Maximum Rate Of Depolarization Research Articles

Cardiac cellular actions of hydrochlorothiazide.

In long term treatment, thiazide diuretics such as hydrochlorothiazide (HCTZ) lower blood pressure by decreasing peripheral resistance rather than by their diuretic effect. This action has been attributed to the opening of Ca2+-activated K+ channels in vascular smooth muscle cells. However, little is known about their cardiac cellular actions. Here we investigated the possible actions of HCTZ on action potential and contraction of rat ventricular muscle strips and on the ionic currents of isolated rat ventricular cardiomyocytes. HCTZ depressed ventricular contraction with an IC30 of 1.85 microM (60% decrease at 100 microM). Action potential duration at -60 mV and maximal rate of depolarization were, however, only slightly decreased by 12% and 22%, respectively, at 100 microM. At the single cell level, HCTZ (100 microM) depressed the fast Na+ current (INa) and the L-type Ca2+ current (ICaL) by 30% and 20%, respectively. The effects on ICaL were not voltage-or frequency-dependent. In cells intracellularly perfused with 50 microM cyclic adenosine, monophosphate HCTZ reduced ICaL by 33%. The transient (Ito), the delayed rectifier and the inward rectifier potassium currents were decreased by 20% at 100 microM HCTZ. The effects on Ito were voltage-dependent. In conclusion, HCTZ at high concentrations possesses a negative inotropic action that could be in part due to its blocking action on INa and ICaL. The actions of HCTZ on multiple cardiac ionic currents could explain its weak effect on action potential duration.

Chronic amiodarone effects on epicardial conduction and repolarization in the isolated porcine heart.

Amiodarone is a potent antiarrhythmic agent with complex chronic effects, notably on repolarization and conduction, that are not fully understood. Its low arrhythmogenic potential has been related to a lack of increase in repolarization dispersion. Since its effects are not documented in pigs we conducted a mapping study of activation and repolarization in isolated perfused porcine hearts. Amio20 female pigs (n = 7) received amiodarone 20 mg/kg per day over 4 weeks while Amio50 female pigs (n = 7) received 50 mg/kg per day over 4 weeks. Concentrations of the drug encompassed values found in clinical studies. Then, activation patterns and activation-to-recovery intervals (ARI) were mapped epicardially from 128 unipolar electrograms in isolated perfused hearts in corroboration of epicardial action potential recordings. Mean ARI was longer in Amio20 experiments compared to the seven control hearts (325 +/- 11 ms vs 288 +/- 5 ms at 1,000 ms), whereas ARI dispersion was not different, being comprised between 7 and 11 ms and generating smooth gradients. In Amio50 experiments, mean ARI was further prolonged (390 +/- 10 ms at 1,500 ms) with an exaggerated reverse rate dependence concomitant with a depressant effect on the plateau of the action potential. Again, ARI dispersion did not differ from controls. Finally, the drug depressed the maximal rate of depolarization (Vmax) and slowed conduction in a rate dependent and concentration dependent fashion. In conclusion, chronic amiodarone induces Class I and Class III antiarrhythmic effects in ventricular porcine epicardium that are concentration dependent but does not affect dispersion of repolarization. This may partly explain its low arrhythmogenic potential.

Electrophysiological changes in rat ventricular and atrial myocardium at different stages of experimental diabetes.

Action potential configuration in ventricular and atrial myocardium, as well as rate-dependent changes in ventricular action potential duration (APD) were studied and compared in healthy and diabetic rats. Diabetes was induced by a single injection of streptozotocin (STZ, 65 mg kg(-1) i.v.). Conventional microelectrode techniques were applied to record action potentials after the establishment of diabetes (2, 6, 10 and 18 weeks after STZ-treatment). Untreated age-matched animals were used as controls. Both depolarization and repolarization were significantly retarded following STZ-treatment. However, the time course of development of diabetic changes in atrial and ventricular myocardium was different. APD was significantly lengthened from week 2 of diabetes in ventricular, but only from week 6 in atrial preparations. In atrial myocardium, lengthening of APD was more pronounced at early rather than late phases of repolarization. The maximum rate of depolarization (Vmax) was significantly reduced from the 6th week of diabetes in both preparations. No differences were observed in action potential amplitude (except at week 18) and in the resting membrane potential in diabetic rats. Diabetic ventricular preparations showed a positive APD-frequency relationship at any level of repolarization, in contrast to control muscles, where APD25 and APD50 values lengthened. But APD75 and APD90 values were not changed significantly with increase in the pacing frequency. The results indicate that development of diabetic alterations are not fully identical in atrial and ventricular myocardium of the rat, probably owing to differences in density and kinetics of ionic currents responsible for atrial and ventricular action potentials.

Post-partum prolongation of the atrial repolarization in rabbit.

Female sexual steroids are known to modify the expression of various K+ channels and thus they can alter cardiac repolarization. In the present work, using conventional microelectrode techniques, action potential characteristics were studied in atrial myocardium isolated from virgin, late pregnant, early (1-3 days) post-partum and late (2-3 weeks) post-partum rabbits. No changes in action potential configuration were observed during pregnancy. However, the duration, overshoot and amplitude of action potentials were significantly increased in the early (1-3 days) post-partum period. Resting potential and maximum rate of depolarization remained unchanged. The observed changes were transient, normal action potential characteristics were obtained at weeks 2-3 post-partum. 4-aminopyridine (1 mmol L(-1)). caused a marked lengthening of action potential duration in all preparations obtained from non-pregnant and pregnant rabbits, whereas this 4-aminopyridine-induced prolongation was moderate in those preparations excised from the hearts of early post-partum animals. Action potential configuration was not affected by pinacidil (10 micromol L(-1)) or glibenclamide (5 micromol L(-1)) in non-pregnant or pregnant animals. In preparations obtained from early post-partum rabbits, pinacidil significantly shortened action potential duration, which was reverted by glibenclamide. The lengthening of action potential duration together with the decreased sensitivity to 4-aminopyridine observed in early post-partum animals may probably be caused by reduction of the transient outward K+ current at this stage. The results also suggest that electrophysiological alterations in the early post-partum period may probably be more pronounced than those associated with pregnancy itself.

Differential effects of quinidine, flecainide, and cibenzoline on anisotropic conduction in the isolated porcine heart.

Anisotropic conduction characteristics, expressed as the ratio of conduction velocities in the longitudinal (Vl) and transverse (Vt) fiber directions, may play a role in the mechanism of some ventricular tachycardias and is influenced by pharmacologic interventions. Discrepancies exist among the reported orientation-dependent effects of available Class I antiarrhythmic drugs. The aim of this study was to assess the respective effects of quinidine (Class IA), flecainide (Class IC), and cibenzoline (not subclassified) on the anisotropic conduction of porcine hearts, in corroboration of their effects on ventricular action potentials. We studied the actions of 3 and 10 microM quinidine, 1 and 3 microM flecainide, and 0.3 and 1 microM cibenzoline on Vl and Vt determined from 128 electrograms recorded with a plaque electrode on the anterior left ventricle of isolated perfused hearts (spacing 2.5 mm). Vl and Vt were computed from isochronal maps displaying ellipsoid activation during stimulation from the center of the plaque. The effects on the maximal rate of depolarization (Vmax) of action potentials were obtained from ventricular muscle exposed to the same drugs. Flecainide [1 microM] and cibenzoline [0.3 microM] did not alter Vl and Vt significantly. Quinidine [3 microM] predominantly depressed Vl at rapid pacing rates, but the Vl/Vt ratio was not significantly altered. Quinidine [10 microM] and flecainide [3 microM] reduced Vl and Vt in a frequency-dependent fashion. Conversely, cibenzoline [1 microM] mostly decreased Vl and thus decreased the Vl/Vt ratio and increased the Vl/Vt at all pacing rates. This different effect was not related to a greater depressant effect on Vmax Quinidine and flecainide act similarly on the anisotropic pattern of conduction (both drugs increase the Vl/Vt ratio), whereas cibenzoline exerts opposite effects. Orientation-dependent effects are different among Class I antiarrhythmic drugs and may be of importance in their therapeutic efficacy or proarrhythmic potential.

Modulation of procainamide's effect on conduction by cellular uncoupling in perfused rabbit hearts.

How cell-to-cell electrical coupling influences an antiarrhythmic agent's effect on conduction is largely unknown. To investigate this, we evaluated the effects of procainamide on myocardial conduction at decreasing degrees of cell-to-cell electrical coupling induced by graded doses of heptanol. Electrograms were recorded from 50 ventricular epicardial sites in a 1 cm x 0.5 cm area during pacing to produce conduction longitudinal or transverse to myocardial fiber orientation in Langendorff-perfused rabbit hearts. The effects of procainamide (15 mg/L) on conduction velocity were determined in the presence of increasing doses of heptanol (0.2, 0.5, and 1.0 mM). In addition, using standard microelectrode techniques in isolated superfused rabbit myocardium, intracellular potentials were recorded in the presence of 15 mg/L procainamide and heptanol (1.0 mM). In the absence of heptanol, procainamide slowed conduction velocity. In the presence of increasing doses of heptanol, procainamide's contribution to the depressant effect on conduction velocity was attenuated and reversed at the highest dose. The latter effect was preferentially seen for conduction longitudinal to myocardial fiber orientation. Heptanol had no effect on action potential amplitude or maximum rate of depolarization in the presence of procainamide. Procainamide's effect on conduction velocity is influenced by the underlying degree of cell-to-cell electrical coupling. The present model should be useful in evaluating the relative ability of other pharmacologic agents to modulate conduction under conditions of changing cell coupling.

Electrophysiological actions of ryanodine on single rabbit sinoatrial nodal cells

1. 1. Effects of ryanodine on the action potentials and the ionic currents in spontaneously beating single rabbit sinoatrial (SA) nodal cells were examined using current-clamp and whole-cell voltage-clamp techniques. 2. 2. Cumulative administrations of ryanodine (10 −8 to 10 −4 M) caused a negative chronotropic effect in a concentration-dependent manner; the effect was not modified by atropine (10 −7 M). At 10 −6 M, ryanodine increased the action potential amplitude and the maximum rate of depolarization, and prolonged the duration of action potentials, significantly. The maximum diastolic potential was unaffected. 3. 3. No arrhythmia occurred in the presence of ryanodine (10 −6 M) alone, but addition of either caffeine (10 mM) or high Ca 2+ (10.8 mM) elicited arrhythmias. The incidence increased with an increase in extracellular Ca 2+ concentration. 4. 4. Ryanodine, at 10 −6 M, enhanced the Ca 2+ current but, at 10 −5 M, inhibited it. Ryanodine inhibited the delayed rectifier K + current and the hyperpolarization-activated inward current in a concentration-dependent manner. 5. 5. In addition, ryanodine actually elevated the cytosolic Ca 2+ level in the SA nodal cells loaded with Ca 2+-sensitive fluorescent dye (fura-2). 6. 6. These results indicate that ryanodine modulates the ionic currents (presumably dependent on cellular Ca 2+ concentration), suggesting similar pharmacological properties to caffeine.

Electrophysiological effects of clentiazem, a new Ca2+ antagonist, on rabbit hearts.

Electrophysiological effects of clentiazem, a new 1,5-benzothiazepine type Ca(2+) antagonist, were examined in comparison with those of diltiazem in excised rabbit heart preparations. In Langendorff-perfused hearts electrically driven at basic cycle lengths of 400-500 ms, clentiazem (10(-8)-10(-6)M) and diltiazem (10(-8)-10(-6)M) caused a concentration-dependent prolongation of the atrio-His bundle conduction time (A-H interval) without affecting the His bundle-ventricular conduction time (H-V interval). The effects of clentiazem were equivalent to those of diltiazem. In isolated rabbit atrioventricular (A-V) node preparations electrically driven at 400- to 500-ms intervals, clentiazem and diltiazem at >10(-6)M concentrations produced concentration-dependent decreases in action potential amplitude (APA), maximum rate of depolarization (V max), and shortened action potential duration at 20 and 50% repolarization (APD(20) and APD(50)), whereas APD(90) was little affected. Application of 10(-6)M clentiazem prolonged effective refractory period (ERP) of the A-V node by approximately 7% of the control, an effect similar to that of diltiazem. In spontaneously beating sinoatrial (S-A) node preparations, clentiazem l0(-6)M or the higher concentration significantly decreased APA, V(max), and slope of slow diastolic depolarization, while reducing the maximum diastolic potential. The inhibitory effects of clentiazem showed strong suppression of APA and V(max) by 31.1 and 47.2% of the control, respectively, whereas both clentiazem (10(-7)-10(-5)M) and diltiazem (10(-7)-10(-5)M) had no effects on parameters of ventricular APs. These results suggest that dentiazem, like diltiazem, has a preferential inhibitory action on cardiac slow Ca(2+) channels.

Myocardial Depressant Effects of Sevoflurane

The effects of anesthetic concentrations of sevoflurane were studied in isolated myocardial tissue to delineate the mechanisms by which cardiac function is altered. Isometric force of isolated guinea pig ventricular papillary muscle was studied at 37 degrees C in normal and 26 mM K+ Tyrode's solution at various stimulation rates. Normal and slow action potentials were evaluated using conventional microelectrodes. Effects of sevoflurane on sarcoplasmic reticulum function in situ were also evaluated by its effect on rapid cooling contractures, which are known to activate Ca2+ release from the sarcoplasmic reticulum, and on concentrations of rat papillary muscle. Finally, Ca2+ and K+ currents of isolated guinea pig ventricular myocytes were examined using the whole-cell patch clamp technique. Sevoflurane equivalent to 1.4% and 2.8% depressed guinea pig myocardial contractions to approximately 85 and approximately 65% of control, respectively, although the maximum rate of force development at 2 or 3 Hz and force in rat myocardium after rest showed less depression. In the partially depolarized, beta-adrenergically stimulated myocardium, sevoflurane selectively depressed late peak force without changing early peak force, whereas it virtually abolished rapid cooling contractures. Sevoflurane did not alter the peak amplitude or maximum depolarization rate of normal and slow action potentials, but action potential duration was significantly prolonged. In isolated guinea pig myocytes at room temperature, 0.7 mM sevoflurane (equivalent to 3.4%) depressed peak Ca2+ current by approximately 25% and increased the apparent rate of inactivation. The delayed outward K+ current was markedly depressed, but the inwardly rectifying K+ current was only slightly affected by 0.35 mM sevoflurane. These results suggest that the direct myocardial depressant effects of sevoflurane are similar to those previously described for isoflurane. The rapid initial release of Ca2+ from the sarcoplasmic reticulum is not markedly decreased, although certain release pathway, specifically those induced by rapid cooling, appear to be depressed. Contractile depression may be partly related to the depression of Ca2+ influx through the cardiac membrane. The major electrophysiologic effect of sevoflurane seems to be a depression of the delayed outward K+ current, which appears to underlie the increased action potential duration.

Actions of dolasetron and its major metabolite on guinea‐pig papillary muscle fibres and the α‐subunit of human heart sodium channels expressed in Xenopus oocytes

The present study evaluated the effects of the anti-emetic 5-HT3 antagonist dolasetron and its major metabolite MDL 73,405 on guinea-pig papillary muscle fibres and human heart sodium channels expressed in Xenopus oocytes. Dolasetron and MDL 73,405 (3–10 μM) reduced the maximum depolarization rate during phase I of the action potential (Vmax) in papillary muscle fibres without significantly changing the action potential duration. The reduction in Vmax was both use- and concentration-dependent. In Xenopus oocytes expressing the α-subunit of human heart sodium channels, extracellular application of dolasetron or MDL 73,405 induced only a low affinity tonic block of the sodium channel; the apparent Kd values were 1.1 and 1.2 mM, respectively. In contrast to guinea-pig sodium channels, neither compound induced a marked use-dependent block of the human channel α-subunit at a concentration of 100 μM (3 and 7% block, respectively). Flecainide produced a reversible, tonic, and use-dependent block in the concentration range of 10 to 100 μM; the Kd value for tonic block was 50 μM. The present results demonstrate that dolasetron and MDL 73,405 do not act directly on the human heart sodium channel α-subunit and further emphasise the need to study drug action on human ionic channels and/or receptors. © 1996 Wiley-Liss, Inc.

Actions of dolasetron and its major metabolite on guinea-pig papillary muscle fibres and the ?-subunit of human heart sodium channels expressed inXenopus oocytes

The present study evaluated the effects of the anti-emetic 5-HT3 antagonist dolasetron and its major metabolite MDL 73,405 on guinea-pig papillary muscle fibres and human heart sodium channels expressed in Xenopus oocytes. Dolasetron and MDL 73,405 (3–10 μM) reduced the maximum depolarization rate during phase I of the action potential (Vmax) in papillary muscle fibres without significantly changing the action potential duration. The reduction in Vmax was both use- and concentration-dependent. In Xenopus oocytes expressing the α-subunit of human heart sodium channels, extracellular application of dolasetron or MDL 73,405 induced only a low affinity tonic block of the sodium channel; the apparent Kd values were 1.1 and 1.2 mM, respectively. In contrast to guinea-pig sodium channels, neither compound induced a marked use-dependent block of the human channel α-subunit at a concentration of 100 μM (3 and 7% block, respectively). Flecainide produced a reversible, tonic, and use-dependent block in the concentration range of 10 to 100 μM; the Kd value for tonic block was 50 μM. The present results demonstrate that dolasetron and MDL 73,405 do not act directly on the human heart sodium channel α-subunit and further emphasise the need to study drug action on human ionic channels and/or receptors. © 1996 Wiley-Liss, Inc.

The cellular electrophysiological effects of tedisamil in human atrial and ventricular fibers

The cellular electrophysiological effects of 1 microM tedisamil (KC 8857) were studied in human atrial and ventricular fibers. Conventional microelectrode technique was applied to record the transmembrane action potentials at stimulation frequency of 100 per min and 37 degrees C. Tedisamil lengthened action potential duration (APD) more in atrial than in ventricular muscle fibers; prolongation of APD90 was 28.9 +/- 3.3% (n = 6; p < 0.05) for atrial and 13.3 +/- 5.2% (n = 6; P < 0.05) for ventricular tissue. The maximal rate of depolarization was depressed slightly, but significantly by 1 microM tedisamil only in ventricular fibers (12.9 +/- 6.5%, n = 6, P < 0.05). From these cellular electrophysiological data it is concluded that the bradycardic/antiischemic agent tedisamil possesses marked Class III properties not only in cardiac tissues of experimental animals but also those of man.

The cellular electrophysiological effects of tedisamil in human atrial and ventricular fibers

The cellular electrophysiological effects of 1 μM tedisamil (KC 8857) were studied in human atrial and ventricular fibers. Conventional microelectrode technique was applied to record the transmembrane action potentials at stimulation frequency of 100 per min and 37 °C. Tedisamil lengthened action potential duration (APD) more in atrial than in ventricular muscle fibers; prolongation of APD90 was 28.9 ± 3.3% (n = 6; p < 0.05) for atrial and 13.3 ± 5.2% (n = 6; P < 0.05) for ventricular tissue. The maximal rate of depolarization was depressed slightly, but significantly by 1 μM tedisamil only in ventricular fibers (12.9 ± 6.5%, n = 6, P < 0.05). From these cellular electrophysiological data it is concluded that the bradycardic/antiischemic agent tedisamil possesses marked Class III properties not only in cardiac tissues of experimental animals but also those of man.

Electrophysiological and arrhythmogenic effects of the histamine type 1-receptor antagonist astemizole on rabbit Purkinje fibers: clinical relevance.

Astemizole is a potent histamine H1-antagonist that has been associated with cases of life-threatening cardiac arrhythmias, including torsade de pointes and atrioventricular (AV) block. However, its effects on cardiac action potential (AP) has not been described. We examined the electrophysiological effects of astemizole on rabbit Purkinje fibers using conventional glass microelectrodes in parallel with the effects of the widely used histamine H2-antagonist cimetidine, selected because it has no known cardiac arrhythmic toxicity. Astemizole (0.01-3 microM) exerted a concentration-dependent prolonging effect on final repolarization that did not reach steady state after 3 h of exposure. This effect was more pronounced at low stimulation frequency and was less marked at high stimulation frequency. In addition, early afterdepolarizations (EADs) occurred in one third of the fibers. Increasing extracellular concentration of KCl (2.7-5.4 mM) or MgCl2 (1-5 mM) suppressed EADs and reversed the prolonging effect that was conversely exaggerated by decreasing KCl (4-2.7 mM) or MgCl2 (1-0.5 mM) concentration. At higher concentrations (3-30 microM), astemizole induced an increasing depressant effect on the maximal rate of depolarization (Vmax) that became more pronounced with high stimulation frequency. All parameters were strongly depressed at 10 microM astemizole, leading to cellular inexcitability in 5 of 12 fibers when exposed to 30 microM astemizole. In comparison, cimetidine induced minor changes on AP characteristics, i.e., a prolongation in plateau duration at high (30-100 microM) concentrations. These results provide evidence that astemizole exerts quinidine-like effects on cardiac APs that are compatible with the occurrence of the clinically observed arrhythmias.

Electrophysiological demonstration and activation of mu-opioid receptors in the rabbit sinoatrial node.

To investigate the presence of opioid receptors and their physiological role in cardiac pacemaker cells, we studied electrophysiological effects of fentanyl citrate, an activator of the mu-opioid receptors, on the spontaneous action potential (AP) and membrane currents, using small preparations (0.2 x 0.2 x 0.1 mm) of rabbit sinoatrial (SA) node (SAN). Fentanyl (0.1-3 microM) progressively decreased the AP amplitude (APA), maximal rate of depolarization (MRD), and spontaneous firing frequency (SFF) and prolonged the AP duration (APD) and diastolic interval in a concentration-dependent manner. At 1 microM, the spontaneous activity ceased in two of the eight preparations. These actions were blocked by a mu-opioid receptor antagonist, beta-funaltrexamine (beta-FNA), but were not modified by either kappa-opioid receptor antagonist nor-binaltorphimine (nor-BNI), or delta-opioid receptor antagonist ICI-174864. In voltage-clamp experiments using double microelectrode techniques, 1 microM fentanyl reduced the Ca2+ current (ICa) obtained on step depolarization from -40 to 0 mV by 19.9 +/- 9.3% (p < 0.05, n = 5), the fast and slow components of the delayed rectifying K+ current (IKfast, IKslow) tail obtained on repolarization from 10 to -60 mV by 54.7 +/- 4.7 and 41.4 +/- 2.4% (p < 0.05, n = 4), and the hyperpolarization-activated inward current at -90 mV by 12.6 +/- 0.5% (p < 0.05, n = 7), respectively. The gating kinetics of ICa and IKslow were not altered.(ABSTRACT TRUNCATED AT 250 WORDS)

Different Actions Between Pindolol and Propranolol on the Ionic Currents in Spontaneously Beating Rabbit Sino-Atrial Node Cells

Electrophysiological effects of beta-adrenoceptor blocking drugs (pindolol and propranolol) on the action potentials (APs) and the membrane currents in spontaneously beating rabbit sino-atrial (SA) node cells were examined by the two-microelectrode voltage-clamp technique. Cumulative administrations of both blocking drugs (10(-7)-10(-6) M) prolonged the AP duration (APD) and caused a negative chronotropic effect in a concentration-dependent manner. Propranolol exhibited stronger responses than pindolol. At 10(-6) M, propranolol depolarized the maximum diastolic potential (MDP) and pindolol decreased the maximum rate of depolarization significantly, but the drugs had little or no effect on the AP amplitude (APA). In voltage-clamp experiments, both blockers (10(-7)-10(-6) M) inhibited the slow inward and the time-dependent outward currents in a concentration-dependent manner. Holding potential was -40 mV. The hyperpolarization-activated inward current was not affected by pindolol, but it was concentration-dependently inhibited by propranolol. The inactivation curve of Isi was not modified by either pindolol or propranolol. The activation curve of IK was unaffected by pindolol, but was shifted by propranolol 10(-6) M by approximately 11 mV in the depolarizing direction. These results indicate that both beta-adrenoceptor blocking drugs (pindolol and propranolol) inhibit the slow inward and the time-dependent outward currents, but produce different actions on the hyperpolarization-activated inward current and the activation kinetic of IK due to their pharmacological properties, thereby resulting in difference in decrease in the spontaneous activity of rabbit SA node cells.

Role of T-type Ca 2+ channel inhibitors in the pacemaker depolarization in rabbit sino-atrial nodal cells

1. 1. Effects of T-type Ca 2+ channel inhibitors, Ni 2+ and tetramethrin, on the spontaneous action potentials in rabbit sino-atrial nodal cells were examined. 2. 2. The firing rate of spontaneous activity was 201 ± 11 beats/min ( n = 18). Experiments were performed at 36°C. 3. 3. Ni 2+ (10 −5 to 10 −4 M) and tetramethrin (10 −7 to 5 × 10 −5M) caused a negative chronotropic effect. Both inhibitors did not affect the maximum diastolic potential, and slowed the rate of depolarization during the diastole. 4. 4. In the presence of TTX (10 −7 M), both inhibitors caused a stronger negative chronotropic effect, and hyperpolarized the maximum diastolic potential. The maximum rate of depolarization was enhanced, and the action potential duration (at 50% repolarization) was prolonged. The action potential amplitude was unaffected. Ni 2+ had more potent actions than tetramethrin. 5. 5. T-type and other Ca 2+ channel inhibitors affected only the late phase of pacemaker potential, resulting in a negative chronotropic effect. 6. 6. These results indicate that T-type Ca 2+ channel inhibitors (Ni 2+ and tetramethrin) slow the pacemaker depolarization at the late phase (but not at the initial phase), resulting in a negative chronotropic effect in the sino-atrial nodal cells.

Electrophysiologic alterations in the rabbit nodal cells induced by membrane lipid peroxidation

To investigate cellular electrophysiologic alterations due to lipid peroxidation of the cell membrane by free radicals as a possible cause of coronary reperfusion arrhythmias, we studied the effects of t-butyl hydroperoxide on the spontaneous action potential and membrane currents of the rabbit sinoatrial and atrioventricular node preparations (0.2 × 0.2 × 0.1 mm). 1–5 min of superfusion with t-butyl hydroperoxide (100–500 μM) caused a transient increase in the spontaneous firing frequency by 9%, accompanied by a 4% increase in the action potential amplitude and a 33% increase in the maximal rate of depolarization ( P<0.05, n = 6). t-Butyl hydroperoxide then gradually suppressed physiological automaticity, but induced abnormal repetitive firing due to early and delayed after-depolarizations. 15 min of superfusion with t-butyl hydroperoxide caused a complete standstill of nodal cells at a resting potential of −46 ± 3 mV ( n = 12). Such effects of t-butyl hydroperoxide on the spontaneous action potential were attenuated by pretreating the cells with butylated hydroxytoluene, a lipid peroxidation inhibitor. Voltage clamp experiments using double microelectrode methods revealed that t-butyl hydroperoxide transiently increased the Ca 2+ current by 22% after 5 min of superfusion but subsequently reduced it to 46% of the control value after 15 min ( P<0.05, n = 6). Similar biphasic changes were observed in the delayed rectifying K + current and hyperpolarization-activated inward current ( n = 6). Background current was progressively increased without any change in its reversal potential ( n = 6). These results suggest that membrane lipid peroxidation may accelerate or suppress physiological automaticity and induce abnormal automaticity by both impairing cellula metabolic function and damaging the lipid membrane structure as well as ionic channel protein.

Electrophysiological Actions of Taurine on Spontaneously Beating Rabbit Sino-Atrial Nodal Cells

Effects of taurine on the spontaneous action potentials in rabbit sino-atrial nodal cells were examined at different extracellular Ca2+ concentrations ([Ca]o). Experiments were performed at 36 degrees C. The firing rate of spontaneous activity was 132.5 +/- 12.1 beats/min (n = 18) in normal Tyrode's solution ([Ca]o = 1.8 mM). Increasing [Ca]o level from 0.9 to 10.8 mM significantly changed the maximum rate of depolarization. Other parameters of the action potentials were unaffected. When [Ca]o was 0.9 mM, application of taurine (1 to 20 mM) tended to cause a positive chronotropic effect and hyperpolarized the maximum diastolic potential. In the normal solution (at 1.8 mM [Ca]o), taurine significantly enhanced only the maximum rate of depolarization. In contrast, under high [Ca]o (5.4 and 10.8 mM), taurine at 1 and 5 mM had a negative chronotropic effect, but 20 mM taurine had a positive chronotropic effect. Also, taurine shortened the action potential duration and hyperpolarized the maximum diastolic potential. The maximum rate of depolarization was inhibited. In 10.8 mM [Ca]o solution, irregular spontaneous activity (dysrhythmias) occurred in 4 of 6 preparations, and addition of taurine (1 to 20 mM) abolished it. These results indicate that taurine modulates the action potential configuration in the sino-atrial nodal cells dependent on [Ca]o.

Chronic versus acute effects of amiodarone on the Vmax-conduction velocity relationship and on the space constant in canine myocardium.

Isolated tissue experiments (canine ventricular epicardium) and computer simulations were used to characterize the relationship between changes in maximum rate of depolarization during upstroke (Vmax) and the conduction velocity (theta) induced after long-term therapy with amiodarone and to compare these data with those obtained after acute superfusion with either desethylamiodarone or the parent compound. After chronic amiodarone, the changes in Vmax were linearly related to the square of the changes in theta during longitudinal propagation (LP) (slope = 0.93, r = 0.93, p = NS with respect to slope = 1), whereas during transverse propagation (TP), the slope of the relationship between both variables was slightly decreased (slope = 0.88, r = 0.96, p < 0.05 with respect to slope = 1). Similar results were observed after acute superfusion with desethylamiodarone (LP), (slope = 1, r = 0.05, p = NS with respect to slope = 1). In contrast, a significant increase in slope (p < 0.05 with respect to slope = 1) was observed after acute superfusion (slope = 1.45, r = 0.85 and slope = 1.48, r = 0.75 during LP and TP, respectively). In addition, the space constant (lambda) after chronic amiodarone (1.05 +/- 0.06 mm) was not significantly different from control (0.98 +/- 0.04 mm), but was slightly though significantly increased after acute amiodarone administration (1.07 +/- 0.03 mm, p < 0.03). Data are mean +/- SEM. Experimental data from chronic amiodarone were well fitted in a one-dimensional Beeler-Reuter-based discrete cable by reducing sodium conductance (GNa) exclusively. In contrast, data from acute superfusion were fitted only when junctional resistance (rj) and GNa were simultaneously reduced. These data suggest that acute amiodarone may modify both active and passive membrane properties whereas chronic amiodarone appears to alter only the active properties; the data further indicate that desethylamiodarone may play an important role in the mechanism of action of chronic amiodarone treatment.