Strontium, nifedipine and 4-aminopyridine modify the time course of the action potential in cells from rat ventricular muscle

Activation recovery interval as an electrocardiographic repolarization index to detect doxorubicin-induced cardiotoxicity

BACKGROUND

It has been reported that early detection and treatment of cancer therapy- related cardiac dysfunction (CTRCD) improves its prognosis. The detailed relationships between electrocardiographic repolarization indices and decreased left ventricular function in CTRCD have not been elucidated. We closely assessed such relationships in patients with doxorubicin (DOX)-induced CTRCD.

METHODS

This retrospective, single-center, cohort study included 471 consecutive patients with malignant lymphoma who received chemotherapy including DOX. Of them, 17 patients with CTRCD and 68 patients without CTRCD who underwent 12‑lead electrocardiogram and an echocardiogram before and after chemotherapy were eventually analyzed. The fluctuations of the following electrocardiographic repolarization indices were evaluated in lead V5: QT, JT, T peak to T end interval (Tp-e), and activation recovery interval (ARI). These indices were corrected by heart rate with the Fridericia formula.

RESULTS

The median period from the end of chemotherapy to the diagnosis of the CTRCD group was 346 days (IQR 170-1283 days). After chemotherapy, the QT interval was significantly prolonged in both with and without CTRCD groups compared with that before chemotherapy (pre QTc vs. post QTc in CTRCD group, 386 ± 27 ms vs. 411 ± 37 ms, p = 0.03, pre QTc vs. post QTc in non-CTRCD group, 388 ± 24 ms vs. 395 ± 25 ms, p = 0.04, respectively). ARIc after chemotherapy was characteristically observed only in the CTRCD group (pre ARIc vs. post ARIc in CTRCD group, 258 ± 53 ms vs. 211 ± 28 ms, p = 0.03, pre ARIc vs. post ARIc in non-CTRCD group, 221 ± 19 ms vs. 225 ± 23 ms, NS, respectively) and had negative correlations with left ventricular ejection fraction (r = -0.56, p < 0.001). Using the receiver-operating characteristic curve, the relationship between ARIc and CTRCD morbidity was examined. The optimal cut-off point of ARIc prolongation between before and after chemotherapy was 18 ms (sensitivity 75 %, specificity 79 %, area under the curve 0.76).

CONCLUSIONS

ARIc prolongation may be useful in the early detection of developing late-onset chronic DOX-induced CTRCD and lead to early treatment for cardiac protection.

Dynamic Action Potential Restitution Contributes to Mechanical Restitution in Right Ventricular Myocytes From Pulmonary Hypertensive Rats

We investigated the steepened dynamic action potential duration (APD) restitution of rats with pulmonary artery hypertension (PAH) and right ventricular (RV) failure and tested whether the observed APD restitution properties were responsible for negative mechanical restitution in these myocytes. PAH and RV failure were provoked in male Wistar rats by a single injection of monocrotaline (MCT) and compared with saline-injected animals (CON). Action potentials were recorded from isolated RV myocytes at stimulation frequencies between 1 and 9 Hz. Action potential waveforms recorded at 1 Hz were used as voltage clamp profiles (action potential clamp) at stimulation frequencies between 1 and 7 Hz to evoke rate-dependent currents. Voltage clamp profiles mimicking typical CON and MCT APD restitution were applied and cell shortening simultaneously monitored. Compared with CON myocytes, MCT myocytes were hypertrophied; had less polarized diastolic membrane potentials; had action potentials that were triggered by decreased positive current density and shortened by decreased negative current density; APD was longer and APD restitution steeper. APD90 restitution was unchanged by exposure to the late Na⁺-channel blocker (5 μM) ranolazine or the intracellular Ca²⁺ buffer BAPTA. Under AP clamp, stimulation frequency-dependent inward currents were smaller in MCT myocytes and were abolished by BAPTA. In MCT myocytes, increasing stimulation frequency decreased contraction amplitude when depolarization duration was shortened, to mimic APD restitution, but not when depolarization duration was maintained. We present new evidence that the membrane potential of PAH myocytes is less stable than normal myocytes, being more easily perturbed by external currents. These observations can explain increased susceptibility to arrhythmias. We also present novel evidence that negative APD restitution is at least in part responsible for the negative mechanical restitution in PAH myocytes. Thus, our study links electrical restitution remodeling to a defining mechanical characteristic of heart failure, the reduced ability to respond to an increase in demand.

Epac activator critically regulates action potential duration by decreasing potassium current in rat adult ventricle

Sympathetic stimulation is an important modulator of cardiac function via the classic cAMP-dependent signaling pathway, PKA. Recently, this paradigm has been challenged by the discovery of a family of guanine nucleotide exchange proteins directly activated by cAMP (Epac), acting in parallel to the classic signaling pathway. In cardiac myocytes, Epac activation is known to modulate Ca(2+) cycling yet their actions on cardiac ionic currents remain poorly characterized. This study attempts to address this paucity of information using the patch clamp technique to record action potential (AP) and ionic currents on rat ventricular myocytes. Epac was selectively activated by 8-CPT-AM (acetoxymethyl ester form of 8-CPT). AP amplitude, maximum depolarization rate and resting membrane amplitude were unaltered by 8-CPT-AM, strongly suggesting that Na(+) current and inward rectifier K(+) current are not regulated by Epac. In contrast, AP duration was significantly increased by 8-CPT-AM (prolongation of duration at 50% and 90% of repolarization by 41±10% and 43±8% respectively, n=11). L-type Ca(2+) current density was unaltered by 8-CPT-AM (n=16) so this cannot explain the action potential lengthening. However, the steady state component of K(+) current was significantly inhibited by 8-CPT-AM (-38±6%, n=15), while the transient outward K(+) current was unaffected by 8-CPT-AM. These effects were PKA-independent since they were observed in the presence of PKA inhibitor KT5720. Isoprenaline (100nM) induced a significant prolongation of AP duration, even in the presence of KT5720. This study provides the first evidence that the cAMP-binding protein Epac critically modulates cardiac AP duration by decreasing steady state K(+) current. These observations may be relevant to diseases in which Epac is upregulated, like cardiac hypertrophy.

Quantification of calcium entry at the T-tubules and surface membrane in rat ventricular myocytes

The action potential of cardiac ventricular myocytes is characterized by its long duration, mainly due to Ca flux through L-type Ca channels. Ca entry also serves to trigger the release of Ca from the sarcoplasmic reticulum. The aim of this study was to investigate the role of cell membrane invaginations called transverse (T)-tubules in determining Ca influx and action potential duration in cardiac ventricular myocytes. We used the whole cell patch clamp technique to record electrophysiological activity in intact rat ventricular myocytes (i.e., from the T-tubules and surface sarcolemma) and in detubulated myocytes (i.e., from the surface sarcolemma only). Action potentials were significantly shorter in detubulated cells than in control cells. In contrast, resting membrane potential and action potential amplitude were similar in control and detubulated myocytes. Experiments under voltage clamp using action potential waveforms were used to quantify Ca entry via the Ca current. Ca entry after detubulation was reduced by approximately 60%, a value similar to the decrease in action potential duration. We calculated that Ca influx at the T-tubules is 1.3 times that at the cell surface (4.9 vs. 3.8 micromol/L cytosol, respectively) during a square voltage clamp pulse. In contrast, during a cardiac action potential, Ca entry at the T-tubules is 2.2 times that at the cell surface (3.0 vs. 1.4 micromol/L cytosol, respectively). However, more Ca entry occurs per microm(2) of junctional membrane at the cell surface than in the T-tubules (in nM/microm(2): 1.43 vs. 1.06 during a cardiac action potential). This difference is unlikely to be due to a difference in the number of Ca channels/junction at each site because we estimate that the same number of Ca channels is present at cell surface and T-tubule junctions ( approximately 35). This study provides the first evidence that the T-tubules are a key site for the regulation of action potential duration in ventricular cardiac myocytes. Our data also provide the first direct measurements of T-tubular Ca influx, which are consistent with the idea that cardiac excitation-contraction coupling largely occurs at the T-tubule dyadic clefts.

Ascending aortic stenosis selectively increases action potential-induced Ca2+ influx in epicardial myocytes of the rat left ventricle

A decrease of the transient outward potassium current (Ito) has been observed in cardiac hypertrophy and contributes to the altered shape of the action potential (AP) of hypertrophied ventricular myocytes. Since the shape and duration of the ventricular AP are important determinants of the Ca2+ influx during the AP (QCa), we investigated the effect of ascending aortic stenosis (AS) on QCa in endo- and epicardial myocytes of the left ventricular free wall using the AP voltage-clamp technique. In sham-operated animals, QCa was significantly larger in endocardial compared to epicardial myocytes (803 +/- 65 fC pF(-1), n = 27 vs. 167 +/- 32 fC pF(-1), n = 38, P < 0.001). Ascending aortic stenosis significantly increased QCa in epicardial myocytes (368 +/- 54 fC pF(-1), n = 42, P < 0.05), but did not alter QCa in endocardial myocytes (696 +/- 65 fC pF(-1), n = 26). Peak and current-voltage relation of the AP-induced Ca2+ current were unaffected by AS. However, the time course of the current-voltage relation was significantly prolonged in epicardial myocytes of AS animals. Model calculations revealed that the increase in QCa can be ascribed to a prolonged opening of the activation gate, whereas an increase in inactivation prevents an excessive increase in QCa. In conclusion, AS significantly increased AP-induced Ca2+ influx in epicardial but not in endocardial myocytes of the rat left ventricle.

L-type Ca2+ channels serve as a sensor of the SR Ca2+ for tuning the efficacy of Ca2+-induced Ca2+ release in rat ventricular myocytes

In cardiac excitation-contraction coupling, Ca2+-induced Ca2+ release (CICR) from ryanodine receptors (RyRs), triggered by Ca2+ entry through the nearby L-type Ca2+ channel, induces Ca2+-dependent inactivation (CDI) of the Ca2+ channel. Aiming at elucidating the physiological role of CDI produced by CICR (CICR-dependent CDI), we investigated the contribution of the CICR-dependent CDI to action potential (AP) waveform and the amount of Ca2+-influx through Ca2+ channels during AP in rat ventricular myocytes. The elimination of the CICR-dependent CDI, by depletion of the SR Ca2+ with thapsigargin, significantly prolonged AP duration (APD). APD changed in parallel with the magnitude of CICR during the recovery of the SR Ca2+ content after transient depletion by caffeine. Such CICR-dependent change of APD persisted under the highly Ca2+ buffered condition where the Ca2+ signalling was restricted to nanoscale domains. Blockers of the Ca2+-dependent Cl- channel or the BK channel did not affect AP waveform. The amount of Ca2+-influx through Ca2+ channels during the SR-depleted type AP waveform, measured in the SR-depleted myocyte, was increased by 40 % over that during the SR-intact type AP waveform measured in the SR-intact myocyte. The protein kinase A stimulation further enhanced the Ca2+-influx during AP under the SR-depleted condition to 70 % of that under the SR-intact condition. These results indicate that the CICR-dependent CDI of L-type Ca2+ channels, under control of the privileged cross-signalling between L-type Ca2+ channels and RyRs, play important roles for monitoring and tuning the SR Ca2+ content via changes of AP waveform and the amount of Ca2+-influx during AP in ventricular myocytes.

Positive and negative contractile effects of somatostatin-14 on rat ventricular cardiomyocytes

Somatostatin-14 elicits negative inotropic and chronotropic actions in atrial myocardium. Less is known about the effects of somatostatin-14 in ventricular myocardium. The direct contractile effects of somatostatin-14 were assessed using ventricular cardiomyocytes isolated from the hearts of adult rats. Cells were stimulated at 0.5 Hz with CaCl2 (2 mM) under basal conditions and in the presence of the beta-adrenoceptor agonist, isoprenaline (1 nM), or the selective inhibitor of the transient outward current (Ito), 4-aminopyridine (500 microM). Somatostatin-14 did not alter basal contractile response but it did inhibit (IC50 = 13 nM) the response to isoprenaline (1 nM). In the presence of 4-aminopyridine (500 microM), somatostatin-14 stimulated a positive contractile response (EC50 = 118 fM) that was attenuated markedly by diltiazem (100 nM). These data indicate that somatostatin-14 exerts dual effects directly in rat ventricular cardiomyocytes: (1) a negative contractile effect, observed in the presence of isoprenaline (1 nM), coupled to activation of Ito; and (2) a previously unreported and very potent positive contractile effect, unmasked by 4-aminopyridine (500 microM), coupled to the influx of calcium ions via L-type calcium channels. The greater potency of somatostatin-14 for producing the positive contractile effect indicates that the peptide may exert a predominantly stimulatory influence on the resting contractility of ventricular myocardium in vivo, whereas the negative contractile effect, observed at much higher concentrations, could indicate that localized elevations in the concentration of the peptide may serve as a negative regulatory influence to limit the detrimental effects of excessive stimulation of cardiomyocyte contractility.

Mechanical and electrophysiological effects of thiopental on rat cardiac left ventricular papillary muscle

Thiopental induces a negative inotropic effect on mammalian heart muscle, where it decreases Ca2+ current and Ca2+ release from the sarcoplasmic reticulum and reduces K+ currents. We analysed the effects of thiopental on the mechanical and electrical activities of rat myocardium, which differ markedly from those of other mammals. The effects of thiopental on mechanical parameters and on the transmembrane resting (RP) and action (AP) potentials of rat left ventricular papillary muscle were investigated. These effects were also studied in the presence of atenolol, a beta-blocking agent, and 4-aminopyridine (4-AP), a blocker of the transient outward K+ current. Thiopental (3.8 x 10(-6), 3.8 x 10(-5) and 1.1 x 10(-4) M) induced a dose-dependent positive inotropic effect. This positive inotropic effect persisted in the presence of atenolol (1 x 10(-6) M) but did not develop in the presence of 1 mM 4-AP; 4-AP had a positive inotropic effect but not in the presence of thiopental. Moreover, thiopental (3.8 x 10(-5) M) lengthened the plateau and the slow repolarizing phase of the AP, while 1 mM 4-AP only prolonged the plateau duration. In rat myocardium, the positive inotropic effect of thiopental in part mimics that of 4-AP, and in part may be explained by the lengthening of the slow repolarizing phase of the AP.

Effects of intracellular calcium elevation on action potential and L-type calcium current of normal and chronically infarcted rat ventricles

The present work investigated the effects of raising [Ca+2]i levels on action potential (AP) and L-type calcium current (I(Ca.L)) of normal and chronically infarcted rat ventricles. Experiments were performed by conventional electrophysiology and whole-cell patch-clamp techniques. In the former, APs were recorded in ventricular strips subjected to different pacing rates or elevation of [Ca+2]o levels. In the latter, I(Ca.L) was studied in isolated myocytes in the absence of an intracellular Ca+2 chelator. The acceleration of heart rate (6 to 240 beats/min) reduced AP duration measured at 20%, 50%, and 90% repolarization (APD20, APD50, and APD90) in the infarcted group, and increased APD20 and APD50 in the control group. Rising [Ca+]o (1.25 to 5.0 mmol/L) induced a decrease of APD20 and APD50 in both groups. Voltage clamp revealed a smaller I(Ca.L) density at approximately -17 mV in myocytes from infarcted ventricles (-1.86 +/- 0.37 vs -3.98 +/- 0.65 pA/pF, P < .05), and the appearance of a non-K+ outward current coupled to I(Ca.L). The results suggest the participation of a Ca+2-activated outward current in the repolarization of normal and infarcted rat ventricles.

Electrophysiological properties of ventricular muscle obtained from spontaneously diabetic mice

The electrophysiological properties of cardiac muscle in KK/Ta mouse (hereafter referred to as KK mouse), an animal model of human non-insulin-dependent diabetes mellitus, were investigated, and the findings compared with those obtained from a non-diabetic control mouse (C57BL/6J mouse; referred to as B6 mouse). The ages of the B6 mice were 23.9 +/- 5.4 weeks (n = 24) and those of the KK mice used were 25.7 +/- 10.8 weeks (n = 34). The KK mice had mild obesity, hyperglycemia and hyperinsulinemia. Ventricular muscles from both mice were examined by light microscopy. Partial myocardial fibrosis and filament disorder in the ventricular muscles were found only in the KK mice. The resting membrane potential of the ventricular muscle was less negative in the KK mice than in the control mice. The maximum rate of rise in the upstroke of the action potential was significantly decreased in the KK mice compared with that of the control mice. These suggest a decrease in a time-independent K+ current (IK1) in the KK mice. The duration of the action potential (APD) at all levels of repolarization was significantly longer in the KK mice than in the B6 mice. A blocker of transient outward current (I(to)), 4-aminopyridine, significantly prolonged the APD of the B6 mice, but failed to prolong it in the KK mice, suggesting that Ito in the diabetic mice is very small. A Ca2+ channel blocker, CoCl2, dramatically lengthened all levels of APD in both groups, suggesting that there is no difference between B6 mice and KK mice in L-type Ca2+ current via Ca2+ channels. These suggest the malfunction or deficiency of ionic channels which carry, at least Ito and IK1 in diabetic mice.

Relationship between transient outward K+ current and Ca2+ influx in rat cardiac myocytes of endo- and epicardial origin

1. The transient outward K+ current (Ito) is a major repolarizing ionic current in ventricular myocytes of several mammals. Recently it has been found that its magnitude depends on the origin of the myocyte and is regulated by a number of physiological and pathophysiological signals. 2. The relationship between the magnitude of Ito, action potential duration (APD) and Ca2+ influx (QCa) was studied in rat left ventricular myocytes of endo- and epicardial origin using whole-cell recordings and the action potential voltage-clamp method. 3. Under control conditions, in response to a depolarizing voltage step to +40 mV, Ito averaged 12.1 +/- 2.6 pA pF-1 in endocardial (n = 11) and 24.0 +/- 2.6 pA pF-1 in epicardial myocytes (n = 12; P < 0.01). APD90 (90 % repolarization) was twice as long in endocardial myocytes, whereas QCa inversely depended on the magnitude of Ito. L-type Ca2+ current density was similar in myocytes from both regions. 4. To determine the effects of controlled reductions of Ito on QCa, recordings were repeated in the presence of increasing concentrations of the Ito inhibitor 4-aminopyridine. 5. Inhibition of Ito by as little as 20 % more than doubled QCa in epicardial myocytes, whereas it had only a minor effect on QCa in myocytes of endocardial origin. Further inhibition of Ito led to a progressive increase in QCa in epicardial myocytes; at 90 % inhibition of Ito, QCa was four times larger than the control value. 6. We conclude that moderate changes in the magnitude of Ito strongly affect QCa primarily in epicardial regions. An alteration of Ito might therefore allow for a regional regulation of contractility during physiological and pathophysiological adaptations.

Age and gender differences in excitation-contraction coupling of the rat ventricle

1. The objective of this study was to determine potential post-pubertal gender-specific differences in the contractility of papillary muscles, the electrophysiological properties and Ca2+ transients of freshly dissociated ventricular myocytes from the rat heart. 2. The contractions of rat papillary muscles from 2- to 14-month-old male and female rats were studied under isometric and isotonic conditions (29 degrees C). While the hearts of young (2-4 months) male and female rats displayed a similar contractile profile, papillary muscles of female rats aged 6 months and older exhibited smaller isometric and isotonic contractions, smaller maximal rates of tension and shortening development and decline (+/-DT/dt and +/-DL/dt) velocities during both the onset and relaxation phases, and shorter contractions than age-matched males. 3. To explore the possible cellular basis accounting for these differences, action potentials and macroscopic currents were recorded from freshly dissociated myocytes using the whole-cell patch clamp technique (35 C). Action potentials from male and female myocytes of 3- and 9-month-old rats did not vary as a function of age or gender. Consistent with these results, the magnitude (expressed in pA pF-1), voltage-dependence and kinetics of the inward rectifier (IK1), transient outward (Ito) and sustained (IK) K+ currents displayed little, if any dependence on age or gender. 4. L-type Ca2+ current (ICa(L)) measured in caesium-loaded myocytes (35 C) from male and female rats of 3, 6 and 9 months of age exhibited similar characteristics. In contrast, while Ca2+ transients measured with indo-1 were similar between 3-month-old male and female rat myocytes, Ca2+ transients of 10-month-old female myocytes were significantly reduced and showed a diminished rate of relaxation in comparison with those recorded in male rats of similar age. 5. These results suggest that important gender-related changes in excitation-contraction coupling occur following puberty, probably due to differences in Ca2+ handling capabilities at the level of the sarcoplasmic reticulum.

Intracellular calcium and electrical restitution in mammalian cardiac cells

The role of calcium current and changes in intracellular calcium concentration ([Ca2+]i) in regulation of action potential duration (APD) during electrical restitution process was studied in mammalian ventricular preparations. Properly timed action potentials were recorded from multicellular preparations and isolated cardiomyocytes using conventional microelectrodes and EGTA-containing patch pipettes. APD increased monotonically in canine and guinea pig ventricular preparations with increasing diastolic interval (DI), while in rabbit papillary muscles the restitution process was biphasic: APD first lengthened, then shortened as the DI increased. When the restitution process was studied in single cardiomyocytes using EGTA-containing patch pipettes, the restitution pattern was similar in the three species studied. Similarly, no difference was observed in the recovery time constant of calcium current (/Ca-L) measured under these conditions in voltage clamped myocytes. Loading the myocytes with the [Ca2+]i-chelator BAPTA-AM had adverse effects in rabbit and canine cells. In rabbit myocytes steady-state APD lengthened and the late shortening component of restitution was abolished in BAPTA-loaded cells. In canine myocytes BAPTA-load shortened steady-state APD markedly, and during restitution, APD decreased with increasing DI. The late shortening component of restitution, observed in untreated rabbit preparations, was greatly reduced after nifedipine treatment, but remained preserved in the presence of 4-aminopyridine or nicorandil. Beat to beat changes in APD, peak/Ca-L and [Ca2+]i, measured using the fluorescent dye, Fura-2, were monitored in rabbit ventricular myocytes after a 1-min period of rest. In these cells, the shortening of APD was accompanied by a gradual reduction of the peak/Ca-L and elevation of diastolic [Ca2+]i during the initial eight post-rest action potentials. It is concluded that elevation of [Ca2+]i shortens, while reduction of [Ca2+]i lengthens APD in rabbit, but not in canine ventricular myocytes. These differences may probably be related to different distributions of [Ca2+]i-dependent ion currents and/or to differences in calcium handling between the two species.

ECG changes during furosemide-induced hypokalemia in the rat

Electrolyte abnormalities have become an increasingly important cause of arrhythmias owing to the widespread use of high-potency diuretics. Hypokalemia is one of the common complications of diuretic use. Although some studies of hypokalemia induced by furosemide as well as of potassium-deficient diets in the rat have been reported, the electrocardiographic (ECG) changes during hypokalemia in the rat are poorly understood. This study was designed to examine such changes. For this purpose, hypokalemia was induced by furosemide administration, and the diagnostic criteria for ECG manifestations of hypokalemia were determined. During hypokalemia, conduction in most parts of the heart was suppressed to an extent depending on plasma potassium concentration. Prolongation of the QT interval was also observed, which agrees with findings in humans and dogs. Furthermore, prolonged durations of the P wave and QRS complex were observed during hypokalemia in the rat. The extent of alteration of the PR interval induced by hypokalemia was less significant than that of P wave and QRS complex durations. These results suggest that the excitabilities of the myocardium in the atria and ventricles may be affected by extracellular potassium level rather than by the atrioventricular conduction system in the rat. Wave amplitude, except that of the P wave, was decreased by severe hypokalemia. These changes were not dependent on the plasma potassium concentration. Typical T wave changes observed with hypokalemia in humans and dogs did not occur in the rat. The ECG manifestations of acute hypokalemia in the rat did not include the typical T wave changes seen in species with ST-segment type ECGs; however, other ECG parameter changes occurring with hypokalemia were qualitatively similar to those in other species. These results may be useful for testing the toxicity of potassium-depleting drugs in the rat.

Ionic mechanism of the effects of hydrogen peroxide in rat ventricular myocytes

1. Whole-cell and amphotericin-perforated patch-clamp techniques have been used to study the effects of hydrogen peroxide (H2O2) on action potentials and underlying ionic currents in single myocytes from the ventricles of adult rat hearts. 2. The results obtained differed markedly depending on the recording method utilized. Conventional whole-cell recordings, in which the myoplasm is dialysed with the contents of the pipette, failed to show any significant effects of H2O2 on the action potential or cell shortening. In contrast, when action potentials were recorded with the amphotericin-perforated patch method, H2O2 (50-200 microM) produced a marked prolongation of the action potential and an increase in cell shortening. 3. Voltage-clamp recordings with the amphotericin-perforated patch method showed that H2O2 caused no significant changes in either the Ca(2+)-independent transient outward K+ current (Ito) or the inwardly rectifying K+ current (IK1). 4. Application of tetrodotoxin (TTX; 8 x 10(-6) M), a Na+ channel blocker, largely inhibited the effects of H2O2 on the action potential. Moreover, anthopleurin A (4 x 10 (-7) M), which augments Na+ current (INa) by slowing its inactivation, mimicked the effects of H2O2 on the action potential of ventricular myocytes. These effects on INa were also blocked almost completely by TTX. 5. The hypothesis that H2O2 can augment INa by slowing its kinetics of inactivation was tested directly using ensemble recordings from cell-attached macropatches. These results demonstrated a significant enhancement of late opening events when H2O2 (200 microM) was included in the recording pipette. A corresponding slowing of inactivation of the ensemble INa was observed. 6. The possibility that protein kinase C (PKC) is an intracellular second messenger for the observed effects of H2O2 was examined using the blocker bisindolylmaelimide (BIS; 10(-7) M). Bath application of BIS prior to H2O2 exposure significantly delayed and also attenuated the development of the action potential prolongation. 7. These results demonstrate marked electrophysiological effects of H2O2 in rat ventricle. The dependence of these effects on recording methods suggests involvement of an intracellular second messenger, and the results with the PKC inhibitor, BIS, support this possibility. The most prominent effect of H2O2 on the ionic currents which underlie the action potential is a slowing of inactivation of the TTX-sensitive INa. Recent molecular studies have demonstrated a PKC phosphorylation site on the rat cardiac Na+ channel isoform and have also shown that PKC activation can slow inactivation of INa.

Which cardiac potassium channel subtype is the preferable target for suppression of ventricular arrhythmias?

Prolongation of the cardiac action potential duration is the hallmark of Class III antiarrhythmic activity. Action potential duration prolongation may be achieved by several means: enhancement of inward current and, more commonly, blockade of one or more of the many outward currents that are carried by K+. However, it is far from clear whether blockade of one particular K+ channel is more efficacious than blockade of another. The objective of this review is to consider this question with particular reference to ischaemic heart disease, a condition for which effective prevention of ventricular arrhythmias continues to be sought.

The effect of extracellular Ca2+ concentration on the negative staircase of Ca2+ transient in field-stimulated rat ventricular cells

We performed experiments using the Ca2+ indicator dye, fura-2 to investigate the effect of extracellular Ca2+ concentration ([Ca2+]o) on sarcoplasmic reticulum (SR) Ca2+ release and loading in single rat ventricular cells. In normal Tyrode solution (1.8 mM [Ca2+]o) repetitive stimulation (0.5 Hz) resulted in a gradual decrease in calcium transients (the negative staircase phenomenon) without being accompanied by a gradual decrease in diastolic intracellular Ca2+ concentration. The rate of the slow decline in calcium transient was faster in lower [Ca2+]o. However, the peak of the first calcium transient was relatively invariant over a wide range of [Ca2+]o (0.5-5 mM). The size of the calcium transient elicited by field stimulation was proportional to that induced by 10 mM caffeine, applied following the field stimulation. These results suggest that the size of calcium transients depends mainly on the Ca2+ content of the SR. The quiescent period favoured the replenishment of the SR and this effect was promoted further by increasing the driving force for Ca2+ entry across the sarcolemma during this period. We conclude that in low [Ca2+]o, short stimulation interval may limit Ca2+ influx across the sarcolemma during the quiescent period to cause a gradual reduction in calcium content of the SR and thus the calcium transient.

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.

Influence of Bistramide A on the twitch tension in rat atrial heart muscle

Bistramide A, a new toxin isolated from the Urochordate Lissoclinum bistratum Sluiter, was applied to rat auricular heart muscle bundles. At a stimulation frequency of 0.2 Hz, the toxin induces a dose-dependent reduction of the stimulated twitch tension force; it decreases Vmax and shortens the duration of the plateau and the slow repolarizing phase of the action potential. In the control solution, switching from a stimulation frequency of 0.2 Hz to 1 Hz decreases the force with which a positive potentiation develops either at a maintained high frequency or after switching from 1 Hz to 0.2 Hz. Bistramide A reduces both the force evoked at 1 Hz and the potentiation. The data suggest that Bistramide A blocks Na+ conductance; inhibits Ca++ channels in a time- and frequency-dependent manner; reduces Na(+)-Ca++ exchange activity; but does not modify the ability of the sarcoplasmic reticulum to be refilled although the rate of Ca++ accumulation is decreased.

Quantitative analysis of potassium channel mRNA expression in atrial and ventricular muscle of rats

The expression of 15 different potassium channel genes in rat atrial and ventricular muscle was quantitatively compared by use of an RNase protection assay. Of these genes, only five, Kv1.2, Kv1.4, Kv1.5, Kv2.1, and Kv4.2, were expressed at significant levels in cardiac muscle. In comparisons of atrial and ventricular RNA samples, transcripts from the Kv1.2 and Kv4.2 genes showed the largest differences in relative abundance. There was an approximately twofold decrease in total Kv4 subfamily mRNA expression in atrial muscle relative to ventricular muscle and a 70% increase in total Kv1 subfamily mRNA. Variation of potassium channel mRNA expression within the left ventricular wall was also examined. There was a large gradient of Kv4.2 expression across the ventricular wall, and Kv4.2 expression in epicardial muscle was more than eight times higher than in papillary muscle. Other potassium channel genes were expressed at relatively uniform levels across the ventricular wall. The results suggest that transcriptional regulation makes a significant contribution to the control of potassium channel expression in cardiac muscle and to the variation of the electrophysiological phenotype of myocytes from different regions of the myocardium.

NPY-(18-36) and pertussis toxin distinguish between the negative and positive contractile effects of neuropeptide Y (NPY) in ventricular cardiomyocytes

The mechanism of action of NPY to produce contractions of adult rat ventricular myocytes was investigated. Positive effects were mediated through an L-type Ca2+ channel. Negative effects, linked through an inhibitory G-protein to a transient outward K+ channel, could be antagonised by NPY-(18-36).

Characterization of two distinct depolarization-activated K+ currents in isolated adult rat ventricular myocytes

Depolarization-activated outward K+ currents in isolated adult rat ventricular myocytes were characterized using the whole-cell variation of the patch-clamp recording technique. During brief depolarizations to potentials positive to -40 mV, Ca(2+)-independent outward K+ currents in these cells rise to a transient peak, followed by a slower decay to an apparent plateau. The analyses completed here reveal that the observed outward current waveforms result from the activation of two kinetically distinct voltage-dependent K+ currents: one that activates and inactivates rapidly, and one that activates and inactivates slowly, on membrane depolarization. These currents are referred to here as Ito (transient outward) and IK (delayed rectifier), respectively, because their properties are similar (although not identical) to these K+ current types in other cells. Although the voltage dependences of Ito and IK activation are similar, Ito activates approximately 10-fold and inactivates approximately 30-fold more rapidly than IK at all test potentials. In the composite current waveforms measured during brief depolarizations, therefore, the peak current predominantly reflects Ito, whereas IK is the primary determinant of the plateau. There are also marked differences in the voltage dependences of steady-state inactivation of these two K+ currents: IK undergoes steady-state inactivation at all potentials positive to -120 mV, and is 50% inactivated at -69 mV; Ito, in contrast, is insensitive to steady-state inactivation at membrane potentials negative to -50 mV. In addition, Ito recovers from steady-state inactivation faster than IK: at -90 mV, for example, approximately 70% recovery from the inactivation produced at -20 mV is observed within 20 ms for Ito; IK recovers approximately 25-fold more slowly. The pharmacological properties of Ito and IK are also distinct: 4-aminopyridine preferentially attenuates Ito, and tetraethylammonium suppresses predominantly IK. The voltage- and time-dependent properties of these currents are interpreted here in terms of a model in which Ito underlies the initial, rapid repolarization phase of the action potential (AP), and IK is responsible for the slower phase of AP repolarization back to the resting membrane potential, in adult rat ventricular myocytes.

Inward current generated by Na-Ca exchange during the action potential in single atrial cells of the rabbit

To investigate the underlying ionic mechanism of the late plateau phase of the action potential in rabbit atrium the whole-cell patch-clamp technique with intracellular perfusion was used. We recorded the inward current during repolarizations following a brief 2 ms depolarizing pulse to +40 mV from a holding potential of between -70 and -80 mV. The development of this current coincides with the onset of the late plateau phase of the action potential. Peak activation of the current occurs about 10 ms from the beginning of the depolarizing pulse, and it decays spontaneously with a slow timecourse. Its voltage dependency from -40 mV to +40 mV shows very steep activation (-40 to -20 mV) and shows almost the same maximum magnitude between -10 mV and +40 mV. This behaviour is quite different from that of the calcium current. The inward current and the late plateau phase of the action potential were both abolished by the application of 5 mM EGTA, 1 microM ryanodine and by reducing the Na+ gradient. The fully activated current-voltage relation of the inward current was plotted as the difference current before and after treatment with Ryanodine, Diltiazem, 20 mM Na+ inside or 30% Na+ outside and shows an exponential voltage dependence with the largest magnitude of the current occurring at negative potentials. The current-voltage (I-V) curve was well fitted by the Na-Ca exchange equation, i = A exp (-(1 - r)EF/RT). The results suggest that the inward current contributes to the generation of the late plateau phase of the rabbit atrial action potential, and is activated by intracellular calcium released from the sarcoplasmic reticulum. Sarcoplasmic reticulum calcium release appears to be triggered both by the membrane voltage and by the calcium current. It is concluded that the inward current is generated by Na-Ca exchange.

Calcium movements during each heart beat

Transsarcolemmal calcium movements are closely related to force generation in the heart. It is important to understand the transport pathways that control these movements of calcium across the sarcolemmal membrane. In the normal, beating heart, sodium-calcium exchange appears to be an important mechanism for the extrusion of calcium from the cell. The kinetics of this exchange are dependent upon the characteristics of the cell action potential. Calcium efflux via sodium-calcium exchange may be sufficient to balance calcium entry through calcium channels during the action potential.

The effects of calcium antagonists on calcium overload contractures in embryonic chick myocytes induced by ouabain and veratrine

1. The protective effects of some calcium antagonists against different forms of calcium overload contracture were investigated in embryonic chick cardiac myocytes. 2. Tetrodotoxin-sensitive sodium currents were recorded from the myocytes by the whole-cell voltage-clamp technique. Although the peak current was attenuated by veratrine, the inactivation process was markedly inhibited, resulting in a large increase in the total inward current. Action potentials were prolonged by veratrine, automaticity was inhibited and the membrane potential depolarized from -79 to around -45 mV. 3. Measurements of contraction were made from aggregates of myocytes using a video edge detection technique which quantified edge movement. Veratrine caused an initial positive inotropism then inhibited automaticity of aggregates with subsequent development of a tonic contracture to around 300% of the twitch contraction. 4. Veratrine-induced contractures were not significantly affected by 10 microM diltiazem or verapamil. Nifedipine (5 microM), nimodipine (5 microM) and ryanodine (5 microM) also had little effect whilst nicardipine and flunarizine caused a concentration-dependent inhibition of veratrine-induced contractures with IC50s of 3 microM and 2 microM respectively. 5. Veratrine-induced contractures were found to be very sensitive to extracellular calcium concentration with an EC50 of 32 microM. Edge movement associated with beating of the myocytes was much less sensitive to calcium (EC50 = 1 mM). Submaximal veratrine contractures in 20-50 microM extracellular calcium were not potentiated by 1 microM Bay K 8644. 6. Tetrodotoxin also inhibited veratrine-induced contractures but did not affect contractions induced by ouabain in the presence of 10 microM diltiazem. 7. Ouabain-induced contractures were also inhibited by nicardipine and flunarizine indicating that these drugs can protect against calcium overload in embryonic chick heart by a mechanism independent of the normal form of voltage-sensitive sodium or calcium channels.

Sodium-calcium exchange during the action potential in guinea-pig ventricular cells

1. Slow inward tail currents attributable to electrogenic sodium-calcium exchange can be recorded by imposing hyperpolarizing voltage clamp pulses during the normal action potential of isolated guinea-pig ventricular cells. The hyperpolarizations return the membrane to the resting potential (between -65 and -88 m V) allowing an inward current to be recorded. This current usually has peak amplitude when repolarization is imposed during the first 50 ms after the action potential upstroke, but becomes negligible once the final phase of repolarization is reached. The envelope of peak current tail amplitudes strongly resembles that of the intracellular calcium transient recorded in other studies. 2. Repetitive stimulation producing normal action potentials at a frequency of 2 Hz progressively augments the tail current recorded immediately after the stimulus train. Conversely, if each action potential is prematurely terminated at 0.1 Hz, repetitive stimulation produces a tail current much smaller than the control value. The control amplitude of inward current is only maintained if interrupted action potentials are separated by at least one full 'repriming' action potential. These effects mimic those on cell contraction (Arlock & Wohlfart, 1986) and suggest that progressive changes in tail current are controlled by variations in the amplitude and time course of the intracellular calcium transient. 3. When intracellular calcium is buffered sufficiently to abolish contraction, the tail current is abolished. Substitution of calcium with strontium greatly reduces the tail current. 4. The inward tail current can also be recorded at more positive membrane potentials using standard voltage clamp pulse protocols. In this way it was found that temperature has a large effect on the tail current, which can change from net inward at 22 degrees C to net outward at 37 degrees C. The largest inward currents are usually recorded at about 30 degrees C. It is shown that this effect is attributable predominantly to the temperature sensitivity of activation of the delayed potassium current, iK, whose decay can then mask the slow tail current at high temperatures. 5. Studies of the relationship between the tail current and the membrane calcium current, iCa, have been performed using a method of drug application which is capable of perturbing iCa in a very rapid and highly reversible manner. Partial block of iCa with cadmium does not initially alter the size of the associated inward current tail. When iCa is increased by applying isoprenaline, the percentage augmentation of the associated tail current is much greater but occurs more slowly.(ABSTRACT TRUNCATED AT 400 WORDS)

The effect of strontium on the drug-receptor interaction on cholinergic drugs

The effect of replacing calcium with strontium in the perfusion fluid was qualitatively and quantitatively studied in the isolated longitudinal muscle of the guinea-pig ileum. In the presence of strontium hyoscine could be considered a competitive antagonist of acetylcholine for the acetylcholine receptor of the longitudinal muscle of the guinea-pig ileum; dibenamine still blocked this receptor in an irreversible way. The equilibrium constants for acetylcholine (KA) and hyoscine (KI) were obtained in the presence of calcium (KA = 3.16 +/- 0.63 microM; KI = 0.38 +/- 0.07 nM), and strontium (KA = 7.00 +/- 0.89 microM; KI = 0.93 +/- 0.16 nM). The results show a decrease in the affinity of both drugs for the muscarinic receptor in the presence of strontium.

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.

Effect of ischemia on calcium-dependent fluorescence transients in rabbit hearts containing indo 1. Correlation with monophasic action potentials and contraction

The effects of acute global ischemia on cytosolic calcium transients were studied in perfused rabbit hearts loaded with the fluorescent calcium indicator indo 1. Indo 1-loaded hearts were illuminated at 360 nm, and fluorescence was recorded simultaneously at 400 and 550 nm from the epicardial surface of the left ventricle. The F400/F550 ratio was calculated by an analog circuit, which allowed cancellation of optical motion artifact. Resulting calcium transients demonstrated a rapid upstroke and slow decay similar to those recorded in isolated ventricular myocytes. Global ischemia rapidly suppressed contraction, but it produced a concurrent increase in the systolic and diastolic levels of the calcium transients, together with an increase in the duration of the peak. The effects of ischemia were reversed by reperfusion, inhibited by verapamil, and mimicked by perfusion of nonischemic hearts with acidified (CO2-rich) solution. In addition to elevation of the calcium transients, ischemia caused a pattern of intracellular calcium alternans that was discernible after 2-3 minutes. The pattern of alternans was stable at a given epicardial site, but it could be out of phase at different sites. Similar nonuniformities were observed in contraction strength and in the duration of monophasic action potentials recorded immediately adjacent to the fiber-optic probe. Abnormalities in intracellular calcium may be a causal factor in the loss of electrical and mechanical synchrony in the acutely ischemic heart.

Calcium-activated inward current and contraction in rat and guinea-pig ventricular myocytes

1. Single ventricular cells from rat and guinea-pig hearts were voltage clamped, and contraction was monitored with an optical method. 2. In rat cells, short (2-10 ms) depolarizing pulses to 0 mV from a holding potential of -40 mV evoked current carried by calcium, and on repolarization to -40 mV there was a slow 'tail' current which decayed much more slowly than the expected deactivation of calcium current at this potential. 3. When rat cells were loaded with EGTA diffusing into the cytosol from an intracellular electrode, contraction and the tail current were both abolished, whereas the peak calcium current was not reduced. 4. Exposure of rat cells to ryanodine (1-2 microM) suppressed both contraction and the tail current, but not peak calcium current. 5. The tail current was unaffected by tetrodotoxin (10 microM), but was reduced by lowering extracellular sodium to 10% by replacement with lithium or choline. 6. In rat cells, exposure to nifedipine (1-5 microM) initially caused a marked reduction of calcium current while substantial contraction and tail current remained; longer exposure to nifedipine suppressed both contraction and the tail current. Isoprenaline (50-100 nM) caused a marked increase in peak calcium current, while under these conditions there was little or no increase in either contraction or tail current. 7. The amplitude of the tail current in rat cells varied with the duration of the depolarization at 0 mV; the tail current evoked by repolarization to -40 mV reached a peak just as contraction was beginning to develop and was back to undetectable levels just as relaxation became significant, as might be expected if the tail current were determined by the cytosolic calcium transient which triggered contraction. 8. In guinea-pig cells, a tail current was also recorded on repolarization to a holding potential of -40 mV, and, as in rat cells, the tail was suppressed by cytosolic EGTA and reduced by exposure of the cells to low-sodium solution. 9. It is concluded that the tail currents recorded in both rat and guinea-pig cells represent current activated by a rise in cytosolic calcium; in rat cells this is markedly dependent on ryanodine-sensitive release of calcium from internal stores. The origin of this current, and its possible role during the plateaux of action potentials are discussed.

Calcium- and voltage-activated plateau currents of cardiac Purkinje fibers

We have used the two-microelectrode voltage-clamp technique to investigate the components of membrane current that contribute to the formation of the early part of the plateau phase of the action potential of calf cardiac Purkinje fibers. 3,4-Diaminopyridine (50 microM) reduced the net transient outward current elicited by depolarizations to potentials positive to -30 mV but had no consistent effect on contraction. We attribute this effect to the blockade of a voltage-activated transient potassium current component. Ryanodine (1 microM), an inhibitor of sarcoplasmic reticulum calcium release and intracellular calcium oscillations in Purkinje fibers (Sutko, J.L., and J.L. Kenyon. 1983. Journal of General Physiology. 82:385-404), had complex effects on membrane currents as it abolished phasic contractions. At early times during a depolarization (5-30 ms), ryanodine reduced the net outward current. We attribute this effect to the loss of a component of calcium-activated potassium current caused by the inhibition of sarcoplasmic reticulum calcium release and the intracellular calcium transient. At later times during a depolarization (50-200 ms), ryanodine increased the net outward current. This effect was not seen in low-sodium solutions and we could not observe a reversal potential over a voltage range of -100 to +75 mV. These data suggest that the effect of ryanodine on the late membrane current is attributable to the loss of sodium-calcium exchange current caused by the inhibition of sarcoplasmic reticulum calcium release and the intracellular calcium transient. Neither effect of ryanodine was dependent on chloride ions, which suggests that chloride ions do not carry the ryanodine-sensitive current components. Strontium (2.7 mM replacing calcium) and caffeine (10 mM), two other treatments that interfere with sarcoplasmic reticulum function, had effects in common with ryanodine. This supports the hypothesis that the effects of ryanodine may be attributed to the inhibition of sarcoplasmic reticulum calcium release.

Electrophysiological effects of Ca antagonists, tetrodotoxin, [Ca]o and [Na]o on myocardium of hibernating chipmunks: possible involvement of Na-Ca exchange mechanism

The electrophysiological performance of myocardium of hibernating chipmunks was investigated in the presence of Ca antagonists and tetrodotoxin, and the effects of high [Ca]o and low [Na]o were examined. The action potential of the preparations was characterized by the low amplitude of the plateau phase (APp). Ca antagonists, nifedipine (10(-6) M) and nitrendipine (2 X 10(-6) M), did not significantly inhibit this APp or the contraction. These nifedipine-insensitive electromechanical responses were completely abolished by an internal Ca release inhibitor, ryanodine. Both increasing [Ca]o and lowering [Na]o, by replacing Na by lithium or choline, also inhibited APp. Tetrodotoxin (10(-5) M) which markedly inhibited the initial rapid phase of the action potential slightly affected APp. These results suggest that the plateau potential of the present preparations is controlled by a process linked to Ca release from internal stores, most likely the Na-Ca exchange mechanism.

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

Action potentials and membrane currents were recorded from isolated single ventricular cells from rabbit hearts using the suction pipette whole-cell clamp method. Action potentials elicited after short diastolic intervals of less than 2 seconds showed an increase and prolongation of the plateau compared to those elicited after a 10-second rest period. The recovery of the tetrodotoxin-insensitive secondary inward current revealed a transient increase at short diastolic intervals above the level of full recovery (after 10 seconds). The increased secondary inward current recovery, however, was voltage-dependent, and the period of its increase did not cover the entire diastolic intervals of the action potential overshoots, suggesting the contribution of another ionic current to the changes in potential. During depolarizing voltage steps, from + to -20 mV, a rapid activating and then inactivating outward current was elicited, which overlapped the calcium current. This outward current exhibited time- and voltage-dependent properties similar to those of the transient outward current in Purkinje and other cardiac preparations. The recovery of the transient outward current was slow, achieving only 75% of its full level at 2 seconds, whereas the same level of calcium current recovery was achieved at 200 milliseconds. The application of 4-aminopyridine suppressed most of the transient outward current, and the rest of the current was abolished by caffeine or Co2+. The 4-aminopyridine sensitive transient outward current exhibited slow recovery kinetics compared to those of the other or calcium current, and its inhibition caused elimination of the augmented plateau during electrical restitution. The application of verapamil or Co2+ for inhibition of secondary inward current also abolished the action potential overshoot. These results indicate that an increase and prolongation of the plateau at short diastolic intervals are produced by the slower recovery from inactivation in the 4-aminopyridine-sensitive transient outward current than that in the calcium current.

Electrophysiological analysis of the sensitivity to calcium in ventricular muscle from alloxan diabetic rats

The effects of acute and chronic alloxan diabetes on the transmembrane electrical activity of rat heart papillary muscle were investigated. The action potential duration (APD) appeared markedly prolonged in all diabetic papillary muscles, as compared to normal. This prolongation of ADP, with no difference in the resting potential (RP), resulted from both a lengthening of the complex time course plateau and a slower rate of repolarisation. APD0 (at 0 mV) and APD10 (+10 mV from RP) increased, respectively, an average of 50% and 24% in the acute, and 72% and 98% in the chronic diabetics as compared to control, whereas Vmax and overshoot (OS) were unchanged. Varying [Ca]o between 0.5 and 3.5 mM did not induce any change in the RP of either control or diabetic papillary muscles. Conversely, there were differences, within and between groups, in the amplitude of the OS and in Vmax, depending on the [Ca]o concentration. In particular, OS and Vmax of acute diabetics were markedly reduced at 1.5 mM. This reduction was maintained at concentrations of [Ca]o lower than 1.5, attesting to the greater sensitivity of both acutely and chronically diabetic muscles to a decrease in external calcium. Cd, a Ca-channel blocker, reduced in diabetics the duration of both the complex plateau and the repolarisation phase, suggesting that a Ca inward current was maintained throughout these two phases. Direct evidence for elucidating the mechanism(s) of the observed APD change in diabetics will be obtained only by transmembrane current analysis.

A transient outward current in isolated cells from the crista terminalis of rabbit heart

Voltage-clamp experiments were carried out with the objective of identifying and characterizing the time- and voltage-dependent properties of a transient outward current recorded in single myocytes from the crista terminalis region of the rabbit heart. A collagenase enzymic dispersion procedure similar to that described by Desilets & Horackova (1982) was used to obtain these viable individual myocytes. Transmembrane ionic currents were recorded using a single micro-electrode voltage-clamp technique. In experiments aimed at studying a tetrodotoxin-resistant transient inward current, (ICa); a transient outward current was consistently recorded following blockade of ICa with Cd2+ (5 X 10(-4) M). The time and voltage dependence of the activation and inactivation of this current were measured. Its steady-state inactivation curve spans the voltage range -70 to -10 mV, and it is activated between -20 and +10 mV. The reversal potential of this transient outward current is approximately -75 mV in [K+]O 5 mM, suggesting that it is carried mainly by K+. This transient outward current can be inhibited completely by external application of 4-aminopyridine (4-AP, 3 mM). The time- and voltage-dependent properties, the reversal potential, and the sensitivity to 4-AP of this transient outward current are all very similar to those of a transient outward current first identified in molluscan neurones. Hence, we have labelled it, IA. Selective inhibition of IA and knowledge of its voltage- and time-dependent properties yield specific predictions concerning its role in the action potential of isolated crista terminalis cells. Consistent with these predictions, a decrease in stimulus rate is found to decrease the duration of the action potential and vice versa; and application of effective doses of 4-AP results in a substantial lengthening of the action potential. These results are discussed in terms of the possible physiological role of IA in subsidiary or follower pace-maker tissue, and the anatomical and physiological heterogeneity of the sino-atrial node region of the rabbit heart.

Influence of a change in stimulation rate on action potentials, currents and contractions in rat ventricular cells

The effects of a change in stimulation rate on electrical activity and accompanying contraction were investigated in ventricular cells isolated from rat heart; the cells were stimulated to contract either by brief depolarization pulses which evoked action potentials, or, under voltage-clamp conditions, by step depolarizations. An increase in stimulation rate from 0.3 to 3 Hz resulted in a gradual reduction in the amplitude of contraction and attenuation of the late phase of the action potential. These changes were less marked at more depolarized potentials. The ventricular cells were voltage clamped at -40 mV and initially stimulated at 0.3 Hz by step depolarizations to 0 mV for 10 or 100 ms, which activated the second inward current (Isi) and an accompanying contraction. The amplitude and time course of contraction were similar with the two pulse durations. When the duration of the depolarization was 100 ms, an increase in stimulation rate to 3 Hz caused a gradual decline in the amplitude of Isi and of the evoked contraction; at the same time extra contractions and small, transient inward currents appeared in addition to the evoked contractions and Isis. There was a reduction in the early component of decay of Isi at 3 Hz. With a depolarizing pulse duration of 10 ms, an increase in stimulation rate to 3 or to 4.2 Hz did not change the amplitude of the evoked Isi or contraction and no extra contractions or currents appeared. Intracellular EGTA abolished all contractions in the cells and an increase in the rate of stimulation with 100 ms pulses did not then induce transient inward currents. There was some decrease in the Isi amplitude but this was not as marked as in the absence of EGTA and the time course of current decay was similar at the two rates. Ryanodine prevented the appearance of extra contractions and currents when the stimulation rate was increased to 3 Hz and, as in the presence of intracellular EGTA, there was a small decrease in Isi amplitude while the time course of decay was similar at the two stimulation rates. The time course of recovery of Isi from inactivation, as shown by a double-pulse procedure, was altered when the duration of the first pulse was reduced from 100 to 10 ms, an extra inactivation of Isi being seen at pulse intervals of 20-100 ms. This extra component of inactivation was not seen with intracellular EGTA or in the presence of ryanodine.(ABSTRACT TRUNCATED AT 400 WORDS)

The effects of ryanodine, EGTA and low-sodium on action potentials in rat and guinea-pig ventricular myocytes: evidence for two inward currents during the plateau

Action potentials were recorded from single cells isolated from rat and guinea-pig ventricular muscle. In rat cells the repolarization showed two distinct phases, referred to as the early and late phases. In guinea-pig cells there was a maintained plateau. Reducing external sodium by replacement with lithium or choline suppressed the late phase of the action potential in rat cells, and shortened the plateau of the action potential in guinea-pig cells. Intracellular EGTA abolished contraction while suppressing the late phase of the action potential in rat cells, and shortening the plateau in guinea-pig cells. Ryanodine (1 microM), which is thought to inhibit the release of calcium from internal stores, suppressed contraction and the late phase of the action potential in rat cells. In guinea-pig cells, there was no substantial effect of ryanodine (1 microM) on either contraction or the time course of the action potential. The late phase of the action potential in rat cells was suppressed by increasing the external potassium concentration to 12 mM, and enhanced by reducing external potassium to 1.2 mM. It is concluded that an inward current activated by internal calcium contributes to the late phase of the action potential in rat cells, and to the plateau in guinea-pig cells. Two possibilities are a current arising from electrogenic sodium-calcium exchange, and a current through ion channels activated by calcium. The effects of reducing external sodium would be consistent with either mechanism. The contribution of such an inward current would be expected to be modified by outward currents through a rectifying potassium conductance which varies with external potassium concentration. In the rat, but not the guinea-pig, the rise in internal calcium which activates the inward current seems to be largely dependent on ryanodine-sensitive release of calcium from internal stores.

Electrophysiological properties of isolated ventricular myocytes

There is no doubt that major advances have been made in cardiac electrophysiology using single heart cells. Accurate analysis of rapid inward sodium current and the second inward current carried by calcium has resulted in a major re-examination of the steps involved in excitation-contraction coupling in the heart. Future work using glass microelectrodes, suction pipettes and patch pipettes will yield a vast amount of information highly relevant to mechanisms involved in the initiation of the heartbeat.