Biomedical subjects

G J Rozanski

Publications and source records attributed to G J Rozanski.

At least 19 recordsLinked to original sources

PubMed2005

Redox control of K+ channel remodeling in rat ventricle.

Electrical remodeling of the diseased heart contributes to contractile dysfunction and arrhythmias, and is characterized by down-regulation of K(+) channels that control action potential morphology. We have recently shown that remodeling of K(+) channels underlying the transient outward current (I(to)) involves a shift in cell redox balance that is reflected by a depletion of the endogenous redox buffer, glutathione (GSH). This study used a pharmacological model to further examine the role of redox-mediated mechanisms in regulating cardiac K(+) currents. Inhibition of major redox pathways was elicited in normal rats by daily injections of 1,3-bis-(2-chloroethyl)-1-nitrosourea (BCNU), an inhibitor of thioredoxin and glutathione reductases, and buthionine sulfoximine (BSO), a blocker of GSH synthesis. Fluorescence microscopy studies showed that [GSH] in isolated ventricular myocytes was decreased ~50% from control after 3 days of BCNU/BSO treatment (P<0.05), consistent with a shift in cell redox state. In voltage-clamp experiments, maximum I(to) density was decreased 33% from control in left ventricular myocytes from BCNU/BSO-treated rats (P<0.05), while the inward rectifier and steady state outward currents were not significantly altered. Decreased I(to) density correlated with significant decreases in Kv4.2 mRNA and proteins levels of Kv4.2 and Kv1.4. Down-regulation of I(to) in myocytes from BCNU/BSO rats was reversed in vitro by exogenous GSH or N-acetylcysteine, a GSH precursor and antioxidant. I(to) density and [GSH] were also up-regulated by receptor tyrosine kinase activation with insulin or a tyrosine phosphatase inhibitor. The effect of these activators on I(to) was blocked by inhibitors of PI 3-kinase, MEK and p38 MAP kinases. These data suggest that expression of cardiac I(to) channels is regulated by endogenous oxidoreductase systems and that receptor tyrosine kinase signaling functionally impacts K(+) channel remodeling through its control of cell redox state.

Animals↗

PubMed2001

Inhibition of K+ currents by homocysteine in rat ventricular myocytes.

INTRODUCTION: Clinical evidence suggests that increased blood levels of homocysteine may be an independent risk factor for the development of cardiovascular disease, but the functional effects of this sulfhydryl amino acid on the myocardium are poorly understood. The present study was conducted to determine the direct effects of homocysteine on the electrophysiologic properties of the heart. METHODS AND RESULTS: Whole-cell voltage-clamp recordings were made in ventricular myocytes isolated from normal rat hearts to analyze the Ca2+-independent, transient outward K+ current (I(to)), a major repolarizing current in these cells. Maximum I(to) density (measured at +60 mV) was decreased approximately 47% from baseline in the presence of 500 microM homocysteine (P < 0.05), but the amount of block varied in a frequency- and voltage-dependent manner. Decreased I(to) density was not accompanied by significant changes in voltage- or time-dependent properties of the current, nor was it affected by pretreating myocytes with the protein kinase inhibitor staurosporine. Because a portion of total extracellular homocysteine is oxidized, we examined the response to homocystine, the oxidized form of homocysteine. In myocytes superfused with 500 microM homocystine, maximum I(to) density was decreased by approximately 40% from baseline (P < 0.05). In contrast, the thiolactone form of homocysteine did not alter I(to) amplitude. CONCLUSION: These data suggest that homocysteine and its oxidized form homocystine acutely inhibit I(to) channels in ventricular myocytes by mechanisms involving the free thiol or disulfide moieties of these compounds. High homocysteine or homocystine levels may contribute to abnormal repolarization and arrhythmogenic conditions in the intact heart.

Algorithms↗

PubMed1998

K+ current inhibition by amphiphilic fatty acid metabolites in rat ventricular myocytes.

Fatty acid metabolites accumulate in the heart under pathophysiological conditions that affect beta-oxidation and can elicit marked electrophysiological changes that are arrhythmogenic. The purpose of the present study was to determine the impact of amphiphilic fatty acid metabolites on K+ currents that control cardiac refractoriness and excitability. Transient outward (Ito) and inward rectifier (IK1) K+ currents were recorded by the whole cell voltage-clamp technique in rat ventricular myocytes, and the effects of two major fatty acid metabolites were examined: palmitoylcarnitine and palmitoyl-coenzyme A (palmitoyl-CoA). Palmitoylcarnitine (0.5-10 microM) caused a concentration-dependent decrease in Ito density in myocytes internally dialyzed with the amphiphile; 10 microM reduced mean Ito density at +60 mV by 62% compared with control (P < 0.05). In contrast, external palmitoylcarnitine at the same concentrations had no effect, nor did internal dialysis significantly alter IK1. Dialysis with palmitoyl-CoA (1-10 microM) produced a smaller decrease in Ito density compared with that produced by palmitoylcarnitine; 10 microM reduced mean Ito density at +60 mV by 37% compared with control (P < 0.05). Both metabolites delayed recovery of Ito from inactivation but did not affect voltage-dependent properties. Moreover, the effects of palmitoylcarnitine were relatively specific, as neither palmitate (10 microM) nor carnitine (10 microM) alone significantly influenced Ito when added to the pipette solution. These data therefore suggest that amphiphilic fatty acid metabolites downregulate Ito channels by a mechanism confined to the cytoplasmic side of the membrane. This decrease in cardiac K+ channel activity may delay repolarization under pathophysiological conditions in which amphiphile accumulation is postulated to occur, such as diabetes mellitus or myocardial infarction.

Animals↗

PubMed1998

Altered K+ current of ventricular myocytes in rats with chronic myocardial infarction.

The aim of the present study was to define the cellular mechanisms underlying changes in K+ channel function in the failing heart after myocardial infarction. Rats with left coronary artery ligation were prepared and allowed to recover for 16 wk before study. Animals with chronic infarction exhibited marked cardiac hypertrophy and signs of heart failure, as indicated by a nearly twofold increase in heart weight- and lung weight-to-body weight ratios, respectively, compared with time-matched controls. Cardiac hypertrophy was also evident by a 49% increase in whole cell capacitance of isolated left ventricular myocytes (P < 0.05). Voltage-clamp experiments revealed that the maximum density of the Ca(2+)-independent, transient outward current (I.t.o.), measured at +60 mV, was 42% less in myocytes from infarcted hearts than in myocytes from control hearts (P < 0.05), whereas the inward rectifier current (IK1) density was not different between groups. The reduced Ito density in the infarcted group was reversed, however, in 4-5 h by treatment with exogenous dichloroacetate or pyruvate, both activators of pyruvate dehydrogenase. Moreover, control myocytes incubated for 6 h in the presence of an inhibitor of pyruvate dehydrogenase, 3-bromopyruvate, exhibited a concentration-dependent decrease in Ito density compared with untreated cells. The present data demonstrate that Ito density is reversibly decreased in surviving myocytes from infarcted hearts and suggest that mechanisms related to glucose metabolism via pyruvate dehydrogenase may be involved. These postinfarction changes in myocyte Ito channel function may relate to impaired contractility and arrhythmogenesis, which are characteristic of the intact, failing heart.

Animals↗

PubMed1997

Proton inhibition of transient outward potassium current in rat ventricular myocytes.

Acidosis elicited during myocardial ischemia is a significant pathophysiological condition markedly affecting the electrical and contractile properties of heart muscle. We examined the effects of protons on K channel activity in rat ventricular myocytes by recording transient outward (Ito) and inward rectifier (IKl) K+ currents using the whole cell, voltage clamp technique. Proton concentration was controlled by independently varying the pH of HEPES-buffered external (pHo) or pipette (pHp) solutions. Mean Ito density in myocytes preconditioned in acidic external solution (pHo 6.0) for 15-20 min was significantly less than control cells equilibrated at physiological pHo. In contrast, IKl was not changed during this period of acidosis. External acidification did not decrease Ito when initiated after intracellular dialysis with standard pHp 7.2. However, when myocytes were dialyzed with acidic pHp, Ito density was significantly less than control, while alkaline pHp had little effect. Despite marked reduction in current density produced by low pHp solutions, steady-state activation and inactivation parameters of Ito were not significantly altered. In addition, the reversal potential of this current, kinetics of inactivation, and recovery from inactivation were not significantly affected by acidic or alkaline pHp solutions. Acidic pHp alone did not change IKl density compared with control, but when combined with Na+/H+ exchange blockade with 5-(N,N-dimethyl)-amiloride or Na(+)-free external solution, IKl density was significantly reduced. Our data suggest that protons inhibit Ito predominantly from the intracellular side of the channel, possibly by altering its conductance or gating properties. Moreover, intracellular protons differentially affect Ito and IKl channels, with the former exhibiting greater sensitivity for a given level of acidosis.

Animals↗

PubMed1997

Electrophysiology of rabbit ventricular myocytes following sustained rapid ventricular pacing.

The present study examined changes in electrophysiological properties of ventricular myocytes isolated from rabbit hearts after 2-3 weeks of rapid ventricular pacing. Left ventricular end-diastolic pressure at completion of the pacing period was nearly four-fold greater than in age-matched controls, although there was no significant change in heart weight/body weight ratio. Action potentials recorded in current-clamp mode at low stimulation frequencies were significantly longer in duration and phase 1 diminished in isolated myocytes from paced hearts compared with control. In voltage-clamp experiments. L-type Ca2+ current (ICa) density was not different between groups of myocytes, but the maximum current (at + 10 mV) elicited by 10 microM isoproterenol was approximately 40% less in myocytes from paced hearts. In contrast, maximum ICa elicited by 10 microM forskolin was similar in both groups. The 4-aminopyridine-sensitive transient outward current (Ito) was 65% less (at + 60 mV) in myocytes from paced hearts than from control. However, after approximately 24 h in culture, Ito density in these myocytes returned toward control values. Despite marked reduction in Ito density, the inward rectifier current (IK1) was not different between groups. These data demonstrate that Ito is significantly and reversibly decreased in myocytes from rapidly paced hearts, which may partly account for marked changes in action potential morphology. Although basal ICa was not altered in this group of myocytes compared with control, its modulation by beta-agonists was markedly blunted, probably through a decrease in receptor density or coupling to adenylyl cyclase. These changes in myocyte K+ and Ca2+ channel behavior in paced hearts may relate to impaired contractility and arrhythmogenesis that is characteristic of the intact failing heart.

Action Potentials↗

PubMed1996

Intracellular protons inhibit transient outward K+ current in ventricular myocytes from diabetic rats.

This study examined the effects of protons on cardiac ion channel function in early stages of diabetes mellitus. Transient outward (I(to)) and inward rectifier K+ (IK1) currents were recorded by the whole cell, voltage-clamp technique in ventricular myocytes isolated from hearts of streptozotocin-induced diabetic and control rats. Proton concentration was controlled by independently varying the pH of buffered external or pipette (pHp) solutions. External acidification did not alter I(to) in diabetic rat myocytes when initiated after intracellular dialysis with standard pHp 7.2, but when these cells were dialyzed with acidic pHp (6.6 or 6.0), I(to) density was significantly reduced. Low pHp also reduced I(to) density more in cells from diabetic rats than in controls, whereas alkaline pHp had no effect on either group of cells compared with standard pHp 7.2. In control myocytes dialyzed with pHp 6.0, block of Na+/H+ exchange with 5-(N,N-dimethyl)-amiloride (DMA) or Na(+)-free external solution further reduced I(to) density compared with pHp 6.0 alone, whereas these treatments had less effect on acid-dialyzed cells from diabetic rats. Dialysis with pHp to 6.0 did not alter IK1 in either group of cells compared with standard pHp 7.2, but when done in the presence of DMA or Na(+)-free conditions, IK1 density in both groups was significantly reduced by nearly the same amount. We conclude that intracellular protons inhibit I(to) channels in ventricular myocytes from diabetic and control rats, but that for a given acid load, inhibition is markedly greater in diabetics. This difference may be explained by a diabetes-induced decrease in Na+/H+ exchange that limits proton extrusion during intracellular acidosis. Moreover, acidosis may differentially suppress I(to) and IK1, suggesting that these K+ channels exhibit dissimilar sensitivities to intracellular protons.

Acidosis↗

PubMed1996

Metabolic basis of decreased transient outward K+ current in ventricular myocytes from diabetic rats.

The purpose of this study was to examine the mechanisms of alterations in cardiac K+ channel function in early stages of experimental diabetes mellitus induced by streptozotocin. Transient outward (Ito) and inward rectifier (IK1) K+ currents were recorded by the whole cell voltage-clamp technique in ventricular myocytes isolated from hearts of 2- to 4-wk diabetic and age-matched control rats. Ito density in myocytes from diabetic rats was approximately 30% less than control (at +60 mV; P < 0.01) under basal recording conditions in the presence of 18 mM external glucose, whereas IK1 density was not different between groups. When external glucose concentration was decreased to 5 mM for 4-6 h, basal Ito density was not changed in either group of myocytes. To further examine the possible metabolic basis of reduced Ito density in myocytes from diabetic rats, we separately tested three structurally different compounds that affect substrate utilization in cardiac myocytes: insulin (0.1 microM), dichloroacetate (1.5 mM), and L-carnitine (10 mM). Each compound completely normalized Ito density in myocytes from diabetic rats treated in vitro for 4-6 h. The same agents had no effect on Ito density in control myocytes, nor was IK1 altered in either group of cells. These data provide the first evidence to support the hypothesis that there is a metabolic basis for decreased Ito density in diabetic rat ventricular myocytes in early stages of this model. Furthermore, our data suggest that depressed glucose metabolism in the diabetic heart may be a key factor underlying changes in Ito channel function, because agents that increase glucose utilization normalize Ito density within a short time period.

Animals↗

PubMed1995

Acidosis masks beta-adrenergic control of cardiac L-type calcium current.

The beta-adrenergic control of the L-type Ca2+ current (ICa) was examined as a function of extracellular pH (pHo) in guinea-pig ventricular myocytes using the whole-cell voltage-clamp technique. ICa was elicited in Cs(+)-loaded myocytes by depolarizing pulses from a holding potential of -40 mV. The maximum ICa density in response to 0.01 or 1 microM isoproterenol was significantly less in myocytes pretreated with acidic external solution (pHo 6.6 or 5.8) compared with cells studied at control pHo 7.4. This acidosis-induced decrease in beta-responsiveness was also accompanied by a similar reduction in basal current density. Myocytes studied under alkaline conditions (pHo 8.2) also had reduced beta-responsiveness although basal ICa density tended to be greater than control. In addition to the diminished effects of isoproterenol, acidic myocytes had smaller responses to extracellular forskolin and internally applied adenosine 3',5'-cyclic monophosphate, compared with control. The blunted responses to these latter stimuli were similar in magnitude to that observed with 1 microM isoproterenol. These findings suggest that protons interfere with the beta-adrenergic control of ICa primarily by a direct inhibition of the Ca2+ channel which independently masks the effects of the adenylyl cyclase cascade.

Adrenergic beta-Agonists↗

PubMed1994

Interleukin-1 enhances beta-responsiveness of cardiac L-type calcium current suppressed by acidosis.

Modulation of the beta-adrenergic control of the cardiac L-type Ca2+ current (ICa) by human recombinant interleukin-1 beta (IL-1) was examined in guinea pig ventricular myocytes using the whole cell voltage-clamp technique. ICa was evoked in Cs(+)-loaded myocytes by depolarizing pulses from a holding potential of -40 mV. In the presence of an acidic external solution (pH 5.8), the response of ICa to isoproterenol (Iso; 0.01 and 1 microM) was markedly decreased compared with control myocytes studied at pH 7.4. However, when cells were pretreated with 1 ng/ml IL-1 and then exposed to acid media, beta-responsiveness was significantly increased compared with untreated cells. Despite this effect of IL-1, maximum ICa density with 0.01 and 1 microM Iso was still 51 and 58%, respectively, less than that measured at pH 7.4. The enhanced beta-responsiveness produced by IL-1 was eliminated by adding amiloride to block Na+/H+ exchange or protein kinase C inhibitors staurosporine (10 nM) and calphostin C (50 nM). However, a direct activator of protein kinase C, phorbol 12-myristate 13-acetate, did not mimic the effects of the cytokine. These data demonstrate that IL-1 partially restores the beta-adrenergic control of cardiac Ca2+ channels suppressed under acidic conditions. Moreover, they suggest that IL-1 acts by enhancing Na+/H+ exchange through a second messenger pathway that may involve protein kinase C. These cellular mechanisms may play a role in altering ventricular function during cytokine-mediated inflammatory processes that are initiated by myocardial ischemia.

Acidosis↗

PubMed1994

IL-1 inhibits beta-adrenergic control of cardiac calcium current: role of L-arginine/nitric oxide pathway.

Modulation of the beta-adrenergic control of cardiac L-type Ca2+ current (Ica) by human recombinant interleukin-1 beta (IL-1) was examined in adult guinea pig ventricular myocytes using the whole cell voltage-clamp technique. ICa was elicited in Cs(+)-loaded myocytes by depolarizing pulses from a holding potential of -40 mV. Isoproterenol (0.01 and 1 microM) exposed to myocytes pretreated with 1 ng/ml IL-1 evoked a significantly smaller increase in ICa density compared with control cells. This IL-1-mediated decrease in beta-responsiveness was usually observed with pretreatment periods of > 1 h and varied as a function of the L-arginine concentration of the pretreatment medium. In addition, it was prevented by 1) IL-1 receptor antagonist, 2) substituting D-arginine for L-arginine, or 3) incubating cells with the nitric oxide synthase inhibitor NG-monomethyl-L-arginine. Thus the present data illustrate that IL-1 significantly alters the beta-adrenergic control of cardiac Ca2+ channels by cellular mechanisms that involve the activation of nitric oxide synthase. These mechanisms may play a role in altering ventricular function during cytokine-mediated inflammatory processes affecting the heart.

Adrenergic beta-Antagonists↗

PubMed1993

Effects of human recombinant interleukin-1 on electrical properties of guinea pig ventricular cells.

OBJECTIVE: To determine whether cytokines alter the electrical properties of heart cells, the effects of human recombinant interleukin-1 beta (IL-1) were examined in excised tissues and dissociated myocytes from guinea pig ventricles. METHODS: In a first series of experiments, transmembrane potentials were recorded from isolated papillary muscles superfused with 1 ng.ml-1 IL-1 in the absence and presence of blockers of arachidonic acid metabolism. Secondly, to examine the ionic mechanisms underlying the response to IL-1, ventricular myocytes were dissociated from collagenase perfused hearts and studied using the whole cell configuration of the patch clamp technique under conditions designed to isolate the L-type Ca2+ current (ICa). RESULTS: In excised papillary muscles, IL-1 significantly prolonged action potential duration (measured at 90% repolarisation) by 24.2(SEM 2.2) ms and effective refractory period by 22.9(2.3) ms (both p < 0.001; n = 44). Other measured variables were not affected. Treatment of muscles with cyclo-oxygenase inhibitors, indomethacin (1 x 10(-5) M) or acetyl salicylic acid (2 x 10(-4) M), abolished the prolongation of action potential duration elicited by IL-1. However, the effects of IL-1 were also blocked by the lipoxygenase inhibitor nordihydroguaiaretic acid (2 x 10(-5) M) or by treating tissues with the leukotriene receptor blocker, ICI198615 (1 x 10(-8) M). In isolated myocytes, 1 ng.ml-1 IL-1 increased ICa density in 44 of 78 cells by 33.6(7.5)% [11.7(0.6) v 14.6(0.7) pA.pF-1; p < 0.001] during voltage steps from -40 to 0 mV. CONCLUSIONS: IL-1 modifies electrical properties of cardiac cells via lipid second messengers generated by cyclo-oxygenase and lipoxygenase pathways. Voltage clamp analyses suggest that these effects are mediated, at least in part, by changes in the conductance of calcium channels.

Action Potentials↗

PubMed1993

Alterations in repolarization of cardiac Purkinje fibers recovering from ischemic-like conditions: genesis of early afterdepolarizations.

INTRODUCTION: Triggered activity initiated from delayed after-depolarizations has been proposed as a possible cause of arrhythmias during reperfusion of ischemic myocardium. However, the potential for abnormal repolarization and early afterdepolarizations (EADs) to develop under similar conditions has not been fully explored. METHODS AND RESULTS: Repolarization of the cell membrane during recovery from ischemic-like conditions was analyzed from transmembrane recordings in isolated rabbit Purkinje fibers paced at different basic cycle lengths. Preparations were exposed to conditions of hypoxia (defined as oxygen tension < 30 mmHg, high potassium, and zero substrate) plus lactic acidosis (pH 6.7) for 45 minutes followed by recovery in normal Tyrode's solution. Compared to control, action potentials during recovery at basic cycle length of 3,000 msec (n = 11) were characterized by a: (1) -7.2 +/- 2.1 mV shift in plateau potential (P < 0.01); (2) 126.1 +/- 63.6 msec increase in plateau duration (P < 0.05); and (3) 0.29 +/- 0.07 V/sec slowing of the maximum rate of terminal repolarization (phase 3; P < 0.01). These changes were larger when 10 to 20 microM amiloride was added to the hypoxic, acidotic test solution but were smaller when tissues were conditioned with hypoxia alone (zero lactate, pH 7.4). Following hypoxia plus acidosis, with or without amiloride, repolarization at long basic cycle lengths was often accompanied by EADs predominantly generated from potentials positive to -40 mV. These afterpotentials were blocked by Ca2+ channel antagonists and exhibited an activation threshold of -26.3 +/- 1.8 mV (n = 7). CONCLUSION: These data are consistent with the hypothesis that the combined negative voltage shift in the plateau and increase in its duration lead to the genesis of low membrane potential EADs by allowing reactivation of Ca2+ channels. Moreover, these results suggest that bradycardia-dependent EADs in Purkinje tissue may underlie arrhythmias in the intact heart during reperfusion of ischemic myocardium by mechanisms that are related in part to the acidosis established during the preceding ischemic conditions.

Acidosis↗

PubMed1993

Vagal-mediated atrial premature beats: a computer model.

A computer algorithm is described that used experimental data to model the arrhythmogenic interaction of phasic vagal stimuli and atrial ectopic pacemakers. The model consisted of a dominant sinus node and a single ectopic pacemaker center separated by conducting atrial tissue. Its primary operation was to predict the timing and incidence of atrial premature beats resulting from transient escape of ectopic automatic impulses when vagal-induced entrance block of the sinus impulse was simulated near the ectopic focus. These predictions were based on a series of experimentally derived phase-response and corrected recovery time curves, describing the modulation of ectopic pacemaker periodicity by vagal input and overdrive suppression, respectively. Depending on the combination of curves tested, the model predicted premature beats to develop only with critically timed vagal stimuli. The coupling intervals of vagal-induced premature beats were > 300 ms and varied as a function of vagal timing and sinus cycle length. The model suggests therefore that phasic vagal stimuli within the atrium may transiently protect ectopic pacemaker foci from conducted sinus impulses and mediate the genesis of atrial extrasystoles with long coupling intervals.

Animals↗

PubMed1993

Effects of flecainide on ectopic atrial automaticity and conduction.

BACKGROUND: Previous studies have shown that class Ic antiarrhythmic agents are effective in suppressing ectopic atrial rhythms and accessory pathway conduction. METHODS AND RESULTS: To explore the potential mechanisms for their effectiveness, we investigated the concentration-dependent effects of the Ic agent flecainide acetate (0.5 to 10 micrograms/mL) on atrial ectopic automaticity and exit conduction in isolated rabbit tricuspid valves. This experimental model consists of three major cell types as defined anatomically and by intracellular recordings: pacemaker, transitional, and working atrial muscle. Simultaneous recordings from these cell types before and during flecainide superfusion (n = 7) showed that the drug produced a slight, concentration-dependent slowing of pacemaker-transitional conduction but elicited third-degree transitional-working atrial muscle block in six of seven preparations at 10 micrograms/mL. Flecainide caused a significant dose-dependent reduction in the initial phase of diastolic depolarization of pacemaker cells but produced only a small, biphasic change in spontaneous pacemaker cycle length. It also caused a significant prolongation in action potential duration in pacemaker and transitional cells and reduction in upstroke velocity in atrial cells. Of note in four additional preparations, flecainide caused a concentration-dependent upward shift in the strength-duration curve for atrial fibers. CONCLUSIONS: These data suggest that flecainide has little direct effect on ectopic atrial automaticity but rather causes exit conduction slowing and block between transitional and atrial muscle fibers. The mechanism for the induction of block is likely due to a decrease in atrial excitability creating a greater electrical load on generated impulses.

Animals↗

PubMed1991

Atrial ectopic pacemaker escape mediated by phasic vagal nerve activity.

Effects of vagal nerve activity on atrial ectopic pacemaker foci were studied in vitro in strips of rabbit tricuspid valve. Transmembrane potentials were recorded from pacemaker and working atrial fibers superfused with Tyrode solution containing propranolol. Tissues were paced from the atrial muscle end at cycle lengths of 90, 70, or 50% of the intrinsic pacemaker cycle, and postganglionic vagal nerve endings were stimulated with brief trains of pulses (200 Hz; 100-200 microseconds) through a second electrode near the pacemaker. Vagal trains scanning diastole hyperpolarized pacemaker and surrounding fibers to a maximum membrane potential of -74.7 +/- 1.8 mV (normal maximum diastolic potential = -75.5 +/- 1.6 mV) and elicited a period of inexcitability lasting 217.9 +/- 27.3 ms (drive cycle = 90% of pacemaker cycle). Inexcitability was evident at critical diastolic intervals where vagal input prevented atrial impulses from activating the pacemaker allowing spontaneous discharges to occur, i.e., escape, late in diastole. Besides inexcitability, incidence and timing of escape impulses were determined by cumulative effects of drive cycle length, vagal stimulus, and subthreshold electrotonic input on intrinsic pacemaker cycle. These data suggest that phasic vagal stimuli may transiently protect atrial ectopic pacemaker foci from conducted sinus impulses by rendering pacemaker and surrounding fibers inexcitable. In the setting of a long sinus (drive) cycle length, phasic vagal activity may result in spontaneous discharges manifest as late atrial premature beats.

Animals↗

PubMed1991

Early afterdepolarizations and triggered activity in rabbit cardiac Purkinje fibers recovering from ischemic-like conditions. Role of acidosis.

BACKGROUND: The mechanisms underlying repetitive activity during reperfusion of ischemic myocardium are thought to include triggered responses elicited at short pacing cycle lengths. The potential to generate repetitive responses at longer pacing cycle lengths under similar conditions, however, has not been explored. Thus, the present study examined the role of cycle length on the cellular electrical changes produced during recovery from ischemic-like conditions and identified the major component precipitating nondriven, repetitive activity. METHODS AND RESULTS: Transmembrane potentials were recorded in vitro from isolated rabbit Purkinje fibers exposed to hypoxia (defined as PO2 less than 30 mm Hg, high [K+]o, and zero glucose) plus lactic acidosis (pH 6.7) for 45 minutes and during recovery in normal Tyrode's solution (pH 7.4). Compared with control, action potential duration (90% repolarization) during recovery increased transiently by 40.9 +/- 11.8 and 241.0 +/- 51.1 msec at respective basic cycle lengths of 1,000 and 3,000 msec (both p less than 0.005). In 81% of preparations, action potential prolongation was accompanied by early afterdepolarizations and triggered activity generated from low (positive to -40 mV) or high (negative to -40 mV) membrane potentials. In 62% of experiments, brief periods of abnormal automaticity also occurred. Triggered responses were 1) unaffected by 1 microM ryanodine, 2) abolished by pacing at short basic cycle lengths or by exposing tissues to 2.5 micrograms/ml lidocaine, and 3) more easily induced at long basic cycle lengths or by superfusing 2.5 micrograms/ml quinidine. When tissues were conditioned with hypoxia alone (pH 7.4), action potential prolongation on recovery was comparatively small, and nondriven responses did not develop. Conversely, addition of 10-20 microM amiloride to the hypoxic, acidic test solution augmented recovery-induced action potential prolongation. CONCLUSIONS: We conclude that acidosis, as a component of ischemia, plus slow pacing frequencies may mediate the genesis of early afterdepolarizations and triggered activity in Purkinje fibers on recovery, long after extracellular pH has been restored to normal. These data may have clinical relevance to the mechanisms of reperfusion arrhythmias in the intact human heart.

Acidosis, Lactic↗

PubMed1990

Electrophysiologic effects of exogenous phosphocreatine in cardiac tissue: potential antiarrhythmic actions.

The cellular electrophysiologic effects of exogenous phosphocreatine (PCr) were analyzed to ascertain its purported antiarrhythmic properties during myocardial ischemia and reperfusion. Transmembrane potentials were recorded from isolated guinea pig papillary muscles and Purkinje fibers studied in vitro. Under control, normoxic conditions, 10 mmol/L PCr significantly increased the action potential duration (measured at 90% of repolarization) in ventricular muscle by 14.6 +/- 3.3 msec and the effective refractory period by 11.5 +/- 3.8 msec (both p less than 0.01). Under ischemic-like conditions (hypoxia, lactic acidosis, elevated [K+]o, zero substrate) PCr had no effect. Phosphocreatinine, a related compound that is not a direct substrate in the creatine kinase reaction, acted similarly to PCr suggesting that alterations induced by PCr did not involve a change in the energy state of cells. However, PCr reduced free [Ca2+]o by nearly 20%, and its electrical effects under normoxic conditions could be largely reversed by a concomitant 20% increase in [Ca2+]o. In Purkinje fibers superfused with low [K+]o-Tyrode's solution to elicit conditions of Ca2+ overload, delayed afterdepolarizations and triggered responses were reversibly inhibited by PCr. These data suggest that the antiarrhythmic effects of PCr in situ may involve prolongation of the effective refractory period in nonischemic tissue or attenuation of membrane changes elicited by Ca2+ overload in ischemic cells. The mechanism by which PCr produces these effects may be related in part to changes in extracellular Ca2+ composition.

Action Potentials↗