Therapeutic potential of modulating potassium currents in the diseased myocardium

Effects of IGF-1 on I(K) and I(K1) Channels via PI3K/Akt Signaling in Neonatal Cardiac Myocytes

Previous studies suggest that sarcolemmal potassium currents play important roles in cardiac hypertrophy. IGF-1 contributes to cardiac hypertrophy via activation of PI3K/Akt signaling. However, the relationships between IGF-1, PI3K/Akt signaling and sarcolemmal potassium currents remain unknown. Therefore, we tested the hypothesis that IGF-1 and PI3K/Akt signaling, independently, decrease sarcolemmal potassium currents in cardiac myocytes of neonatal rats. We compared the delayed outward rectifier (I_K) and the inward rectifier (I_K) current densities resulting from IGF-1 treatments to those resulting from simulation of PI3K/Akt signaling using adenoviral (Ad) BD110 and wild-type Akt and to those resulting from inhibition of PI3K signaling by LY294002. Ad.BD110 and Ad.Akt decreased I_K and these decrements were attenuated by LY 294002. The IGF-1 treatments decreased both I_K and I_K1 but only the I_K decrement was attenuated by LY294002. These findings demonstrate that IGF-1 may contribute to cardiac hypertrophy by PI3K/Akt signal transduction mechanisms in neonatal rat cardiomyocytes. Failure of LY294002 to effectively antagonize IGF-1 induced decrements in I_K1 suggests that a signal pathway adjunct to PI3K/Akt contributes to IGF-1 protection against arrhythmogenesis in these myocytes. Our findings imply that sarcolemmal outward and inward rectifier potassium channels are substrates for IGF-1/PI3K/Akt signal transduction molecules.

Potassium channel remodeling in cardiac hypertrophy

Cardiac hypertrophy is an adaptive process against increased work loads; however, hypertrophy also presents substrates for lethal ventricular arrhythmias, resulting in sudden arrhythmic deaths that account for about one third of deaths in cardiac hypertrophy. To maintain physiological cardiac function in the face of increased work loads, hypertrophied cardiomyocytes undergo K(+) channel remodeling that provides a prolongation in action potential duration and an increase in Ca(2+) entry. Increased Ca(2+) entry, in turn, activates signaling mechanisms including a calcineruin/NFAT pathway to permit remodeling of the K(+) channels. This results in a positive feedback loop between the K(+) channel remodeling and altered Ca(2+) handling; this loop may represent a potential therapeutic target against sudden arrhythmic deaths in cardiac hypertrophy. The purposes of this review are to: (1) discuss types of K(+) channels and their mRNA that undergo remodeling in cardiac hypertrophy; (2) report on recent research on molecular mechanisms of K(+) channel remodeling; and (3) address physiological events underlying new therapeutic modalities to ameliorate arrhythmias and sudden death in cardiac hypertrophy.

Ketanserin, a 5-HT2 antagonist, directly inhibits the ATP-sensitive potassium channel in mouse ventricular myocytes

The effects of ketanserin, a 5-HT2 antagonist, on the ATP-sensitive K+ (K(ATP)) channels were studied in mouse ventricular myocytes using patch clamp technique. Under the whole-cell voltage clamp conditions, ketanserin (1-100 microM) reversibly inhibited pinacidil-induced K(ATP) current in a concentration-dependent fashion with a Ki value of 9.36 microM and the Hill coefficient was 0.67. This inhibition was developed even with the presence of 5-hydroxytryptamine (100 microM) in the bath. Prazosin, a selective alpha1-antagonist, also failed to mimic the effect of ketanserin. Ketanserin did not affect the channel activity in inside-out configuration under the ATP-free internal solution. Furthermore, ketanserin applied to the external solution did not affect the pinacidil-induced channel activity in the cell-attached patches, but did inhibit it when applied into the pipette. These results suggest that the inhibitory action of ketanserin observed in this study was probably due to a direct action on the K(ATP) channel rather than to an action through the 5-HT2 receptor or alpha1-adrenoceptor blockade, and that the antiarrhythmic activity of ketanserin against cardiac arrhythmias induced in the ischemic/reperfused heart is at least in part attributable to its inhibition of the K(ATP) channel.

Aprikalim reduces the Na+-Ca2+ exchange outward current enhanced by hyperkalemia in rat ventricular myocytes

BACKGROUND

[corrected] Aprikalim, an adenosine triphosphate (ATP) sensitive K+ (K(ATP)) channel opener, attenuates the elevation of intracellular Ca2+ concentration ([Ca2+]i) and improves the contractile functions after hyperkalemic and hypothermic cardioplegia. There is evidence that cardioplegia increases the Na+-Ca2+ exchange activity without affecting Ca2+ influx through L-type Ca2+ channels or Ca2+ content in the sarcoplasmic reticulum, the intracellular Ca2+ store.

METHODS

We measured the Na+-Ca2+ exchange outward current with the patch-clamp technique in single rat ventricular myocytes exposed to hyperkalemia and hypothermia in the presence of aprikalim. The intracellular calcium concentration ([Ca2+]i) during cardioplegia, and the contractile function and [Ca2+]i transients induced by electrical stimulation or caffeine during rewarming and reperfusion in single ventricular myocytes were also determined. Contraction and [Ca2+]i were determined with video tracking and spectrofluorometry, respectively.

RESULTS

Aprikalim, 100 micromol/L, the effect of which was blocked by glibamclamide, a K(ATP) inhibitor, significantly attenuated the hyperkalemia-elevated Na+-Ca2+ exchange current by 26% and 11% at 22 degrees C and 4 degrees C, respectively. Aprikalim also attenuated significantly the [Ca2+]i elevated during cardioplegia. Furthermore aprikalim significantly attenuated the reduction in amplitude and prolongation in duration of contraction of myocytes after cardioplegia. The effects of aprikalim mimicked those of nickle (Ni2+), a Na+-Ca2+ exchange blocker. The electrically or caffeine-induced [Ca2+]i transients were unaltered by cardioplegia or aprikalim.

CONCLUSIONS

Aprikalim attenuates the Na+-Ca2+ exchange outward current elevated by hyperkalemia, which may attenuate the [Ca2+]i elevation during hyperkalemia and improve the contractile function after cardioplegia in the ventricular myocyte. The study provides further support that addition of a K(ATP) channel opener to the cardioplegic solution may produce beneficial effects in open heart surgery.

Analysis of the electrophysiological effects of ambasilide, a new antiarrhythmic agent, in canine isolated ventricular muscle and purkinje fibers

The aim of the study was to determine the in vitro rate-dependent cellular electrophysiological effects of ambasilide (10 and 20 microM/l), a new investigational antiarrhythmic agent, in canine isolated ventricular muscle and Purkinje fibers by applying the standard microelectrode technique. At the cycle length (CL) of 1000 ms, ambasilide significantly prolonged the action potential duration measured at 90% repolarization (APD(90)) in both ventricular muscle and Purkinje fibers. Ambasilide (10 microM/l) produced a more marked prolongation of APD(90) at lower stimulation frequencies in Purkinje fibers (at CL of 2000 ms = 56.0 +/- 16.1%, n = 6, versus CL of 400 ms = 15.1 +/- 3.7%, n = 6; p < 0.05), but, in 20 microM/l, this effect was considerably diminished (15.2 +/- 3.6%, n = 6, versus 7.3 +/- 5.1%, n = 6, p < 0.05). In ventricular muscle, however, both concentrations of the drug induced an almost frequency-independent lengthening of APD(90) in response to a slowing of the stimulation rate (in 20 microM/l at CL of 5000 ms = 19.0 +/- 1.5%, n = 9, versus CL of 400 ms = 16.9 +/- 1.4%, n = 9). Ambasilide induced a marked rate-dependent depression of the maximal rate of rise of the action potential upstroke (V(max)) (in 20 microM/l at CL of 300 ms = -45.1 +/- 3.9%, n = 6, versus CL of 5000 ms = -8.5 +/- 3.9%, n = 6, p < 0. 05, in ventricular muscle) and the corresponding recovery of V(max) time constant was tau = 1082.5 +/- 205.1 ms (n = 6). These data suggest that ambasilide, in addition to its Class III antiarrhythmic action, which is presumably due to its inhibitory effect on the delayed rectifier potassium current, possesses I/B type antiarrhythmic properties as a result of the inhibition of the fast sodium channels at high frequency rate with relatively fast kinetics. This latter effect may play an important role in its known less-pronounced proarrhythmic ("torsadogenic") potential.

Dispersion of ventricular repolarization in left ventricular hypertrophy: influence of afterload and dofetilide

INTRODUCTION

Increased dispersion of ventricular repolarization is observed in cardiac hypertrophy and is associated with sudden cardiac death. At present, there is little information about the effects of cardiac hemodynamics and antiarrhythmic drugs on dispersion of repolarization in disease states. We compared the effects of increasing afterload and the Class III antiarrhythmic drug, dofetilide, on dispersion of ventricular repolarization in hypertrophied rabbit hearts to normal rabbit hearts.

METHODS AND RESULTS

Cardiac hypertrophy was induced in rabbits by abdominal aortic banding. Isolated hearts were studied 49+/-4 days postsurgery in the working heart mode using a blood-buffer perfusate. The action potential duration (APD) was measured from eight sites on the epicardium of the heart at low (50+/-7 mmHg) afterload and high afterload (97+/-12 mmHg) at baseline and during dofetilide perfusion. APD dispersion, determined as the difference between the maximal and minimal APD, was greater in hypertrophied hearts (42+/-8 msec) compared with control hearts (26+/-8 msec, P < 0.05) at baseline and low afterload. Increasing afterload caused a decrease in APD dispersion in hypertrophied hearts (P < 0.05) but not in control hearts, and APD dispersion was similar in hypertrophied hearts (31+/-9 msec) compared with control hearts (30+/-9 msec, P = NS). During dofetilide perfusion, APD dispersion remained greater in hypertrophied hearts (60+/-39 msec) compared with control hearts (30+/-13 msec, P < 0.05) at low afterload but not high afterload. Increasing afterload caused shortening of the APD in most regions of the control hearts, whereas APD did not shorten significantly in hypertrophied hearts at baseline and tended to increase during dofetilide perfusion. During dofetilide perfusion, the maximal change in APD recorded from the posterior wall of the left ventricle following an increase in afterload was -18+/-21 msec in control hearts and 7+/-21 ms in hypertrophied hearts (P < 0.05).

CONCLUSION

Epicardial APD dispersion decreases in hypertrophied hearts following an increase in afterload, and this response is mediated in part by the absence of afterload-induced shortening of the APD. This effect may be due in part to altered responses of the delayed rectifying current to cardiac loading conditions in the setting of cardiac hypertrophy.

LOE 908 blocks delayed rectifier type potassium channels in PC12 cells and cortical neurons in culture

The effects of (R,S)-(3,4-dihydro-6,7-dimethoxy-isoquinoline-1-yl)-2- phenyl-N,N-di-[2-(2,3,4-trimethoxyphenyl)ethyl]-acetamide (LOE 908) were studied on K+ currents in undifferentiated cells from a phaeochromocytoma cell line (PC12), in cortical neurons from rat in primary culture, in a rat blood lymphoma cell line (RBL-1) and in a kidney cell line (BHK21). In PC12 cells delayed rectifier K+ currents measured in the whole-cell mode of the patch clamp technique were almost completely blocked by 10 microM LOE 908. The IC50 value was 0.7 microM and the Hill coefficient 0.8. After washout of the inhibitor about 80% of the current recovered. In rat cortical neurons in primary culture LOE 908 inhibited tetraethylammonium (TEA, 10 mM)-sensitive delayed rectifying K+ currents (LOE 908: 1 microM, 61 +/- 25% inhibition; 10 microM 103 +/- 19% inhibition). In contrast to the inhibitory action of LOE 908 on delayed rectifying K+ currents, Ca(2+)-activated potassium currents in BHK21 cells were only inhibited by 25 +/- 5% (10 microM LOE 908, n = 5) and no effect of LOE 908 was found on inward-rectifying K+ currents in RBL-1 cells. We conclude that LOE 908 is a K+ channel blocker with selectivity for delayed outward rectifying K+ channels.

Electrophysiological basis for the antiarrhythmic action and positive inotropy of HA-7, a furoquinoline alkaloid derivative, in rat heart

1. HA-7, a new synthetic derivative of furoquinoline alkaloid, increased the contractile force of right ventricular strips and effectively suppressed the ischaemia-reperfusion induced polymorphic ventricular tachyrhythmias in adult rat heart (EC50 = 2.8 microM). 2. In rat ventricular myocytes, HA-7 concentration-dependently prolonged the action potential duration (APD) and decreased the maximal rate of rise of the action potential upstroke (Vmax). The action potential amplitude and resting membrane potential were also reduced, but to a smaller extent. The prolongation of APD by HA-7 was prevented by pretreating the cells with 1 mM 4-AP. 3. Voltage clamp experiments revealed that HA-7 decreased the maximal current amplitude of I(Na) (IC50 = 4.1 microM) and caused a negative shift of its steady-state inactivation curve and slowed its rate of recovery from inactivation. The use-dependent inhibition of I(Na) by HA-7 was enhanced at a higher stimulation rate. The L-type Ca2+ current (I(Ca)) was also reduced, but to a lesser degree (IC50 = 5.3 microM, maximal inhibition = 31.8%). 4. This agent also influenced the time- and voltage-dependent K currents. The prolongation of APD was associated with an inhibition of a 4-AP sensitive transient outward K current (I(to)) (IC50 = 2.9 microM) and a slowly inactivating, steady-state outward current (I(SS)) (IC50 = 2.5 microM). The inhibition of I(to) by HA-7 was associated with an acceleration of its time constant of inactivation. HA-7 suppressed I(to) in a time-dependent manner and caused a significant negative shift of the voltage-dependent steady-state inactivation curve but did not affect its rate of recovery from inactivation. 5. At higher concentrations, the inward rectifier K+ current (I(KI)) was also inhibited but to a less extent. Its slope conductance after 3, 10 and 30 microM HA-7 was decreased by 24+/-4%, 41+/-5% and 54+/-8%. respectively. 6. We conclude that HA-7 predominantly blocks I(to) and Na+ channels and that it also weakly blocks Ca2+ and I(KI) channels. These changes alter the electrophysiological properties of the heart and terminate the ischaemia reperfusion induced ventricular arrhythmia. The significant I(to) inhibition and minimal I(Ca) suppression may afford an opportunity to develop an effective antiarrhythmic agent linked with positive inotropy.

Cloned potassium channels from eukaryotes and prokaryotes

Potassium channels contribute to the excitability of neurons and signaling in the nervous system. They arise from multiple gene families including one for voltage-gated potassium channels and one for inwardly rectifying potassium channels. Features of potassium permeation, channel gating and regulation, and subunit interaction have been analyzed. Potassium channels of similar design have been found in animals ranging from jellyfish to humans, as well as in plants, yeast, and bacteria. Structural similarities are evident for the pore-forming alpha subunits and for the beta subunits, which could potentially regulate channel activity according to the level of energy and/or reducing power of the cell.

In swine myocardium, the infarct size reduction induced by U-89232 is glibenclamide sensitive: evidence that U-89232 is a cardioselective opener of ATP-sensitive potassium channels

We determined whether U-89232, a derivative of the ATP-sensitive potassium (KATP) channel opener cromakalim, is cardioselective and whether its action on the myocardium is still sensitive to glibenclamide. Experiments were performed in open-chest pigs subjected to a 60-min occlusion of the left anterior descending coronary artery (LADCA) and to 2 h of reperfusion. Four groups of animals were studied (n = 6 each). Animals received either U-89232, 3 mg/kg i.v. over a 15-min period (U), or glibenclamide, a selective KATP channel blocker, 1 mg/kg i.v. over a 15-min period (GLI) before the LADCA occlusion. In the GLI + U group, first glibenclamide (1 mg/kg/15 min) and then U-89232 (3 mg/kg/15 min) were infused before the 60 min of ischemia. Saline-treated animals served as controls (CON). Hemodynamic parameters were continuously monitored. Regional contractile wall function was quantified with ultrasonic crystals aligned to measure wall thickening. At the end of the protocol, infarct size (IS, as percentage of risk region) was determined by incubating the myocardium with p-nitrobluetetrazolium. With comparable myocardium at risk, infusion of U-89232 before 60 min of LADCA occlusion significantly reduced infarct size (IS, 18.5 +/- 3.7%; p < 0.001 vs. 63.2 +/- 3.3% for the controls), whereas glibenclamide had no effect on infarct size (IS, 69.5 +/- 4.4%). The administration of glibenclamide before U-89232 infusion blocked the infarct size-reducing effect of U-89232 [IS, 61.2 +/- 9.1 (NS) vs. controls and p < 0.001 vs. U]. Infusion of U-89232 had no effect on hemodynamic parameters or on regional wall function. At least in a pig model, U-89232 appears to be a cardioselective KATP channel opener, because in the absence of hemodynamic alterations, it exhibits a profound cardioprotective effect, which is fully reversible by blocking KATP channels.

Quinidine pharmacodynamics in normal and isoproterenol-induced hypertrophied blood-perfused working rabbit hearts

Ventricular hypertrophy is associated with several electrophysiologic abnormalities. However, little is known about the pharmacodynamics of antiarrhythmic drugs in the setting of ventricular hypertrophy. We studied the myocardial accumulation and pharmacodynamics of quinidine in 10 control rabbit hearts and 10 with isoproterenol-induced hypertrophy. Hearts were perfused in the working heart configuration. Electrophysiologic measurements were made at low afterload (30 cm H2O) and high afterload (60 cm H2O) at baseline and during quinidine perfusion (972 ng/ml). The myocardial quinidine concentration measured at the end of each experiment was significantly lower in the hypertrophied hearts (25.0 +/- 11.7 micrograms/g) as compared with the control hearts (51.2 +/- 12.7 micrograms/g, p < 0.001). The left ventricular (LV) monophasic action potential (MAP) duration was significantly shorter in the hypertrophied hearts as compared with control hearts at low afterload (166 +/- 27 vs. 192 +/- 24 ms, p < 0.01) and at high afterload (141 +/- 7 vs. 171 +/- 24 ms, p < 0.01). Quinidine prolonged MAP duration to a similar extent in both hypertrophied and control hearts; the MAP prolongation occurred at both low (192 +/- 21 vs. 223 +/- 25 ms, p < 0.02) and high afterloads (179 +/- 15 vs. 216 +/- 20 ms, p < 0.01) in the hypertrophied and control hearts, respectively. However, the ratios of the changes in electrophysiologic parameters to quinidine myocardial concentrations were greater in the hypertrophied hearts than in control hearts (p < 0.05). Therefore, AP duration (APD) is significantly shortened in isoproterenol-induced hypertrophy. The magnitude of quinidine effects on MAP duration and ventricular effective refractory period (VERP) are similar in hypertrophied hearts and control hearts, but the myocardial concentration-effect relations are increased significantly in hypertrophied hearts.

Effects of tedisamil (KC-8857) on cardiac electrophysiology and ventricular fibrillation in the rabbit isolated heart

1. The direct cardiac electrophysiological and antifibrillatory actions of tedisamil (KC-8857) were studied in rabbit isolated hearts. 2. Tedisamil (1, 3, and 10 microM), prolonged the ventricular effective refractory period (VRP) from 120 +/- 18 ms (baseline) to 155 +/- 19, 171 +/- 20, and 205 +/- 14 ms, respectively. Three groups of isolated hearts (n = 6 each) were used to test the antifibrillatory action of tedisamil. Hearts were perfused with 1.25 microM pinacidil, a KATP channel activator. Hearts were subjected to hypoxia for 12 min followed by 40 min of reoxygenation. Ventricular fibrillation (VF) developed during hypoxia and reoxygenation in both the control and 1 microM tedisamil-treated groups (5/6 and 4/6, respectively). Tedisamil (3 microM) reduced the incidence of VF (0/6, P = 0.007 vs. control). 3. In a separate group of hearts, VF was initiated by electrical stimulation. The administration of 0.3 ml of 10 mM tedisamil, via the aortic cannula, terminated VF in all hearts, converting them to normal sinus rhythm. 4. Tedisamil (3 microM) reversed pinacidil-induced negative inotropic effects in rabbit isolated atrial muscle which were equilibrated under normoxia, as well as in atrial muscle subjected to hypoxia and reoxygenation. 5. The results demonstrate a direct antifibrillatory action of tedisamil in vitro. The mechanism responsible for the observed effects may involve modulation by tedisamil of the cardiac ATP-regulated potassium channel, in addition to its antagonism of IK and Ito.

Electrophysiological basis for antiarrhythmic efficacy, positive inotropy and low proarrhythmic potential of (-)-caryachine

1. (-)-Caryachine, isolated from the plant (Cryptocarya chinensis), increased the contractility of atrial and right ventricular strips and significantly suppressed the reperfusion arrhythmias in adult rabbit heart (ED50 = 1.27 microM). 2. Data obtained by the whole-cell voltage clamp technique has shown that (-)-caryachine causes a negative shift of the steady-state Na channel inactivation and a slower rate of recovery from inactivation. The maximal Na current amplitude decreased to 67 +/- 7%, 29 +/- 8% and 12 +/- 5% after 0.5, 1.5 and 4.5 microM (-)-caryachine, respectively. 3. This agent also had effects on the time- and voltage-dependent K currents. (-)-Caryachine markedly suppressed the 4-AP-sensitive transient outward current (I10). However, it produced very little voltage-dependent shift in inactivation. After 0.5, 1.5 and 4.5 microM of the compound, the respective value of I10 elicited at +60 mV was 80 +/- 7%, 45 +/- 8% and 15 +/- 3%. At higher concentrations, the inward rectifier K current (IK1) was also inhibited but to a much smaller extent. Its slope conductance after 0.5, 1.5 and 4.5 microM (-)-caryachine was reduced to 71 +/- 9%, 51 +/- 12% and 42 +/- 11%, respectively. The outward hump of inward rectification was not changed. 4. In contrast, the L-type Ca current was not significantly changed by (-)-caryachine. 5. Electrophysiological studies in perfused whole heart preparations revealed that (-)-caryachine increased the intra-atrial conduction interval and also prolonged the atrial refractory period. No proarrhythmic effects were induced during the infusion of this compound (up to 13.5 microM). 6. We conclude that (-)-caryachine predominantly blocks the Na and I10 currents. These changes alter the electrophysiological properties of the heart and terminate the induced ventricular arrhythmias. The relatively selective I10 inhibition, safety margin of Ik1 suppression and lack of effect on Ica-L will provide an opportunity to develop an effective antiarrhythmic agent with positive inotropy as well as low proarrhythmic potential.

Basic concepts in cellular cardiac electrophysiology: Part II: Block of ion channels by antiarrhythmic drugs

Antiarrhythmic drugs have relative specificity for blocking each of the major classes of ion channels that control the action potential. The kinetics of block is determined by the state of the channel. Those channel states occupied at depolarized potentials generally have greater affinity for the blocking drugs. The kinetics of the drug-channel interaction is important in determining the blocking profile observed clinically. The increased mortality resulting from drug treatment in CAST and several atrial fibrillation trials has resulted in a shift in antiarrhythmic drug development from the Na+ channel blocking (Class I) drugs to the K+ channel blocking (Class III) drugs. While both Classes of drugs have a proarrhythmic potential, this may be less for the Class III agents. Their lack of negative inotropy also make them more attractive. It is important that the potential advantages of these agents be evaluated in controlled clinical trials. In several laboratories, the techniques of molecular biology and biophysics are being combined to determine the block site of available drugs. This information will aid in the future development of agents with greater specificity, and hopefully greater efficacy and safety than those currently in clinical use.

Effects of potassium channel openers on Na+ and K+ currents in rabbit sinus node and atrial myocytes

The effects of the potassium channel openers (KCOs) Cromakalim and Lemakalim on rabbit sinoatrial and atrial myocytes were examined by means of the whole-cell patch-clamp technique. Lemakalin (up to 100 microM) had no effect on potassium current in sinoatrial cells. Both Lemakalim and Cromakalim (100 microM) displayed a two-fold action on atrial myocytes: (1) they increased an outwardly rectifying conductance at potentials positive to EK and, (2) they markedly decreased a TTX-sensitive Na+ current active in the voltage range -50/-30 mV. This novel action on TTX-sensitive currents is of particular interest since these two benzopyrans have been thought to specifically target potassium channels.

Time-dependent fading of the activation of KATP channels, induced by aprikalim and nucleotides, in excised membrane patches from cardiac myocytes

1. The effects of the potassium channel opener (KCO) aprikalim (RP 52891) on the nucleotide-induced modulation of ATP-sensitive K+ (KATP) channels in freshly dissociated ventricular myocytes of guinea-pig heart, were studied by use of the inside-out patch-clamp technique. The internal surface of the excised membrane patch was initially bathed with a standard solution (Mg(2+)-free with EDTA), then sequentially superfused with solutions containing nucleoside diphosphates (NDPs: 200 microM ADP and 50 microM GDP) and NDPs plus 1 mM MgCl2 (with EGTA; referred to as Mg-NDP solution). 2. The normalized concentration-response (channel closing) relationship to ATP was shifted to the right when the standard solution was replaced by the Mg-NDP solution. Hence, the internal concentration of ATP ([ATP]i) inhibiting the channel activity by half (Ki) increased from 56 microM to 180 microM, with an apparently constant slope factor (s = 2.37). NDPs in the absence of Mg2+ did not decrease the sensitivity of the channels to ATP. 3. In standard solution, aprikalim (100 microM) activated KATP channels in the presence of a maximally inhibitory [ATP]i (500 microM). This effect was strongly enhanced when aprikalim was applied to patches exposed to Mg-NDP solution, as demonstrated by the 9 fold increase in Ki for [ATP]i (from 180 microM to 1.5 mM and s = 2.37). 4. The ability of aprikalim to overcome the channel closing effects of ATP in Mg-NDP solution waned rapidly. Similarly, the NDP-induced activation of ATP-blocked channels was also time-dependent. Both activation processes disappeared before the channel run-down phenomenon appeared in ATP-free conditions. 5. In conclusion, aprikalim is much more potent in opening KATP channels in membrane patches bathed in Mg-NDP solution than in standard solution. However, under the former experimental conditions, the effect of aprikalim waned rapidly. It is proposed that the waning phenomenon results from changes in the intrinsic enzymatic activity of the KATP channel protein (possibly linked to the experimental conditions) which lead to the channel closure.

Quinidine-induced open channel block of K+ current in rat ventricle

1. The effects of quinidine on calcium-independent outward K+ currents in rat ventricular myocytes were studied using whole-cell patch clamp techniques. 2. Quinidine sulphate (6 microM) significantly prolonged repolarization of the ventricular action potential. This effect was larger during early repolarization (25% level) than at later times (90% level). 3. Quinidine reduced the amplitude of a transient outward current, and accelerated its rate of decay by approximately 4 fold at membrane potentials between 0 to +50 mV. Quinidine also reduced the amplitude of a slowly inactivating, tetraethylammonium-sensitive 'pedestal' component of the outward current. 4. The quinidine-induced block of the transient outward current was dependent on time and membrane potential. Maximal block occurred with depolarizations of about 100 ms duration, and longer depolarizations (up to 1.5 s) produced little additional block. The membrane potential dependence of quinidine-induced block was very similar to the membrane potential dependence of activation of the transient outward current. The membrane potential dependence of steady-state inactivation of the transient outward current was not significantly affected by quinidine. 5. These results show that quinidine blocks outward K+ currents in rat ventricular cells. The time and potential dependence of this block suggests that quinidine blocks the transient outward K+ current by acting primarily on the open state of these channels.

Ischemic myocardial cell protection conferred by the opening of ATP-sensitive potassium channels

The responses of the cardiac myocyte to a potentially injurious ischemic stress are multiple. The opening of the ATP-sensitive K+ channels may constitute one such response. These channels are present in the plasmalemma at very elevated density and have a large unitary conductance. Consequently, the opening of a small fraction (0.01-0.1%) of these channels during ischemia can help to drive the myocyte into an "emergency" state, in which its syncytial functions become rapidly downregulated and strategies appropriate to preserving cell viability are implemented. Thus, ATP-sensitive K+ channels in cardiac myocytes would appear to be an efficient and apparently redundant natural means of defense against metabolic stress. These channels can undergo physiologic modulation, as occurs during cardiac ischemic preconditioning in several species, including humans. The term cardioprotection refers to an endogenous cardioprotective strategy, whereby the myocardium slows its energy demands, produces fewer toxic glycolytic products, and exhibits reduced injury following a potentially lethal ischemic stress. Openers of cardiac ATP-sensitive K+ channels, a class of drugs that includes, in particular, aprikalim and nicorandil, also afford cardioprotection by reducing the functional and biochemical damage produced by ischemia. Hence, these compounds can improve the recovery of cardiac contractility, reduce the extracellular leakage of intracellular enzymes, delay the loss of ATP, and preserve the cell ultrastructure in isolated heart preparations subjected to transient ischemic conditions. Furthermore, when segmental contractility has been strongly depressed by a stunning insult, nicorandil and aprikalim can accelerate recovery at the reperfusion. Finally, nicorandil and aprikalim decrease substantially the size of the necrotic region that results from a prolonged ischemic insult followed by reperfusion. All of these desirable effects of K(+)-channel openers can be abolished by blockers of ATP-sensitive K+ channels, such as glibenclamide. The fundamental mechanism of the myocyte viability protection conferred by K(+)-channel openers is not yet clear. It may exploit some of the same pathways that mediate cardiac ischemic preconditioning. If this suggestion holds true, drugs opening cardiac ATP-sensitive K+ channels would mimic, exploit, or intensify those cardioprotective means that are naturally available to the cardiac myocyte for overcoming metabolic stress.

Metabolic oscillations in heart cells

Oscillatory rhythms underlie biological processes as diverse and fundamental as neuronal firing, secretion, and muscle contraction. We have detected periodic changes in membrane ionic current driven by intrinsic oscillations of energy metabolism in guinea pig heart cells. Withdrawal of exogenous substrates initiated oscillatory activation of ATP-sensitive potassium current and cyclical suppression of depolarization-evoked intracellular calcium transients. The oscillations in membrane current were not driven by pacemaker currents or by alterations in intracellular calcium and thus represent a novel cytoplasmic cardiac oscillator. The linkage to energy metabolism was demonstrated by monitoring oscillations in the oxidation state of pyridine nucleotides. Interventions which altered the rate of glucose metabolism modulated the oscillations, suggesting that the rhythms originated at the level of glycolysis. The metabolic oscillations produced cyclical changes in electrical excitability, underscoring the potential importance of this intrinsic oscillator in the genesis of cardiac arrhythmias.

Cloning and functional expression of a rat heart KATP channel

Potassium channels that are ATP-sensitive (KATP) couple membrane potential to the metabolic status of the cell. KATP channels are inhibited by intracellular ATP and are stimulated by intracellular nucleotide diphosphates. KATP channels are important regulators of secretory processes and muscle contraction, and are targets for therapeutic treatment of type II diabetes by the inhibitory sulphonylureas and for hypertension by activators such as pinacidil. In cardiac tissue, KATP channels are central regulators of post-ischaemic cardioprotection. Electrophysiological and pharmacological characteristics vary among KATP channels recorded from diverse tissues suggesting extensive molecular heterogeneity. A complementary DNA encoding a KATP channel was isolated from rat heart using the polymerase chain reaction. We report here that the expressed channels possess all of the essential features of native cardiac KATP channels, including sensitivity to intracellular nucleotides. In addition the cloned channels are activated by the potassium channel opener, pinacidil, but are not inhibited by the sulphonylurea, glibenclamide.

Potassium channel openers and other regulators of KATP channels

1. Interest in ATP-sensitive K (KATP) channels first arose when it was shown that hypoglycaemic sulphonylureas, such as glibenclamide, closed these channels in pancreatic beta-cells to cause insulin release. The demonstration that certain smooth muscle relaxants (K channel openers) may exert their actions through opening a similar channel in vascular smooth muscle fueled further investigation of these channels and their physiological role in a variety of tissue types, including various types of smooth muscle, cardiac and skeletal muscle and neural and endocrine organ function. 2. The K channel openers have a variety of potential therapeutic applications, including disorders of smooth muscle hyperreactivity, such as hypertension, and a great deal of research has focused on this field. More recently, attention has turned to the cardiac actions of these compounds and this area is discussed in detail. One of the current problems is the lack of selectivity of KATP channel regulators. However, there have been a number of recent encouraging reports suggesting that, under certain pathophysiological conditions, the action of the K channel openers may be enhanced, conferring upon them some degree of selectivity. 3. A number of endogenous regulators of these channels have been identified, particularly in the category of endogenous openers of these channels. At present though, the physiological role of these channels and the endogenous regulators identified, is unclear. 4. It is evident that, although advances have been made, much work is still required to increase our understanding and ultimately to allow selective pharmacological manipulation of these channels to become a therapeutic reality.

Triamterene inhibits the delayed rectifier potassium current (IK) in guinea pig ventricular myocytes

In humans, proarrhythmia during therapy with action potential-prolonging drugs can be associated with hypokalemia often provoked by concomitant administration of diuretic agents. Consequently, therapy with class III antiarrhythmics and K(+)-sparing diuretics, such as triamterene, may be indicated. Triamterene, along with its K(+)-sparing properties, exhibits other pharmacological effects. In the heart, it can increase action potential duration (guinea pig atria and papillary muscles), protect against reperfusion-induced arrhythmias (rat), and increase the QT interval (humans). Therefore, studies were undertaken to assess effects of triamterene on cardiac K+ repolarizing currents. Guinea pig ventricular myocytes were superfused at 30 degrees C with Cd(2+)-containing solution to block Isi and held at -40 mV to inactivate INa. Currents were measured in the whole-cell configuration of the patch-clamp technique. The delayed rectifier outward current (IK) was elicited by short (250-millisecond) and long (5000-millisecond) depolarizing pulses, and time-independent currents were assessed by a rapid ramp test protocol. After high-voltage long pulses (+50 mV; 5000 milliseconds), tail current amplitude of the slow component of IK (IKs) was decreased 36 +/- 6% (n = 6) and 51 +/- 8% (n = 6) by triamterene 10(-5) and 10(-4) mol/L, respectively. After low-voltage short pulses (-20 mV; 250 milliseconds), tail current amplitude corresponding essentially to the rapid component of IK (IKr) was decreased only 14 +/- 11% (n = 9) and 19 +/- 10% (n = 10) by triamterene 10(-5) and 10(-4) mol/L, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of the class III antiarrhythmic, dofetilide (UK-68,798) on the heart rate of midgestation rat embryos, in vitro

Gestation day 11 (GD11) and 14 (GD14) embryos were cultured for up to 4 hours in the presence of Dofetilide (0.01-0.50 microgram/ml), a potent Class III Antiarrhythmic which selectively inhibits the rapid component of the time dependent outward potassium current (IKr). Significant (P < or = 0.05) reductions in heart rate (HR) as measured over a 4 hour period were dose dependent and reversible. The sensitivity of the GD11 embryos was greater than GD14 embryos (14-64% decrease in HR vs. an 11-43% decrease in HR, respectively) at the same concentrations tested. These in vitro results support the hypothesis that the embryo-lethality of Class III Antiarrhythmics observed in vivo may be a class effect of the IKr subtype potassium channel blockers. The data suggest a possible mechanism of embryotoxicity is to lower embryonic HR resulting in subsequent hypoxia and death. Dofetilide's effects on GD11 HR were partially reversible by the sequential addition of Isoproterenol or Theophylline.

Clinical relevance of cardiac arrhythmias generated by afterdepolarizations. Role of M cells in the generation of U waves, triggered activity and torsade de pointes

Recent findings point to an important heterogeneity in the electrical behavior of cells spanning the ventricular wall as well as important differences in the response of the various cell types to cardioactive drugs and pathophysiologic states. These observations have permitted a fine tuning and, in some cases, a reevaluation of basic concepts of arrhythmia mechanisms. This brief review examines the implications of some of these new findings within the scope of what is already known about early and delayed afterdepolarizations and triggered activity and discusses the possible relevance of these mechanisms to clinical arrhythmias.

Enhancement of ATP-sensitive potassium current in cat ventricular myocytes by beta-adrenoreceptor stimulation

1. To address the questions of whether beta-adrenoreceptor stimulation can augment ATP-sensitive potassium current (IK(ATP)), and what the mechanism of such an effect might be, action potentials and whole-cell ionic currents were recorded from adult cat cardiac ventricular myocytes using a conventional whole-cell patch technique. 2. An outwardly directed, ohmic, non-inactivating, glyburide (10 microM)-sensitive current reversing near the reversal potential for potassium (EK) developed slowly (10-25 min) in cells dialysed with an ATP-free pipette (intracellular) solution. During this time, action potential duration markedly decreased while the resting membrane potential hyperpolarized closer to EK. Extended (> 30 min) periods of internal dialysis with ATP-free solution eventually resulted in run-down of the outward current. 3. Externally applied isoprenaline (1 microM) caused a rapidly developing (< or = 60 s), sustained enhancement of a glyburide (10 microM)-sensitive IK(ATP) in cells internally dialysed with ATP-free solution. IK(ATP) remained elevated even after the isoprenaline was removed, and subsequent applications of the beta-agonist failed to increase IK(ATP) further. Half-maximal isoprenaline stimulation of IK(ATP) occurred at a concentration of approximate of 1.5 nM. 4. Pretreatment with propranolol (1 microM) prevented the enhancement of IK(ATP) by a beta-agonist. 5. Isoprenaline-induced IK(ATP) could be blocked by either internal application of GDP-beta-S (2-5 mM) or pretreatment with cholera toxin (1-10 microgram ml-1, > 18 h). Pretreatment with pertussis toxin (1-2 microgram ml-1, > 18 h) did not attenuate the isoprenaline response, whereas internally applied GTP-gamma-S (100 microM) or F- (20 mM) caused IK(ATP) to increase rapidly in the absence of the beta-agonist. 6. Although externally applied forskolin (10 microM) also stimulated IK(ATP), neither 1,9-dideoxyforskolin (10 microM) nor 8-(4-chlorophenylthio)-cAMP (200 microM) had any effect on the current. Internal application of the adenylate cyclase inhibitor 2'-deoxyadenosine-3'-monophosphate (100 microM) resulted in a reduction in the response to isoprenaline, while internal application of a protein kinase A inhibitor (PKI5-24, 22.5 microM) did not attenuate the response to the beta-agonist. 7. IK(ATP) developed slowly during internal dialysis with ATP-free solution.(ABSTRACT TRUNCATED AT 400 WORDS)

Pinacidil-induced electrical heterogeneity and extrasystolic activity in canine ventricular tissues. Does activation of ATP-regulated potassium current promote phase 2 reentry?

BACKGROUND

Pinacidil is known to augment a time-independent outward current in cardiac tissues by activating the ATP-regulated potassium channels. Activation of this current, IK-ATP, is thought to be responsible for increased potassium permeability in ischemia. The contribution of IK-ATP activation to arrhythmogenesis and the role of activation of this current in suppression of arrhythmias are areas of great interest and debate. Because electrical depression attending myocardial ischemia is more accentuated in ventricular epicardium than in endocardium, we endeavored to contrast the effects of pinacidil-induced IK-ATP activation on the electrophysiology of canine ventricular epicardium and endocardium.

METHODS AND RESULTS

Standard microelectrode techniques were used. Pinacidil (1 to 5 mumol/L) produced a marked dispersion of repolarization and refractoriness in isolated canine ventricular epicardium as well as between epicardium and endocardium. In endocardium, pinacidil abbreviated action potential duration (APD90) and refractoriness by 8.0 +/- 2.3%. In epicardium, the effects of pinacidil were nonhomogeneous. At some sites, pinacidil induced an all-or-none repolarization at the end of phase 1 of the action potential, resulting in 55.5 +/- 8.7% abbreviation of APD90 and refractoriness. Adjacent to these were sites at which the dome was maintained with only minor changes in APD and refractoriness. Extrasystolic activity displaying features of reentry was observed in isolated sheets of epicardium (63.2%) after exposure to pinacidil (1 to 5 mumol/L) but never in its absence. Dispersion of repolarization and ectopic activity was most readily induced in epicardium by a slowing of the stimulation rate in the presence of pinacidil. Electrical homogeneity was restored and arrhythmias abolished after washout of pinacidil or addition of either a transient outward current blocker, 4-aminopyridine, or a blocker of the ATP-regulated potassium channels, glybenclamide.

CONCLUSIONS

Our data suggest that the activation of IK-ATP can produce a marked dispersion of repolarization and refractoriness in epicardium as well as between epicardium and endocardium, leading to the development of extrasystolic activity via a mechanism that we have called phase 2 reentry. The available data also suggest that blockade of the transient outward current and/or the ATP-regulated potassium channels may be useful antiarrhythmic interventions under ischemic or "ATP depleted" conditions.

Antiarrhythmic drugs, clofilium and cibenzoline are potent inhibitors of glibenclamide-sensitive K+ currents in Xenopus oocytes

1. The novel K+ channel opener, Y-26763 induced outward K+ currents in voltage-clamped follicle-enclosed Xenopus oocytes in a concentration-dependent manner with an EC50 value of 58 microM. 2. The Y-26763-induced K+ current was completely and reversibly blocked by glibenclamide (an ATP-sensitive K+ channel blocker) in a concentration-dependent manner (IC50 140 nM). Effects of several antiarrhythmic drugs on Y-26763-induced glibenclamide-sensitive K+ currents were investigated. 3. (+/-)-Cibenzoline, RS-2135, pirmenol, lorcainide and KW-3407 (class I antiarrhythmic drugs, Na+ channel blockers) suppressed Y-26763 responses in a concentration-dependent manner with IC50 values (in microM) of 6.6, 54, 68, 71 and 370, respectively. 4. Clofilium, E-4031, MS-551 and bretylium (class III antiarrhythmic drugs which increase the action potential duration) also suppressed Y-26763 responses concentration-dependently, IC50 values (in microM) were 3.3, 660, 980 and > or = 2000, respectively. N-acetylprocainamide (class III antiarrhythmic drug) scarcely suppressed Y-26763 responses. 5. The glibenclamide-sensitive K+ currents elicited by KRN2391 were also suppressed by all these antiarrhythmic drugs. 6. The antiarrhythmic drugs, clofilium and (+/-)-cibenzoline block glibenclamide-sensitive K+ channels in Xenopus oocytes at concentrations comparable to their therapeutic plasma levels.