Decrease In Action Potential Duration Research Articles

Opposite effects of halothane on guinea-pig ventricular action potential duration

Halothane protects the heart against the reperfusion injury observed after an ischemia. In ischemic or anoxic conditions, a large ATP-sensitive K + (K ATP) conductance is supposed to provide an endogenous protection to the myocardium. In this study, we tested the possibility that halothane acted by modulating this conductance. Isolated guinea-pig cardiomyocytes were successively studied in current clamp and in voltage-clamp conditions. Action potentials regulation by halothane was tested in control conditions and in situations where the K ATP channels were activated. In control conditions, halothane decreased action potential duration of myocytes but did not significantly alter the inward rectifying K + current. Conversely, halothane lengthened action potential of cells in which the K ATP conductance was activated, by inhibiting the K ATP current. In ischemic conditions, simultaneous shortening of long action potentials and lengthening of shortened ones would be expected to homogenize the absolute refractory period at the border between normoxic and anoxic zones. This effect, together with a decrease in calcium load, could protect the myocardium against re-entrant arrhythmias.

Diazepam reduces action potential duration in guinea-pig papillary muscle by a cAMP-dependent mechanism

In this study, we have analyzed the role of cyclic AMP (cAMP) as the mediator of the decrease in action potential duration induced by diazepam. Diazepam (1–100 μM) reduced, in a dose-dependent manner, the duration of intracellular action potential recorded in the papillary muscle obtained from the right ventricle of the guinea pig heart. This effect was mimicked by the analog of cyclic AMP, 8-Br-cAMP (100 μM), but not by γ-amino-butyric acid (GABA). Also, the selective antagonist of the benzodiazepine receptors, flumazenil did not modify the effect of diazepam. The diazepam-induced shortening of action potential duration was partially antagonized by the inhibitor of cAMP synthesis carbachol (1 μM) of the blocker of the cAMP-dependent protein kinase A, Rp-cAMP[S] (1 μM). These results indicate that cyclic AMP is involved in the diazepam-induced shortening of the action potential duration of the guinea pig papillary muscle.

Contribution of L-type Ca2+ current to electrical activity in sinoatrial nodal myocytes of rabbits.

The role of L-type calcium current (ICa,L) in impulse generation was studied in single sinoatrial nodal myocytes of the rabbit, with the use of the amphotericin-perforated patch-clamp technique. Nifedipine, at a concentration of 5 microM, was used to block ICa,L. At this concentration, nifedipine selectively blocked ICa,L for 81% without affecting the T-type calcium current (ICa,T), the fast sodium current, the delayed rectifier current (IK), and the hyperpolarization-activated inward current. Furthermore, we did not observe the sustained inward current. The selective action of nifedipine on ICa,L enabled us to determine the activation threshold of ICa,L, which was around -60 mV. As nifedipine (5 microM) abolished spontaneous activity, we used a combined voltage- and current-clamp protocol to study the effects of ICa,L blockade on repolarization and diastolic depolarization. This protocol mimics the action potential such that the repolarization and subsequent diastolic depolarization are studied in current-clamp conditions. Nifedipine significantly decreased action potential duration at 50% repolarization and reduced diastolic depolarization rate over the entire diastole. Evidence was found that recovery from inactivation of ICa,L occurs during repolarization, which makes ICa,L available already early in diastole. We conclude that ICa,L contributes significantly to the net inward current during diastole and can modulate the entire diastolic depolarization.

Increased calcium-dependent K + channel activity contributes to the maturation of cellular firing patterns in developing cerebellar Purkinje neurons

Developmental changes in neuronal excitability reflect the regulated expression of ion channels and receptors. Purkinje neurons of the rat cerebellum progress from slow irregular firing to a fast pacemaker-like pattern during postnatal development in vivo. In this study, a comparable period of development in culture was investigated at the protein level using cell-attached single channel recordings to quantify the abundance of active calcium-dependent (K Ca) and delayed rectifier (K D) potassium channels. In control cultures, K Ca channel activity increased whereas K D channel activity was not significantly different with developmental age. The increase in active K Ca channels was antagonized by chronic treatment with the blocker, tetraethylammonium (TEA, 1 mM), which also retarded the normal development of cellular firing patterns. The consequences of chronic TEA treatment were assessed in cultures after thorough washout of the TEA-containing culture medium. Current clamp analyses (nystatin-perforated patches) showed that control Purkinje neurons progressed from a single spike mode to a repetitive firing mode, with a concomitant decrease in action potential duration and an increase in maximal firing rate. Chronic TEA treatment prevented these changes; Purkinje neurons retained the slow firing rate and long duration action potentials that are typical of the immature state. These data suggest that the developmental increase in K Ca channel activity may be required for the maturation of cellular firing patterns in cerebellar Purkinje neurons.

Accentuated antagonism in canine subendocardium is not altered by chronic exercise.

Acetylcholine often affects cardiac action potential repolarization only during augmented adrenergic tone, i.e., the phenomenon of accentuated antagonism. Since chronic exercise involves repeated changes in autonomic outflow, we determined whether it also influenced adrenergic/cholinergic interactions in isolated canine cardiac tissue. Using standard micro-electrode techniques in thin ventricular subendocardial slices isolated from exercised (EX: 8-10 wk daily exercise) and sedentary (SED): 8-10 wk cage rest) dogs, we examined transmembrane potential responses to isoproterenol (ISO: 10(-8), 10(-7), 10(-6) M) and to ISO in the presence of ACH (10(-5) M). Control transmembrane characteristics at BCL = 500 ms were similar for EX (N = 8 dogs) and SED (N = 9 dogs). ISO (10(-6) M) decreased action potential duration at 50% repolarization (APD50): EX = -29 +/- 15 ms; SED = 11 ms and at 90% repolarization (APD90): EX = -37 +/- 17 ms; and SED = -24 +/- 14 ms (P > 0.05, EX vs SED). ACH alone did not alter APD. With ACH (10(-5) M), delta APD50 with ISO (10(-6) M) was -5 +/- ms and 0 +/- 5 ms for EX and SED, respectively; delta APD90 was -8 +/- 4 ms and -8 +/- 7 ms for EX and SED, respectively (P > 0.05, EX vs SED). Thus, ACH antagonized ISO-mediated acceleration of repolarization equally in both groups. Chronic daily exercise does not influence adrenergic/cholinergic interactions at the cellular level.

Antiarrhythmic and Arrhythmogenic Actions of Varying Levels of Extracellular Magnesium: Possible Cellular Basis for the Differences in the Efficacy of magnesium and Lidocaine in Torsade de Pointes.

BACKGROUND: In recent years, there has been an increasing use of antiarrhythmic drugs that act predominantly by prolonging myocardial repolarization. An inevitable electrophysiologic consequence of these drugs is the development of torsade de pointes as a proarrhythmic reaction. Both intravenous lidocaine and magnesium sulphate have been used in the acute control of such a proarrhythmia. Their electrophysiologic mechanisms in this setting are not well defined. METHODS AND RESULTS: Using the standard microelectrode techniques, the effects of magnesium (Mg) and lidocaine on action potential duration (APD), and on barium-induced spontaneous action potentials, were studied in canine Purkinje fiber preparations. The objective was to clarify the direct and indirect effects of magnesium on triggered activities due to early afterdepolarizations. Superfusion in media with 0.1 mM Mg and 2.5 mM K produced more pronounced increases in APD measured at -20mV repolarization time [APD(20)] than those in a solution with 5 mM K. This effect was further enhanced at lower stimulation frequencies. The striking prolongation of APD(20) by solutions with low potassium concentrations diminished as the Mg concentration was increased. In solutions with 2.5 mM K, Mg produced concentration-dependent decreases in APD(20). This effect was greater at lower stimulation frequencies. Lidocaine at 4.0 x 10(_5) M produced a marked shortening of the APD in the entire firing frequency of the abnormal automaticity in a concentration-dependent manner. With 10 mM Mg, such action potentials appeared only sporadically. Magnesium also decreased the amplitude and the maximum upstroke velocity of these action potentials. In contrast, lidocaine at 4.0 x 10(-5) M exhibited no significant effects on action potentials due to barium-induced abnormal automaticity, or on additional depolarizations developing from the repolarization phase of these action potentials. CONCLUSIONS: The data indicate that (i) hypomagnesemia may be arrhythmogenic when combined with hypokalemia and bradycardia leading to a prolongation of the plateau phase of the action potential, (ii) magnesium administration may suppress triggered activities mainly by a direct inhibition of the development of triggered potentials, and (iii) lidocaine may suppress triggered potentials only indirectly by preventing the development of early afterdepolarizations due to the shortening effect on the APD. These findings are consistent with the clinical observation of a high incidence of torsade de pointes in the setting of hypokalemia and hypomagnesemia introduced by a chronic diuretic therapy. They are also consistent with the marked effectiveness of intravenous Mg relative to the inconsistent clinical effects of lidocaine in controlling torsade de pointes.

Two parallel signaling pathways couple 5HT1A receptors to N- and L-type calcium channels in C-like rat dorsal root ganglion cells.

The coupling of serotonin receptors to Ca2+ channels was studied in a subpopulation of acutely isolated rat dorsal root ganglion (DRG) cell bodies (type 1 DRG cells), which have membrane properties similar to C-type nociceptive sensory neurons. In these cells, serotonin (5HT) inhibited high-threshold Ca2+ channel current and decreased action potential duration. The inhibitory effects of 5HT and the 5HT1A agonist 8-OH-DPAT were shown to be antagonized by the 5HT1A antagonists spiperone and pindolol, respectively, indicating involvement of a 5HT1A receptor. Several observations suggest that 5HT1A receptors couple to N- and L-type Ca2+ channels by two different signaling pathways in type 1 DRG cells. The inhibition of Ca2+ channel currents produced by 10 microM 5HT occurred in two phases, an initial slowing of current activation rate (kinetic slowing), which was complete within 10 s, and a simultaneous reduction in steady state current amplitude (steady state inhibition), which peaked in approximately 1 min. The kinetic slowing, but not steady state inhibition, was reversed by a positive prepulse to +70 mV (prepulse). Blockade of N-type Ca2+ channels selectively reduced the kinetic slowing and its reversal by prepulses. Chelation of intracellular Ca2+ or blockade of L-type Ca2+ channels selectively reduced the steady state inhibition. Recordings using the cell-attached patch configuration suggest that steady state inhibition required a component that was diffusible in the cytosol, while kinetic slowing occurred via a membrane delimited pathway. The application of 5HT to the cell body outside the patch pipette reduced macroscopic Ca2+ channel currents in 33% of small-diameter DRG cells tested, indicating the participation of a cytosolic diffusible component. Application of 5HT (a membrane impermeant compound) outside the patch pipette produced steady state inhibition only, whereas similar application of membrane permeant 5HT1A agonists, 8-OH-DPAT or 5-methoxy-N,N-dimethyl-tryptamine, produced kinetic slowing and steady state inhibition. Together these data suggest that 5HT1A receptors couple negatively to Ca2+ channels via two pathways: a membrane-delimited pathway that couples to N-channels and actuates voltage-sensitive kinetic slowing and a pathway dependent on a cytosolic diffusible component and free intracellular Ca2+, which couples to L channels and actuates steady state inhibition.

Epinephrine Facilitates Cardiac Fibrillation by Shortening Action Potential Refractoriness

Epinephrine released during ventricular tachycardia (VT) or early fibrillation (VF) appears to be instrumental in stabilizing fibrillation. However, mechanisms remain unclear. Effects of epinephrine on refractory period at normal sinus rates depend on basic cycle length, but effects at short cycle lengths, typical of VT/VF, are unknown. Therefore, the goal of this study was to determine whether epinephrine shortens action potential duration and refractoriness at these short cycle lengths. To simulate early VT/VF, myocardial cell aggregates (n=35) were paced using field stimulation (5 ms rectangular waveform) at cycle lengths of 200, 180, 160 and 140 ms, which occur duringin situfibrillation; normal sinus rhythm was simulated by pacing at 600 and 400 ms intervals. Action potentials and excitation threshold were recorded with intracellular microelectrodes under control conditions, with 0.9μm/l epinephrine, and with 0.9μm/l epinephrine and 0.5μm/l propranolol. At short cycle lengths, epinephrine significantly shortened action potential duration and refractoriness compared to control. At a cycle length of 160 ms, action potential duration was reduced by 14 ms at 60% repolarization (P<0.0002) and stimulation threshold by 18% (P<0.02). Epinephrine also allowed pacing at a cycle length of 140 ms, not achievable under control conditions. Because epinephrine decreases action potential duration at short cycle lengthin situ, re-entry wavefronts are less likely to encounter refractory tissue; fibrillation is more likely to occur and to remain stabilised. Reduction in action potential duration and excitation threshold were reversed by propranolol, suggesting that epinephrine effects are produced byβ-stimulation.

Mechanism of lethal proarrhythmia observed in the Cardiac Arrhythmia Suppression Trial: role of adrenergic modulation of drug binding.

A variety of recent in vivo studies have sought to clarify the mechanism underlying the proarrhythmic response of flecainide in the Cardiac Arrhythmia Suppression Trial (CAST). Increased inducibility of relatively stable ventricular arrhythmias in subacute and chronic postinfarction models has been universally observed. The arrhythmogenesis has been explained in part by drug induced modulation of anisotropic conduction in persistently ischemic tissue, increased durations of vulnerable windows, enhanced generation of unidirectional block with the introduction of extrastimuli, variability of repolarization within the ventricular wall, and the creation of stable reentrant circuits with narrow central zones of propagation. While these data explain arrhythmogenesis in general, malignant ventricular arrhythmia capable of producing the excess sudden or arrhythmic death mortality in the CAST trial have not been universally observed, nor have the proported beneficial effects of beta-blockade seen in the CAST trial and other studies been explained. Additional studies examining the adrenergic modulation of flecainide binding have shown reversal of flecainide effects in normal tissue, but paradoxical amplification of flecainide induced conduction slowing in depolarized tissue. This variable effect in normal versus abnormal tissue produces significant dispersions of conduction with an expected increased propensity for conduction failure in response to ectopy, increased liminal length for impulse propagation, enhanced vulnerability to premature extrastimuli, and completed reentrant circuits in regions of depressed membrane potentials. This, along with the decrease in action potential duration and accompanying refractoriness in the setting of adrenergic modulation may favor more malignant double wavelet or unstable ventricular arrhythmias.

Cardioprotective effects of NIP-121, a novel ATP-sensitive potassium channel opener, during ischemia and reperfusion in coronary perfused guinea pig myocardium.

We investigated the effect of NIP-121, a novel ATP-sensitive K+ channel opener, on myocardial damage during ischemia/reperfusion. The action potential and contractile force of coronary-perfused guinea pig right ventricular walls were recorded. The preparations were subjected to 30-min no-flow ischemia with or without NIP-121 or glibenclamide, followed by 60-min reperfusion. In untreated tissues, decreases in action potential duration (APD) and contractile force and an increase in resting tension were observed during the no-flow period. On reperfusion, transient arrhythmias were observed and resting or contractile force returned to <50% of preischemic values. NIP-121, at 0.3 microM, a concentration showing only a slight negative inotropic effect, caused a faster decrease in APD and contractile force but abolished the increase in resting tension (RT) during the no-flow period. On reperfusion, no arrhythmia was observed in NIP-121-treated preparations, and contractile force recovered to approximately 80% of the preischemic value. Glibenclamide 1 microM attenuated the decrease in APD but affected neither the decrease in contractile force nor the increase in RT during the no-flow period. On reperfusion, the incidence of arrhythmia was increased in glibenclamide-treated preparations, and the recovery of basal tension and contractile force was inhibited: Contractile force recovered to only approximately 15% of the preischemic value. NIP-121 was also shown to attenuate the decrease in tissue ATP during ischemia and reperfusion. We demonstrated that NIP-121 may have protective effects against myocardial injury during ischemia and reperfusion. Activation of ATP-sensitive K+ current may be an adaptive mechanism for cardioprotection under compromised blood flow.

Regional differences in the negative inotropic effect of acetylcholine within the canine ventricle.

1. Regional differences in the effects of ACh on sub-epicardial, mid-wall and sub-endocardial cells of the dog left ventricle have been studied. 2. ACh produced a dose-dependent, atropine-sensitive negative inotropic effect that was greatest in sub-epicardial cells and small or absent in sub-endocardial cells. 3. In sub-epicardial (but not sub-endocardial) cells, ACh also resulted in a dose-dependent decrease in action potential duration. The inotropic effect of ACh on sub-epicardial cells was primarily the result of the decrease of action potential duration, because during trains of voltage clamp pulses the inotropic effect of ACh was reduced or abolished. At a holding potential of -80 mV, 10(-5)M ACh decreased L-type Ca2+ current by approximately 8% and this is thought to be responsible for the small inotropic effect during trains of pulses. 4. Although 4-AP, a blocker of the transient outward current (I(to)), abolished the "spike and dome' morphology of the sub-epicardial action potential, it had little or no effect on the actions of ACh on sub-epicardial cells. ACh had no effect on I(to) in sub-epicardial cells in voltage clamp experiments. 5. ACh activated a Ba(2+)-sensitive outward current (IK,ACh) in sub-epicardial cells, but little or no such current in sub-endocardial cells. In sub-epicardial cells, ACh also inhibited the inward rectifier current, IK,1. 6. It is concluded that in left ventricular sub-epicardial cells, ACh activates IK,ACh. This results in a shortening of the action potential and, therefore, a negative inotropic effect. In subendocardial cells, ACh activates little or no IK,ACh and, therefore, it has little or no negative inotropic effect. This may result from a regional variation in the expression of the muscarinic K+ channel.

The effects of sevoflurane on contractile and electrophysiologic properties in isolated guinea pig papillary muscles.

We examined, in guinea pig papillary muscles, whether the negative inotropic effect of sevoflurane is due to the depression of the influx of extracellular Ca2+ or to inhibition of the availability of intracellularly stored Ca2+. Sevoflurane decreased action potential duration and contractile force in a concentration-dependent fashion in normally polarized guinea pig papillary muscles. Sevoflurane produced a depression of contractile force with different rates or patterns of stimulation in the rested state and at low stimulation frequencies. In a potentiated state, sevoflurane did not depress contractile forces. Although sevoflurane decreased action potential duration and contractile force in a concentration-dependent fashion in normal Tyrode's solution, in high K+ Tyrode's solution, it caused a depression of contractile force without a shortening of action potential duration. Sevoflurane also depressed contractile force in normal and high K+ Tyrode's solution with ryanodine 1 microM. Our results suggest that in myocardial contractile force the negative inotropic effect of sevoflurane might be caused by depression of transsarcolemmal Ca2+ influx, accompanied by shortening of the action potential duration.

Endothelin-1 partially inhibits ATP-sensitive K+ current in guinea pig ventricular cells.

To clarify the pathophysiological significance of endothelin (ET) in the ischemic myocardium, we examined the effect of endothelin-1 (ET-1) on the ATP-sensitive K+ current (IK.ATP) and compared it with that of ET-3 in guinea pig ventricular cells using conventional microelectrode and patch clamp techniques. In isolated guinea pig papillary muscles, ET-1 (30 nM) markedly increased developed tension (DT), with little influence on action potential duration (APD), whereas ET-3 at the same concentration failed to affect DT or APD. Both nicorandil (1 mM) and cromakalim (30 microM) markedly shortened APD and decreased DT in papillary muscles. ET-1, but not ET-3, partially reversed the nicorandil-induced decreases in APD and DT in a concentration-dependent manner. ET-1 also attenuated the cromakalim-induced decreases in APD and DT. In single ventricular myocytes, both nicorandil and cromakalim increased a steady-state outward current, which was sensitive to 1 microM glibenclamide, suggesting that these drugs activate IK.ATP. ET-1 (30 nM) significantly inhibited the IK.ATP, whereas ET-3 failed to affect it. The ET-1 induced inhibition of IK.ATP was abolished by BQ-485 (100 nM), an ETA receptor-selective antagonist. Neither the protein kinase C (PKC) inhibitor staurosporine (20 nM) nor the calmodulin antagonist W-7 (50 microM) affected the inhibitory action of ET-1 on the nicorandil-induced IK.ATP. In pertussis toxin (PTX)-treated cells, the inhibitory action of ET-1 on IK.ATP was augmented rather than attenuated. These results suggest that ET-1 partially inhibits the IK.ATP through the activation of ETA receptors, although the precise intracellular mechanism remains to be clarified. Because activation of the ATP-sensitive K+ channels is considered to protect the ischemic myocardium, the partial inhibition of IK.ATP by ET-1 may lead to the aggravation of myocardial injury, potentially due to an increase in transmembrane Ca2+ influx.

Acute and chronic effects of amiodarone on mammalian ventricular cells.

This article reviews experimental studies on the electrophysiological effects of amiodarone on mammalian hearts. Acute application of amiodarone (0.1-10 microM) to papillary muscles or single ventricular cells isolated from rabbits or guinea pigs caused a significant decrease in action potential duration (APD) as well as a marked use-dependent inhibition of the maximum upstroke velocity (Vmax) of action potential with fast recovery kinetics. Acute amiodarone also caused a concentration-dependent decrease in the calcium current (ICa) and the delayed-rectifier potassium current (IK). Action potentials recorded from papillary muscles or single ventricular cells isolated from the rabbits treated with oral amiodarone (100 mg/kg daily, 4 weeks) were characterized by a moderate frequency-independent prolongation of APD. There was no use-dependent Vmax inhibition. The ventricular cells treated with chronic amiodarone showed a significant decrease in the current density of ICa, IK and Ito. The amount of mRNA for Kv1.5, a Shaker-related potassium channel from the rats treated with chronic oral amiodarone was significantly lower than that from control rats. These results suggest that the major and consistent effects of chronic amiodarone is repolarization delay (Class III action) through a decrease in IK and Ito density, probably due to a modulation of gene expression of potassium channels. When amiodarone above a certain concentration is present in the extracellular space, fast kinetic Class I and Class IV actions would be added as acute effects.

COMPARATIVE STUDY ON THE ELECTROPHYSIOLOGICAL EFFECTS OF MONENSIN ON ATRIAL AND VENTRICULAR MUSCLE PREPARATIONS OF GUINEA-PIG HEART

The effects of monensin (10 µmol/L) alone and in presence of verapamil (3 µmol/L) or ouabuin (1 µmol/L) or glibenclamide (10 µmol/L) or BaCl2 separately were studied on the electrophysiological properties represented by force of contraction, duration of action potential, and membrane resting potential of isolated atrial and ventricular muscle of guinea –pig heart. Monensin induced a transient increase in the force of contraction of atrial and ventricular muscles. It caused a decrease in action potential duration of papillary muscle, this effect was reversed by glibenclamide. Monensin increased the action potential duration of atrial muscle in absence or presence of glibenclamide. In presence of ouabain monensin induced a significant depolarization of resting membrane potential in both atrial and ventricular muscles. Verapanil did not antagonize the effects monensin. Monensin induced an increase in Na+ a transient increase in Ca2+ and a decrease in K+ contents of atrial and ventricular tissues. From the data of the present work it could be concluded that, Na+ pumped out the cell by Na+/K+ pump was less in atria then in ventricles in presence of moninsin. Less consumption of intracellular ATP in ventricular cells than in atrial cells, thus the ATP of ventricular cells did not reach the critical concentration level which open ATP dependent K+ channels, The study also showed absence of Na+ dependent K+ channels in guinea-pig atria.

Mechanical and Electrophysiological Effects of Preconditioning in Isolated Ischemic/Reperfused Rat Hearts

Changes in action potential duration (APD) were studied during ischemic/reperfusion injury preceded or not by preconditioning in isolated rat hearts. Hearts were perfused on a Langendorff apparatus with Krebs-Henseleit carbonate buffer and submitted to 25-min global low-flow ischemia (coronary flow, 0.3 mol x min-1) followed by 30-min reperfusion. In hearts that had been preconditioned, two intermittent periods of total ischemia for 5 min each, separated by 5-min reflow, were performed before low-flow ischemia. At the end of the ischemic period, APs were significantly prolonged in nonpreconditioned hearts; this prolongation was abolished by preconditioning. Moreover, preconditioning increased the recovery of the contractile function. Therefore, ischemia can widen APD. The results also showed that in rats, preconditioning can be produced in a manner qualitatively similar to preconditioning in other species. Verapamil (3 x 10(-9) mol x min(-1)) or 4-aminopyridine (4-AP, 3 x 10(-6) mol x min(-1)) applied exclusively during low-flow ischemia significantly improved postischemic contractile function in nonpreconditioned hearts (25.9 +/- 4.4. and 37.9 +/- 2.4 vs. 12.9 +/- 5.3%, respectively) as well as in preconditioned hearts (61.8 +/- 4.2 and 55.5 +/- 4.7 vs. 36.0 +/- 1.4%, respectively). With verapamil, this protection was associated with a decrease in APD at 90% of repolarization in the nonpreconditioned hearts (APD90 32.2 +/- 0.1 vs. 71.1 +/- 6.7 ms at the end of ischemia). With 4-AP, this same protection was associated with an increase in APD in the preconditioned hearts (APD90 67.7 +/- 0.7 vs. 48.5 +/- 2.6 ms at the end of ischemia). Both agents given during a 25-min ischemic challenge improved myocardial recovery in nonpreconditioned and preconditioned hearts, despite discordant effects on the AP. Furthermore, the action of these agents was cumulative with the effect of preconditioning.

Palmitoyl-L-carnitine acts like ouabain on voltage, current, and contraction in guinea pig ventricular cells

We previously showed that palmitoyl-L-carnitine (L-PC) inhibits the Na/K pump current (INa/K). In the present report, we test the hypothesis that L-PC, like ouabain, should increase myocyte shortening. Membrane potentials or ionic currents were recorded simultaneously with cell shortening in single guinea pig ventricular myocytes at room temperature (22 degrees C). Like ouabain, L-PC (1 microM) reversibly depolarized the resting membrane, decreased action potential duration, and increased the amplitude of myocyte contractions. Neither L-PC nor ouabain had a significant effect on Ca current (ICa). When L-PC increased cell shortening during ramp voltage clamp, membrane current shifted inward at voltages negative to -20 mV and shifted outward at more positive voltages. Similar to toxic concentrations of ouabain, L-PC induced transient inward currents and aftercontractions. At concentrations that inhibit INa/K, L-PC acted like ouabain to produce characteristic effects on membrane potentials, currents, and cell contractions that were unrelated to significant changes in ICa. L-PC reduces surface negative charge of erythrocytes and myocytes (C. Gruver and A. J. Pappano, J. Mol. Cell. Cardiol. 25: 1275-1284, 1993), and we speculate that L-PC inhibits INa/K by this mechanism.

Actions of the Novel Vasodilator, Flosequinan, in Isolated Ventricular Cardiomyocytes

Although the potent vasodilating effect of flosequinan is well characterised, the positive inotropic action reported is more varied and less well understood. We examined the contractile and electrophysiologic effects of flosequinan and its metabolite, BTS 53554, in cardiomyocytes from either adult male Sprague-Dawley rats (200-250 g) or New-Zealand White rabbits (2-2.5 kg) and compared the effects with those of sulmazole and enoximone [selective phosphodiesterase (PDE) III inhibitors], Ro 20-1724 and rolipram (selective PDE IV inhibitors) and 3-isobutyl-1-methylxanthine (IBMX, nonselective PDE inhibitor). Flosequinan and BTS 53554 had positive contractile effects (p < 0.05) in both rat and rabbit ventricular cardiomyocytes only at the maximum concentration (10(-3) M). Differences were noted between species, however. Flosequinan 10(-3) M had a greater contractile effect than BTS 53554 (10(-3) M) in rabbit cardiomyocytes, but not in rat cardiomyocytes. We studied the interaction of flosequinan or the metabolite with other PDE inhibitors in rat cardiomyocytes. Contractile amplitudes were not significantly different with equimolar concentrations (3 x 10(-4) M) of Ro 20-1724, flosequinan, or BTS 53554 alone (15 +/- 6, 18 +/- 4, and 32 +/- 10%, respectively, greater than the mean basal dL value of 7.38 +/- 0.12%, mean +/- SE error). However, the combinations of Ro 20-1724 with flosequinan and Ro 20-1724 with BTS 53554 produced synergistic responses: 71 +/- 10 and 72 +/- 14%, respectively, greater than the mean basal dL value (p < 0.05). In contrast, the combinations of either flosequinan or BTS 53554 with IBMX or sulmazole produced no further increase in contractile amplitude. Neither flosequinan nor BTS 53554 produced any detectable increase in cyclic AMP, whereas significant increases were noted with Ro 20-1724, IBMX, and sulmazole (p < 0.05) in rat cardiomyocytes. Flosequinan increased beating frequency in rat isolated right auricles concentration dependently and was significant over the concentration range of 10(-5)-3 x 10(-4) M; flosequinan 3 x 10(-4) M maximally increased the mean frequency of beating by 35% of the predrug value (255 +/- 15 beats/min). Flosequinan had no effect on resting membrane potential, amplitude, or maximum upstroke velocity in rat isolated left ventricular (LV) papillary muscle, but at the maximum concentration (10(-3) M), flosequinan decreased action potential duration (APD) at 10, 50, and 75% of repolarization (p < 0.05). BTS 53554 produced no changes in AP characteristics over the concentration range of 10(-5)-10(-3) M.(ABSTRACT TRUNCATED AT 400 WORDS)

Cellular Electrophysiological Changes Induced In Vitro by Radiofrequency Current: Comparison with Electrical Ablation

The purpose of this study was to examine the cellular electrophysiological effects of radiofrequency energy delivery in an in vitro canine epicardial preparation and compare the effects of those of high energy electrical ablation in a similar preparation. Ten joules of direct current energy or 40 volts of radiofrequency energy were delivered by a 6 French 2-mm tip catheter to the epicardial surface of 2 x 3 cm epicardial strips superfused with Tyrode's solution. Direct current energy delivery produced a crater and central zone of necrosis surrounded by a border zone of viable but damaged tissue that extended up to 10-12 mm from the site of energy delivery. Cellular electrophysiological abnormalities that included a less negative resting membrane potential, decreased peak dV/dT, decreased action potential amplitude, and decreased action potential duration (APD) were approximately linearly related to the distance from the crater edge. In addition, viable and inexcitable cells were frequently interspersed. Between 2 and 5 mm from the crater edge, 36.4% of the cells were inexcitable whereas others displayed normal action potential characteristics. In contrast, radiofrequency current produced a central zone of necrosis surrounded by a smaller border zone. Cellular damage that was qualitatively similar to that produced by direct current energy extended only up to 6-8 mm from the edge of the crater. In addition, severe abnormalities were noted in intracellular potentials recorded within 2 mm of the ablation site, and only minor abnormalities further away. Lesions were relatively homogeneous. Between 2 and 5 mm from the ablation site only 2.6% of the cells were inexcitable (P < 0.05 vs direct current). In conclusion, radiofrequency current produces lesions that are smaller and more homogeneous than those produced by direct current ablation. Although the border zone is small, a region of partially depolarized but viable myocardium is present after radiofrequency current energy delivery. These findings provide a cellular basis for several clinical observations that have been made following radiofrequency current energy delivery.

Differential responsiveness of atrial and ventricular myocytes to potassium channel openers.

We examined the effects of the potassium channel openers (PCOs) pinacidil and lemakalim (BRL 38227) on action potential (AP) configurations and outward currents in atrial and ventricular myocytes isolated from rabbit and guinea pig hearts, using the whole-cell configuration of the patch clamp technique at 33 degrees-35 degrees C. The PCOs known to activate ATP-sensitive K+ current (IKATP) in various tissues, induced this current and decreased AP duration (APD) in rabbit ventricular myocytes. In contrast, in rabbit atrial myocytes, PCOs either had no effect or increased duration and plateau amplitude of the AP. The predominant outward current in rabbit atrial myocytes is a 4-aminopyridine (4-AP)-sensitive transient outward current (Ito). The PCOs caused a decrease in Ito without inducing IKATP in rabbit atria. In identical experimental conditions, PCOs activated IKATP in both guinea pig atrial and ventricular myocytes. Our results suggest that (a) a species as well as cardiac tissue difference exists in responsiveness to PCOs, and (b) the decrease in Ito without concomitant induction of IKATP can lead to changes in AP configuration opposite to that expected from IKATP activation. Different effects of PCOs on distinct parts of the heart could lead to disparity in APD and refractoriness that may contribute to arrhythmogenesis.