Balance Between Rapid Delayed Rectifier K+ Current and Late Na+ Current on Ventricular Repolarization: An Effective Antiarrhythmic Target?

BACKGROUND

Rapid delayed rectifier K+ current (IKr) and late Na+ current (INaL) significantly shape the cardiac action potential (AP). Changes in their magnitudes can cause either long or short QT syndromes associated with malignant ventricular arrhythmias and sudden cardiac death.

METHODS

Physiological self AP-clamp was used to measure INaL and IKr during the AP in rabbit and porcine ventricular cardiomyocytes to test our hypothesis that the balance between IKr and INaL affects repolarization stability in health and disease conditions.

RESULTS

We found comparable amount of net charge carried by IKr and INaL during the physiological AP, suggesting that outward K+ current via IKr and inward Na+ current via INaL are in balance during physiological repolarization. Remarkably, IKr and INaL integrals in each control myocyte were highly correlated in both healthy rabbit and pig myocytes, despite high overall cell-to-cell variability. This close correlation was lost in heart failure myocytes from both species. Pretreatment with E-4031 to block IKr (mimicking long QT syndrome 2) or with sea anemone toxin II to impair Na+ channel inactivation (mimicking long QT syndrome 3) prolonged AP duration (APD); however, using GS-967 to inhibit INaL sufficiently restored APD to control in both cases. Importantly, INaL inhibition significantly reduced the beat-to-beat and short-term variabilities of APD. Moreover, INaL inhibition also restored APD and repolarization stability in heart failure. Conversely, pretreatment with GS-967 shortened APD (mimicking short QT syndrome), and E-4031 reverted APD shortening. Furthermore, the amplitude of AP alternans occurring at high pacing frequency was decreased by INaL inhibition, increased by IKr inhibition, and restored by combined INaL and IKr inhibitions.

CONCLUSIONS

Our data demonstrate that IKr and INaL are counterbalancing currents during the physiological ventricular AP and their integrals covary in individual myocytes. Targeting these ionic currents to normalize their balance may have significant therapeutic potential in heart diseases with repolarization abnormalities. Visual Overview: A visual overview is available for this article.

[Effects of allitridum on rapidly delayed rectifier potassium current in HEK293 cell line]

OBJECTIVE

To study the effect of allitridum on rapidly delayed rectifier potassium current (IKr) in HEK293 cell line.

METHODS

HEK293 cells were transiently transfected with HERG channel cDNA plasmid pcDNA3.1 via Lipofectamine. Allitridum was added to the extracellular solution by partial perfusion after giga seal at the final concentration of 30 µmol/L. Whole-cell patch clamp technique was used to record the HERG currents and gating kinetics before and after allitridum exposure at room temperature.

RESULTS

The amplitude and density of IHERG were both suppressed by allitridum in a voltage-dependent manner. In the presence of allitridum, the peak current of IHERG was reduced from 73.5∓4.3 pA/pF to 42.1∓3.6 pA/pF at the test potential of +50 mV (P<0.01). Allitridum also concentration-dependently decreased the density of the IHERG. The IC50 of allitridum was 34.74 µmol/L with a Hill coefficient of 1.01. Allitridum at 30 µmol/L caused a significant positive shift of the steady-state activation curve of IHERG and a markedly negative shift of the steady-state inactivation of IHERG, and significantly shortened the slow time constants of IHERG deactivation.

CONCLUSION

Allitridum can potently block IHERG in HEK293 cells, which might be the electrophysiological basis for its anti-arrhythmic action.

Suppressive effects of diltiazem and verapamil on delayed rectifier K(+)-channel currents in murine thymocytes

BACKGROUND

Lymphocytes predominantly express delayed rectifier K(+)-channels (Kv1.3) in their plasma membranes, and these channels play crucial roles in the lymphocyte activation and proliferation. Since diltiazem and verapamil, which are highly lipophilic Ca(2+) channel blockers (CCBs), exert relatively stronger immunomodulatory effects than the other types of CCBs, they would affect the Kv1.3-channel currents in lymphocytes.

METHODS

Employing the standard patch-clamp whole-cell recording technique in murine thymocytes, we examined the effects of these drugs on the channel currents and the membrane capacitance.

RESULTS

Both diltiazem and verapamil significantly suppressed the peak and the pulse-end currents of the channels, although the effects of verapamil were more marked than those of diltiazem. Both drugs significantly lowered the membrane capacitance, indicating the interactions between the drugs and the plasma membranes.

CONCLUSIONS

This study demonstrated for the first time that CCBs, such as diltiazem and verapamil, exert inhibitory effects on Kv1.3-channels expressed in lymphocytes. The effects of these drugs may be associated with the mechanisms of immunomodulation by which they decrease the production of inflammatory cytokines.

Role of slow delayed rectifying potassium current in dynamics of repolarization and electrical memory in swine ventricles

Dynamics of repolarization, quantified as restitution and electrical memory, impact conduction stability. Relatively less is known about role of slow delayed rectifying potassium current, I(Ks), in dynamics of repolarization and memory compared to the rapidly activating current I(Kr). Trans-membrane potentials were recorded from right ventricular tissues from pigs during reduction (chromanol 293B) and increases in I(Ks) (mefenamic acid). A novel pacing protocol was used to explicitly control diastolic intervals to quantify memory. Restitution hysteresis, a consequence of memory, increased after chromanol 293B (loop thickness and area increased 27 and 38 %) and decreased after mefenamic acid (52 and 53 %). Standard and dynamic restitutions showed an increase in average slope after chromanol 293B and a decrease after mefenamic acid. Increase in slope and memory are hypothesized to have opposite effects on electrical stability; therefore, these results suggest that reduction and enhancement of I(Ks) likely also have offsetting components that affect stability.

[Electrophysiology mechanisms of 4-butyl-alpha-agarofuran: a new anxiolytic and antidepressant drug]

To investigate the electrophysiology mechanisms of new anxiolytic and antidepressant drug: 4-butyl-alpha-agarofuran (AF-5), patch clamp-recording was used to test the effects of AF-5 on voltage-dependent sodium currents, voltage-dependent potassium currents, L-type voltage-dependent calcium currents and GABA dependent Cl(-) currents in primary cultured rat cortical neurons. Effects of AF-5 on Kv2.1 currents, expressed stably in HEK293 cells, were also tested. Our results showed that, delayed rectifier potassium currents (I(K(DR, L-type voltage-dependent calcium currents (I(LC-ca)) in primary cultured rat cortical neurons and Kv2.1 currents in HEK293 cells were significantly inhibited by AF-5, with IC50 as 6.17, 4.4 and 5.29 micromol x L(-1) respectively. However, voltage-dependent sodium currents (I(Na)), GABA dependent Cl(-) currents and transient outward potassium currents (I(K(A)) in primary cultured rat cortical neurons were not significantly blocked by AF-5. Our results concluded that, blocked I(K(DR)) and I(L-Ca) currents may be one of the mechanisms of anxiolytic and antidepression actions of AF-5.

Suppression of phosphoinositide 3-kinase signaling and alteration of multiple ion currents in drug-induced long QT syndrome

Many drugs, including some commonly used medications, can cause abnormal heart rhythms and sudden death, as manifest by a prolonged QT interval in the electrocardiogram. Cardiac arrhythmias caused by drug-induced long QT syndrome are thought to result mainly from reductions in the delayed rectifier potassium ion (K(+)) current I(Kr). Here, we report a mechanism for drug-induced QT prolongation that involves changes in multiple ion currents caused by a decrease in phosphoinositide 3-kinase (PI3K) signaling. Treatment of canine cardiac myocytes with inhibitors of tyrosine kinases or PI3Ks caused an increase in action potential duration that was reversed by intracellular infusion of phosphatidylinositol 3,4,5-trisphosphate. The inhibitors decreased the delayed rectifier K(+) currents I(Kr) and I(Ks), the L-type calcium ion (Ca(2+)) current I(Ca,L), and the peak sodium ion (Na(+)) current I(Na) and increased the persistent Na(+) current I(NaP). Computer modeling of the canine ventricular action potential showed that the drug-induced change in any one current accounted for less than 50% of the increase in action potential duration. Mouse hearts lacking the PI3K p110α catalytic subunit exhibited a prolonged action potential and QT interval that were at least partly a result of an increase in I(NaP). These results indicate that down-regulation of PI3K signaling directly or indirectly via tyrosine kinase inhibition prolongs the QT interval by affecting multiple ion channels. This mechanism may explain why some tyrosine kinase inhibitors in clinical use are associated with increased risk of life-threatening arrhythmias.

[Effect of GBE50 on delayed rectifier potassium current of ventricular myocytes in ischemic guinea pig]

OBJECTIVE

Ginkgo biloba extract 50 (GBE50) is a new multicomponent drug with a polyvalent action extracted from the leave of Ginkgo biloba. The aim of this experiment was to study the effects of GBE50 on delayed rectifier potassium current (I(K)) in ventricular myocytes under normal and simulated ischemia conditions in guinea pigs.

METHODS

Single ventricular myocytes were isolated by an enzymatic dissociation method. I(K) were recorded by whole-cell patch clamp technique in voltage clamp mode. GBE50 was added to the perfusion chamber from low to high concentrations (25, 50,100 mg/L) in normal condition. Different concentrations of GBE50 (25, 50, 100 mg/L) were prepared with simulated ischemic fluid.

RESULTS

(1) Under normal condition, 100 mg/L GBE50 decreased I(K) (n = 7, P < 0.05). (2) Under ischemia condition, it was observed that I(K) was inhibited (n = 8, P < 0.05). (3) Perfusion with ischemia solution containing 50 mg/L (n = 8, P > 0.05) and 100 mg/L GBE50 (n = 6, P > 0.05) could reverse the decrease of I(K).

CONCLUSION

GBE50 significantly decreased I(K) in a concentration-dependent manner. GBE50 could alleviate the electrophysiological heterogeneity of myocardium to prevent ischemic myocardium from arrhythmia.

Effect of etomidate on voltage-dependent potassium currents in rat isolated hippocampal pyramidal neurons

BACKGROUND

Previous studies demonstrated general anesthetics affect potassium ion channels, which may be one of the mechanisms of general anesthesia. Because the effect of etomidate on potassium channels in rat hippocampus which is involved in memory function has not been studied, we investigated the effects of etomidate on both delayed rectifier potassium current (I(K(DR))) and transient outward potassium current (I(K(A))) in acutely dissociated rat hippocampal pyramidal neurons.

METHODS

Single rat hippocampal pyramidal neurons from male Wistar rats of - 10 days were acutely dissociated by enzymatic digestion and mechanical dispersion according to the methods of Kay and Wong with slight modification. Voltage-clamp recordings were performed in the whole-cell patch clamp configuration. Currents were recorded with a List EPC-10 amplifier and data were stored in a computer using Pulse 8.5. Student's paired two-tail t test was used for data analysis.

RESULTS

At the concentration of 100 micromol/L, etomidate significantly inhibited I(K(DR)) by 49.2% at +40 mV when depolarized from -110 mV (P < 0.01, n = 8), while did not affect I(K(A)) (n = 8, P > 0.05). The IC(50) value of etomidate for blocking I(K(DR)) was calculated as 5.4 micromol/L, with a Hill slope of 2.45. At the presence of 10 micromol/L etomidate, the V1/2 of activation curve was shifted from (17.3 +/- 1.5) mV to (10.7 +/- 2.9) mV (n = 8, P < 0.05), the V1/2 of inactivation curve was shifted from (-18.3 +/- 2.2) mV to (-45.3 +/- 9.4) mV (n = 8, P < 0.05). Etomidate 10 micromol/L shifted both the activation curve and inactivation curve of I(K(DR)) to negative potential, but mainly affected the inactivation kinetics.

CONCLUSIONS

Etomidate potently inhibited I(K(DR)) but not I(K(A)) in rat hippocampal pyramidal neurons. I(K(DR)) was inhibited by etomidate in a concentration-dependent manner, while I(K(A)) remained unaffected.

[The effects of Sphing-1-phosphate(S1P) on the potassium channel of the ventricular myocytes]

AIM

To study the effect of Sphingosine-1-phosphate on the delayed rectifier potassium current (IK) and the inward rectifier potassium current (IK1) of guinea pig isolated ventricular myocytes.

METHODS

Whole cell patch clamp technique was applied to record the delayed rectifier potassium current and the delayed rectifier potassium current of guinea pig isolated ventricular myocytes.

RESULTS

(1) IK of S1P (1.1 micromol/L) decreased from (1.2 +/- 0.26)nA to (0.95 +/- 0.23)nA. While IK of S1P (2.2 micromol/L) decreased from (1.43 +/- 0.31)nA to (1.02 +/- 0.28)nA. There was significant difference compared to control group (P < 0.01, n = 6). The IK peak value was decreased from (1.29 +/- 0.26) nA to (1.26 +/- 0.37)nA at the group of S1P (1.1 micromol/L) plus suramin (200 micromol/L) and showed no significant difference compared to control group (P > 0.05, n = 6). (2) IK1 of S1P (1.1 micromol/L) decreased from (-8.94 +/- 2.01)nA to (-8.81 = 1.55)nA. While IK of S1P (2.2 micromol/L) decreased from (-8.86 +/- 1.59)nA to (-8.55 +/- 1.39)nA. There was no significant difference compared to control group (P > 0.05, n = 6).

CONCLUSION

S1P inhibits delayed rectifier potassium current of ventricular myocyte in guinea pig remarkably, S1P shows no effects on delayed rectifier potassium current of ventricular myocyte in guinea pig.

Delayed rectifier potassium channels are involved in SO2 derivative-induced hippocampal neuronal injury

Recent studies implicate the possible neurotoxicity of SO(2), however, its mechanisms remain unclear. In the present study, we investigated SO(2) derivative-induced effect on delayed rectifier potassium channels (I(K)) and cellular death/apoptosis in primary cultured hippocampal neurons. The results demonstrate that SO(2) derivatives (NaHSO(3) and Na(2)SO(3), 3:1M/M) effectively augmented I(K) and promoted the activation of delayed rectifier potassium channels. Also, SO(2) derivatives increased neuronal death percentage and contributed to the formation of DNA ladder in concentration-dependent manners. Interestingly, the neuronal death and DNA ladder formation, caused by SO(2) derivatives, could be attenuated by the delayed rectifier potassium channel blocker (tetraethylammonium, TEA), but not by the transient outward potassium channel blocker (4-aminopyridine, 4-AP). It implies that stimulating delayed rectifier potassium channels were involved in SO(2) derivative-caused hippocampal neuronal insults, and blocking these channels might be one of the possibly clinical treatment for SO(2)-caused neuronal dysfunction.

Evidence for state-dependent block of DPI 201-106, a synthetic inhibitor of Na+ channel inactivation, on delayed-rectifier K+ current in pituitary tumor (GH3) cells

DPI 201-107 (DPI), a diphenylpiperazinylindole derivative, was reported to be a cardio-selective modifier of voltage-gated Na+ channels. It remains unclear whether DPI has any effects on ion currents. The effects of DPI on ion currents and membrane potential in pituitary tumor (GH3) cells were investigated in this study. DPI (1-100 microM) suppressed the amplitude of delayed-rectifier K+ current (I(K(DR))) in a concentration-dependent manner with an IC(50) value of 9.4 microM. The presence of DPI also enhanced the rate and extent of I(K(DR)) inactivation. Recovery from block by DPI (10 microM) was fitted by a single exponential. Crossover of tail currents during the exposure to DPI was also observed. Under current-clamp recordings, DPI prolonged action potential duration in GH3 cells. With a minimal binding scheme, DPI-induced block of I(K(DR))) was quantitatively provided. The exposure to DPI also blocked I(K(DR))) with a concomitant increase in current inactivation in NG108-15 neuronal cells. Taken together, the results imply that DPI acts as an open-channel blocker of delayed-rectifier K+ channels in these cells. The widening of action potentials induced by DPI in these cells may be explained mainly by its block of I(K(DR))) in a state-dependent manner.

Olanzapine prolongs cardiac repolarization by blocking the rapid component of the delayed rectifier potassium current

Prolongation of the QT interval has been observed during treatment with olanzapine, a thienobenzodiazepine antipsychotic agent. Our objectives were 1) to characterize the effects of olanzapine on cardiac repolarization and 2) to evaluate effects of olanzapine on the major time-dependent outward potassium current involved in cardiac repolarization, namely I(Kr) (I(Kr): rapid component of the delayed rectifier potassium current).Isolated, buffer-perfused guinea pig hearts (n = 40) were stimulated at different pacing cycle lengths (150-250 msec) and exposed to olanzapine at concentrations ranging from 1 to 100 microM. Olanzapine increased monophasic action potential duration measured at 90% repolarization (MAPD90) in a concentration-dependent manner by 6.7 +/- 0.7 msec at 3 microM but by 26.0 +/- 4.3 msec at 100 microM (250 msec cycle length). Increase in MAPD(90) was also reverse frequency dependent; 30 microM olanzapine increased MAPD90 by 28.0 +/- 6.2 msec at a pacing cycle length of 250 msec but by only 18.9 +/- 2.2 msec at a pacing cycle length of 150 msec. Experiments in HERG-transfected (HERG: human ether-a-gogo-related gene) HEK293 cells (n = 36) demonstrated concentration-dependent block of the rapid component (I(Kr)) of the delayed rectifier potassium current: tail current was decreased 50% at olanzapine 3.8 microM. Olanzapine possesses direct cardiac electrophysiological effects similar to those of class III anti-arrhythmic drugs. These effects were observed at concentrations that can be measured in patients under conditions of impaired drug elimination such as renal or hepatic insufficiency, during co-administration of other CYP1A2 substrates/inhibitors or after drug overdose. These results offer a new potential explanation for QT prolonging effects observed during olanzapine treatment in patients.

The plateau outward current in canine ventricle, sensitive to 4-aminopyridine, is a constitutive contributor to ventricular repolarization

BACKGROUND AND PURPOSE

I(Kur) (Ultra-rapid delayed rectifier current) has microM sensitivity to 4-aminopyridine (4-AP) and is an important modulator of the plateau amplitude and action potential duration in canine atria. Kv1.5 encodes I(Kur) and is present in both atria and ventricles in canines and humans. We hypothesized that a similar plateau outward current with microM sensitivity to 4-AP is present in canine ventricle.

EXPERIMENTAL APPROACH

We used established voltage clamp protocols and used 4-AP (50 and 100 microM) to measure a plateau outward current in normal canine myocytes isolated from the left ventricular mid-myocardium.

KEY RESULTS

Action potential recordings in the presence of 4-AP showed significant prolongation of action potential duration at 50 and 90% repolarization at 0.5 and 1 Hz (P<0.05), while no prolongation occurred at 2 Hz. Voltage clamp experiments revealed a rapidly activating current, similar to current characteristics of canine atrial I(Kur), in approximately 70% of left ventricular myocytes. The IC(50) of 4-AP for this current was 24.2 microM. The concentration of 4-AP used in our experiments resulted in selective blockade of an outward current that was not I(to) or I(Kr). Beta-adrenergic stimulation with isoprenaline significantly increased the 4-AP sensitive outward current density (P<0.05), suggesting a role for this current during increased sympathetic stimulation. In silico incorporation into a canine ventricular cell model revealed selective AP prolongation after current blockade.

CONCLUSIONS AND IMPLICATIONS

Our results support the existence of a canine ventricular plateau outward current sensitive to micromolar 4-AP and its constitutive role in ventricular repolarization.

Activation of BKCa channels via cyclic AMP- and cyclic GMP-dependent protein kinases by eugenosedin-A in rat basilar artery myocytes

BACKGROUND AND PURPOSE

The study investigated whether eugenosedin-A, a 5-hydroxytryptamine and alpha/beta adrenoceptor antagonist, enhanced delayed-rectifier potassium (K(DR))- or large-conductance Ca(2+)-activated potassium (BK(Ca))-channel activity in basilar artery myocytes through cyclic AMP/GMP-dependent and -independent protein kinases.

EXPERIMENTAL APPROACH

Cerebral smooth muscle cells (SMCs) were enzymatically dissociated from rat basilar arteries. Conventional whole cell, perforated and inside-out patch-clamp electrophysiology was used to monitor K(+)- and Ca(2+)-channel activities.

KEY RESULTS

Eugenosedin-A (1 microM) did not affect the K(DR) current but dramatically augmented BK(Ca) channel activity in a concentration-dependent manner. Increased BK(Ca) current was abolished by charybdotoxin (ChTX, 0.1 microM) or iberiotoxin (IbTX, 0.1 microM), but not affected by a small-conductance K(Ca) blocker (apamin, 100 microM). BK(Ca) current activation by eugenosedin-A was significantly inhibited by an adenylate cyclase inhibitor (SQ 22536, 10 microM), a soluble guanylate cyclase inhibitor (ODQ, 10 microM), competitive antagonists of cAMP and cGMP (Rp-cAMP, 100 microM and Rp-cGMP, 100 microM), and cAMP- and cGMP-dependent protein kinase inhibitors (KT5720, 0.3 microM and KT5823, 0.3 microM). Eugenosedin-A reversed the inhibition of BK(Ca) current induced by the protein kinase C activator, phorbol myristyl acetate (PMA, 0.1 microM). Eugenosedin-A also prevented BK(Ca) current inhibition induced by adding PMA, KT5720 and KT5823. Moreover, eugenosedin-A reduced the amplitude of voltage-dependent L-type Ca(2+) current (I(Ca,L)), but without modifying the voltage-dependence of the current.

CONCLUSIONS AND IMPLICATIONS

Eugenosedin-A enhanced BK(Ca) currents by stimulating the activity of cyclic nucleotide-dependent protein kinases. Physiologically, this activation would result in the closure of voltage-dependent calcium channels and thereby relax cerebral SMCs.

Basal and angiotensin II-inhibited neuronal delayed-rectifier K+ current are regulated by thioredoxin

In previous studies, we determined that macrophage migration inhibitory factor (MIF), acting intracellularly via its intrinsic thiol-protein oxidoreductase (TPOR) activity, stimulates basal neuronal delayed-rectifier K+ current ( IKv) and inhibits basal and angiotensin (ANG) II-induced increases in neuronal activity. These findings are the basis for our hypothesis that MIF is a negative regulator of ANG II actions in neurons. MIF has recently been recategorized as a member of the thioredoxin (Trx) superfamily of small proteins. In the present study we have examined whether Trx influences basal and ANG II-modulated IKv in an effort to determine whether the Trx superfamily can exert a general regulatory influence over neuronal activity and the actions of ANG II. Intracellular application of Trx (0.8–80 nM) into rat hypothalamic/brain stem neurons in culture increased neuronal IKv, as measured by voltage-clamp recordings. This effect of Trx was abolished in the presence of the TPOR inhibitor PMX 464 (800 nM). Furthermore, the mutant protein recombinant human C32S/C35S-Trx, which lacks TPOR activity, failed to alter neuronal IKv. Trx applied at a concentration (0.08 nM) that does not alter basal IKv abolished the inhibition of neuronal IKv produced by ANG II (100 nM). Given our observation that ANG II increases Trx levels in neuronal cultures, it is possible that Trx (like MIF) has a negative regulatory role over basal and ANG II-stimulated neuronal activity via modulation of IKv. Moreover, these data suggest that TPOR may be a general mechanism for negatively regulating neuronal activity.

Substance P provides neuroprotection in cerebellar granule cells through Akt and MAPK/Erk activation: Evidence for the involvement of the delayed rectifier potassium current

In the current study, we have evaluated the ability of substance P (SP) and other neurokinin 1 receptor (NK1) agonists to protect, in a dose- and time-dependent manner, primary cultures of rat cerebellar granule cells (CGCs) from serum and potassium deprivation-induced cell death (S-K5). We also established the presence of SP high affinity NK1 transcripts and the NK1 protein localization in the membrane of a sub-population of CGCs. Moreover, SP significantly and dose-dependently reduced the Akt 1/2 and Erk1/2 dephosphorylation induced by S-K5 conditions, as demonstrated by Western blot analysis. Surprisingly, in SP-treated CGCs caspase-3 activity was not inhibited, while the calpain-1 activity was moderately reduced. Corroborating this result, SP blocked calpain-mediated cleavage of tau protein, as demonstrated by the reduced appearance of a diagnostic fragment of 17 kDa by Western blot analysis. In addition, SP induced a significant reduction of the delayed rectifier K+ currents (Ik) in about 42% of the patched neurons, when these were evoked with depolarizing potential steps. Taken together, the present results demonstrate that the activation of NK1 receptors expressed in CGCs promote the neuronal survival via pathways involving Akt and Erk activation and by inhibition of Ik which can contribute to the neuroprotective effect of the peptide.

Temperature acclimation modifies sinoatrial pacemaker mechanism of the rainbow trout heart

The hypothesis of pacemaker level origin of thermal compensation in heart rate was tested by recording action potentials (AP) in intact sinoatrial tissue and enzymatically isolated pacemaker cells of rainbow trout acclimated at 4°C (cold) and 18°C (warm). With electrophysiological recordings, the primary pacemaker was located at the base of the sinoatrial valve, where a morphologically distinct ring of tissue comprising myocytes and neural elements was found by histological examination. Intrinsic beating rate of this pacemaker was higher in cold-acclimated (46 ± 6 APs/min) than warm-acclimated trout (38 ± 3 APs/min; P < 0.05), and a similar difference was seen in beating rate of isolated pacemaker cells (44 ± 6 vs. 38 ± 6 APs/min; P < 0.05), supporting the hypothesis that thermal acclimation modifies the intrinsic pacemaker mechanism of fish heart. Inhibition of sarcoplasmic reticulum (SR) with 10 μM ryanodine and 1 μM thapsigargin did not affect heart rate in either warm- or cold-acclimated trout at 11°C but reduced heart rate in warm-acclimated trout from 74 ± 2 to 42 ± 6 APs/min ( P < 0.05) at 18°C. At 11°C, a half-maximal blockade of the delayed rectifier K+current ( IKr) with 0.1 μM E-4031 reduced heart rate more in warm-acclimated (from 45 ± 1 to 24 ± 5 APs/min) than cold-acclimated trout (56 ± 3 vs. 48 ± 2 APs/min), whereas IKrdensity was higher and AP duration less in cold-acclimated trout ( P > 0.05). Collectively, these findings suggest that a cold-induced increase in AP discharge frequency is at least partly due to higher density of the IKrin the cold-acclimated trout, whereas contribution of SR Ca2+release to thermal compensation of heart rate is negligible.

Blockade of atrial-specific K+-currents increases atrial but not ventricular contractility by enhancing reverse mode Na+/Ca2+-exchange

BACKGROUND

AVE0118 (2'-{[2-(4-Methoxy-phenyl)-acetylamino]-methyl}-biphenyl-2-carboxylic acid (2-pyridin-3-yl-ethyl)-amide) blocks atrial ultrarapid delayed rectifier currents (I(Kur)) and prolongs the atrial action potential (AP) plateau without affecting ventricular repolarisation. In patients with atrial contractile dysfunction due to atrial tachyarrhythmias, this response might increase atrial contractility without risk of ventricular proarrhythmia. This study was designed to evaluate the inotropic mechanisms of AVE0118.

METHODS AND RESULTS

In isometrically contracting atrial trabeculae, AVE0118 increased contractile force by 55.4% in sinus rhythm patients (n = 9) and by 107.4% in patients with atrial fibrillation (n = 8). In freshly isolated canine atrial myocytes studied under perforated patch current clamp (37 degrees C), AVE0118 increased myocyte fractional shortening from 3.8+/-0.6 to 9.6+/-0.8% and prolonged action potential duration at 30% repolarisation from 9+/-2 to 102+/-11 ms. Clamping cells to an AP waveform recorded during exposure to AVE0118 produced the same inotropic response as the drug itself. In action potential clamp, peak Ca2+ inward current (I(CaL)) current declined from 5.5+/-1.3 pA/pF during control to 4.1+/-0.7 pA/pF when an AP recorded in the presence of AVE0118 was used as command waveform. However, I(CaL) was more sustained with AVE0118 and the time integral did not change (135+/-37 vs. 173+/-30 pA/pFms, p = ns). Importantly, blockade of reverse mode Na+/Ca2+-exchanger activity with 5 microM KBR7943 or using a Na+-free pipette solution abolished the positive inotropic effect of the AP recorded in the presence of AVE0118. In ventricular myocytes AVE0118 did not elicit a positive inotropic response.

CONCLUSIONS

Block of I(Kur) by AVE0118 enhances atrial contractility both in patients with sinus rhythm and atrial fibrillation. The positive inotropic effect is atrial-specific and due to the changes of the action potential configuration which enhances Ca2+ entry via reverse mode Na+/Ca2+ exchange.

Imipramine inhibits A-type delayed rectifier and ATP-sensitive K+ currents independent of G-protein and protein kinase C in murine proximal colonic myocytes

The effects of imipramine on A-type delayed rectifier K+ currents and ATP-sensitive K+ (KATP) currents were studied in isolated murine proximal colonic myocytes using the whole-cell patch-clamp technique. Depolarizing test pulses between -80 mV and +30 mV with 10 mV increments from the holding potential of -80 mV activated voltage-dependent outward K+ currents that peaked within 50 ms followed by slow decreasing sustained currents. Early peak currents were inhibited by the application of 4-aminopyridine, whereas sustained currents were inhibited by the application of TEA. The peak amplitude of A-type delayed rectifier K+ currents was reduced by external application of imipramine. The half-inactivation potential and the half-recovery time of A-type delayed rectifier K+ currents were not changed by imipramine. With 0.1 mM ATP and 140 mM K+ in the pipette and 90 mM K+ in the bath solution and a holding potential of -80 mV, pinacidil activated inward currents; this effect was blocked by glibenclamide. Imipramine also inhibited KATP currents. The inhibitory effects of imipramine in A-type delayed rectifier K+ currents and KATP currents were not changed by guanosine 5-O-(2-thiodiphosphate) (GDPbetaS) and chelerythrine, a protein kinase C inhibitor. These results suggest that imipramine inhibits A-type delayed rectifier K+ currents and KATP currents in a manner independent of G-protein and protein kinase C.

Inhibition by bis(7)-tacrine of native delayed rectifier and KV1.2 encoded potassium channels

Bis(7)-tacrine [bis(7)-tetrahydroaminacrine] acts as an AChE inhibitor and also exerts modulatory effects on many ligand-gated ion channels and voltage-gated Ca(2+) and K(+) channels. It has been reported previously that tacrine and some other AChE inhibitors suppressed I(K(A)) in central and peripheral neurons. The present study aimed to explore whether bis(7)-tacrine could modulate the function of native delayed rectifier potassium channels in DRG neurons and K(V)1.2 encoded potassium channels expressed in oocytes. We found that both delayed rectifier potassium currents (I(K(DR))) in rat DRG neurons and the currents recorded from oocytes expressing K(V)1.2 (I(K(K(V)1.2))) were suppressed by bis(7)-tacrine, the potency of which was two orders greater than that of tacrine. The IC(50) values for bis(7)-tacrine and tacrine inhibition of I(K(KD)) in DRG neurons were 0.72+/-0.05 and 58.3+/-3.7 microM, respectively; while the two agents inhibited I(K(K(V)1.2)) in oocytes with an IC(50) of 0.24+/-0.06 and 102.1+/-21.5 microM, respectively. The possible mechanism for bis(7)-tacrine inhibition of I(K(A)) and I(K(K(V)1.2)) was identified as the suppression of their activation, inactivation.

Reduction of repolarization reserve by halothane anaesthesia sensitizes the guinea-pig heart for drug-induced QT interval prolongation

The utility of halothane-anaesthetized guinea-pigs as an in vivo model for predicting the clinical potential of a drug to induce QT interval prolongation was assessed using the electrocardiogram and monophasic action potential (MAP) recordings with electrical ventricular pacing. Intravenous administration of D-sotalol (0.3 mg kg(-1)) and terfenadine (0.3 mg kg(-1)), blockers of a rapid component of delayed rectifier potassium currents, prolonged the QT interval by 32+/-7 and 23+/-6 ms, respectively, whereas chromanol 293B (1 mg kg(-1)), a blocker of a slow component of delayed rectifier potassium currents, lengthened it by 33+/-8 ms. The extent of the QT interval prolongation by these drugs was greater than those in previous reports using pentobarbital-anaesthetized guinea-pigs. The MAP duration at the control was shortened by decreasing the pacing cycle length from 400 to 200 ms, but the MAP duration at each cycle length was prolonged by D-sotalol. The formulas of Van de Water, Matsunaga, Fridericia and Bazett showed good correlation of the repolarization period when compared with the MAP duration at a pacing cycle length of 400 ms. The halothane-anaesthetized guinea-pig model may possess enough sensitivity to detect drug-induced QT interval prolongation, indicating that halothane anaesthesia can reduce the repolarization reserve of the heart in vivo.

Effects of carvedilol on transient outward and ultra-rapid delayed rectifier potassium currents in human atrial myocytes

Carvedilol is a beta- and alpha(1)-adrenoceptor antagonist. It is widely used in the treatment of cardiovascular diseases including atrial arrhythmias. However, it is unclear whether carvedilol may affect the repolarization currents, transient outward K(+) current (I(to)) and ultra-rapid delayed rectifier K(+) current (I(Kur)) in the human atrium. The present study evaluated effects of carvedilol on I(to) and I(Kur) in isolated human atrial myocytes by whole-cell patch-clamp recording technique. We found that carvedilol reversibly inhibited I(to) and I(Kur) in a concentration-dependent manner. Carvedilol (0.3 microM) suppressed I(to) from 9.2+/-0.5 pA/pF to 4.8+/-0.5 pA/pF (P<0.01) and I(Kur) from 3.6+/-0.5 pA/pF to 1.9+/-0.3 pA/pF (P<0.01) at +50 mV. I(to) was inhibited in a voltage-dependent manner, being significantly attenuated at test potentials from +10 to +50 mV, whereas the inhibition of I(Kur) was independent. The concentration giving a 50% inhibition was 0.50 microM for I(to) and 0.39 microM for I(Kur). Voltage-dependence of activation, inactivation and time-dependent recovery from inactivation of I(to) were not altered by carvedilol. However, time to peak and time-dependent inactivation of I(to) were significantly accelerated, indicating an open channel blocking action. The findings indicate that carvedilol significantly inhibits the major repolarization K(+) currents I(to) and I(Kur) in human atrial myocytes.

AVE0118, Blocker of the Transient Outward Current ( I to ) and Ultrarapid Delayed Rectifier Current ( I Kur ), Fully Restores Atrial Contractility After Cardioversion of Atrial Fibrillation in the Goat

Background— The loss of atrial contractile function after cardioversion of atrial fibrillation (AF) contributes to the thromboembolic risk associated with AF. The newly developed blocker of the transient outward current ( I to ) and ultrarapid delayed rectifier current ( I Kur ) AVE0118 prolongs atrial action potential duration and might therefore enhance atrial contractility. We compared the ability of AVE0118 to restore atrial contraction after cardioversion of AF with the efficacy of conventional positive inotropic compounds in the goat model of AF.

Methods and Results— Eighteen goats were chronically instrumented with epicardial electrodes, a pressure transducer in the right atrium, and piezoelectric crystals to measure right atrial diameter. Atrial contractility and refractoriness and QT duration were measured before and after 1 week (3 to 8 days) of AF induced by repetitive burst pacing. The measurements were repeated after administration of digoxin (0.02 mg/kg), dobutamine (5 μg · kg −1 · min −1 ), the Ca 2+ sensitizer EMD57033 (1 mg · kg −1 · min −1 ), the L-type Ca 2+ channel agonist BayY5959 (0.1 mg · kg −1 · min −1 ), and AVE0118 (0.01 to 0.2 mg · kg −1 · min −1 ). The effect of AVE0118 on the configuration of atrial monophasic action potentials was determined for comparison. After 1 week of AF, atrial contractility during sinus rhythm or slow atrial pacing was reduced to <10%. Digoxin and dobutamine failed to increase atrial contractility. EMD57033 restored 41% and BayY5959 restored 48% of atrial contractility at baseline. BayY5959 significantly prolonged QT duration by 24.7%. AVE0118 enhanced atrial contraction to 156% of the baseline value. The positive inotropic effect was accompanied by a pronounced prolongation of atrial action potential duration and refractoriness, whereas QT duration remained unchanged.

Conclusions— Conventional positive inotropic drugs showed limited effect on atrial contractility after cardioversion of AF or produced QT prolongation. In contrast, the I to / I Kur blocker AVE0118 fully restored atrial contraction without proarrhythmic effects on the ventricle.

An M2-like muscarinic receptor enhances a delayed rectifier K+ current in rat sympathetic neurones

BACKGROUND AND PURPOSE

Resting superior cervical ganglion (SCG) neurones are phasic cells that switch to a tonic mode of firing upon muscarinic receptor stimulation. This effect is partially due to the muscarinic inhibition of the M-current. Because delayed rectifier K+ channels are essential to sustain tonic firing in central neurones, we asked whether the delayed rectifier current IKV in SCG neurones was modulated by the muscarinic receptors expressed in these cells.

EXPERIMENTAL APPROACH

Whole-cell patch-clamp records of M-current and IKV were done in cultured or acutely dissociated rat SCG neurones. To characterize the receptor that regulates IKV, cells were bathed with muscarinic agonists and antagonists, relatively specific for receptor subtypes.

KEY RESULTS

The muscarinic agonist oxotremorine-M (Oxo-M) enhanced IKV by approximately 46% relative to its basal value. This effect remained unaltered when M-current was suppressed by linopirdine or Ba2+. Enhancement of IKV was insensitive to the M1-antagonist pirenzepine, whereas it was inhibited (approximately 60%) by the M2/4-antagonist himbacine. Further, the relatively specific M2-agonist bethanechol was as potent as Oxo-M in enhancing IKV. The modulation of IKV was insensitive to pertussis toxin (PTX), but was severely attenuated when internal ATP was replaced by its non-hydrolysable analogue AMP-PNP.

CONCLUSIONS AND IMPLICATIONS

These results suggest that an M2-like muscarinic receptor couples to a PTX-insensitive G-protein and to an ATP-dependent pathway to enhance IKV. Modulation of IKV must be taken into consideration in order to understand more precisely how muscarinic receptors acting on different ion channels regulate sympathetic excitability.

Long QT 1 Mutation KCNQ1A344VIncreases Local Anesthetic Sensitivity of the Slowly Activating Delayed Rectifier Potassium Current

Background

Anesthesia in patients with long QT syndrome (LQTS) is a matter of concern. Congenital LQTS is most frequently caused by mutations in KCNQ1 (Kv7.1), whereas drug-induced LQTS is a consequence of HERG (human ether-a-go-go-related gene) channel inhibition. The aim of this study was to investigate whether the LQT1 mutation A344V in the S6 region of KCNQ1, at a position corresponding to the local anesthetic binding site in HERG, may render drug insensitive KCNQ1 channels into a toxicologically relevant target of these pharmacologic agents. This may suggest that LQTS constitutes not only a nonspecific but also a specific pharmacogenetic risk factor for anesthesia.

Methods

The authors examined electrophysiologic and pharmacologic properties of wild-type and mutant KCNQ1 channels. The effects of bupivacaine, ropivacaine, and mepivacaine were investigated using two-electrode voltage clamp and whole cell patch clamp recordings.

Results

The mutation A344V induced voltage-dependent inactivation in homomeric KCNQ1 channels and shifted the voltage dependence of KCNQ1/KCNE1 channel activation by +30 mV. The mutation furthermore increased the sensitivity of KCNQ1/KCNE1 channels for bupivacaine 22-fold (KCNQ1wt/KCNE1: IC50 = 2,431 +/- 582 microM, n = 20; KCNQ1A344V/KCNE1: IC50 = 110 +/- 9 microM, n = 24). Pharmacologic effects of the mutant channels were dominant when mutant and wild-type channels were coexpressed. Simulation of cardiac action potentials with the Luo-Rudy model yielded a prolongation of the cardiac action potential duration and induction of early afterdepolarizations by the mutation A344V that were aggravated by local anesthetic intoxication.

Conclusions

The results indicate that certain forms of the LQTS may constitute a specific pharmacogenetic risk factor for regional anesthesia.

Acute estradiol application increases inward and decreases outward whole-cell currents of neurons in rat hypothalamic ventromedial nucleus

Acute estradiol (E2) can potentiate the excitatory responses of hypothalamic ventromedial nucleus (VMN) neurons to neurotransmitters. To investigate the mechanism(s) underlying the potentiation, the whole-cell patch voltage clamp technique was used to study VMN neurons in hypothalamic slices prepared from female juvenile (3-5 weeks) rats. A voltage step and/or ramp was applied every 5 min to evoke whole-cell currents before, during and after a treatment with E2 (10 nM), corticosterone (10 nM) or vehicle for up to 20 min. Acute E2 increased inward currents in 38% of neurons tested. Their average peak inward current amplitudes started to increase within 5 min and reached the maximum of 163% of pretreatment level (Pre) at 20 min of treatment before recovering toward Pre. These increases are significantly greater than the Pre and corresponding vehicle controls and non-responsive neurons. Outward currents were decreased significantly by E2 in 27% of E2-treated cells, down to 60% of Pre levels. E2 also appeared to affect the kinetics of the inward and outward currents of estrogen-responsive neurons. Whenever observed, the effects of acute E2 were reversible after a 5- to 10-min washing. Probability analysis indicates that E2 affected the inward and the outward currents independently. The E2 effects are specific in that they were not produced by similar treatment with vehicle or corticosterone. Pharmacological characterizations using ion replacement and channel blockers showed that the inward currents were mediated practically all by Na(+) and the outward currents mainly by K(+). Thus, acute E2 can enhance inward Na(+) and attenuate outward K(+) currents. Since both effects will lead to an increase in neuronal excitability, they may explain our previous observation that E2 potentiates the excitation of VMN neurons.

3-Benzidino-6(4-chlorophenyl) pyridazine blocks delayed rectifier and transient outward potassium current in acutely isolated rat hippocampal pyramidal neurons

3-[(beta-morpholinoethyl)amino]-4-methyl-6-phenylpyridazine (minaprine) is an acetylcholinesterase (AChE) inhibitor. 3-Benzidino-6(4-chlorophenyl) pyridazine (BCP) and minaprine have a central pyridazine ring in common. In this study, we investigated the effects of BCP on delayed rectifier potassium current (IK(DR)) and transient outward potassium current (IK(A)) in acutely isolated rat hippocampal pyramidal neurons by using whole-cell patch-clamp technique. IK(DR) and IK(A) were inhibited by BCP (0.01-500 microM) in a concentration-dependent and voltage-dependent manner. The IC50 value for the blocking action of BCP on IK(DR) and IK(A) was calculated as 7.13+/-0.18 microM and 0.55+/-0.11 microM, respectively. At the concentration of 10 microM, BCP shifted the activation curve of IK(DR) to positive potential by 29.09 mV. Meanwhile, at the concentration of 10 microM, BCP also shifted the activation and inactivation curve of IK(A) to positive potential by 34.18 and 22.47 mV, respectively. In conclusion, BCP potently inhibits IK(DR) and IK(A) in rat hippocampal pyramidal neurons.

Effects of Liuwei Dihuang decoction on ion channels and synaptic transmission in cultured hippocampal neuron of rat

The effect of Liuwei Dihuang decoction (LW), a traditional Chinese medicine (TCM) prescription, on voltage-dependent currents and synaptic transmission were investigated in cultured hippocampal neurons of rat by whole-cell patch clamp recording technique. After application with serum from LW-treated rats, termed LW-containing serum (LWCS) for 48 h, the amplitude of delay rectifying K+ current (IK) and voltage-gated Ca2+ current (ICa) decreased. While the frequency of spontaneous excitatory post-synaptic current (sEPSC) and miniature excitatory post-synaptic current (mEPSC) increased significantly. Yet the amplitude of voltage-depended Na+ current (INa) and transient outward K+ current (IA), membrane capacitance and resistance remained unchanged. The results indicated that LWCS possessed the effect of modulating or improving neuronal and synaptic function, which possibly contribute to the cognition enhancing effect of LW.

Apolipoprotein E4 suppresses delayed-rectifier potassium channels in membrane patches excised from hippocampal neurons

Recent studies show a clear association between Alzheimer's disease (AD) and the apolipoprotein E epsilon 4 allele (APOE4). The mechanisms underlying apoE4-mediated detrimental effects have not been well-clarified. The present study investigates possible effects of apoE4 on the delayed-rectifier potassium (IK) channels in inside-out membrane patches excised from rat hippocampal neurons. Acute application of apoE4 (0.5 microM) to the inside of the membrane patches markedly and reversibly suppressed the single IK channel activities. The average open probability and open frequency of IK channels decreased by (92.6+/-7.1)% and (88.6+/-3.2)%, respectively. The mean open time of IK channels decreased by (81.6+/-6.7)%, and the mean closed-time of them increased by 6.9+/-1.9 fold. Meanwhile, the mean current amplitude of IK channels was not significantly affected. In contrast, application of apolipoprotein A (apoA, 0.5 microM), another member of apolipoprotein family with similar molecular weight and amino acid sequence to apoE4, did not exhibit any effects on IK currents. These results indicate that apoE4 molecules can rapidly suppress the activities of IK channels in hippocampal neurons when they act on the inner side of the neuronal membrane. We propose that the overproduction of apoE4 in neurons may suppress normal IK channel activities and thus be responsible for the late-developed neuronal damages related to the pathogenesis of AD.

Contribution of I Ks to ventricular repolarization in canine myocytes

The role of the slow delayed rectifier K(+) current (I (Ks)) in cardiac repolarization seems to be largely influenced by the experimental conditions including the species and tissue studied. The aim of this study was to determine the contribution of I (Ks) to repolarization in canine ventricular myocytes by measuring the frequency dependent action potential lengthening effect of 10 microM chromanol 293B using sharp microelectrodes. Pretreatment with isoproterenol (2 nM), E-4031 (1 microM), and injection of inward current pulses were applied to modify action potential configuration. Chromanol alone caused moderate but statistically significant lengthening of action potentials at cycle lengths longer than 500 ms. The lengthening effect of chromanol, which was strongly enhanced in the presence of either isoproterenol or E-4031, was proportional to the amplitude of plateau, whereas poor correlation was found with action potential duration. Similar results were obtained when action potential configuration was modified by injection of depolarizing current pulses. Computer simulations revealed that activation of I (Ks) is a sharp function of the plateau amplitude within the physiological range, while elongation of repolarization may enhance I (Ks) only when it is excessive. It was concluded that the effect of I (Ks) on ventricular repolarization critically depends on the level of action potential plateau; however, other factors, like action potential duration, cycle length, or suppression of other K(+) currents can also influence its contribution.

Blockers of the delayed-rectifier potassium current in pancreatic beta-cells enhance glucose-dependent insulin secretion

Delayed-rectifier K+ currents (I(DR)) in pancreatic beta-cells are thought to contribute to action potential repolarization and thereby modulate insulin secretion. The voltage-gated K+ channel, K(V)2.1, is expressed in beta-cells, and the biophysical characteristics of heterologously expressed channels are similar to those of I(DR) in rodent beta-cells. A novel peptidyl inhibitor of K(V)2.1/K(V)2.2 channels, guangxitoxin (GxTX)-1 (half-maximal concentration approximately 1 nmol/l), has been purified, characterized, and used to probe the contribution of these channels to beta-cell physiology. In mouse beta-cells, GxTX-1 inhibits 90% of I(DR) and, as for K(V)2.1, shifts the voltage dependence of channel activation to more depolarized potentials, a characteristic of gating-modifier peptides. GxTX-1 broadens the beta-cell action potential, enhances glucose-stimulated intracellular calcium oscillations, and enhances insulin secretion from mouse pancreatic islets in a glucose-dependent manner. These data point to a mechanism for specific enhancement of glucose-dependent insulin secretion by applying blockers of the beta-cell I(DR), which may provide advantages over currently used therapies for the treatment of type 2 diabetes.

L-364,373 fails to activate the slow delayed rectifier K+ current in canine ventricular cardiomyocytes

Activators of the slow delayed rectifier K+ current (I(Ks)) are promising tools to suppress ventricular arrhythmias originating from prolongation of action potentials. A recently synthesized compound, L-364,373, was shown to activate I(Ks) in ventricular cells isolated from guinea pigs and rabbits. Due to the interspecies differences known to exist in the properties of the delayed rectifier K+ currents, the effect of L-364,373 on I(Ks) was studied and compared with that of another I(Ks) activator mefenamic acid in canine ventricular myocytes. Mefenamic acid (100 microM) significantly increased the amplitude of the fully activated I(Ks) current, as well as the I(Ks) current tails, by shifting the voltage dependence of its activation towards negative voltages and increased the time constant for deactivation. In contrast, L-364,373, up to concentrations of 3 microM, failed to augment I(Ks) at any membrane potential studied, but slightly increased the time constant of deactivation. It is concluded that human studies are required to evaluate the therapeutically beneficial effects of I(Ks) activators. Rodent cardiac tissues are not suitable for this purpose.

Calcium- and metabolic state-dependent modulation of the voltage-dependent Kv2.1 channel regulates neuronal excitability in response to ischemia

Ischemic stroke is often accompanied by neuronal hyperexcitability (i.e., seizures), which aggravates brain damage. Therefore, suppressing stroke-induced hyperexcitability and associated excitoxicity is a major focus of treatment for ischemic insults. Both ATP-dependent and Ca2+-activated K+ channels have been implicated in protective mechanisms to suppress ischemia-induced hyperexcitability. Here we provide evidence that the localization and function of Kv2.1, the major somatodendritic delayed rectifier voltage-dependent K+ channel in central neurons, is regulated by hypoxia/ischemia-induced changes in metabolic state and intracellular Ca2+ levels. Hypoxia/ischemia in rat brain induced a dramatic dephosphorylation of Kv2.1 and the translocation of surface Kv2.1 from clusters to a uniform localization. In cultured rat hippocampal neurons, chemical ischemia (CI) elicited a similar dephosphorylation and translocation of Kv2.1. These events were reversible and were mediated by Ca2+ release from intracellular stores and calcineurin-mediated Kv2.1 dephosphorylation. CI also induced a hyperpolarizing shift in the voltage-dependent activation of neuronal delayed rectifier currents (IK), leading to enhanced IK and suppressed neuronal excitability. The IK blocker tetraethylammonium reversed the ischemia-induced suppression of excitability and aggravated ischemic neuronal damage. Our results show that Kv2.1 can act as a novel Ca2+- and metabolic state-sensitive K+ channel and suggest that dynamic modulation of IK/Kv2.1 in response to hypoxia/ischemia suppresses neuronal excitability and could confer neuroprotection in response to brief ischemic insults.

Macrophage Migration Inhibitory Factor Increases Neuronal Delayed Rectifier K+ Current

Macrophage migration inhibitory factor (MIF) has widespread actions in the immune, endocrine, and nervous systems. Previously, we reported that increases in the intracellular levels of MIF depress the firing of hypothalamus/brain stem neurons in culture, including the chronotropic actions of angiotensin II. The objective of this study was to investigate the effects of MIF on delayed rectifier K+ current ( IKv), one of the component currents whose activity contributes to neuronal firing. Intracellular perfusion of MIF (80 nM) into Sprague–Dawley rat neuronal cultures caused a significant increase in IKv, as measured by patch-clamp recordings. This effect was apparent by 3 min, and was maximal after 20–30 min. IKv current density (pA/pF) increased from 31.58 ± 2.36 in controls to 41.88 ± 3.76 in MIF-treated neurons (mean ± SE; n = 9; P < 0.01). MIF that had been inactivated by boiling did not alter IKv, and MIF-neutralizing antibodies abolished the action of recombinant MIF (rMIF). The stimulatory effect of MIF on IKv current density was mimicked by intracellular application of either P1S-MIF (80 nM) or the peptide MIF-(50–65) (0.8–8 μM), both of which harbor the thiol-protein oxidoreductase (TPOR) activity of the MIF molecule. Conversely, neither C60S-MIF (80 nM) nor the MIF homologue D-dopachrome tautomerase (80 nM), both of which lack TPOR activity, altered IKv. Finally, the increase in IKv produced by rMIF was abolished by the superoxide scavenger Tiron (1 mM). These studies indicate that the neuronal action of MIF includes a stimulatory action on IKv that may be mediated by a TPOR/superoxide-scavenging mechanism.

Differential mechanisms underlying the modulation of delayed-rectifier K+ channel in mouse neocortical neurons by nitric oxide

The modulatory effects of nitric oxide (NO) on voltage-dependent K+ channels are intricate. In our present study, the augmentation and reduction of K+ currents by NO donor S-nitro-N-acetylpenicillamine (SNAP) and pure dissolved NO was observed in dissociated neurons from mice neocortex with both whole cell and cell-attached patch clamp. By using a specific electrochemical sensor, the critical concentrations of NO that increased or reduced the channel activities were accurately quantified. Low concentrations of SNAP (20 microM) or NO solution (0.1 microM) enhanced whole cell delayed rectifier K+ -current (IK) and left the fast inactivating A current (IA) unchanged. However, high concentrations of SNAP (100 microM) and NO (0.5 microM) reduced both IK and IA currents. In cell-attached experiments, a significant increase in channel open probability (NP0) was observed when using low concentrations of SNAP or NO. High concentrations of SNAP or NO dramatically decreased NP0. The increase in channel activities by low concentrations of SNAP was abolished in the presence of either inhibitors of soluble guaylate cyclase or inhibitors of cGMP-dependent protein kinase G, suggesting a link to the NO-cGMP signaling cascade. The reduction of channel activities by high concentrations of SNAP was reversed by the reducing agent dithiothreitol, implying a redox reaction mechanism. Thus both NO-cGMP signaling and a redox mechanism are involved in the modulation of IK channel activity for neuron excitability.

Cocaine elicits action potential bursts in a central snail neuron: The role of delayed rectifying K+ current

The effects of cocaine were studied in an identifiable RP4 neuron of the African snail, Achatina fulica Ferussac, using the two-electrode voltage-clamp method. The RP4 neuron generated spontaneous action potentials and bath application of cocaine (0.3-1 mM) reversibly elicited action potential bursts of the central RP4 neuron in a concentration-dependent manner. The action potential bursts were not blocked when neurons were immersed in high-Mg(2+)solution, Ca(2+)-free solution, nor after continuous perfusion with atropine, d-tubocurarine, propranolol, prazosin, haloperidol, or sulpiride. Similarly, the action potential bursts were not abolished by pretreatment with N-[2-(p-bromocinnamylamino)ethyl]-5-isoquinolinesulfonamide dihydrochloride, (9S,10S,12R)-2,3,9,10,11,12-hexahydro-10-hydroxy-9-methyl-1-oxo-9,12-epoxy-1H-diindolo[1,2,3-fg:3',2',1'-kl]pyrrolo[3,4-i][1,6]benzodiazocine-10-carboxylic acid hexyl ester or anisomycin. Injection of hyperpolarizing current at an intensity of greater than 2 nA effectively suppressed the cocaine-elicited action potential bursts and no postsynaptic potentials were observed under these conditions. These results suggest that the generation of action potential bursts elicited by cocaine was not due to (1) the synaptic effects of neurotransmitters, (2) the cholinergic, adrenergic or dopaminergic receptors of the excitable membrane, or (3) the cAMP second messengers and new protein synthesis of the RP4 neuron. Notably, the induction of action potential bursts was blocked by pretreatment with 1-[6-[((17beta)-3-methoxyestra-1,3,5[10]-trien-17-yl)amino]hexyl]-1H-pyrrole-2,5-dione. Voltage-clamp studies conducted on the RP4 neuron revealed that cocaine at 0.3 mM decreased (1) the Ca(2+) current, (2) the delayed rectifying K(+) current, (3) the fast-inactivating K(+) current and (4) the Ca(2+)-activated K(+) current, but had no remarkable effects on the Na(+) current. Perfusion with Ca(2+)-free solution, which may abolish the Ca(2+) current and Ca(2+)-activated K(+) current, did not cause any bursts of action potentials in control RP4 neurons. Application of 4-aminopyridine, an inhibitor of fast-inactivating K(+) current, and paxilline, an inhibitor of Ca(2+)-activated K(+) current, failed to elicit action potential bursts, whereas tetraethylammonium chloride, a blocker of Ca(2+)-activated K(+) current and delayed rectifying K(+) current, and tacrine, an inhibitor of delayed rectifying K(+) current, successfully elicited action potential bursts. Further, while 1-[6-[((17beta)-3-methoxyestra-1,3,5[10]-trien-17-yl)amino]hexyl]-1H-pyrrole-2,5-dione did not affect the delayed rectifying K(+) current of the RP4 neuron, 1-[6-[((17beta)-3-methoxyestra-1,3,5[10]-trien-17-yl)amino]hexyl]-1H-pyrrole-2,5-dione decreased the inhibitory effect of cocaine on the delayed rectifying K(+) current. It is concluded that cocaine elicits action potential bursts in the central snail RP4 neuron and that the effect is closely related to the inhibitory effects on the delayed rectifying K(+) current.

ANEPIII, a new recombinant neurotoxic polypeptide derived from scorpion peptide, inhibits delayed rectifier, but not A-type potassium currents in rat primary cultured hippocampal and cortical neurons

A new recombinant neurotoxic polypeptide ANEPIII (BmK ANEPIII) derived from Scorpion peptide, which was demonstrated with antineuroexcitation properties in animal models, was examined for its action on K+ currents in primary cultured rat hippocampal and cortical neurons using the patch clamp technique in the whole-cell configuration. The delayed rectifier K+ current (I(k)) was inhibited by externally applied recombinant BmK ANEPIII, while the transient A-current (I(A)) remained virtually unaffected. BmK ANEPIII 3 microM, reduced the delayed rectifier current by 28.2% and 23.6% in cultured rat hippocampal and cortical neurons, respectively. The concentration of half-maximal block was 155.1 nM for hippocampal neurons and 227.2 nM for cortical neurons, respectively. These results suggest that BmK ANEPIII affect K+ currents, which may lead to a reduction in neuronal excitability.

Electrocardiographic method for identifying drug-induced repolarization abnormalities associated with a reduction of the rapidly activating delayed rectifier potassium current

Several important non-cardiac drugs have been removed from the market after revealing harmful effect that was not identified during prior safety-assessment studies. We developed a new technique for the measurements of repolarization abnormalities from surface ECGs; this method improves sensitivity and specificity of the current technique used to identify the presence of abnormal ion current kinetics in the myocardial cells namely a prolongation of the QT interval on the surface ECG signal. We described in this paper the method and preliminary results, revealing the superiority of our technique that may play a role in the future of drug-safety assessment.

[Inhibition of salicylate on potassium channels in rat inferior colliculus neurons]

OBJECTIVE

To understand what role of the transient outward potassium channels and the delayed rectifier potassium channels play in the mechanism of salicylate-induced tinnitus.

METHODS

The effects of salicylate on the transient outward potassium channels and the delayed rectifier potassium channels in freshly dissociated inferior colliculus neurons of rats were studied, using the whole-cell voltage clamp method.

RESULTS

Salicylate blocked the transient outward potassium current (I(K(A and the delayed rectifier potassium current (I(K(DR in concentration-dependent manner (0.1-1 mmol/L). The IC50 values for the blocking action of salicylate on I(K(A)) and I(K(DR)) were 2.27 and 0.80 mmol/L, respectively. At a concentration of 1 mmol/L, salicylate did not shift the activation and inactivation curves of I(K(A)), but significantly shifted the activation and inactivation curves of I(K(DR)) negatively by approximately 11 mV and 24 mV.

CONCLUSIONS

Salicylate inhibits both I(K(A)) and I(K(DR)) in rat inferior colliculus neurons but only significantly affects the activation and inactivation kinetics of I(K(DR)). Effects of I(K(A)) and I(K(DR)), especially I(K(DR)), by salicylate may play an important role in salicylate-induced tinnitus.

Isolation and characterization of I(Kr) in cardiac myocytes by Cs+ permeation

Isolation of the rapidly activating delayed rectifier potassium current (I(Kr)) from other cardiac currents has been a difficult task for quantitative study of this current. The present study was designed to separate I(Kr) using Cs+ in cardiac myocytes. Cs+ have been known to block a variety of K+ channels, including many of those involved in the cardiac action potential such as inward rectifier potassium current I(K1) and the transient outward potassium current I(to). However, under isotonic Cs+ conditions (135 mM Cs+), a significant membrane current was recorded in isolated rabbit ventricular myocytes. This current displayed the voltage-dependent onset of and recovery from inactivation that are characteristic to I(Kr). Consistently, the current was selectively inhibited by the specific I(Kr) blockers. The biophysical and pharmacological properties of the Cs+-carried human ether-a-go-go-related gene (hERG) current were very similar to those of the Cs+-carried I(Kr) in ventricular myocytes. The primary sequence of the selectivity filter in hERG was in part responsible for the Cs+ permeability, which was lost when the sequence was changed from GFG to GYG, characteristic of other, Cs+-impermeable K+ channels. Thus the unique high Cs+ permeability in I(Kr) channels provides an effective way to isolate I(Kr) current. Although the biophysical and pharmacological properties of the Cs+-carried I(Kr) are different from those of the K+-carried I(Kr), such an assay enables I(Kr) current to be recorded at a level that is large enough and sufficiently robust to evaluate any I(Kr) alterations in native tissues in response to physiological or pathological changes. It is particularly useful for exploring the role of reduction of I(Kr) in arrhythmias associated with heart failure and long QT syndrome due to the reduced hERG channel membrane expression.

Inhibitory action of L-type Ca2+ current by paeoniflorin, a major constituent of peony root, in NG108-15 neuronal cells

The effects of paeoniflorin, a glycoside isolated from the root of Paeonia lactiflora, on ion currents in a mouse neuroblastoma and rat glioma hybrid cell line, NG108-15 were investigated. Paeoniflorin (1-300 microM) reversibly produced an inhibition of L-type voltage-dependent Ca2+ current (I(Ca,L)) in a concentration-dependent manner. Paeoniflorin caused no change in the overall shape of the current-voltage relationship of I(Ca,L). The IC50 value of paeoniflorin-induced inhibition of I(Ca,L) was 14 microM. However, neither adenosine deaminase (1 U/ml) nor 8-cyclopentyl-1, 3-dipropylxanthine (10 microM) could reverse the inhibition by paeoniflorin of I(Ca,L). Paeoniflorin (30 microM) shifted the steady-state inactivation curve of I(Ca,L) to more negative membrane potentials by approximately -10 mV. It also prolonged the recovery of I(Ca,L). The inhibitory effect of paeoniflorin on I(Ca,L) exhibited tonic and use-dependent characteristics. Paeoniflorin could effectively suppress I(Ca,L) evoked by action potential waveforms. Paeoniflorin at a concentration of 30 microM produce a slight inhibition of voltage-dependent Na+ current and delayed rectifier K+ current. Under current-clamp configuration, unlike adenosine, this compound decreased the firing of action potentials. Taken together, this study indicates that paeoniflorin can block L-type Ca2+ channels in NG108-15 cells in a mechanism unlinked to the binding to adenosine receptors. The effects of paeoniflorin on ion currents may partly, if not entirely, contribute to the underlying mechanisms through which it affects neuronal or neuroendocrine function.

[Emodin blocks voltage dependent potassium channels in rat proximal colon smooth muscle cells]

AIM

To investigate the effect of emodin on the voltage dependent potassium (K(V)) currents in rat proximal colon smooth muscle cells.

METHODS

Whole cell patch clamp technique was used to record potassium currents including fast transient outward current (I(KA)) and delayed rectifier current (I(Kdr)). Contamination of calcium-dependent potassium currents was minimized with CdCl2 in external solution and EGTA in pipette solution.

RESULTS

Emodin (1-30 micromol x L(-1)) reversibly and dose-dependently reduced the amplitude of I(Kdr) with an K(d) value of (1.9 +/- 0.1) micromol x L(-1). I(KA) was also inhibited with 30 micromol x L(-1) emodin to a lesser extent. Although acceleration of the decay rate of the K(V) currents was observed, the block by emodin was not through open block mechanism because a steady state level of inhibition of I(Kdr) was achieved during the first pulse from holding potential -70 mV to + 50 mV after the cells were holding at -70 mV for a three minutes interval in the presence of emodin. Emodin (5 micromol x L(-1)) had no effect on the steady-state activation and inactivation kinetics of K(V) currents, but 30 micromol x L(-1) of emodin produced a positive shift of the voltage dependence of activation, and an increase in the steepness of activation gating as well as shifted the voltage dependence of inactivation to positive direction.

CONCLUSION

Emodin, not through open block mechanism, markedly reduced the amplitude of I(KA) and I(Kdr) and modulated the gating properties of K(V) channels in a reversible and dose-dependent manner.

[The effect of ginkgolide B on action potential, L-type calcium current and delayed rectifier potassium current in ischemic guinea pig ventricular myocytes]

AIM

To study the effect of ginkgolide B from Ginkgo leave on action potential (AP), L-type calcium current (I(Ca) - L) and delayed rectifier potassium current (I(K)) in normal and ischemic guinea pig ventricular myocytes.

METHODS

With the standard microelectrode technique to record action potential and whole-cell variant patch-clamp technique to record calcium and potassium current.

RESULTS

(1) Under normal condition, ginkgolide B shortened APD and had no effect on RP, AP and V(max). Ginkgolide B also increased I(K) in a concentration dependent manner and had no significant effect on I(Ca) - L (2) Under ischemia condition, it was observed that shortening of APD, APA, decrease V(max) and depolarization of RP was induced by ischemia, but ginkgolide B could attenuate above--mentioned changes. (3) Under ischemia condition, I(Ca) - L and I(K) were inhibited, perfusion with ischemia solution containing ginkgolide B could reverse the decrease of I(Ca) - L and I(K).

CONCLUSION

Ginkgolide B had protective effect on ischemic myocardium to prevent ischemic arrhythmia.

[Effect of corticosterone on the delayed rectified potassium currents of cultured rat hippocampal neurons in vitro]

AIM

The effect of stress hormone corticosterone on hippocampal neurons delayed rectified potassium currents was probed.

METHODS

The potassium currents of rat hippocampal neurons in primary culture were measured with patch clamp whole-cell recording.

RESULTS

It showed that the amplitudes of the potassium currents in hippocampal neurons decreased. However, the threshold potential of potassium currents increased.

CONCLUSION

It is speculated that excessive corticosterone may hurt the hippocampal neurons via its effects on the potassium currents of rat hippocampal cells.