Mechanism of increased amplitude and duration of the plateau with sudden shortening of diastolic intervals in rabbit ventricular cells

Differential Expression and Remodeling of Transient Outward Potassium Currents in Human Left Ventricles

BACKGROUND

Myocardial, transient, outward currents, I_to, have been shown to play pivotal roles in action potential (AP) repolarization and remodeling in animal models. The properties and contribution of I_to to left ventricular (LV) repolarization in the human heart, however, are poorly defined.

METHODS AND RESULTS

Whole-cell, voltage-clamp recordings, acquired at physiological (35°C to 37°C) temperatures, from myocytes isolated from the LV of nonfailing human hearts identified 2 distinct transient currents, I_to,fast (I_to,f) and I_to,slow (I_to,s), with significantly (P<0.0001) different rates of recovery from inactivation and pharmacological sensitives: I_to,f recovers in ≈10 ms, 100× faster than I_to,s, and is selectively blocked by the Kv4 channel toxin, SNX-482. Current-clamp experiments revealed regional differences in AP waveforms, notably a phase 1 notch in LV subepicardial myocytes. Dynamic clamp-mediated addition/removal of modeled human ventricular I_to,f, resulted in hyperpolarization or depolarization, respectively, of the notch potential, whereas slowing the rate of I_to,f inactivation resulted in AP collapse. AP-clamp experiments demonstrated that changes in notch potentials modified the time course and amplitudes of voltage-gated Ca²⁺ currents, I_Ca. In failing LV subepicardial myocytes, I_to,f was reduced and I_to,s was increased, notch and plateau potentials were depolarized (P<0.0001) and AP durations were prolonged (P<0.001).

CONCLUSIONS

I_to,f and I_to,s are differentially expressed in nonfailing human LV, contributing to regional heterogeneities in AP waveforms. I_to,f regulates notch and plateau potentials and modulates the time course and amplitude of I_Ca. Slowing I_to,f inactivation results in dramatic AP shortening. Remodeling of I_to,f in failing human LV subepicardial myocytes attenuates transmural differences in AP waveforms.

Prominent J-wave and T-wave alternans associated with mechanical alternans in a patient with takotsubo cardiomyopathy

A 74-year-old woman with takotsubo cardiomyopathy developed polymorphic ventricular tachycardia during the acute phase. She exhibited prominent J-wave and T-wave alternans preceding ventricular tachycardia. These abnormalities disappeared after recovery from myocardial stunning.

Usability of QTc dispersion for the prediction of orthostatic intolerance syndromes

BACKGROUND

Syncope is defined as transient loss of consciousness and muscle tone, usually of short duration. Noncardiac causes of syncope are classified as orthostatic intolerance syndromes (OIS). QT and QTc (corrected QT) dispersions are the measurements of myocardial instability and show predisposition to arrhythmias. In this study; clinical findings, QT and QTc dispersions of the patients who were diagnosed as OIS were evaluated retrospectively. Also, the aim of the study is to clarify the association of clinical characteristics of unexplained syncope with the outcome of the QT and QTc dispersions in children.

METHODS

We designed a retrospective study including 152 children and adolescents who had repeated unexplained syncope or presyncope between June 2002 and August 2010. Head-up Tilt table test (HUTT) were performed for all patients. Control group consisted of 67 healthy children. The QT and QTc dispersions were measured from the 12 ECG leads.

RESULTS

Eighty-four (55.2%) patients had positive and 68 (44.8%) had negative response to HUTT. QT and QTc dispersions were significantly higher in HUTT-positive group than in negative (p < 0.01, p < 0.001 respectively). Also, QTc dispersion was significantly higher in both vasovagal syncope and postural orthostatic tachycardia syndrome groups than in HUTT-negative group (p < 0.001, p < 0.05 respectively). Specifity and sensitivity of QTc dispersion for predicting positive HUTT are 76.5% and 59.5% respectively. The positive predictive value of the test calculated as 75.8%.

CONCLUSIONS

These results revealed that we can use QTc dispersion measurement as a noninvasive electrocardiographic test to evaluate OIS for predicting positive result before performing HUTT.

Dynamic changes in the QT-R-R relationship during head-up tilt test in patients with vasovagal syncope

BACKGROUND

QT interval is influenced by preceding R-R intervals and autonomic nervous tone. Changes in QT intervals during vasovagal reflex might reflect autonomic modulation of ventricular repolarization; however, this issue has not been fully elucidated. This study aimed to evaluate dynamic response of QT interval to transient changes in R-R interval during vasovagal syncope (VVS) induced by head-up tilt test.

METHODS

Eighteen patients with VVS and 18 age-and sex-matched controls were studied. All patients with VVS had a positive mixed-type response to head-up tilt and all controls had a negative response. CM5-lead digital electrocardiogram (ECG) was recorded and QT intervals were analyzed using Holter ECG analyzer. Using scatter plots of consecutive QT and the preceding R-R intervals, QT-R-R relations during tilt-up and tilt-back or during vasovagal reflex were independently fitted to an exponential curve: QT (second) = A + B x exp[k x R-R (second)].

RESULTS

During the tilt-up, A, B, and k did not differ between patients with VVS and controls. During the tilt back, k showed equivalent positive value compared to the tilt-up (4.1 +/- 1.3 vs -4.6 +/- 0.9) in controls. However, k remained negative (-1.3 +/- 1.5) during vasovagal reflex in patients with VVS. In six patients, in whom metoprolol was effective in eliminating VVS, QT-R-R relationship during the tilt-back became similar to that in controls.

CONCLUSIONS

In patients with VVS, hysteresis of the QT-R-R relation is similarly shown during tilt-up as in controls, whereas this hysteresis is no longer evident and failure of QT prolongation is observed during VVS.

The Electrical Restitution Curve Revisited:

The electrical restitution curve (ERC) traditionally describes the recovery of action potential duration (APD) as a function of the interbeat interval or, more correctly, the diastolic interval (DI). Often overlooked in modeling studies, the normal ventricular ERC is triphasic, starting with a steep initial recovery at the shortest DIs, a transient decline, and a final asymptotic rise to a plateau phase reached at long DIs. Recent studies have proposed that it would be advantageous to lower the slope of the ERC by drug intervention, as this might reduce the potential for electrical alternans and ventricular fibrillation. This review discusses the pros and cons of a flat versus steep slope of the ERC and draws attention to mechanisms thatjustify the (physiologically) steep slope, rather than a flat slope, as a better design against arrhythmias. Five potential mechanisms are discussed, which allows for a different interpretation of the effect of the slope on arrhythmogenicity. The most important appears to be the physiologic rate adaptive shortening of APD that, by reciprocal lengthening of the DI, allows the subsequent APD to move more quickly from the steep initial ERC phase onto the flat phase. A less steep initial ERC phase would protract the transition toward more fully recovered APD and, in fact, may perpetuate electrical alternans. The triphasic ERC time course in normal myocardium cannot be explained by or fitted to single exponentials or single ion channel recovery kinetics. A simple tri-ionic model is suggested that may help explain the shape of the ERC at various repolarization levels and place APD recovery into perspective with intracellular calcium recycling and recovery of contractile force.

Cellular basis for dispersion of repolarization underlying reentrant arrhythmias

Substantial heterogeneity in ion channel density and expression exists in cells isolated from various regions of the heart. Cell-to-cell coupling in the intact heart, however, is expected to attenuate the functional expression of the ion channel heterogeneities. Due to limitations of conventional electrophysiological recording techniques, the extent to which cellular electrical heterogeneities are functionally present in intact myocardium remains unknown. High-resolution optical mapping with voltage-sensitive dyes was used to measure transepicardial and transmural repolarization gradients in the Langendorff perfused guinea pig ventricle and the canine wedge preperation, respectively. Diversity of repolarization kinetics in the transepicardial direction modulated dispersion of repolarization in a biphasic fashion as premature coupling interval was shortened. Moreover, modulation of repolarization paralleled arrhythmia vulnerability in a predictable fashion. Transmural optical mapping revealed significant gradients of repolarization across the ventricular wall that were markedly increased in a surrogate model of LQTS. Transmural gradients of repolarization in LQTS were associated with an enhanced susceptibility to TdP. Therefore, despite strong cell-to-cell coupling in the normal heart, heterogeneities in the ionic make-up of cells across the epicardial and transmural surfaces result in functional heterogeneities of repolarization leading to arrhythmias.

Differences in the effects of urinary incontinence agents S-oxybutynin and terodiline on cardiac K(+) currents and action potentials

1. The cardiac electrophysiological effects of S-oxybutynin, a single-enantiomer drug under evaluation for the management of urinary incontinence, have been investigated and compared with those of terodiline, an incontinence agent withdrawn following reports of QT lengthening and ventricular tachyarrhythmia. Membrane currents were recorded from whole-cell configured guinea-pig and rabbit ventricular myocytes, and action potentials were recorded from guinea-pig and rabbit papillary muscles. 2. L-type Ca(2+) current (I:(Ca,L)), rapidly-activating K(+) current (I:(Kr)) and slowly-activating K(+) current (I:(Ks)) were unaffected by submicromolar S-oxybutynin and inhibited by higher concentrations; IC(50) values were 17.8 microM for I:(Ca,L), 12 microM for I:(Kr), and 41 microM for I:(Ks). Terodiline IC(50) values were somewhat lower for I:(Ca,L) (15.2 microM) and I:(Ks) (30 microM), but 24 fold lower in the case of I:(Kr) (0.5 microM). 3. The durations of action potentials in guinea-pig and rabbit papillary muscles driven at 1 Hz were unaffected or moderately shortened by 0.1 - 100 microM S-oxybutynin, but lengthened by terodiline. Terodiline (< or =10 microM) also depressed maximal upstroke velocity. 4. The action potential plateau shortened by an average of 23% when control rabbit papillary muscles were driven at 0.4 Hz instead of 1 Hz. Plateau shortening was significantly smaller in the presence of drugs (30 microM S-oxybutynin, 3 and 30 microM terodiline), suggesting that they suppress the transient outward current (I:(to)) involved in rate-dependent shortening. In experiments on rabbit ventricular myocytes, 3 and 30 microM S-oxybutynin inhibited I:(to) by 9+/-2% and 35+/-3%, respectively, whereas 3 and 30 microM terodiline inhibited the current by 31+/-3% and 87+/-3%, respectively. 5. The results indicate that S-oxybutynin has relatively weak non-specific effects on cardiac ion channels, and that clinically relevant submicromolar concentrations are unlikely to have terodiline-like proarrhythmic actions on the myocardium.

Preceding stimulus frequency-dependent potentiation of the postrest shortening of the action potential duration in rabbits

Action potential duration (APD) in rabbit ventricular myocardium shortens after a rest period (postrest shortening). However, the effects of preceding stimulus frequency on the postrest shortening have not been elucidated. We recorded transmembrane action potentials (TAPs) and monophasic action potentials (MAPs) from the rabbit ventricle. In in vitro experiments. repetitive regular stimuli (S1) at cycle lengths ranging between 500 to 3000 ms were followed by a single extrastimulus (S2) at a coupling interval of 5000 ms. A decrease in S1S1 interval resulted in a progressive shortening of the duration of TAP (TAPD) elicited by S2 (S2-TAPD), which was potentiated by increasing extracellular calcium concentration ([Ca2+]o) or application of ouabain and was inhibited by lowering [Ca2+]o or verapamil. Application of ryanodine was most effective in lengthening S2-TAPD following a short S1S1 interval. 4-aminopyridine and E4031 caused marked lengthening of S2-TAPD when S1S1 was long. However, the lengthening effect was attenuated and disappeared with a shorter S1S1 interval. In in vivo experiments, regular ventricular pacing (S1) at cycle lengths ranging between 250 to 1000 ms was followed by a single extrastimulus (S2) with a coupling interval (S1S2) of 1500 ms. A decrease in the S1S1 interval also resulted in progressive shortening of the duration of MAP elicited by S2. Our results indicate that the postrest shortening is potentiated by an increase in the preceding stimulus frequency in the rabbit ventricle, in which the function of sarcoplasmic reticulum may play a significant role.

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.

Paradoxically shortened QT interval after a prolonged pause

We analyzed Holter ECG recordings in 15 patients with episodes of prolonged RR intervals > 2.5 seconds. In 13 patients, the QT interval showed a linear prolongation when RR interval was < 1.5 seconds and became relatively flat at longer RR intervals. In the remaining two patients, the QT and RR intervals were correlated within physiological range of RR intervals. However, at longer RR intervals, the QT interval was unexpectedly shortened and constant. The paradoxically shortened QT interval observed in the present 2 cases may indicate an abnormal adaptation of repolarization time to an abrupt increase in the preceding RR intervals.

Postrest shortening of the action potential duration in rabbits: in vitro and in vivo correlation

Previous evidence has shown that the action potential duration of rabbit ventricular muscle cells shortens after a rest period (postrest shortening). However, there has not been much research on postrest shortening in the intact heart. We recorded transmembrane action potentials (TAPs) of isolated papillary muscle from rabbit ventricle with glass microelectrodes and monophasic action potentials (MAPs) of the rabbit left ventricular endocardium with contact electrodes. In the in vitro experiments, repetitive regular stimuli (S1) at a cycle length of 1 sec were followed by a single extrastimulus (S2) at coupling intervals (S1S2) ranging between 0.5 sec and 8 sec. The increase in the S1S2 interval resulted in a progressive shortening of the duration of TAP elicited by the S2, which was abolished by the simultaneous application of 1 mmol/L 4-aminopyridine and 2 micromol/L ryanodine. In the in vivo experiments, regular right ventricular pacing (S1) at a cycle length of 0.35 sec was followed by a single extrastimulus (S2) with coupling intervals (S1S2) ranging between 0.25 sec and 3 sec. The increase in the S1S2 interval also resulted in a progressive shortening of the duration of MAP elicited by the S2. This is the first report to demonstrate postrest shortening in the intact heart, which probably occurs because of a mechanism analogous to that observed in the isolated ventricular muscle.

Repolarizing K+ currents in rabbit heart Purkinje cells

1. Electrophysiological experiments on single myocytes obtained from Purkinje fibres and ventricular tissue of adult rabbit hearts were done to compare the contributions of three potassium (K+) currents to the action potentials in these two tissues. 2. In Purkinje cells reductions in extracellular potassium, [K+]o, from normal (5.4 mM) to 2.0 mM resulted in a large hyperpolarization and marked lengthening of the action potential. In ventricular myocytes, these changes were much less pronounced. Voltage clamp measurements demonstrated that these differences were mainly due to a much smaller inward rectifier K+ current, IK1, in Purkinje cells than in ventricular myocytes. 3. Application of 4-aminopyridine (4-AP, 2 mM) showed that all Purkinje cells exhibited a very substantial Ca2+-independent transient K+ outward current, It. 4-AP significantly broadened the early, rapid repolarization phase of the action potential. 4. Selective inhibitors of the fast component, IK, r (MK-499, 200 nM) and the slow component IK,s (L-735821 (propenamide), 20 nM) of the delayed rectifier K+ currents both significantly lengthened the action potential, suggesting that these conductances are present, but very small (< 20 pA) in Purkinje cells. Attempts to identify time- and voltage-dependent delayed rectifier K+ current(s) in Purkinje cells failed, although a slow delayed rectifier was observed in ventricular myocytes. 5. These results demonstrate significant differences in action potential waveform, and underlying K+ currents in rabbit Purkinje and ventricular myocytes. Purkinje cells express a much smaller IK1, and a larger It than ventricular myocytes. These differences in current densities can explain some of the most important electrophysiological properties of these two tissues.

Divergent effect of acute ventricular dilatation on the electrophysiologic characteristics of d,l-sotalol and flecainide in the isolated rabbit heart

INTRODUCTION

The interaction between acute ventricular dilatation (AVD) as one aspect of ventricular dysfunction and Class I and III antiarrhythmic drugs is uncertain. We therefore investigated the effects of AVD on the electrophysiologic properties of d,l-sotalol and flecainide.

METHODS AND RESULTS

The isolated rabbit heart was used as a model of AVD. The ventricular size and, therefore, the diastolic pressure were modified by sudden volume changes of a fluid-filled balloon placed in the left ventricle. Pacing was performed alternately using epi- and endocardial monophasic action potential (MAP)-pacing catheters at cycle lengths from 1,000 to 300 msec. d,l-Sotalol (10 microM) resulted in a significant (P < 0.05) lengthening of refractoriness (+13.5% +/- 3.1%), MAP duration (+14.9% +/- 3.2%), and QT interval (+15.5% +/- 4.1%) (mean +/- SEM at 1,000 msec). These effects had a reverse rate-dependence. AVD to a diastolic pressure of 30 mmHg reduced refractoriness and left ventricular MAP duration. In comparison with the control group with the same extent of AVD, d,l-sotalol still led to a significant prolongation of repolarization for all cycle lengths except 300 msec, so that its effects were not absolutely but relatively preserved. In contrast, flecainide (2 microM) had no significant effects on refractoriness or MAP duration. It led to a significant, rate-dependent increase of pacing thresholds (+47.6% +/- 8.2%), prolongation of QRS (+48.8% +/- 5.6%), and conduction time (+78.6% +/- 8.6%) (mean +/- SEM at 300 msec). In the flecainide group, AVD significantly increased the normal rate-dependent prolongation of QRS (+16.7% +/- 5.5%) and conduction time (+17.1% +/- 4.3%).

CONCLUSION

Our data demonstrate that, during AVD, the Class III effect of d,l-sotalol is preserved, whereas flecainide's effect of slowing conduction is exaggerated. This may contribute to flecainide-related proarrhythmia in certain clinical situations.

Effects of development and thyroid hormone on K+ currents and K+ channel gene expression in rat ventricle

1. In rat heart, three K+ channel genes that encode inactivating transient outward (ITO)-like currents are expressed. During development the predominant K+ channel mRNA species switches from Kv1.4 to Kv4.2 and Kv4.3. However, no functional correlate of this isoform switch has been reported. We investigated action potential characteristics and ITO in cultured neonatal rat ventricular myocytes and adult rat hearts. We further examined whether the changes in K+ channel gene expression and the associated electrophysiology that occurs during development could be induced by thyroid hormone. 2. In myocytes isolated from right ventricle of adult rat heart, action potential duration was short and independent of rate of stimulation. The density of ITO was 21.5 +/- 1.8 pA pF-1 (n = 21). Recovery from inactivation was best described by a single exponential (tau fast = 31.7 +/- 2.7 ms, n = 13). The current remaining at the end of a 500 ms pulse (ISUS) was 6.2 +/- 0.5 pA pF-1 (n = 19). 3. In contrast to adult cells, action potential duration was prolonged and was markedly rate dependent in cultured neonatal rat ventricular myocytes. The current density of ITO measured in cultured ventricular myocytes from 1- to 2-day-old rats was 10.1 +/- 1.5 pA pF-1 (n = 17). The recovery from inactivation for ITO was best described by the sum of two exponentials (tau fast = 64.3 +/- 8.8 ms, 54.4 +/- 10.2%; tau slow = 8216 +/- 2396 ms, 37.4 +/- 7.9%; n = 5). ISUS was 4.4 +/- 0.6 pA pF-1 (n = 17). Steady-state activation and inactivation were similar in adult and neonatal ventricular myocytes. 4. In neonatal myocytes treated with thyroid hormone, tri-iodothyronine (T3, 100 nM), action potential duration was abbreviated and independent of stimulation rate. Whilst T3 did not significantly increase ITO density (24.0 +/- 2.9 pA pF-1; n = 21 in T3 treated cells cf. 20.1 +/- 3.0 pA pF-1; n = 37 in untreated controls), the recovery from inactivation of ITO was accelerated (tau fast = 39.2 +/- 3.6 ms, 82.2 +/- 8.9%, n = 9). T3 did however, increase ISUS current density (4.7 +/- 0.77 pA pF-1; n = 37 and 7.0 +/- 0.7 pA pF-1, n = 21, in control and T3 treated cells, respectively. 5. The effects of T3 (100 nM) were associated with a marked decrease in the expression of Kv1.4 at the mRNA and protein level, and an increase in the expression of Kv4.3 without changes in Kv4.2 mRNA levels. 6. The findings of the present study indicate that postnatal development involves a shortening of action potential duration and an increase in the density of ITO. Furthermore, we show that development is also associated with a loss of action potential rate dependence, and an acceleration in the rate of recovery of ITO. We propose that these functional effects occur as a consequence of the previously reported developmental Kv1.4 to Kv4.2/Kv4.3 isoform switch. In cultured neonatal myocytes, T3 induced many of the electrophysiological and molecular changes that normally occur during postnatal development, suggesting that this hormone may play an important role in postnatal electrophysiological development.

In vitro effects of capsaicin: antiarrhythmic and antiischemic activity

The antiarrhythmic effects of vehicle (0.1% dimethyl sulfoxide: DMSO) or capsaicin were evaluated in isolated perfused rat and guinea pig heart preparations. In the rat, capsaicin reduced ischemic ventricular tachycardia from 100% in control to 0%, and ischemic ventricular fibrillation from 60% in control to 0% at 30 microM, and diltiazem reduced the incidence of ischemic ventricular tachycardia and ventricular fibrillation to 55% and 0%, respectively. Reperfusion ventricular fibrillation was reduced from 90% to 20% and 33% for capsaicin and diltiazem, respectively, at these concentrations. In isolated perfused globally ischemic rat hearts, antiischemic efficacy was assessed as a significant extension (36% and 50%) in time to contracture with 30 microM capsaicin and 1 microM diltiazem, respectively. Capsaicin reduced left ventricular developed pressure by 35% in non-ischemic rat hearts, and increased coronary flow by 40%. The increased time to contracture for either compound was not blocked by glyburide (0.1 microM) suggesting a lack of any involvement of ATP-sensitive K+ channels. In isolated guinea pig hearts subjected to global ischemia, capsaicin and diltiazem reduced reperfusion ventricular fibrillation from 100% to 10% and 0% at 30 and 3 microM, respectively. Electrophysiologic evaluation in guinea pig papillary muscles using standard microelectrode techniques demonstrated significant (P < 0.05) action potential durations at 90% repolarization shortening at 1 Hz by 9%, 28% and 39%, and 23%, 37% and 51% at 10, 30, and 100 microM of capsaicin or diltiazem, respectively. Unlike diltiazem, no changes in action potential duration were observed with capsaicin (up to 100 microM) at faster stimulation rates (5 Hz). In conclusion, capsaicin displays both antiarrhythmic and antiischemic efficacy. These data suggest that the effects of capsaicin are mediated primarily through block of Ca2+ channels in these preparations.

Characterization of an E4031-sensitive potassium current in quiescent AT-1 cells

INTRODUCTION

A cardiac culture cell line (AT-1) recently has been generated from transgenic mice. Initial studies have yielded opposing results as to the nature of the major repolarizing current(s) in these cells. The present study describes the ion selectivity, voltage dependence, and E4031 sensitivity of the major time-dependent outward current present in AT-1 cells. In addition, we have determined whether an outward current with the characteristics we observed could be capable of modulating action potential duration in a frequency-dependent manner (for stimulation cycle lengths between 250 and 1000 msec).

METHODS AND RESULTS

Action potentials and membrane currents were recorded from nonconfluent cultures of quiescent AT-1 cells using the "perforated patch" technique. AT-1 cells showed a round appearance 1 or 2 days after plating. An E4031-insensitive transient outward current seemed to be absent in these cells. The main time-dependent outward current was a rapidly activating and rectifying potassium current with properties similar to those of IKr. Most of the potassium current was sensitive to the benzenesulfonamide E4031 (5 microM). The same concentration of E4031 led to a 38% increase in action potential duration. Action potential parameters were independent of the stimulation cycle length within the range of 250 to 1000 msec, thus suggesting that the membrane currents involved in the action potential of AT-1 cells are completely reset within a diastolic interval of approximately 150 msec.

CONCLUSION

AT-1 cells present a unique electrophysiologic phenotype, which is clearly different from that reported for freshly dissociated adult atrial or ventricular myocytes from other species. AT-1 cells may be a good model to study IKr, since there seems to be minimal contamination by other outward conductances (such as IKs). In addition, the feasibility of culturing AT-1 cells provides us with a system where electrophysiologic experiments on IKr currents could be combined with biochemical or molecular biological studies requiring significant periods of incubation in a cell culture system.

Rate-dependent prolongation of action potential duration in single ventricular myocytes obtained from hearts of rats with streptozotocin-induced chronic diabetes sustained for 30-32 weeks

We examined the characteristics of the action potentials of single ventricular myocytes obtained from the hearts of rats with chronically-induced diabetes. Male Wistar rats were made diabetic by injecting streptozotocin (65 mg/kg) and 30-32 weeks later the hearts were excised and used for an electrophysiological study. Action potentials were recorded from isolated right ventricular myocytes by an electrode fabricated for patch clamp in the whole-cell recording configuration. The action potential durations (APDs) of steady state chronic diabetic rat myocytes were longer than those of age-matched normal rat myocytes at all levels of repolarization (APD25, APD50, APD75, and APD90). As the stimulation frequency was increased (0.2-2 Hz), the APDs were lengthened in both diabetic and normal rats, and the difference of APDs between the groups was greater when the stimulation frequency was higher. When we examined alterations of APDs under conditions of train stimulation (2Hz, 20 stimuli), (1) the APDs in both groups were prolonged, and (2) the degree of prolongation of APD was significantly greater and the rate of APD prolongation was significantly faster in myocytes from the diabetic rats. The prolongation of APD in these heart cells is probably secondary to alteration of the transient outward current Ito, and sheds light on repolarization abnormality in cases of diabetic cardiomyopathy.

Ionic basis for OPC-8212-induced increase in action potential duration in isolated rabbit, guinea pig and human ventricular myocytes

Changes in transmembrane ionic currents induced by OPC-8212 (3,4-dihydro-6-[4-(3,4-dimethoxybenzoyl)-1-piperazinyl]-2(1H)-quinoline) , a recently introduced positive inotropic agent which lengthens cardiac action potential duration, were examined using whole-cell voltage-clamp techniques in single rabbit, guinea pig and human ventricular myocytes. In rabbit, OPC-8212 (12 mumol/l) significantly increased membrane action potential duration measured at 90% of repolarization by an average of 88 ms (from 462 +/- 25 to 550 +/- 35 ms, n = 4; P < 0.05). In rabbit this increase in duration was not associated with significant changes in either the inward rectifier or transient outward K+ currents. The magnitude of the secondary inward current evoked from a holding potential of -50 mV was significantly increased by 97 +/- 8% (n = 6; P < 0.01) while a demonstrable delayed rectifier outward current could not be identified in the rabbit myocytes examined at room temperature. In guinea pig ventricular myocytes, where the delayed rectifier was large, 12 mumol/l OPC-8212 significantly depressed the current by 58 +/- 10% (n = 6; P < 0.01). The effects of OPC-8212 in human ventricular myocytes obtained from the explanted heart of a single patient having an idiopathic cardiomyopathy most closely resembled those observed in isolated rabbit ventricular myocytes. Thus, in rabbit and a few human ventricular myocytes examined at room temperature, OPC-8212 appeared to lengthen cardiac membrane action potential duration primarily by increasing the amplitude of the secondary inward current believed to primarily represent current through L-type Ca2+ channels. In guinea pig preparations, OPC-8212 also decreased the delayed rectifier outward K+ current which also would account for an increase in action potential duration. OPC-8212 could not be demonstrated to affect Na+ current inactivation in a manner similar to that produced by 1 mg/l veratrine, a recognized Na+ channel agonist, which dramatically slowed this process.

K+ channels and control of ventricular repolarization in the heart

K+ channels form a large family, in which voltage-operated and ligand-operated channels can be distinguished. Under physiological conditions, four K+ currents contribute to the repolarization process and their role is discussed: i) the transient outward current (ito) is responsible for the rapid initial repolarization process from the crest of the action potential to the plateau level; ii) the delayed K+ current (iK) is involved in the overall repolarization process during the plateau; iii) the inward rectifier (iK1) is responsible for the final rapid repolarization and the maintenance of the resting potential; iv) a ligand-operated channel activated by acetylcholine and adenosine participates in the repolarization process and the maintenance of the resting potential in nodal, atrial and Purkinje cells. In the context of antiarrhythmic interventions, block of outward K+ current and prolongation of refractoriness is currently considered as an alternative to block of the Na+ current and reduction of conduction velocity. Although some of these drugs show use-dependent block, the frequency-dependent changes in current and action potential duration are not ideal.

Effects of long-term oral administration of amiodarone on the electromechanical performance of rabbit ventricular muscle

1. The effects of long-term administration of oral amiodarone on transmembrane action potential and contraction of ventricular muscle were investigated in rabbits. 2. ECGs of rabbits that received oral amiodarone 50 mg or 100 mg kg-1 daily for 4 weeks, showed a significant prolongation of RR, QT and corrected QT (QTc) intervals, whereas PQ and QRS were unaffected. Serum and myocardial tissue amiodarone concentrations were 0.14-0.18 micrograms ml-1 and 1.47-3.63 micrograms g-1 wet wt. respectively. 3. Right ventricular papillary muscles isolated from treated rabbits were characterized by a moderate prolongation of action potential duration (APD) compared with controls. A slight decrease of the maximum upstroke velocity (Vmax) was also observed at the higher dose. The APD prolongation by chronic amiodarone, unlike acute effects of sotalol, E-4031, Cs+ and 4-aminopyridine, did not show marked reverse use-dependence. 4. APD and Vmax restitution following slow basic stimuli (0.03 Hz) were unaffected by chronic treatment with amiodarone. 5. Acute application of amiodarone (10 microM) caused a significant decrease in APD and developed tension, as well as a marked use-dependent Vmax inhibition with fast recovery kinetics. 6. These findings suggest that a major and consistent electro-physiological effect of chronic amiodarone is repolarization delay (Class-III action) showing minimal frequency-dependence. However, when amiodarone above a certain concentration is present in the extracellular space, a fast kinetic Class-I action would be added as an acute effect.

Effects of K+ channel blockers on the action potential of hypoxic rabbit myocardium

1. In order to assess the role of different ionic currents in hypoxia-induced action potential shortening, we investigated the effects of blockers of voltage-dependent and ATP-sensitive K(+)-channel on the membrane potential of hypoxic rabbit hearts and papillary muscles. The response to blocking of the inward rectifier was studied at three external K+ concentration: 2.5, 5, and 7.5 mM. 2. Hypoxia produced a progressive decline in action potential duration (APD) that levelled off after 15 to 20 min. Steady state APD values at 25% and 95% repolarization (APD25 and APD95) were 26.0 +/- 1.9% and 42.2 +/- 2.4% of controls respectively. 3. Tetraethylammonium (TEA, 10 mM) delayed but did not reduce APD shortening at the steady state. 4. Blocking of IK1 with a mixture of 0.2 mM Ba2+ and 4 mM Cs+ lengthened APD in normoxia and prevented APD95 shortening in hypoxia. The APD25 shortening was significantly attenuated at all [K]o. 5. Glibenclamide (Glib, 30 microM) did not prevent APD shortening, but produced a progressive action potential (AP) lengthening after 15 min of hypoxia. Steady levels of 48 +/- 3.5% and 62 +/- 5.0% of controls for APD25 and APD95 respectively were reached after 45 min. 6. The relation between APD25 and pacing rate was determined in normoxic and hypoxic papillary muscles and the effects of 2 mM 4-aminopyridine (4-AP) were examined. Hypoxia attenuated the APD25 shortening currently observed when the stimulation rate was lowered from 1 to 0.1 Hz without altering the plateau reduction occurring at frequencies above 2 Hz. These effects were potentiated by 4-AP.7. Our data suggest that the accelerated AP repolarization in hypoxic rabbit myocardium represents a delicate balance of several outward currents: IKI, IK-ATP. and at least one yet unidentified current component rather insensitive to changes in [K]o and to K+ channel blockers.

The role of intracellular sodium in the control of cardiac contraction

Intracellular sodium was estimated in ventricular myocytes using the new Na-sensitive fluorescent indicator SBFI. Membrane potential and contraction were also measured simultaneously. Using an in situ calibration method, we found that intracellular sodium activity (aiNa) was 2.9 mM in quiescent rabbit cells. When the digitalis analogue strophanthidin inhibited the Na-K pump of myocytes with action potentials (APs), changes of contraction and aiNa were dissociated in time. There was also marked hysteresis between contraction and aiNa. When strophanthidin was applied to the same myocytes under voltage-clamp conditions, temporal dissociation between contraction and aiNa was dramatically reduced. This suggests that much of the dissociation and hysteresis was due the change in AP shape with strophanthidin. A small amount of residual hysteresis still existed even with voltage-clamp, and this persisted when the pump was blocked by removal of external potassium as an alternative method. We suggest that a gradient of sodium concentration from the subsarcolemmal space to the bulk cytoplasm might be responsible for hysteresis. Whereas SBFI probably signals the average Na level of the cytoplasm, subsarcolemmal Na may control Ca influx and contraction via Na-Ca exchange.

Chloride conductance pathways in heart

Nonelectrogenic movement of Cl- is believed to be responsible for the active accumulation of intracellular Cl- in cardiac muscle. The electro-neutral pathways underlying this nonpassive distribution of Cl- are believed to include Cl(-)-HCO3- exchange, Na(+)-dependent cotransport (operating as Na(+)-Cl- and Na(+)-K(+)-2Cl- cotransport), and K(+)-Cl- cotransport. The electrogenic movement of Cl- in cardiac muscle is particularly interesting from a historical perspective. Until recently, there was some doubt as to whether Cl- carried any current in the heart. Early microelectrode experiments indicated that a Cl- conductance probably played an important role in regulating action potential duration and resting membrane potential. Subsequent voltage-clamp experiments identified a repolarizing, transient outward current that was believed to be conducted by Cl-, yet further investigation suggested that this transient outward current was more likely a K+ current, not a Cl- current. This left some doubt as to whether Cl- played any role in regulating membrane potential in cardiac muscle. More recent studies, however, have identified a highly selective Cl- conductance that is regulated by intracellular adenosine 3',5'-cyclic monophosphate, and it appears that this Cl- current may play an important role in the regulation of action potential duration and resting membrane potential.

Effect of sotalol on transmembrane ionic currents responsible for repolarization in cardiac ventricular myocytes from rabbit and guinea pig

The effects of sotalol, a beta-adrenoceptor blocker and class III antiarrhythmic agent, on transmembrane ionic currents were examined in single rabbit and guinea pig ventricular myocytes using whole-cell voltage-clamp techniques. In neither of these species did 60 microM sotalol appreciably effect the inward rectifier, the transient outward or the inward calcium currents. In addition, sotalol did not elicit a slowly inactivating component of the sodium current as did 1 microgram/ml veratrine. In guinea pig ventricular myocytes, sotalol also significantly depressed the outward delayed rectifier current. An outward delayed rectifier current was not observed in rabbit ventricular myocytes examined at room temperature; and, under these conditions sotalol did not lengthen action potential duration. Sotalol induced lengthening of cardiac action potential duration can, therefore, be explained by depression the outward delayed rectifier current.

Intracellular mechanisms for alpha 1-adrenergic regulation of the transient outward current in rabbit atrial myocytes

1. The intracellular mechanism(s) underlying the decrease of a transient outward K+ current (It) induced by alpha 1-adrenergic agonists was studied in isolated adult rabbit atrial myocytes using whole-cell voltage clamp and cell-attached patch clamp techniques. Experiments were carried out at 22-23 degrees C. 2. Application of the specific alpha 1-adrenergic agonist, methoxamine, produced a decrease in It which was irreversible after the non-hydrolysable GTP analogues, GTP gamma S and Gpp(NH)p, had been introduced into cells via the recording micropipette. 3. Pre-treatment of cells with 0.1-0.15 microgram/ml pertussis toxin (PT) for 8-9 h at 30-34 degrees C did not prevent the alpha 1-induced decrease in It. Yet, this protocol, as measured by the PT-catalysed incorporation of [32P]ADP-ribose in membrane-associated 40 and 41 kDa proteins, effectively caused the ADP-ribosylation of approximately 70% of the PT-sensitive GTP-binding proteins (i.e. Gi) in these treated cells. After taking into account the proportion of non-viable cells (20-30%), the effectiveness of this treatment probably approaches 100% in the viable myocytes from which electrophysiological recordings were made. 4. Cell-attached patch recordings showed that bath application of methoxamine altered the single-channel events underlying It by decreasing their opening probability. Averaged currents from ensemble single-channel openings recorded in the presence of 0.2 mM-methoxamine outside the patch reproduced the features of alpha 1-adrenergic modulation of the macroscopic It observed during whole-cell voltage clamp measurements. This observation provides evidence for the involvement of a diffusible intracellular second messenger in the alpha 1-adrenergic modulation of It. 5. The protein kinase C (PKC) activators, 4 beta-phorbol 12-myristate 13-acetate (PMA) and 1-oleoyl-2-acetylglycerol (OAG) increased It, when included in the bath perfusate, whereas the inactive analogues, 4 alpha-phorbol and 4 alpha-phorbol 12,13-didecanoate, had no effect on It. 6. Exposure of cells to the PKC inhibitors, staurosporine and H-7, either by bath superfusion or intracellularly, via the recording micropipette, did not block the decrease in It produced by methoxamine. 7. Prolonged stimulation of atrial myocytes for 7-9 h at 22 degrees C with 500 nM-PMA produced a 'down-regulation' of endogenous PKC activity, as well as a physical loss of the immunoreactive enzyme, as measured by an in vitro assay, and an anti-PKC monoclonal antibody, respectively.(ABSTRACT TRUNCATED AT 400 WORDS)

Aminopyridines block an inactivating potassium current having slow recovery kinetics

The blocking action of aminopyridines on an inactivating K current (lKi) in GH3 pituitary cells was studied before and after altering the macroscopic decay of the current with N-bromoacetamide (NBA). The first depolarizing pulse delivered either seconds or minutes after beginning 4-aminopyridine (4AP) application, elicited a current with both a more rapid decay and a reduced peak amplitude. The rapid decay (or time-dependent block) was especially prominent in NBA-treated cells. With continued drug application, subsequent test pulses revealed a stable block of peak current, greater in NBA-treated than control cells. Recovery from block was enhanced by hyperpolarizing holding potentials and by the first depolarizing pulse delivered after prolonged recovery intervals. Unlike aminopyridine block of other K currents, there was no convincing evidence for voltage shifts in activation or inactivation, or for voltage and frequency-dependent unblock. Increasing the open probability of the channels did, however, facilitate the block. Although the behavior of currents in 4AP was suggestive of "open channel block," the block was not produced by 4-aminopyridine methiodide, a positively charged aminopyridine. Moreover, because partial block and recovery occurred without opening the channels we suggest that aminopyridines bind to, or near, this K channel, that this binding is enhanced by opening the channel, and that a conformational change is induced which mimics inactivation. Because recovery from block is enhanced by negative potentials, we suggest that aminopyridine molecules may become "trapped" by inactivation awaiting the slow process of reactivation to escape their binding sites.

Stimulation-dependent facilitation of the high threshold calcium current in guinea-pig ventricular myocytes

1. Stimulation-dependent modulation of Ca currents was examined in guinea-pig ventricular myocytes at room temperature. Whole-cell recordings of Ca currents were made under conditions which minimized ionic fluxes through other channels. 2. Stimulation from rest at a rate of 2 Hz resulted in a decrease of the low threshold Ca current within one pulse and facilitation of the high threshold Ca current within five pulses. Facilitation was associated with a reduction in the rate of inactivation. 3. Pulse durations as short as 10 ms facilitated the high threshold Ca current in subsequent pulses. Facilitation produced by a single pulse decayed with a half-time of several seconds. 4. Substitution of Ba2+ or Sr2+ for external Ca2+ reduced the rate of inactivation of the high threshold Ca current and abolished facilitation of the current. 5. Facilitation persisted with 40 microM-Ruthenium Red added to the internal solution or 0.2-2 microM-ryanodine added to the bath solution to reduce Ca2+ release from the sarcoplasmic reticulum. 6. Facilitation was modulated by isoprenaline. Low concentrations of isoprenaline (5-10 nM) increased the amount of facilitation. Isoprenaline (1 microM) increased the Ca current approximately 3-fold, however, facilitation was nearly abolished. 7. Caffeine (0.5 and 1 mM) affected the Ca current and facilitation in a manner similar to 1 microM-isoprenaline. It increased the Ca currents approximately 2.5-fold and facilitation was not observed. 8. We conclude that stimulation-dependent facilitation of the high threshold Ca current is mediated by calcium and hypothesize that calcium affects a site near the Ca channel that modifies the rate of inactivation. The common actions of caffeine and high concentrations of isoprenaline suggest that calcium modulates a phosphorylation step.

Characteristics of transient outward currents in single smooth muscle cells from the ureter of the guinea-pig

1. Two kinds of transient outward currents were observed upon depolarization of single smooth muscle cells isolated from guinea-pig ureter. The major transient outward current was through Ca2(+)-activated K+ channels (IK(Ca) which had a large conductance (130 pS; 126 mM [K+]i/5.9 mM [K+]o). 2. The smaller transient outward current (ITO) was pharmacologically separated from other membrane currents in the presence of 1 mM-Cd2+ and 2 mM-tetraethylammonium(TEA+) and was selectively blocked by 3 mM-4-aminopyridine. It peaked (approximately 200 pA) within 10 ms upon depolarization from -80 to +20 mV and its half-inactivation time was approximately 50 ms at +20 mV. Half-maximum voltages (V 1/2) for activation and inactivation were about -8 and -50 mV, respectively, in the presence of 1 mM-Cd2+ and 2 mM-TEA+. The time course of recovery from inactivation of ITO was fitted with a single-exponential function (tau = 100 ms at -80 mV). A tenfold change of [K+]o resulted in a 53 mV change in the reversal potential of the tail of ITO. 3. Cadmium reduced peak ITO and shifted the voltage dependence of activation and inactivation in the positive direction in a concentration-dependent manner. The V 1/2 for inactivation in the absence of Cd2+ was estimated to be approximately -64 mV. 4. Single-channel outward currents which appeared only in the initial part of a depolarizing pulse from about -100 mV were recorded using the cell-attached patch clamp. The decay of the ensemble average of the current was similar to the macroscopic ITO under whole-cell clamp. When the holding potential was less negative, the opening probability of the channel greatly decreased. The channel conductance in normal extracellular medium was 14 pS. 5. In ureter cells ITO resembles A-type current. ITO does not contribute significantly to the repolarization of the action potential but it may regulate membrane excitability by opposing Ca2+ current activated around the threshold of the action potential.

Barucainide, a novel class Ib antiarrhythmic agent with a slow kinetic property: electrophysiologic observations in isolated canine and rabbit cardiac muscle

Electrophysiologic effects of barucainide hydrochloride on the transmembrane potentials of isolated canine and rabbit cardiac muscle were investigated by means of a standard microelectrode technique. Barucainide (10(-6) to 3.0 x 10(-5) mol/L) produced concentration-dependent inhibition of the maximum upstroke velocity (Vmax) in canine Purkinje fibers and ventricular muscle. Tonic block of Vmax was small and negligible. Barucainide produced marked use-dependent block (UDB). The onset rate of UDB and the speed of recovery from UDB were comparable to those of disopyramide. The recovery time constant was 8.0 +/- 0.8 seconds. Membrane depolarization induced by high concentrations of potassium enhanced the inhibitory action of barucainide on Vmax. The enhancement of tonic block was much greater than that of UDB indicating a selective effectiveness of the drug in pathologically depolarized tissues. Barucainide produced marked shortening of the action potential duration (APD), which indicates that barucainide exerts class Ib antiarrhythmic effects. Such shortening effects on the APD were not affected by pretreatment with 1 mmol/L cobalt or with 0.5 mmol/L 4-aminopyridine. In contrast, barucainide had no marked effects on the APD in the presence of 4.0 x 10(-5) mol/L lidocaine, although it produced almost the same extent of Vmax inhibition as did barucainide alone. Frequency of the pacemaker activity in the rabbit sinoatrial tissue and of the automaticity at normal resting potential in Purkinje fibers enhanced by 2.0 X 10(-6) mol/L isoproterenol were significantly suppressed by barucainide. However, barucainide up to 3.0 x 10(-5) mol/L failed to suppress the firing frequency of the abnormal automaticity in canine Purkinje fibers induced by 5 mmol/L barium. These overall findings indicate that barucainide is a novel class Ib antiarrhythmic agent which, unlike other Ib agents, has a slow kinetic property. However, the precise significance of this unique electrophysiologic feature with respect to the antiarrhythmic action of the drug is not known but provides the basis for understanding the relationship of electrophysiologic effects of barucainide in vitro and its potential antiarrhythmic actions in experimental animals and in humans.

A long lasting Ca2+-activated outward current in guinea-pig atrial myocytes

Among other characteristics, the steady-state current-voltage relationship of patch-clamped single atrial myocytes from guinea-pig hearts is defined by an outward current hump in the potential region -15 to +40 mV. This hump was reversibly suppressed by Co2+ (3 mM) or nitrendipine (5 microM) and enhanced by Bay K 8644 (5 microM). The maintained outward current component suppressed by Co2+ extended between -15.2 +/- 1.9 mV and +39.5 +/- 1.7 mV (mean +/- SEM of 14 cells) and has an amplitude of 95.7 +/- 9.4 pA at +10 mV. In isochronal I-V curves, the hump was already visible at 400 ms with essentially the same amplitude as at 1500 ms. The Co2+-sensitive outward current underlying the hump was poorly time-dependent during 1.5 s voltage pulses but slowly relaxed upon repolarization. Tail currents reversed near the K+ equilibrium potential under our experimental conditions. The current hump of the steady-state I-V curve was also abolished by caffeine (10 mM) or ryanodine (3 microM), both drugs that interfere with sarcoplasmic reticulum function. Apamin (1 microM) or quinine (100 microM) but not TEA (5-50 mM) markedly reduced its amplitude. However, at similar concentrations as required to inhibit the hump, both apamin and quinine appeared to be poorly specific for Ca2+-activated K+ currents in heart cells since they also inhibited the L-Type Ca2+ current. It is concluded that a long lasting Ca2+-activated outward current, probably mainly carried by K+ ions but not sensitive to TEA, exists in atrial myocytes which is responsible for the current hump of the background I-V curve.

Calcium-sensitive and insensitive transient outward current in rabbit ventricular myocytes

1. A suction pipette whole-cell voltage-clamp technique was used to record membrane currents and potentials of isolated ventricular myocytes from rabbit hearts. 2. Transient outward current (Ito) was activated by voltage steps positive to -20 mV, increasing in amplitude with further depolarization to reach a maximum around +70 mV. The current attained its peak within 10 ms and then it inactivated for 100-200 ms. 3. A large portion of Ito still remained after the calcium current (ICa) was blocked when depolarizing pulses were applied at a frequency of 0.1 Hz or less. Therefore, this current component is referred to as calcium-insensitive Ito or It. 4. It showed voltage- and time-dependent inactivation similar to that observed in Purkinje fibres and other cardiac preparations. 5. The reversal potential of It depended on external K+ concentration, [K+]o, with a slope of 32 mV per 10-fold change in the presence of a normal [Na+]o (143 mM), while the slope was 48 mV per 10-fold change in low [Na+]o (1.0 mM). 6. It was completely inhibited by 2-4 mM-4-aminopyridine. Ito in the presence of ICa was also partially blocked by 4-aminopyridine and the remainder was abolished by 5 mM-caffeine. 7. The calcium-insensitive and caffeine-sensitive Ito differed in their decay rates as well as in their recovery time courses. The former was predominantly available at a slow pulsing rate, while the latter increased its amplitude with high-frequency depolarization. 8. The caffeine-sensitive Ito was inhibited by a blockade of ICa, by replacing Ca2+ with Sr2+, by external application of ryanodine and by internal application of EGTA. This indicates that the current is calcium-sensitive and is dependent on increased myoplasmic Ca2+ through Ca2+ influx via the sarcolemma and Ca2+ release from the sarcoplasmic reticulum. The current is therefore designated as IK, Ca. 9. The physiological functions of IK, Ca and It are indicated by their contribution to ventricular repolarization at fast and slow heart rates, respectively.

Comparison of potassium currents in rabbit atrial and ventricular cells

1. In rabbit and human hearts there are significant differences in the action potential configuration in atrium and ventricle, and the action potential waveform exhibits marked frequency dependence in both tissues. To study the ionic mechanism(s) of these phenomena, the size and time course of the potassium (K+) currents responsible for repolarization have been recorded from single cells using a whole-cell microelectrode voltage clamp method. 2. At physiological heart rates, the action potential in atrial cells has a short plateau phase; however, the rapid early repolarization is strongly frequency dependent. Ventricular myocytes have a long plateau (400-700 ms at 23 degrees C), and the late repolarizing phase of the action potential is much faster in ventricle than in atrium. 3. In both cell types, four different outward currents can be recorded: (i) a large transient outward current, It; (ii) IK(Ca), a smaller Ca2+-dependent K+ current; (iii) IK, a small, maintained time- and voltage-dependent delayed rectifier K+ current; (iv) IK1, an inwardly rectifying K+ current. 4. It, which is responsible for early repolarization, is much larger in atrium than in ventricle. It has very rapid activation and inactivation kinetics but a very slow time course of recovery from inactivation (tau = 5.4 s at 23 degrees C). Our results show that the reactivation kinetics of It are responsible for the pronounced dependence of the shape of the atrial action potential on stimulus frequency. 5. IK(Ca) is variable from cell to cell and is larger in atrium than in ventricle. In both cell types, IK(Ca) is much smaller than It. 6. The delayed rectifier current, IK, is very small and turns on relatively slowly in both cell types. It is therefore not activated strongly during the relatively short plateau of the atrial action potential. Even in ventricle, it contributes only a small repolarizing current. 7. IK1, the inward rectifier K+ current, is much larger in ventricle than in atrium. The current-voltage relationship for IK1 in ventricle exhibits a negative slope conductance between -50 and 0 mV. IK1 is the outward current which generates the resting membrane potential and it modulates the final repolarization phase of the action potential in both cell types. 8. These data strongly suggest that the action potential configuration and its frequency dependence in rabbit atrial and ventricular cells are mainly due to the differences in sizes and kinetics of It and IK1.

Inactivation, reactivation and pacing dependence of calcium current in frog cardiocytes: correlation with current density

1. Ca2+ currents were measured in single cells isolated from frog ventricle using the whole-cell patch clamp technique and a perfused pipette. K+ currents were blocked with intracellular (120 mM) and extracellular (20 mM) Cs+. 2. A single type of Ca2+ current (ICa) was found in these cells. The current activated at voltages positive to -30 mV, exhibited a symmetrical current-voltage relationship with a peak at 0 mV, and was slowly inactivating with Ba2+ as charge carrier. 3. Large variations in ICa amplitude were observed from cell to cell (ICa at 0 mV = 293.1 +/- 283.3 pA; N = 152). These variations were not due simply to differences in cell membrane area, which was estimated by cell membrane capacitance (Cm), because the density of Ca2+ current (dICa = ICa/Cm) also varied significantly from cell to cell (1.3-28 pA/pF at 0 mV; mean +/- S.D. = 4.49 +/- 3.96; N = 152). 4. The inactivation curve of ICa was a complex function of membrane potential. 200 ms pre-pulses to voltages between -60 and +20 mV progressively inactivated ICa elicited by a subsequent test pulse with half-maximal inactivation occurring for pre-pulses to approximately -40 mV. With pre-pulses positive to +20 mV, ICa elicited by the test pulse became progressively larger. The degree of inactivation induced by a 200 ms depolarization to potentials more positive than +20 mV varied significantly from cell to cell, while no such variations were observed in the negative range of membrane potentials. 5. The time course of reactivation (i.e. removal from inactivation) of ICa at -80 mV often exhibited an overshoot. The amplitude of the overshoot varied between 100% (i.e. no overshoot) and approximately 180% in eighty-one cells. 6. The degree of inactivation at positive potentials (+100 mV) and the amplitude of the overshoot were strongly correlated with the Ca2+ current density. The overshoot was more pronounced, the reactivation was faster, and the inactivation at positive potentials was less in cells with lower ICa density. 7. Increasing the stimulation frequency from 0.125 to 2 Hz induced a positive staircase of ICa in cells with ICa density less than 2 pA/pF and a negative staircase in cells with ICa density greater than 3 pA/pF. 8. Perfusing the patch pipette with 5 mM-BAPTA instead of EGTA reduced the amplitude of the overshoot and slightly slowed the inactivation kinetics. Replacing extracellular Ca2+ ions by Ba2+ ions completely suppressed the overshoot.(ABSTRACT TRUNCATED AT 400 WORDS)