Changes in ventricular repolarization during acidosis and low-flow ischemia

Cardiac repolarisation indices are associated with oxygen consumption during maximal exercise test in highly-trained cross-country skiers

The objective of this study was to test the vectorelectrocardiographic T-wave characteristics for their associations with oxygen consumption (VO2) and physical performance during a maximal cardiopulmonary exercise test (CPET) in highly trained cross-country skiers. Male highly trained cross-country skiers (n = 30) performed the maximal CPET on the bicycle ergospirometric "Oxycon Pro" system with simultaneous oxygen consumption (VO2) and electrocardiogram recording. The measurements were done at rest; the stage preceding anaerobic threshold (preAnT); peak load; and recovery. The anaerobic threshold was estimated by respiratory exchange ratio. Physical performance was estimated by maximal oxygen consumption (VO2max/kg). VECG characteristics were calculated using Kors transformation procedure. During the test, the magnitudes of T-vector, Tx and Ty components decreased until preAnT, then stayed relatively stable until peak load, and reversed during recovery. In univariate linear regression analysis, T-vector amplitude and Tx, Ty and Tz magnitudes were associated with VO2/kg during the test (p < 0.010). The baseline T-vector characteristics were not associated with physical performance. At the preAnT stage, Tx and T-vector amplitude were associated with VO2max/kg (RC 12.70, 95% CI 0.68-24.73, p = 0.039 and RC 10.64, 95% CI 1.62-19.67, p = 0.023, respectively).

Metabolic Acidosis of Chronic Kidney Disease and Cardiovascular Disorders

The burden of chronic diseases, which include both cardiovascular diseases (CVD) and chronic kidney disease (CKD), is constantly growing worldwide. Moreover, bidirectional links between kidney and heart disorders are commonly recognized and the pathogenesis of these interactions is a matter of current interest in medicine. One remarkable aspect, extensively showed by epidemiological studies, is the very high prevalence of CVD in patients with CKD, up to thirty times higher than in the general population. Since the traditional cardiovascular risk factors cannot solely account for this difference, numerous abnormalities due to the decline in glomerular filtration rate were hypothesized to be involved as non-traditional risk factors for CVD. Among them, the metabolic acidosis frequently seen in advanced CKD was studied, but conflicting results were reported. Therefore, we intend to briefly summarize the current knowledge and points of controversy regarding the possible influence of CKD-related chronic metabolic acidosis on cardiovascular diseases.

Metformin prevents ischaemic ventricular fibrillation in metabolically normal pigs

AIMS/HYPOTHESIS

Metformin is the drug most often used to treat type 2 diabetes. Evidence suggests that metformin may reduce mortality of individuals with type 2 diabetes, but the mechanism of such an effect is unknown and outcomes of metformin treatment in people without diabetes have not been determined. If metformin favourably affected mortality of non-diabetic individuals, it might have even broader therapeutic utility. We evaluated the effect of metformin on myocardial energetics and ischaemic ventricular fibrillation (VF) in metabolically normal pigs.

METHODS

Domestic farm pigs were treated with metformin (30 mg kg-1 day-1 orally for 2-3 weeks; n = 36) or received no treatment (n = 37). Under anaesthesia, pigs underwent up to 90 min low-flow regional myocardial ischaemia followed by 45 min of reperfusion. Pigs were monitored for arrhythmia, monophasic action potential morphology, haemodynamics and myocardial substrate utilisation, AMP-activated protein kinase (AMPK) phosphorylation activity and ATP concentration.

RESULTS

Death due to VF occurred in 12% of pigs treated with metformin compared with 50% of untreated controls (p = 0.03). The anti-fibrillatory effect of metformin was associated with attenuation of action potential shortening in ischaemic myocardium (p = 0.02) and attenuation of the difference in action potential duration between ischaemic and non-ischaemic regions (p < 0.001) compared with untreated controls. Metformin had no effect on myocardial contractile function, oxygen consumption, or glucose or lactate utilisation. During ischaemia, however, metformin treatment amplified the activation of AMPK and preserved ATP concentration in myocardium compared with untreated controls (each p < 0.05).

CONCLUSIONS/INTERPRETATION

Chronic treatment of metabolically normal pigs with metformin at a clinically relevant dose reduces mortality from ischaemic VF. This protection is associated with preservation of myocardial energetics during ischaemia. Maintenance of myocardial ATP concentration during ischaemia is likely to prevent action potential shortening, heterogeneity of repolarisation, and propensity for lethal arrhythmia. The findings suggest that metformin might be protective in non-diabetic individuals with coronary heart disease.

Mechanisms Underlying the Emergence of Post-acidosis Arrhythmia at the Tissue Level: A Theoretical Study

Acidosis has complex electrophysiological effects, which are associated with a high recurrence of ventricular arrhythmias. Through multi-scale cardiac computer modeling, this study investigated the mechanisms underlying the emergence of post-acidosis arrhythmia at the tissue level. In simulations, ten Tusscher-Panfilov ventricular model was modified to incorporate various data on acidosis-induced alterations of cellular electrophysiology and intercellular electrical coupling. The single cell models were incorporated into multicellular one-dimensional (1D) fiber and 2D sheet tissue models. Electrophysiological effects were quantified as changes of action potential profile, sink-source interactions of fiber tissue, and the vulnerability of tissue to the genesis of unidirectional conduction that led to initiation of re-entry. It was shown that acidosis-induced sarcoplasmic reticulum (SR) calcium load contributed to delayed afterdepolarizations (DADs) in single cells. These DADs may be synchronized to overcome the source-sink mismatch arising from intercellular electrotonic coupling, and produce a premature ventricular complex (PVC) at the tissue level. The PVC conduction can be unidirectionally blocked in the transmural ventricular wall with altered electrical heterogeneity, resulting in the genesis of re-entry. In conclusion, altered source-sink interactions and electrical heterogeneity due to acidosis-induced cellular electrophysiological alterations may increase susceptibility to post-acidosis ventricular arrhythmias.

Effects of pH on nifekalant-induced electrophysiological change assessed in the Langendorff heart model of guinea pigs

Since information regarding the effects of pH on the extent of nifekalant-induced repolarization delay and torsades de pointes remains limited, we assessed it with a Langendorff heart model of guinea pigs. First, we investigated the effects of pH change from 7.4 to 6.4 on the bipolar electrogram simulating surface lead II ECG, monophasic action potential (MAP), effective refractory period (ERP), and terminal repolarization period (TRP) and found that acidic condition transiently enhanced the ventricular repolarization. Next, we investigated the effects of pH change from 6.4 to 7.4 in the presence of nifekalant (10 μM) on the ECG, MAP, ERP, TRP, and short-term variability (STV) of MAP90 and found that the normalization of pH prolonged the MAP90 and ERP while the TRP remained unchanged, suggesting the increase in electrical vulnerability of the ventricle. Meanwhile, the STV of MAP90 was the largest at pH 6.4 in the presence of nifekalant, indicating the increase in temporal dispersion of repolarization, which gradually decreased with the return of pH to 7.4.Thus, a recovery period from acidosis might be more dangerous than during the acidosis, because electrical vulnerability may significantly increase for this period while temporal dispersion of repolarization remained increased.

Role of CaMKII in post acidosis arrhythmias: a simulation study using a human myocyte model

Postacidotic arrhythmias have been associated to increased sarcoplasmic reticulum (SR) Ca(2+) load and Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) activation. However, the molecular mechanisms underlying these arrhythmias are still unclear. To better understand this process, acidosis produced by CO2 increase from 5% to 30%, resulting in intracellular pH (pHi) change from 7.15 to 6.7, was incorporated into a myocyte model of excitation-contraction coupling and contractility, including acidotic inhibition of L-type Ca(2+) channel (I(CaL)), Na(+)-Ca(2+) exchanger, Ca(2+) release through the SR ryanodine receptor (RyR2) (I(rel)), Ca(2+) reuptake by the SR Ca(2+) ATPase2a (I(up)), Na(+)-K(+) pump, K(+) efflux through the inward rectifier K(+) channel and the transient outward K(+) flow (I(to)) together with increased activity of the Na(+)-H(+) exchanger (I(NHE)). Simulated CaMKII regulation affecting I(rel), I(up), I(CaL), I(NHE) and I(to) was introduced in the model to partially compensate the acidosis outcome. Late Na(+) current increase by CaMKII was also incorporated. Using this scheme and assuming that diastolic Ca(2+) leak through the RyR2 was modulated by the resting state of this channel and the difference between SR and dyadic cleft [Ca(2+)], postacidotic delayed after depolarizations (DADs) were triggered upon returning to normal pHi after 6 min acidosis. The model showed that DADs depend on SR Ca(2+) load and on increased Ca(2+) leak through RyR2. This postacidotic arrhythmogenic pattern relies mainly on CaMKII effect on I(CaL) and I(up), since its individual elimination produced the highest DAD reduction. The model further revealed that during the return to normal pHi, DADs are fully determined by SR Ca(2+) load at the end of acidosis. Thereafter, DADs are maintained by SR Ca(2+) reloading by Ca(2+) influx through the reverse NCX mode during the time period in which [Na(+)]i is elevated.

hERG K+ Channels: Structure, Function, and Clinical Significance

The human ether-a-go-go related gene (hERG) encodes the pore-forming subunit of the rapid component of the delayed rectifier K+ channel, Kv11.1, which are expressed in the heart, various brain regions, smooth muscle cells, endocrine cells, and a wide range of tumor cell lines. However, it is the role that Kv11.1 channels play in the heart that has been best characterized, for two main reasons. First, it is the gene product involved in chromosome 7-associated long QT syndrome (LQTS), an inherited disorder associated with a markedly increased risk of ventricular arrhythmias and sudden cardiac death. Second, blockade of Kv11.1, by a wide range of prescription medications, causes drug-induced QT prolongation with an increase in risk of sudden cardiac arrest. In the first part of this review, the properties of Kv11.1 channels, including biogenesis, trafficking, gating, and pharmacology are discussed, while the second part focuses on the pathophysiology of Kv11.1 channels.

Slowing of Electrical Activity in Ventricular Fibrillation is Not Associated with Increased Defibrillation Energies in the Isolated Rabbit Heart

Prolonged out-of-hospital ventricular fibrillation (VF) arrests are associated with reduced ECG dominant frequency (DF) and diminished defibrillation success. Partial reversal of ischemia increases ECG DF and improves defibrillation outcome. We have investigated the metabolic components of ischemia responsible for the decline in ECG DF and defibrillation success. Isolated Langendorff-perfused rabbit hearts were loaded with the voltage-sensitive dye RH237. Using a photodiode array, epicardial membrane potentials were recorded at 252 sites (15 mm × 15 mm) on the anterior surface of the left and right ventricles. Simultaneously, a global ECG was recorded. VF was induced by burst pacing, and after 60s, perfusion was either reduced to 6 ml/min or the perfusate composition changed to impose hypoxia (95% N(2)/5% CO(2)), pH 6.7 (80% O(2)/20% CO(2)), or hyperkalemia (8 mM). Using fast Fourier transform, power spectra were created from the optical signals and the global ECG. The optical power spectra were summated to give a global power spectrum (pseudoECG). At 600 s the minimum defibrillation voltage (MDV) was determined by step-up protocol. During VF, the ECG and pseudoECG DF were reduced by low-flow ischemia (9.0 ± 1.0 Hz, p < 0.01, n = 5) and raised [K(+)](o) (12.2 ± 1.3 Hz, p < 0.05, n = 7) compared to control (19.2 ± 1.5 Hz, n = 20), but were unaffected by acidic pH(o) (16.7 ± 1.1 Hz, n = 11) and hypoxia (14.0 ± 1.2 Hz, n = 10). In contrast, the MDV was raised by acidic pH (156.1 ± 26.4 V, p < 0.001) and hypoxia (154.1 ± 22.1 V, p < 0.01) compared to control (65.6 ± 2.3 V), but comparable changes were not observed in low-flow ischemia (61.0 ± 0.5 V) or raised [K(+)](o) (56 ± 3 V). In summary, different metabolites are responsible for the reduction in DF and the increase in defibrillation energy during ischemic VF.

Regulation of the voltage-insensitive step of HERG activation by extracellular pH

Human ether-à-go-go-related gene (HERG, Kv11.1, KCNH2) voltage-gated K(+) channels dominate cardiac action potential repolarization. In addition, HERG channels play a role in neuronal and smooth cell excitability as well as cancer pathology. Extracellular pH (pH(o)) is modified during myocardial ischemia, inflammation, and respiratory alkalosis, so understanding the response of HERG channels to changes in pH is of clinical significance. The relationship between pH(o) and HERG channel gating appears complex. Acidification has previously been reported to speed, slow, or have no effect on activation. We therefore undertook comprehensive analysis of the effect of pH(o) on HERG channel activation. HERG channels have unique and complex activation gating characteristics with both voltage-sensitive and voltage-insensitive steps in the activation pathway. Acidosis decreased the activation rate, suppressed peak current, and altered the sigmoidicity of gating near threshold potentials. At positive voltages, where the voltage-insensitive transition is rate limiting, pH(o) modified the voltage-insensitive step with a pK(a) similar to that of histidine. Hill coefficient analysis was incompatible with a coefficient of 1 but was well described by a Hill coefficient of 4. We derived a pH(o)-sensitive term for a five-state Markov model of HERG channel gating. This model demonstrates the mechanism of pH(o) sensitivity in HERG channel activation. Our experimental data and mathematical model demonstrate that the pH(o) sensitivity of HERG channel activation is dominated by the pH(o) sensitivity of the voltage-insensitive step, in a fashion that is compatible with the presence of at least one proton-binding site on each subunit of the channel tetramer.

Arrhythmogenic actions of the Ca2+ channel agonist FPL-64716 in Langendorff-perfused murine hearts

The experiments explored the extent to which alterations in L-type Ca(2+) channel-mediated Ca(2+) entry triggers Ca(2+)-mediated arrhythmogenesis in Langendorff-perfused murine hearts through use of the specific L-type Ca(2+) channel modulator FPL-64716 (FPL). Introduction of FPL (1 microm) resulted in a gradual development (>10 min) of diastolic electrical events and alternans in spontaneously beating hearts from which monophasic action potentials were recorded. In regularly paced hearts, they additionally led to non-sustained and sustained ventricular tachycardia (nsVT and sVT). Programmed electrical stimulation (PES) resulted in nsVT and sVT after 5-10 and >10 min perfusion, respectively. Pretreatments with nifedipine, diltiazem and cyclopiazonic acid abolished arrhythmogenic tendency induced by subsequent introduction of FPL, consistent with its dependence upon both extracellular Ca(2+) entry and the degree of filling of the sarcoplasmic reticular Ca(2+) store. Values for action potential duration at 90% repolarization when any of these agents were applied to FPL-treated hearts became indistinguishable from those shown by untreated control hearts, in contrast to earlier reports of their altering in long QT syndrome type 3 and hypokalaemic murine models for re-entrant arrhythmogenesis. These arrhythmic effects instead correlated with alterations in Ca(2+) homeostasis at the single-cell level found in investigations of the effects of both FPL and the same agents in regularly stimulated fluo-3 loaded myocytes. These findings are compatible with a prolonged extracellular Ca(2+) entry that potentially results in an intracellular Ca(2+) overload and produces the cardiac arrhythmogenecity following addition of FPL.

Increased intracellular Ca2+ and SR Ca2+ load contribute to arrhythmias after acidosis in rat heart. Role of Ca2+/calmodulin-dependent protein kinase II

Returning to normal pH after acidosis, similar to reperfusion after ischemia, is prone to arrhythmias. The type and mechanisms of these arrhythmias have never been explored and were the aim of the present work. Langendorff-perfused rat/mice hearts and rat-isolated myocytes were subjected to respiratory acidosis and then returned to normal pH. Monophasic action potentials and left ventricular developed pressure were recorded. The removal of acidosis provoked ectopic beats that were blunted by 1 muM of the CaMKII inhibitor KN-93, 1 muM thapsigargin, to inhibit sarcoplasmic reticulum (SR) Ca(2+) uptake, and 30 nM ryanodine or 45 muM dantrolene, to inhibit SR Ca(2+) release and were not observed in a transgenic mouse model with inhibition of CaMKII targeted to the SR. Acidosis increased the phosphorylation of Thr(17) site of phospholamban (PT-PLN) and SR Ca(2+) load. Both effects were precluded by KN-93. The return to normal pH was associated with an increase in SR Ca(2+) leak, when compared with that of control or with acidosis at the same SR Ca(2+) content. Ca(2+) leak occurred without changes in the phosphorylation of ryanodine receptors type 2 (RyR2) and was blunted by KN-93. Experiments in planar lipid bilayers confirmed the reversible inhibitory effect of acidosis on RyR2. Ectopic activity was triggered by membrane depolarizations (delayed afterdepolarizations), primarily occurring in epicardium and were prevented by KN-93. The results reveal that arrhythmias after acidosis are dependent on CaMKII activation and are associated with an increase in SR Ca(2+) load, which appears to be mainly due to the increase in PT-PLN.

Heterogeneity of ventricular fibrillation dominant frequency during global ischemia in isolated rabbit hearts

INTRODUCTION

Ventricular fibrillation (VF) studies show that ECG-dominant frequency (DF) decreases as ischemia develops. This study investigates the contribution of the principle ischemic metabolic components to this decline.

METHODS AND RESULTS

Rabbit hearts were Langendorff-perfused at 40 mL/min with Tyrode's solution and loaded with RH237. Epicardial optical action potentials were recorded with a photodiode array (256 sites, 15 x 15 mm). After 60 seconds of VF (induced by burst pacing), global ischemia was produced by low flow (6 mL/min), or the solution changed to impose hypoxia (95% N2/5% CO2), low pH(o) (6.7, 80% O2/20% CO2), or raised [K+](o) (8 mM). DF of the optical signals was determined at each site. Conduction velocity (CV), action potential duration (APD90), effective refractory period (ERP), activation threshold, dV/dt(max), and membrane potential were measured in separate experiments during ventricular pacing. During VF, ischemia decreased DF in the left ventricle (LV) (to [58 +/- 6]%, P < 0.001), but not the right (RV) ([93 +/- 5]%). Raised [K+](o) reproduced this DF pattern (LV: [67 +/- 12]%, P < 0.001; RV: [95 +/- 9]%). LV DF remained elevated in hypoxia or low pH(o). During ventricular pacing, ischemia decreased CV in LV but not RV. Raised [K+](o) did not change CV in either ventricle. Ischemia and raised [K+](o) shortened APD90 without altering ERP. LV activation threshold increased in both ischemia and raised [K+](o) and was associated with diastolic depolarization and decreased dV/dt(max).

CONCLUSIONS

These results suggest that during VF, decreased ECG DF in global ischemia is largely due to elevated [K+](o) affecting the activation thresholds in the LV rather than RV.

Metabolic regulation of potassium channels

Potassium (K+) channels exist in all three domains of organisms: eubacteria, archaebacteria, and eukaryotes. In higher animals, these membrane proteins participate in a multitude of critical physiological processes, including food and fluid intake, locomotion, stress response, and cognitive functions. Metabolic regulatory factors such as O2, CO2/pH, redox equivalents, glucose/ATP/ADP, hormones, eicosanoids, cell volume, and electrolytes regulate a diverse group of K+ channels to maintain homeostasis.

Electrophysiological response of rat atrial myocytes to acidosis

The effect of acidosis on the electrical activity of isolated rat atrial myocytes was investigated using the patch-clamp technique. Reducing the pH of the bathing solution from 7.4 to 6.5 shortened the action potential. Acidosis had no significant effect on transient outward or inward rectifier currents but increased steady-state outward current. This increase was still present, although reduced, when intracellular Ca(2+) was buffered by 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA); BAPTA also inhibited acidosis-induced shortening of the action potential. Ni(2+) (5 mM) had no significant effect on the acidosis-induced shortening of the action potential. Acidosis also increased inward current at -80 mV and depolarized the resting membrane potential. Acidosis activated an inwardly rectifying Cl(-) current that was blocked by 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS), which also inhibited the acidosis-induced depolarization of the resting membrane potential. It is concluded that an acidosis-induced increase in steady-state outward K(+) current underlies the shortening of the action potential and that an acidosis-induced increase in inwardly rectifying Cl(-) current underlies the depolarization of the resting membrane potential during acidosis.

Electrophysiological response of rat ventricular myocytes to acidosis

The effects of acidosis on the action potential, resting potential, L-type Ca(2+) (I(Ca)), inward rectifier potassium (I(K1)), delayed rectifier potassium (I(K)), steady-state (I(SS)), and inwardly rectifying chloride (I(Cl,ir)) currents of rat subepicardial (Epi) and subendocardial (Endo) ventricular myocytes were investigated using the patch-clamp technique. Action potential duration was shorter in Epi than in Endo cells. Acidosis (extracellular pH decreased from 7.4 to 6.5) depolarized the resting membrane potential and prolonged the time for 50% repolarization of the action potential in Epi and Endo cells, although the prolongation was larger in Endo cells. At control pH, I(Ca), I(K1), and I(SS) were not significantly different in Epi and Endo cells, but I(K) was larger in Epi cells. Acidosis did not alter I(Ca), I(K1), or I(K) but decreased I(SS); this decrease was larger in Endo cells. It is suggested that the acidosis-induced decrease in I(SS) underlies the prolongation of the action potential. I(Cl,ir) at control pH was Cd(2+) sensitive but 4,4'-disothiocyanato-stilbene-2,2'-disulfonic acid resistant. Acidosis increased I(Cl,ir); it is suggested that the acidosis-induced increase in I(Cl,ir) underlies the depolarization of the resting membrane potential.

Preconditioning attenuates the shortening of recovery during coronary occlusion in isolated rabbit hearts with D-sotalol-induced long QT intervals

The effects of 20-min ligations of the anterior branch of the left coronary artery were studied in Langendorff-perfused rabbit hearts with 92 microM D-sotalol added to the perfusate to induce long QT intervals and triggered arrhythmias. Epicardial electrograms, a left ventricular endocardial monophasic action potential, and simulated X and Y lead electrocardiograms were used to characterize ventricular conduction and recovery. In contrast to previous work showing that global ischemia eliminated triggered activity, coronary occlusion did not alter its mean incidence. Although the anatomic distribution of earliest sites of epicardial activation by triggered beats was altered, triggered beats still appeared on the epicardial surface in the nonperfused regions. Coronary occlusion had a small and variable effect on epicardial conduction velocity but caused a significantly greater percent shortening of epicardial activation-recovery intervals in the nonperfused region of hearts given D-sotalol than in control hearts. In hearts given D-sotalol, preconditioning significantly attenuated the shortening of epicardial activation-recovery intervals in response to coronary occlusion. However, preconditioning had no effect on the mean incidence of triggered activity during coronary occlusion. Thus, the persistence of triggered activity and the shortened myocardial recovery time associated with coronary occlusion could contribute to increasing the likelihood of occurrence of malignant ventricular arrhythmias. Preconditioning by attenuating the shortening of recovery would be anti-arrhythmic.

Slowed conduction and ventricular tachycardia after targeted disruption of the cardiac sodium channel gene Scn5a

Voltage-gated sodium channels drive the initial depolarization phase of the cardiac action potential and therefore critically determine conduction of excitation through the heart. In patients, deletions or loss-of-function mutations of the cardiac sodium channel gene, SCN5A, have been associated with a wide range of arrhythmias including bradycardia (heart rate slowing), atrioventricular conduction delay, and ventricular fibrillation. The pathophysiological basis of these clinical conditions is unresolved. Here we show that disruption of the mouse cardiac sodium channel gene, Scn5a, causes intrauterine lethality in homozygotes with severe defects in ventricular morphogenesis whereas heterozygotes show normal survival. Whole-cell patch clamp analyses of isolated ventricular myocytes from adult Scn5a(+/-) mice demonstrate a approximately 50% reduction in sodium conductance. Scn5a(+/-) hearts have several defects including impaired atrioventricular conduction, delayed intramyocardial conduction, increased ventricular refractoriness, and ventricular tachycardia with characteristics of reentrant excitation. These findings reconcile reduced activity of the cardiac sodium channel leading to slowed conduction with several apparently diverse clinical phenotypes, providing a model for the detailed analysis of the pathophysiology of arrhythmias.

4-Aminopyridine: effects on electrical activity during ischemia and reperfusion in perfused rat hearts

We investigated the effects of 2 and 4 mM 4-aminopyridine (4-AP,--blocker of the transient outward current I(to) on the electrophysiological response to regional ischemia and reperfusion. Spontaneously beating rat hearts were subjected to coronary occlusion (10 min) followed by reperfusion. The surface electrogram and the membrane potential from subepicardial left ventricular cells were recorded throughout. The basal effect of 4-AP was a dose dependent increase in the action potential duration (APD90) without changes in the resting potential or the heart rate. During early ischemia resting depolarization (from 87.4 +/- 1.9-70.1 +/- 2.5 mV in the controls) was enhanced by 4 mM, 4-AP (84.3 +/- 1.4 mV vs. 61.7 +/- 1.3 mV) whereas APD90 increased by 73.5%. These effects resulted in a marked reduction in the duration of diastolic intervals that led to conduction failure and aborted responses. A partial recovery was found by the end of ischemia concomitant with APD90 shortening in both, control and 4-AP treated hearts. On reperfusion, 4-AP did not influence the initial incidence of ventricular tachyarrhythmias but decreased their duration from 531.5 +/- 56.3-260.7 +/- 100 sec (2 mM) and to 75.6 +/- 10.5 sec (4 mM). These data confirm others obtained by Henry et al. in isolated cells indicating that ischemia induces sequential changes in several K+ conductances. In addition, they show that changes in action potential characteristics may exert beneficial effects on reperfusion arrhythmias by acting on the arrhythmic substrate without suppressing the trigger mechanism.

Glibenclamide improves postischemic recovery of myocardial contractile function in trained and sedentary rats

In this study, we sought to determine whether there was any evidence for the idea that cardiac ATP-sensitive K+ (K(ATP)) channels play a role in the training-induced increase in the resistance of the heart to ischemia-reperfusion (I/R) injury. To do so, the effects of training and an K(ATP) channel blocker, glibenclamide (Glib), on the recovery of left ventricular (LV) contractile function after 45 min of ischemia and 45 min of reperfusion were examined. Female Sprague-Dawley rats were sedentary (Sed; n = 18) or were trained (Tr; n = 17) for >20 wk by treadmill running, and the hearts from these animals used in a Langendorff-perfused isovolumic LV preparation to assess contractile function. A significant increase in the amount of 72-kDa class of heat shock protein was observed in hearts isolated from Tr rats. The I/R protocol elicited significant and substantial decrements in LV developed pressure (LVDP), minimum pressure (MP), rate of pressure development, and rate of pressure decline and elevations in myocardial Ca(2+) content in both Sed and Tr hearts. In addition, I/R elicited a significant increase in LV diastolic stiffness in Sed, but not Tr, hearts. When administered in the perfusate, Glib (1 microM) elicited a normalization of all indexes of LV contractile function and reductions in myocardial Ca(2+) content in both Sed and Tr hearts. Training increased the functional sensitivity of the heart to Glib because LVDP and MP values normalized more quickly with Glib treatment in the Tr than the Sed group. The increased sensitivity of Tr hearts to Glib is a novel finding that may implicate a role for cardiac K(ATP) channels in the training-induced protection of the heart from I/R injury.

Bupivacaine effects on hKv1.5 channels are dependent on extracellular pH

1. Bupivacaine-induced cardiotoxicity increases in hypoxic and acidotic conditions. We have analysed the effects of R(+)bupivacaine on hKv1.5 channels stably expressed in Ltk(-) cells using the whole-cell patch-clamp technique, at three different extracellular pH (pH(o)), 6.5, 7.4 and 10.0. 2. Acidification of the pH(o) from 7.4 to 6.5 decreased 4 fold the potency of R(+)bupivacaine to block hKv1.5 channels. At pH(o) 10.0, the potency of the drug increased approximately 2.5 fold. 3. Block induced by R(+)bupivacaine at pH(o) 6.5, 7.4 and 10.0, was voltage- and time-dependent in a manner consistent with an open state block of hKv1.5 channels. 4. At pH(o) 6.5, but not at pH(o) 7.4 or 10.0, R(+)bupivacaine increased by 95+/-3 % (n=6; P<0.05) the hKv1.5 current recorded at -10 mV, likely due to a drug-induced shift of the midpoint of activation (DeltaV=-8.5+/-1.4 mV; n=7). 5. R(+)bupivacaine development of block exhibited an 'instantaneous' component of block at the beginning of the depolarizing pulse, which averaged 12.5+/-1.8% (n=5) and 4.6+/-1.6% (n=6), at pH(o) 6.5 and 7.4, respectively, and that was not observed at pH(o) 10.0. 6. It is concluded that: (a) alkalinization of the pH(o) increases the potency of block of R(+)bupivacaine, and (b) at pH(o) 6.5, R(+)bupivacaine induces an 'agonist effect' of hKv1.5 current when recorded at negative membrane potentials.