Domperidone should not be considered a no-risk alternative to cisapride in the treatment of gastrointestinal motility disorders.

BACKGROUND: Several cases of QT prolongation and ventricular tachyarrhythmia have been reported with domperidone, a gastrokinetic and antiemetic agent available worldwide but still under investigation in the United States. Although electrolyte disturbances such as hypokalemia could account for some of these events, we hypothesized that domperidone may have unsuspected electrophysiological effects predisposing some patients to proarrhythmia. METHODS AND RESULTS: Studies were undertaken in 9 isolated guinea pig hearts, which demonstrated reverse use-dependent prolongation of cardiac repolarization by 100 nmol/L domperidone. Action potential duration increased 27% from baseline with domperidone (from 114+/-3 to 145+/-2 ms) during pacing at a cycle length of 250 ms, and a 9% increase (from 97+/-2 to 106+/-3 ms) was seen with pacing at a cycle length of 150 ms. Experiments in human ether-a-go-go-related gene (HERG)-transfected Chinese hamster ovary cells (n=32) demonstrated a concentration-dependent block of the rapid component (I(Kr)) of the delayed rectifier potassium current. The tail current decreased by 50% at 162 nmol/L domperidone. CONCLUSIONS: Domperidone possesses cardiac electrophysiological effects similar to those of cisapride and class III antiarrhythmic drugs. These effects are observed at clinically relevant concentrations of the drug. Therefore, domperidone should not be considered a no-risk alternative to cisapride, a drug that was recently withdrawn from the US market.

Sildenafil (Viagra) prolongs cardiac repolarization by blocking the rapid component of the delayed rectifier potassium current.

BACKGROUND-Several cases of unexpected death have been reported with sildenafil in patients predisposed to ischemic cardiac events. Although acute episodes of ischemia could account for some of these deaths, we hypothesized that sildenafil may have unsuspected electrophysiological effects predisposing some patients to proarrhythmia. METHODS AND RESULTS-Studies were undertaken in 10 isolated guinea pig hearts that demonstrated prolongation of cardiac repolarization in a reverse use-dependent manner by sildenafil 30 mcmol/L. Action potential duration increased 15% from baseline 117+/-3 to 134+/-2 ms with sildenafil during pacing at 250 ms cycle length, whereas a 6% increase from 99+/-2 to 105+/-2 ms was seen with pacing at 150 ms cycle length. Experiments in human ether-a-go-go-related gene (HERG)-transfected HEK293 cells (n=30) demonstrated concentration-dependent block of the rapid component (I(Kr)) of the delayed rectifier potassium current: activating current was 50% decreased at 100 mcmol/L. This effect was confirmed using HERG-transfected Chinese hamster ovary (CHO) cells, which exhibit no endogenous I(K)-like current. CONCLUSIONS-Sildenafil possesses direct cardiac electrophysiological effects similar to class III antiarrhythmic drugs. These effects are observed at concentrations that may be found in conditions of impaired drug elimination such as renal or hepatic insufficiency, during coadministration of another CYP3A substrate/inhibitor, or after drug overdose and offer a new potential explanation for sudden death during sildenafil treatment.

Ultradian variations in sensitivity of rat aorta rings to noradrenaline.

The in vitro sensitivity of the rat aorta to the vasoconstrictor effect of noradrenaline was found to vary according to the time the animals were killed, with a minimum at 10:00 h and a maximum at 16:00 h. This ultradian rhythm was not influenced by the presence of the endothelium but was modified by some, as yet unidentified, circulating factor(s). The results also strongly suggested an ultradian rhythm of in vitro sensitivity to the beta-adrenoceptor agonists, salbutamol and isoprenaline. This rhythm appears to be opposite to that for the sensitivity of aortic alpha-adrenoceptors to stimulation. In conclusion, our results demonstrate that the regulation of aortic tone follows an ultradian rhythm.

Vitamin K modulates cardiac action potential by blocking sodium and potassium ion channels.

BACKGROUND: Cardiovascular collapses, syncopes, and sudden deaths have been observed following the rapid administration of intravenous vitamin K. Our objectives were to characterize the effects of vitamin K on cardiac action potentials and to evaluate effects of vitamin K on sodium and potassium currents, namely I(Na), I(Kr), and I(Ks). METHODS AND RESULTS: Guinea pig hearts (n = 21) were paced at a cycle length of 250 msec and exposed to vitamin K at 1.15-4.6 micromol/L (2.5-10 mg/L). Monophasic action potential duration measured at 90% repolarization (MAPD(90)) was not significantly reduced (-1.6 +/- 0.3 msec; P >.05; N.S.) at 1.15 micromol/L, but increased by 6.5 +/- 0.4 msec (P <.05) at 2.3 micromol/L. MAPD(90) was not measurable at 4.6 micromol/L, as a result of inexcitability. Patch-clamp experiments in ventricular myocytes demonstrated a approximately 50% reduction in I(Na) by 10 micromol/L vitamin K and a concentration-dependent reduction of the K(+) current elicited by short depolarizations (250 msec; I(K250)). Estimated IC(50) for I(K250), mostly representing I(Kr), was 2.3 micromol/L. Vitamin K was less potent to block the K(+) current elicited by long depolarizations (5,000 msec; I(K5000)), mostly representing I(Ks), with an estimated IC(50) over 100 micromol/L. CONCLUSIONS: Therapeutic concentrations ( approximately 1.5 micromol/L) of intravenous vitamin K modulate cardiac action potential by blocking ionic currents involved in cardiac depolarization and repolarization.

Pimozide (Orap) prolongs cardiac repolarization by blocking the rapid component of the delayed rectifier potassium current in native cardiac myocytes.

BACKGROUND: Several cases of QT prolongation and ventricular tachyarrhythmia have been reported with pimozide, a potent neuroleptic useful in the management of motor and phonic tics associated with Tourette syndrome. To further elucidate the mechanism underlying these clinical observations, the effects of pimozide on monophasic action potential duration (MAPD(90)) and on potassium currents involved in the repolarization of native isolated ventricular myocytes were examined. METHODS AND RESULTS: Studies were undertaken in eight isolated guinea pig hearts that demonstrated reverse rate-dependent prolongation of cardiac repolarization by pimozide 100 nmol/L. Action potential duration increased 24% from baseline 115 +/- 2 to 142 +/- 4 msec with pimozide 100 nmol/L during pacing at 250 msec cycle length, while a 10% increase from 97 +/- 2 to 107 +/- 3 msec was seen with pacing at a cycle length of 150 msec. Experiments in native isolated ventricular myocytes (n = 20) demonstrated concentration-dependent block of the rapid component (I(Kr)) of the delayed rectifier potassium current: tail current was decreased by 50% at 15 nmol/L. CONCLUSIONS: Pimozide possesses cardiac electrophysiological effects similar to those of class III antiarrhythmic drugs. These effects are concentration-dependent and observed at recommended dosages of the drug. Since pimozide is strongly metabolized by CYP3A4, special care should be taken to avoid potential pharmacokinetic interactions leading to high plasma levels of pimozide and proarrhythmia.

Effects of antiarrhythmic agents on junctional resistance of guinea pig ventricular cell pairs.

Modulation of intercellular coupling through gap junctions can lead to a decrease in conduction velocity and conduction block. Previous studies have suggested that antiarrhythmic agents alter the internal resistance (sum of cytoplasmic and gap junctions resistances) of cardiac fibers. The objective of this study was to directly assess the effect of antiarrhythmic agents on junctional resistance between two isolated cells using the double whole-cell patch-clamp technique. The experimental protocol consisted in holding the membrane potential of each guinea pig ventricular myocyte of a coupled cell pair at 0 mV. Then, a junctional voltage gradient was created by changing membrane potential in only one cell. Voltage gradients were varied between -50 to +50 mV in steps of 20 mV. The extracellular medium was set to minimize trans-sarcolemmal currents and the junctional current was recorded in the cell maintained at 0 mV. Drugs tested were quinidine, lidocaine, procainamide, flecainide, propranolol, sotalol, amiodarone and verapamil. Drugs were superfused after a control period of 5 min. during which junctional resistance was observed to be stable. None of the antiarrhythmic agents tested in this study directly affected junctional resistance, although procainamide slightly increased junctional resistance 110 +/- 8% after 10 min of exposure. In conclusion, drugs tested in this study, chosen among all classes of antiarrhythmic agents, did not affect junctional resistance of cardiac myocyte cell pairs. However, long-term modulation or indirect effects of antiarrhythmic agents on gap junctions under physiological conditions cannot be excluded.

Lysophosphatidylcholine, a metabolite which accumulates early in myocardium during ischemia, reduces gap junctional coupling in cardiac cells.

Lysophosphatidylcholine (LPC) is a metabolite that accumulates rapidly during cardiac ischemia in animal and human hearts. LPC induces electrophysiological changes including action potential alterations and cardiac arrhythmias. Since there is increasing evidence that disrupted cell coupling can contribute to the generation of cardiac arrhythmias under ischemic conditions, this study was designed to assess the effects of LPC on gap junction coupling between cardiac cell pairs using the dual whole-cell voltage-clamp technique. To measure gap junction resistance (r(j)), both cells of a pair were first clamped to a common holding potential and then, one cell was stepped to various voltages (20 mV steps from -50 to +50 mV). Junctional conductance (g(j)=1/r(j)) was derived from the junctional current recorded in the non-stimulated cell divided by the trans-junctional voltage. Extracellular medium was set to minimize non-junctional membrane conductance. LPC induced a decrease in g(j)after about 3, 8 and 12 min superfusion, respectively, for 50, 10 and 5 micromol/l. When LPC was continuously superfused (no washout), no steady-state was observed but a complete uncoupling (i.e. when the junctional resistance is infinitely high) after a delay of 7.3+/-1.2 min, 11.3+/-6.0 min, 15. 2+/-5.5 min and 23.3+/-6.0 min, respectively, for LPC 50 (n=5), 20 (n=4), 10 (n=5) and 5 (n=3) micromol/l. Mg(2+(out))at a concentration of 5 mmol/l exerted protective effects against LPC-induced uncoupling. In conclusion, LPC, at concentrations measured in situ during cardiac ischemia, is a potent inhibitor of gap junction communications between cardiac cells. Impaired junctional communications due to LPC accumulation early during ischemia could decrease electrical conduction and contribute to the genesis of malignant arrhythmias.

Angiotensin II modulates the delayed rectifier potassium current of guinea pig ventricular myocytes.

Amplitude of the delayed rectifier (IK) tail current measured following long (5000msec) depolarizing pulses (-10 to +50mV) was decreased 19 +/- 3% (P < 0.05) in a voltage-independent manner by angiotensin II (AII) 100nM. In contrast, amplitude of tail current measured following short (250msec) depolarizing pulses to potentials > +10mV was increased 13 +/- 3% (P < 0.05) by AII 30nM. Deactivation kinetics of IK measured at -30mV were altered by AII 30nM and 100nM; time constant of the faster deactivating phase (tau 1) was decreased 1.4-fold. In summary, data obtained demonstrated that physiological concentrations of AII modulates major outward potassium currents involved in cardiac repolarization. Results suggest that AII increases amplitude of the rapid component of IK (IKr) but decreases its slow component IKs. Thus, we postulate that modulators of AII effects may exhibit direct cardiac electrophysiological properties.

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 lisinopril on intercellular resistance of guinea pig ventricular myocytes.

Previous reports have suggested that angiotensin converting enzyme (ACE) inhibitors are able to modulate intercellular resistance through an ACE-independent action. Therefore, the aim of our study was to determine whether the ACE inhibitor lisinopril would modulate intercellular resistance of guinea pig ventricular myocytes. Junctional resistance was measured using the double patch-clamp technique in eight cell pairs at baseline and during superfusion with 10(-4) M lisinopril. In these cell pairs, mean junctional resistance (+/-SEM) measured at baseline (14.5 +/- 3.8 M omega) was unchanged (15.0 +/- 3.6 M omega; p > 0.05) during lisinopril even after a 25-min exposure to the drug (n = 3). As well, a 20-min exposure to 10(-6) M lisinopril from the intracellular milieu did not modify the junctional resistance. In conclusion, this study demonstrates that modulation of junctional resistance independent of drug-related ACE inhibition is not observed with lisinopril.

Conduction block in Purkinje fibers by homogeneous versus localized decrease of the gap junction conductance.

Gap junction channels provide the pathway for the cell-to-cell propagation of cardiac action potential. Impairment of junctional conductance decreases conduction velocity and can cause block, two conditions that favor ventricular arrhythmias and fibrillation by re-entrant excitation. These experiments were designed to examine the effects of homogeneous versus localized decrease of the gap junction conductance on propagation of action potential in Purkinje fibers from sheep hearts. The fibers were mounted in a three-compartment chamber, and cell-to-cell conductance was progressively reduced by applying heptanol either over a central 2-mm segment or over the whole fiber length. The internal resistivities (Ri) at which conduction of the action potential became blocked were determined in both cases. With 3.5 mM heptanol in the central compartment, conduction failed when Ri was increased by only 3-4.6 times the control values. In contrast, when the same concentration of heptanol was added simultaneously to all three compartments, Ri had to rise by a factor of 7.5-9.4 before conduction became decremental and was blocked. In both situations, dV/dt(max) at the time of conduction block was similarly decreased to about 50% of the control values. Other parameters being equal, a moderate decrease of the gap junction conductance and of the fast sodium current, insufficient to block propagation of the action potential when they are homogeneously distributed, become sufficient to interrupt conduction if the action potential merges abruptly into a portion of fiber with normal internal conductivity at the outlet of the area of increased resistance. This greater sensitivity to block is accounted for by the increase in electrical load at the discontinuity in the core conductor between the region of increased internal resistance and the normal part of fiber that follows. Areas of steep transition from high to low input resistances of the core conductor, such as may develop in localized ischemia, therefore appear particularly susceptible to conduction failure.

Hydrogen peroxide modifies the kinetics of HERG channel expressed in a mammalian cell line.

Reactive oxygen species such as H2O2 were shown to influence both electrical and contractile properties of the heart. H2O2 modulates action potential duration and leads to reperfusion-induced arrhythmias. As these effects could involve the modulation of repolarizing currents, we assessed effects of H2O2 on HERG (which encodes the cardiac potassium channel I(Kr)) expressed in Chinese hamster ovary cells. HERG currents were recorded using the whole-cell patch-clamp technique. HERG activation and deactivation were accelerated when cells were superfused with 30 microM, 100 microM, or 1 mM H2O2. For example, at 1 mM H2O2, tau(act) was decreased from 862 +/- 178 to 633 +/- 151 ms (P < 0.05; n = 6), and fast tau(deact) was reduced from 286 +/- 47 to 151 +/- 18 ms (P < 0.05; n = 6). A negative shift of V1/2 was also observed (from -1.9 to -13.7 mV with 30 microM H2O2; P < 0.05), reflecting the acceleration of the activating current. Effects of H2O2 superfusion were prevented by intracellular application of catalase but superoxide dismutase prevented only H2O2-induced acceleration of activation. This indicates that H2O2 diffuses intracellularly before acting on HERG and that its effects on activation but not deactivation are mediated by the superoxide anion. Moreover, tau(act) decrease preceded fast tau(deact) decrease by about 4 min, suggesting that these effects were not produced by the same intracellular pathway or at the same site on HERG protein. Acceleration of HERG activation kinetics leads to an increase of outward current during the plateau phase of the action potential. This could suggest a reason for H2O2-induced shortening of the action potential. The faster HERG deactivation could be involved in reperfusion-induced arrhythmias by reducing K+ conductance in the early diastole, thus increasing the risks of premature beats.

Effects of cadmium and nisoldipine on the delayed rectifier potassium current in guinea pig ventricular myocytes.

Block of the slow inward calcium current (Isi) during assessment of the delayed rectifier potassium current (IK) of cardiac ventricular myocytes is commonly achieved by use of either inorganic compounds such as cadmium or dihydropyridine derivatives such as nisoldipine. Effects of these two Isi blockers on IK characteristics of guinea pig ventricular myocytes were compared in this study. Currents were measured in the whole cell configuration of the patch-clamp technique and IK tail amplitudes were measured at -40 mV after depolarizations to various test potentials (voltage steps, -20 to +50 mV) for either 250 (IK250), 450 (IK450) or 5000 (IK5000) msec. Activating and tail currents measured in the presence of cadmium were of greater amplitudes when voltage steps were more positive than 0 mV but were of smaller amplitudes at Vtest < or = 0 mV compared to currents measured in the presence of nisoldipine or Tyrode solution. In the presence of the rapid component of the delayed rectifier E-4031, a blocker of cadmium increased IKs amplitude during high voltage tests and caused a positive shift in the voltage dependence of IKs activation at low depolarizing potentials. In contrast, no effect on IK was observed when nisoldipine was added to Tyrode solution. In conclusion, results obtained in this study suggest that cadmium depresses and/or shifts the activation curve of the rapid component and increases and positively shifts the slow component of IK in guinea pig ventricular myocytes. These observations lead us to propose that nisoldipine may be a better tool to inhibit long lasting inward calcium current during assessment of IK.

Modulation of HERG potassium channel properties by external pH.

We expressed the human eag-related gene (HERG), known to encode for the cardiac potassium channel IKr, in Chinese hamster ovary (CHO) cells. This study investigated effects of external pH (pHo) on HERG current properties using the whole-cell patch-clamp technique. We observed that current amplitude was decreased and kinetics of activation and deactivation were faster when pHo was lowered from 7. 4 to 6.0, while activation was accelerated and V1/2 negatively shifted when pHo was changed from 7.4 to 8.0. These effects can be explained by surface charge screening, amino acid group titration and/or changes in ionic atmosphere. We conclude that alterations of HERG channel by pHo could have consequences for the onset of arrhythmias during cardiac ischemia.

Erythromycin blocks the rapid component of the delayed rectifier potassium current and lengthens repolarization of guinea pig ventricular myocytes.

BACKGROUND: Administration of erythromycin to humans has been associated with lengthening of cardiac repolarization and even proarrhythmia. The objectives of our study were to describe effects of erythromycin on repolarization of isolated hearts and to determine effects of the drug on major K+ currents involved in cardiac repolarization. METHODS AND RESULTS: A first set of experiments was conducted in isolated, buffer-perfused guinea pig hearts electrically stimulated at a basic cycle length of 250 ms. In this model, erythromycin 10(-4) mol/L increased monophasic action potential duration measured at 90% repolarization (MAPD90) by 40 +/- 7 ms. Increase in MAPD90 was reproducibly observed in seven hearts studied. To study the mechanism of these effects on cardiac repolarization, a second set of experiments was performed in isolated guinea pig ventricular myocytes using the whole cell configuration of the patch-clamp technique. In these cells, erythromycin 10(-4) mol/L decreased by about 40% (P < .05 versus baseline) the time-dependent outward K+ current elicited by short depolarizations (250 ms) to low depolarizing voltages (-20 to 0 mV). In contrast, the drug was without significant effects on the time-dependent K+ current elicited by long pulses (5000 ms) to high depolarizing voltages (+10 to +50 mV), on the time-independent background current (mostly IKl), and on the slow inward calcium current. CONCLUSIONS: The outward time-dependent K+ current blocked by erythromycin in isolated guinea pig ventricular myocytes had characteristics similar to those described for IKr. Selective block of this component of IK gives an explanation for the effects of erythromycin on cardiac repolarization. These effects were observed at clinically relevant concentrations reached after intravenous administration of the drug and warn for potential interactions with other action potential-lengthening drugs.

Mechanism of sodium channel block by venlafaxine in guinea pig ventricular myocytes.

Venlafaxine is a newly introduced antidepressant agent. The drug causes selective inhibition of neuronal reuptake of serotonine and norepinephrine with little effect on other neurotransmitter systems. Cases of seizures, tachycardia, and QRS prolongation have been observed following drug overdose in humans. The clinical manifestations of cardiac toxicity suggest that venlafaxine may exhibit cardiac electrophysiological effects on fast conducting cells. Consequently, studies were undertaken to characterize effects of venlafaxine on the fast inward sodium current (I(Na)) of isolated guinea pig ventricular myocytes. Currents were recorded with the whole-cell configuration of the patch-clamp technique in the presence of Ca(2+) and K(+) channel blockers. Results obtained demonstrated that venlafaxine inhibits peak I(Na) in a concentration-dependent manner with an estimated IC(50) of 8. 10(-6) M. Inhibition was exclusively of a tonic nature and rate-independent. Neither kinetics of inactivation (tau(inac)= 0.652 +/- 0.020 ms, under control conditions; tau(inac)= 0.636 +/- 0.050, in the presence of 10(-5) M venlafaxine; n = 5 cells isolated from five animals) nor kinetics of recovery from inactivation of the sodium channels (tau(re)= 58.7 +/- 1.6 ms, under control conditions; tau(re)= 54.4 +/- 1.8, in the presence of 10(-5) M venlafaxine; n = 10 cells isolated from six animals) were significantly altered by 10(-5) M venlafaxine. These observations led us to conclude that venlafaxine blocks I(Na) following its binding to the resting state of the channel. Thus, the characteristics of block of I(Na) by venlafaxine are different from those usually observed with most tricyclic antidepressants or conventional class I antiarrhythmic drugs.

Preconditioning in the absence or presence of sustained ischemia modulates myocardial Cx43 protein levels and gap junction distribution.

In the heart, brief repeated episodes of ischemia prior to a sustained occlusion (ischemic preconditioning; PC) significantly delay the onset of necrosis and arrhythmogenesis. Ischemia has been reported to influence gap junction organization and connexin43 (Cx43) content, but whether PC affects these structures is not known. We investigated the effect of PC (2 cycles of 5-min ischemia plus 10-min reperfusion) followed by prolonged reperfusion without concomitant regional coronary occlusion on the myocardial Cx43 content and its spatial distribution in rabbit hearts. We also compared the effect of sustained ischemia with or without PC on Cx43 spatial distribution. In experiments with PC only, there was an initial decrease in Cx43 levels within the ischemic zone followed by a progressive increase after 48 h reperfusion. End-to-end immunolabeling of Cx43 was augmented in the ischemic region between 24 and 48 h reperfusion; labeling was not uniquely confined to myocyte abutments, but was also dispersed along the sarcolemma. Cx43 immunolabelling was more intense and diffuse in hearts subjected to PC before sustained coronary occlusion (compared to non-PC). These data indicate that gap junctions are significantly altered during brief episodes of ischemia. Reorganization of the gap junction complex could contribute to PC-mediated reductions in cardiac arrhythmias.

Ethanol delays and reverses lysophosphatidylcholine-induced calcium overload in neonatal rat heart cells.

Increased lysophosphatidylcholine (LPC) production by the ischemic heart is associated with tissue damage. In vitro, LPC produces an increase in cytosolic [Ca2+], usually followed by cell contracture and lysis. Since ethanol reportedly protect cells during ischemia-reperfusion, we wished to determine whether ethanol could protect heart cells against LPC-induced Ca2+ overload. Newborn rat heart cells in culture were loaded with Fura-2 and [Ca2+]i recorded in individual cells. The presence of 22 or 44 mM ethanol increased the time required for 10 microM x L-palmitoyl-LPC to produce maximal Ca2+ accumulation from 8.4+/-0.4 min (n=47) to 21.1+/-2.1 x min (n=32; P<0.01) and 23.8+/-1.8 min (n=10; P<0.01) respectively. The onset of the [Ca2+]i increase could be reversed partially by the addition of ethanol (44 or 88 mM). After the addition of 22 mM ethanol, the cells retained the Fura-2 three times longer than under control conditions. Ethanol (88 mM) decreased the critical micelle concentration of LPC, thus decreasing the LPC monomer concentration in this solution. La3+ also protected the cells against LPC but no further protection was afforded by the addition of ethanol. Our results suggest that ethanol concentrations commonly found in the blood of social drinkers protect heart cells against the deleterious effect of LPC.

Block of the rapid component of the delayed rectifier potassium current by the prokinetic agent cisapride underlies drug-related lengthening of the QT interval.

BACKGROUND: Lengthening of the QT interval and torsades de pointes resulting in cardiac arrests and deaths have been noticed during treatment with cisapride, a newly developed gastrointestinal prokinetic agent. The rapid (I[Kr]) and slow (I[Ks]) components of the delayed rectifier current (I[K]) are candidate ionic currents to explain cisapride-related toxicity because of their role in repolarization of cardiac ventricular myocytes. Our objectives were to (1) characterize effects of cisapride on two major time-dependent outward potassium currents involved in the repolarization of cardiac ventricular myocytes, I(Kr) and I(Ks), and (2) determine action potential-prolonging effects of cisapride on isolated hearts. METHODS AND RESULTS: A first set of experiments was performed in isolated guinea pig ventricular myocytes with the whole-cell configuration of the patch-clamp technique. Cells were held at -40 mV while time-dependent outward currents were elicited by depolarizing pulses lasting either 250 ms (I[K250]) or 5000 ms (I[K5000]). Effects of cisapride on the I(Kr) component were assessed by measurement of time-dependent activating currents elicited by short pulses (250 ms; I[K250]) to low depolarizing potentials (-20, -10, and 0 mV). Time-dependent activating currents elicited by long pulses (5000 ms; I[K5000]) to positive potentials (>+30 mV) were recorded to assess effects of the drug on the I(Ks) component. A second set of experiments was conducted in isolated guinea pig hearts buffer-perfused in the Langendorff mode to assess effects of the drug on monophasic action potential duration measured at 90% repolarization (MAPD90). Hearts were exposed to cisapride 100 nmol/L at decremental pacing cycle lengths of 250, 225, 200, 175, and 150 ms to determine reverse frequency-dependent effects of the drug. Overall, 112 myocytes were exposed to seven concentrations of cisapride (10 nmol/L to 10 micromol/L). Cisapride inhibited I(Kr), the major time-dependent outward current elicited by short pulses (I[K250]) to low depolarizing potentials, in a concentration-dependent manner with an IC50 of 15 nmol/L (therapeutic levels, 50 to 200 nmol/L). Conversely, block of I(Ks) by the drug was less potent (estimated IC50 >10 micromol/L). In isolated hearts (n=9 experiments), cisapride 100 nmol/L increased MAPD90 by 23+/-3 (P<.05) at a basic cycle length of 250 ms but by only 7+/-1 ms (P<.05) at a basic cycle length of 150 ms. CONCLUSIONS: Block of I(Kr) gives an explanation to lengthening of cardiac repolarization observed in isolated guinea pig hearts. Potent block of I(Kr) is also likely to underlie prolongation of the QT interval observed in patients receiving clinically recommended doses of cisapride as well as severe cardiac toxicity (torsades de pointes) observed in patients with increased plasma concentrations of the drug.

Block of IKs, the slow component of the delayed rectifier K+ current, by the diuretic agent indapamide in guinea pig myocytes.

There is a high incidence of diuretic use among patients who develop exaggerated QT prolongation and polymorphic ventricular tachycardia (torsade de pointes) during treatment with action potential-prolonging agents. Diuretic-induced hypokalemia is thought to be the usual mechanism, but a direct effect of diuretic drugs on repolarizing currents is an additional possibility. Therefore, in this study, we examined the effects of the diuretic agents chlorthalidone and indapamide on the cardiac delayed rectifier current. In guinea pig ventricular myocytes, this current is made up of two components: IKr, a rapidly activating, inwardly rectifying current blocked by most action potential-prolonging antiarrhythmics, and IKs, a slowly activating component. In this preparation, indapamide blocked outward current in a time-, voltage- and concentration-dependent fashion, whereas chlorthalidone (1 mmol/L) was without effect. The following features of the effect of indapamide strongly suggest selective block of IKs: (1) Indapamide block was significantly greater with 5000-millisecond activating pulses (-43 +/- 5% at +50 mV [100 mumol/L indapamide]) than with 225-millisecond ones (-20 +/- 5%; n = 5, P < .01), and the signature of the indapamide-sensitive current was a slowly activating delayed rectifier current. (2) The voltage dependence of indapamide block (EC50, 101 mumol/L at +50 mV and 196 mumol/L at +10 mV) was consistent with preferential block of IKs relative to IKr. (3) In the presence of indapamide, an envelope-of-tails test for IKr was satisfied. The drug-insensitive current had rectifying properties similar to those described for IKr in these cells.(ABSTRACT TRUNCATED AT 250 WORDS)