currents obtained by whole-cell recording were substantially inhibited after addition of carvedilol with an IC₅₀ value of 5.1 μM. This drug also led to a largely hyperpolarizing shift (30 mV) of the inactivation curve, which may contribute to the blocking action due to more inactivation of Kv2.1 currents occurred in depolarization potentials. Mutations at Y380 (a putative TEA binding site) and K356 (a K⁺ binding site) in the outer vestibule of Kv2.1 channels significantly eliminated the inhibitory effects of carvedilol and prevented the leftward shift of inactivation. Moreover, mutations at above positions modulated the effects of carvedilol on the deactivation but not activation kinetics of Kv2.1 channels. Collected data indicate that carvedilol can exert a blocking effect on the closed-state of Kv2.1 channels, and specific residues within the S5-P and P-S6 linkers in the outer vestibule may play crucial roles in carvedilol-induced blocking for Kv2.1 channels.

Acidosis slows electrical conduction through the atrio-ventricular node

Acidosis affects the mechanical and electrical activity of mammalian hearts but comparatively little is known about its effects on the function of the atrio-ventricular node (AVN). In this study, the electrical activity of the epicardial surface of the left ventricle of isolated Langendorff-perfused rabbit hearts was examined using optical methods. Perfusion with hypercapnic Tyrode's solution (20% CO₂, pH 6.7) increased the time of earliest activation (T_act) from 100.5 ± 7.9 to 166.1 ± 7.2 ms (n = 8) at a pacing cycle length (PCL) of 300 ms (37°C). T_act increased at shorter PCL, and the hypercapnic solution prolonged T_act further: at 150 ms PCL, T_act was prolonged from 131.0 ± 5.2 to 174.9 ± 16.3 ms. 2:1 AVN block was common at shorter cycle lengths. Atrial and ventricular conduction times were not significantly affected by the hypercapnic solution suggesting that the increased delay originated in the AVN. Isolated right atrial preparations were superfused with Tyrode's solutions at pH 7.4 (control), 6.8 and 6.3. Low pH prolonged the atrial-Hisian (AH) interval, the AVN effective and functional refractory periods and Wenckebach cycle length significantly. Complete AVN block occurred in 6 out of 9 preparations. Optical imaging of conduction at the AV junction revealed increased conduction delay in the region of the AVN, with less marked effects in atrial and ventricular tissue. Thus acidosis can dramatically prolong the AVN delay, and in combination with short cycle lengths, this can cause partial or complete AVN block and is therefore implicated in the development of brady-arrhythmias in conditions of local or systemic acidosis.

Modulation of ventricular transient outward K⁺ current by acidosis and its effects on excitation-contraction coupling

The contribution of transient outward current (Ito) to changes in ventricular action potential (AP) repolarization induced by acidosis is unresolved, as is the indirect effect of these changes on calcium handling. To address this issue we measured intracellular pH (pHi), Ito, L-type calcium current (ICa,L), and calcium transients (CaTs) in rabbit ventricular myocytes. Intracellular acidosis [pHi 6.75 with extracellular pH (pHo) 7.4] reduced Ito by ~50% in myocytes with both high (epicardial) and low (papillary muscle) Ito densities, with little effect on steady-state inactivation and activation. Of the two candidate α-subunits underlying Ito, human (h)Kv4.3 and hKv1.4, only hKv4.3 current was reduced by intracellular acidosis. Extracellular acidosis (pHo 6.5) shifted Ito inactivation toward less negative potentials but had negligible effect on peak current at +60 mV when initiated from -80 mV. The effects of low pHi-induced inhibition of Ito on AP repolarization were much greater in epicardial than papillary muscle myocytes and included slowing of phase 1, attenuation of the notch, and elevation of the plateau. Low pHi increased AP duration in both cell types, with the greatest lengthening occurring in epicardial myocytes. The changes in epicardial AP repolarization induced by intracellular acidosis reduced peak ICa,L, increased net calcium influx via ICa,L, and increased CaT amplitude. In summary, in contrast to low pHo, intracellular acidosis has a marked inhibitory effect on ventricular Ito, perhaps mediated by Kv4.3. By altering the trajectory of the AP repolarization, low pHi has a significant indirect effect on calcium handling, especially evident in epicardial cells.

Extracellular proton depression of peak and late Na⁺ current in the canine left ventricle

Cardiac ischemia reduces excitability in ventricular tissue. Acidosis (one component of ischemia) affects a number of ion currents. We examined the effects of extracellular acidosis (pH 6.6) on peak and late Na(+) current (I(Na)) in canine ventricular cells. Epicardial and endocardial myocytes were isolated, and patch-clamp techniques were used to record I(Na). Action potential recordings from left ventricular wedges exposed to acidic Tyrode solution showed a widening of the QRS complex, indicating slowing of transmural conduction. In myocytes, exposure to acidic conditions resulted in a 17.3 ± 0.9% reduction in upstroke velocity. Analysis of fast I(Na) showed that current density was similar in epicardial and endocardial cells at normal pH (68.1 ± 7.0 vs. 63.2 ± 7.1 pA/pF, respectively). Extracellular acidosis reduced the fast I(Na) magnitude by 22.7% in epicardial cells and 23.1% in endocardial cells. In addition, a significant slowing of the decay (time constant) of fast I(Na) was observed at pH 6.6. Acidosis did not affect steady-state inactivation of I(Na) or recovery from inactivation. Analysis of late I(Na) during a 500-ms pulse showed that the acidosis significantly reduced late I(Na) at 250 and 500 ms into the pulse. Using action potential clamp techniques, application of an epicardial waveform resulted in a larger late I(Na) compared with when an endocardial waveform was applied to the same cell. Acidosis caused a greater decrease in late I(Na) when an epicardial waveform was applied. These results suggest acidosis reduces both peak and late I(Na) in both cell types and contributes to the depression in cardiac excitability observed under ischemic conditions.

Docosahexaenoic acid has influence on action potentials and transient outward potassium currents of ventricular myocytes

Background

There are many reports about the anti-arrhythmic effects of ω-3 polyunsaturated fatty acids, however, the mechanisms are still not completely delineated. The purpose of this study was to investigate the characteristics of action potentials and transient outward potassium currents (I_to) of Sprague-Dawley rat ventricular myocytes and the effects of docosahexaenoic acid (DHA) on action potentials and I_to.

Methods

The calcium-tolerant rat ventricular myocytes were isolated by enzyme digestion. Action potentials and I_to of epicardial, mid-cardial and endocardial ventricular myocytes were recorded by whole-cell patch clamp technique.

Results

1. Action potential durations (APDs) were prolonged from epicardial to endocardial ventricular myocytes (P < 0.05). 2. I_to current densities were decreased from epicardial to endocardial ventricular myocytes, which were 59.50 ± 15.99 pA/pF, 29.15 ± 5.53 pA/pF, and 12.29 ± 3.62 pA/pF, respectively at +70 mV test potential (P < 0.05). 3. APDs were gradually prolonged with the increase of DHA concentrations from 1 μmol/L to 100 μmol/L, however, APDs changes were not significant as DHA concentrations were in the range of 0 μmol/L to 1 μmol/L. 4. I_to currents were gradually reduced with the increase of DHA concentrations from 1 μmol/L to 100 μmol/L, and its half-inhibited concentration was 5.3 μmol/L. The results showed that there were regional differences in the distribution of action potentials and I_to in rat epicardial, mid-cardial and endocardial ventricular myocytes. APDs were prolonged and I_to current densities were gradually reduced with the increase of DHA concentrations.

Conclusion

The anti-arrhythmia mechanisms of DHA are complex, however, the effects of DHA on action potentials and I_to may be one of the important causes.

The effects of over-expression of the FK506-binding protein FKBP12.6 on K(+) currents in adult rabbit ventricular myocytes

This study examines the effects of the intracellular protein FKBP12.6 on action potential and associated K⁺ currents in isolated adult rabbit ventricular cardiomyocytes. FKBP12.6 was over-expressed by ~6 times using a recombinant adenovirus coding for human FKBP12.6. This over-expression caused prolongation of action potential duration (APD) by ~30%. The amplitude of the transient outward current (I_to) was unchanged, but rate of inactivation at potentials positive to +40 mV was increased. FKBP12.6 over-expression decreased the amplitude of the inward rectifier current (I_K1) by ~25% in the voltage range −70 to −30 mV, an effect prevented by FK506 or lowering intracellular [Ca²⁺] below 1 nM. Over-expression of an FKBP12.6 mutant, which cannot bind calcineurin, prolonged APD and affected I_to and I_K1 in a similar manner to wild-type protein. These data suggest that FKBP12.6 can modulate APD via changes in I_K1 independently of calcineurin binding, suggesting that FKBP12.6 may affect APD by direct interaction with I_K1.

Acidosis inhibits spontaneous activity and membrane currents in myocytes isolated from the rabbit atrioventricular node

Recent evidence from intact hearts suggests that the function of cardiac nodal tissue may be particularly susceptible to acidosis. Little is currently known, however, about the effects of acidosis on the cellular electrophysiology of the atrioventricular node (AVN). This study was conducted, therefore, to determine the effect of acidosis on the spontaneous activity and membrane currents of myocytes isolated from the rabbit AVN, recorded at 35-37 degrees C using whole-cell patch-clamp. Reduction of extracellular pH (pH(e); from 7.4 to 6.8 or 6.3) produced pH-dependent slowing of spontaneous action potential rate and upstroke velocity, and reductions in maximum diastolic potential and action potential amplitude. Ionic current recordings under voltage-clamp indicated that acidosis (pH(e) 6.3) decreased L-type Ca current (I(Ca,L)), without significant changes in voltage-dependent activation or inactivation. Acidosis reduced the E-4031-sensitive, rapid delayed rectifier current (I(Kr)) tail amplitude at -40 mV following command pulses to between -30 and +50 mV, and accelerated tail-current deactivation. In contrast, the time-dependent hyperpolarisation-activated current, I(f), was unaffected by acidosis. Background current insensitive to E-4031 and nifedipine was reduced by acidosis. Measurement of intracellular pH (pH(i)) from undialysed cells using BCECF showed a reduction in mean pH(i) from 7.24 to 6.45 (n=17) when pH(e) was lowered from 7.4 to 6.3. We conclude that I(f) is unlikely to be involved in the response of the AVN to acidosis, whilst inhibition of I(Ca,L) and I(Kr) by acidosis are likely to play a significant role in effects on AVN cellular electrophysiology.

Phosphorylation of serine 1928 in the distal C-terminal domain of cardiac CaV1.2 channels during beta1-adrenergic regulation

During the fight-or-flight response, epinephrine and norepinephrine released by the sympathetic nervous system increase L-type calcium currents conducted by Ca(V)1.2a channels in the heart, which contributes to enhanced cardiac performance. Activation of beta-adrenergic receptors increases channel activity via phosphorylation by cAMP-dependent protein kinase (PKA) tethered to the distal C-terminal domain of the alpha(1) subunit via an A-kinase anchoring protein (AKAP15). Here we measure phosphorylation of S1928 in dissociated rat ventricular myocytes in response to beta-adrenergic receptor stimulation by using a phosphospecific antibody. Isoproterenol treatment increased phosphorylation of S1928 in the distal C-terminal domain, and a similar increase was observed with a direct activator of adenylyl cyclase, forskolin, confirming that the cAMP and PKA are responsible. Pretreatment with selective beta1- and beta2-adrenergic antagonists reduced the increase in phosphorylation by 79% and 42%, respectively, and pretreatment with both agents completely blocked it. In contrast, treatment with these agents in the presence of 1,2-bis(2-aminophenoxy)ethane-N',N'-tetraacetic acid (BAPTA)-acetoxymethyl ester to buffer intracellular calcium results in only beta1-stimulated phosphorylation of S1928. Whole-cell patch clamp studies with intracellular BAPTA demonstrated that 98% of the increase in calcium current was attributable to beta1-adrenergic receptors. Thus, beta-adrenergic stimulation results in phosphorylation of S1928 on the Ca(V)1.2 alpha1 subunit in intact ventricular myocytes via both beta1- and beta2-adrenergic receptor pathways, but the beta2-dependent increase in phosphorylation depends on elevated intracellular calcium and does not contribute to regulation of whole-cell calcium currents at basal calcium levels. Our results correlate phosphorylation of S1928 with beta1-adrenergic functional up-regulation of cardiac calcium channels in the presence of BAPTA in intact ventricular myocytes.

Actions of the anti-oestrogen agent clomiphene on outward K+ currents in rat ventricular myocytes

1. The effects of clomiphene (CLM) on cardiac outward K+ current components from rat isolated ventricular myocytes were investigated using the whole-cell patch-clamp technique. Clomiphene (10 micromol/L) significantly inhibited both peak (Ipeak) and end-pulse (Ilate) outward currents (elicited by a 500 msec voltage step from -40 to +50 mV in the presence of K+-containing intracellular and extracellular solutions) by approximately 37% (n = 6; P < 0.01) and 49% (n = 6; P < 0.01), respectively. In contrast, CLM had no effect on outward currents when K+-free solutions were used. 2. A double-pulse protocol and Boltzmann fitting were used to separate individual K+ current components on the basis of their voltage-dependent inactivation properties. At potentials positive to -80 mV, two inactivating transient outward components (Ito) and (IKx) and a non-inactivating steady state component (Iss) could be distinguished. 3. Clomiphene inhibited both Ito and Iss. The maximal block of Ito and Iss induced by CLM (100 micromol/L) was approximately 61% (n = 5) and 43% (n = 5) with IC50 values of 1.54 +/- 0.39 and 2.2 +/- 0.4 micromol/L, respectively. In contrast, the peak magnitude of IKx was unaltered by CLM, although its time-course of inactivation was accelerated. 4. Further experiments whereby myocytes were superfused with the vasoactive peptide endothelin (ET)-1 (20 nmol/L) revealed that CLM (10 micro mol/L) completely abolished the ET-1-sensitive component of Iss. 5. Our findings demonstrate, for the first time, the effects of CLM on distinct cardiac K+ current components and show that CLM modulates the voltage-gated K+ current components Ito and IKx and inhibits the steady state outward current Iss in rat ventricular myocytes.

Beta-adrenergic regulation requires direct anchoring of PKA to cardiac CaV1.2 channels via a leucine zipper interaction with A kinase-anchoring protein 15

Activation of beta-adrenergic receptors and consequent phosphorylation by cAMP-dependent protein kinase A (PKA) greatly increases the L-type Ca2+ current through CaV1.2 channels in isolated cardiac myocytes. A kinase-anchoring protein 15 (AKAP15) coimmunoprecipitates with CaV1.2 channels isolated from rat heart membrane extracts and transfected cells, and it colocalizes with CaV1.2 channels and PKA in the transverse tubules of isolated ventricular myocytes. Site-directed mutagenesis studies reveal that AKAP15 directly interacts with the distal C terminus of the cardiac CaV1.2 channel via a leucine zipper-like motif. Disruption of PKA anchoring to CaV1.2 channels via AKAP15 using competing peptides markedly inhibits the beta-adrenergic regulation of CaV1.2 channels via the PKA pathway in ventricular myocytes. These results identify a conserved leucine zipper motif in the C terminus of the CaV1 family of Ca2+ channels that directly anchors an AKAP15-PKA signaling complex to ensure rapid and efficient regulation of L-type Ca2+ currents in response to beta-adrenergic stimulation and local increases in cAMP.

Extracellular acidosis modulates drug block of Kv4.3 currents by flecainide and quinidine

INTRODUCTION

As a molecular model of the effect of ischemia on drug block of the transient outward potassium current, the effect of acidosis on the blocking properties of flecainide and quinidine on Kv4.3 currents was studied.

METHODS AND RESULTS

Kv4.3 channels were stably expressed in Chinese hamster ovary cells. Whole-cell, voltage clamp techniques were used to measure the effect of flecainide and quinidine on Kv4.3 currents in solutions of pH 7.4 and 6.0. Extracellular acidosis attenuated flecainide block of Kv4.3 currents, with the IC50 for flecainide (based on current-time integrals) increasing from 7.8 +/- 1.1 microM at pH 7.4 to 125.1 +/- 1.1 microM at pH 6.0. Similar effects were observed for quinidine (IC50 5.2 +/- 1.1 microM at pH 7.4 and 22.1 +/- 1.3 microM at pH 6.0). Following block by either drug, Kv4.3 channels showed a hyperpolarizing shift in the voltage sensitivity of inactivation and a slowing in the time to recover from inactivation/block that was unaffected by acidosis. In contrast, acidosis attenuated the effects on the time course of inactivation and the degree of tonic- and frequency-dependent block for both drugs.

CONCLUSION

Extracellular acidosis significantly decreases the potency of blockade of Kv4.3 by both flecainide and quinidine. This change in potency may be due to allosteric changes in the channel, changes in the proportion of uncharged drug, and/or changes in the kinetics of drug binding or unbinding. These findings are in contrast to the effects of extracellular acidosis on block of the fast sodium channel by these agents and provide a molecular mechanism for divergent modulation of drug block potentially leading to ischemia-associated proarrhythmia.

Two pore residues mediate acidosis-induced enhancement of C-type inactivation of the Kv1.4 K(+) channel

Acidosis inhibits current through the Kv1.4 K(+) channel, perhaps as a result of enhancement of C-type inactivation. The mechanism of action of acidosis on C-type inactivation has been studied. A mutant Kv1.4 channel that lacks N-type inactivation (fKv1.4 Delta2-146) was expressed in Xenopus oocytes, and currents were recorded using two-microelectrode voltage clamp. Acidosis increased fKv1.4 Delta2-146 C-type inactivation. Replacement of a pore histidine with cysteine (H508C) abolished the increase. Application of positively charged thiol-specific methanethiosulfonate to fKv1.4 Delta2-146 H508C increased C-type inactivation, mimicking the effect of acidosis. Replacement of a pore lysine with cysteine (K532C) abolished the acidosis-induced increase of C-type inactivation. A model of the Kv1.4 pore, based on the crystal structure of KcsA, shows that H508 and K532 lie close together. It is suggested that the acidosis-induced increase of C-type inactivation involves the charge on H508 and K532.

Electrophysiological response of rat atrial myocytes to acidosis

The effect of acidosis on the electrical activity of isolated rat atrial myocytes was investigated using the patch-clamp technique. Reducing the pH of the bathing solution from 7.4 to 6.5 shortened the action potential. Acidosis had no significant effect on transient outward or inward rectifier currents but increased steady-state outward current. This increase was still present, although reduced, when intracellular Ca(2+) was buffered by 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA); BAPTA also inhibited acidosis-induced shortening of the action potential. Ni(2+) (5 mM) had no significant effect on the acidosis-induced shortening of the action potential. Acidosis also increased inward current at -80 mV and depolarized the resting membrane potential. Acidosis activated an inwardly rectifying Cl(-) current that was blocked by 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS), which also inhibited the acidosis-induced depolarization of the resting membrane potential. It is concluded that an acidosis-induced increase in steady-state outward K(+) current underlies the shortening of the action potential and that an acidosis-induced increase in inwardly rectifying Cl(-) current underlies the depolarization of the resting membrane potential during acidosis.

Electrophysiological response of rat ventricular myocytes to acidosis

The effects of acidosis on the action potential, resting potential, L-type Ca(2+) (I(Ca)), inward rectifier potassium (I(K1)), delayed rectifier potassium (I(K)), steady-state (I(SS)), and inwardly rectifying chloride (I(Cl,ir)) currents of rat subepicardial (Epi) and subendocardial (Endo) ventricular myocytes were investigated using the patch-clamp technique. Action potential duration was shorter in Epi than in Endo cells. Acidosis (extracellular pH decreased from 7.4 to 6.5) depolarized the resting membrane potential and prolonged the time for 50% repolarization of the action potential in Epi and Endo cells, although the prolongation was larger in Endo cells. At control pH, I(Ca), I(K1), and I(SS) were not significantly different in Epi and Endo cells, but I(K) was larger in Epi cells. Acidosis did not alter I(Ca), I(K1), or I(K) but decreased I(SS); this decrease was larger in Endo cells. It is suggested that the acidosis-induced decrease in I(SS) underlies the prolongation of the action potential. I(Cl,ir) at control pH was Cd(2+) sensitive but 4,4'-disothiocyanato-stilbene-2,2'-disulfonic acid resistant. Acidosis increased I(Cl,ir); it is suggested that the acidosis-induced increase in I(Cl,ir) underlies the depolarization of the resting membrane potential.

Inhibition of the K+ channel kv1.4 by acidosis: protonation of an extracellular histidine slows the recovery from N-type inactivation

1. Acidosis alters the transient outward current, ito, in the heart. We have studied the mechanism underlying the effect of acidosis on one of the K+ channels, Kv1.4 (heterologously expressed in Xenopus laevis oocytes), known to underlie ito. 2. At pH 6.5, wild-type Kv1.4 current was inhibited during repetitive pulsing, in part as a result of a slowing of recovery from N-type inactivation. 3. Acidosis still caused slowing of recovery after deletion of just one (either the first or second) of the N-terminal inactivation ball domains. However, deletion of both the N-terminal inactivation ball domains greatly reduced the inhibition. 4. As well as the N-terminus, other parts of the channel are also required for the effect of acidosis, because, whereas the transfer of the N-terminus of Kv1.4 to Kv1.2 conferred N-type inactivation, it did not confer acidosis sensitivity. 5. Replacement of an extracellular histidine with a glutamine residue (H508Q) abolished the slowing of recovery by acidosis. Reduction of C-type inactivation by raising the bathing K+ concentration or by the mutation K532Y also abolished the slowing. 6. It is concluded that binding of protons to H508 enhances C-type inactivation and this causes a slowing of recovery from N-type inactivation and, thus, an inhibition of current during repetitive pulsing.