Electromechanical effects of caffeine in isolated human atrial fibres

Antiarrhythmic effects of dehydroevodiamine in isolated human myocardium and cardiomyocytes

ETHNOPHARMACOLOGICAL RELEVANCE

Dehydroevodiamine alkaloid (DeHE), a bioactive component of the Chinese herbal medicine Wu-Chu-Yu (Evodiae frutus), exerted antiarrhythmic effect in guinea-pig ventricular myocytes. We further characterize the electromechanical effects of DeHE in the human atrial and ventricular tissues obtained from hearts of patients undergoing corrective cardiac surgery or heart transplantation.

MATERIALS AND METHODS

The transmembrane potentials of human myocardia were recorded with a traditional microelectrode technique while sarcolemmal Na(+) and Ca(2+) currents in single human cardiomyocytes were measured by a whole-cell patch-clamp technique. The intracellular pH (pHi) and Na(+)-H(+) exchanger (NHE) activity were determined using BCECF-fluorescence in human atria.

RESULTS

In human atria, DeHE (0.1-0.3 μM) depressed upstroke velocity, amplitude of action potential, and contractile force, both in slow and fast response action potential. Moreover, the similar depressant effects of DeHE were found in human ventricular myocardium. Both in isolated human atrial and ventricular myocytes, DeHE (0.1-1 μM) reversibly, concentration-dependently decreased the Na(+) and Ca(2+)currents. Moreover, DeHE (0.1 and 0.3 μM) suppressed delayed afterdepolarizations and aftercontractions, induced by epinephrine and high [Ca(2+)]o in atria. In human ventricular myocardium, the strophanthidin-induced triggered activities were attenuated by pretreating DeHE (0.3 μM). The resting pHi and NHE activity were also significantly increased by DeHE (0.1-0.3 μM).

CONCLUSIONS

We concluded for the first time that, in the human hearts, DeHE could antagonize triggered arrhythmias induced by cardiotonic agents through a general reduction of the Na(+) and Ca(2+) inward currents, while increase of resting pHi and NHE activity.

Caffeine prevents protection in two human models of ischemic preconditioning

OBJECTIVES

We studied whether caffeine impairs protection by ischemic preconditioning (IP) in humans.

BACKGROUND

Ischemic preconditioning is critically dependent on adenosine receptor stimulation. We hypothesize that the adenosine receptor antagonist caffeine blocks the protective effect of IP.

METHODS

In vivo ischemia-reperfusion injury was assessed in the thenar muscle by 99mTc-annexin A5 scintigraphy. Forty-two healthy volunteers performed forearm ischemic exercise. In 24 subjects, this was preceded by a stimulus for IP. In a randomized double-blinded design, the subjects received caffeine (4 mg/kg) or saline intravenously before the experiment. At reperfusion, 99mTc-annexin A5 was administered intravenously. Targeting of annexin was quantified by region-of-interest analysis, and expressed as percentage difference between experimental and contralateral hand. In vitro, we assessed recovery of contractile function of human atrial trabeculae, harvested during heart surgery, as functional end point of ischemia-reperfusion injury. Field-stimulated contraction was quantified at baseline and after simulated ischemia-reperfusion, in a paired approach with and without 5 min of IP, in the presence (n=13) or absence (n = 17) of caffeine (10 mg/l).

RESULTS

Ischemic preconditioning reduced annexin targeting in the absence of caffeine (from 13 +/- 3% to 7 +/- 1% at 1 h, and from 19 +/- 2% to 9 +/- 3% at 4 h after reperfusion, p = 0.006), but not after caffeine administration (targeting 11 +/- 2% and 16 +/- 3% at 1 and 4 h). In vitro, IP improved post-ischemic functional recovery in the control group, but not in the caffeine group (8 +/- 3% vs. -8 +/- 5%, p=0.003).

CONCLUSIONS

Caffeine abolishes IP in 2 human models at a dose equivalent to the drinking of 2 to 4 cups of coffee. (The Effect of Caffeine on Ischemic Preconditioning; http://clinicaltrials.gov/ct/show/NCT00184912?order=1; NCT00184912).

Effects of alcohol on intracellular pH regulators and electromechanical parameters in human myocardium

BACKGROUND

Disturbances in intracellular pH (pHi) of the heart can trigger major changes in the strength and rhythm of the heartbeat. It is well known that two extruders, Na+/H+ exchange (NHE) and Na+/HCO3- symporter (NHS), and a monocarboxylic acid transporter (MCT) are involved in acid-equivalent extruding in the human heart. Drinking alcohol has been proven to affect blood pressure and heart contractility and, sometimes, causes cardiac arrhythmia. To assess the effects of alcohol on pHi regulators and electromechanical parameters, various concentrations of alcohol were superfused into human myocardium in the present study.

METHODS

Human atrial myocardium was obtained from hearts of patients undergoing corrective cardiac surgery. Institutional rules for the protection of human subjects were observed. In the whole study, pHi was measured by an epifluorescent, ratiometric microspectrofluorimetry technique with the dye BCECF, while electrophysiological experiments were performed by traditional micropipette. NHE and NHS activities were measured after pHi recovery from intracellular acidosis induced by NH4Cl prepulse, while MCT activity was measured by a lactate adding/removing technique.

RESULTS

In pHi experiments, we demonstrated that alcohol could induce a biphasic, concentration-dependent (30-1000 mM) pHi change (i.e., alkalosis after acidosis) in human atrium in HEPES-buffered Tyrode solution. To a smaller extent, similar results were found when the superfusate was replaced by HCO3- -buffered Tyrode solution. NHE activity was increased by a moderate concentration of alcohol (30 mM), while it was inhibited in a concentration-dependent manner by higher concentrations of alcohol (>100 mM). On the contrary, 30-1000 mM alcohol increased the activity of NHS in a concentration-dependent manner. Surprisingly, MCT activity was not affected by alcohol. In electromechanical experiments, we found that alcohol (30-1000 mM) had a notable concentration-dependent inhibitory effect on the contractile force, while higher concentrations of alcohol (>100 mM) decreased the action potential amplitude, upstroke velocity, duration of repolarization, and force of contractions in a concentration-dependent way. All these alcohol-induced pHi changes and electromechanical inhibitions were reversible.

CONCLUSIONS

To our knowledge, this study provides the first evidence that alcohol can affect pHi in human myocardial tissue by changing the activity of acid extruders (i.e., NHE and NHS).

Calcium overload and cardiac function

The changes in cardiac function caused by calcium overload are reviewed. Intracellular Ca(2+) may increase in different structures [e.g. sarcoplasmic reticulum (SR), cytoplasm and mitochondria] to an excessive level which induces electrical and mechanical abnormalities in cardiac tissues. The electrical manifestations of Ca(2+) overload include arrhythmias caused by oscillatory (V(os)) and non-oscillatory (V(ex)) potentials. The mechanical manifestations include a decrease in force of contraction, contracture and aftercontractions. The underlying mechanisms involve a role of Na(+) in electrical abnormalities as a charge carrier in the Na(+)-Ca(2+) exchange and a role of Ca(2+) in mechanical toxicity. Ca(2+) overload may be induced by an increase in [Na(+)](i) through the inhibition of the Na(+)-K(+) pump (e.g. toxic concentrations of digitalis) or by an increase in Ca(2+) load (e.g. catecholamines). The Ca(2+) overload is enhanced by fast rates. Purkinje fibers are more susceptible to Ca(2+) overload than myocardial fibers, possibly because of their greater Na(+) load. If the SR is predominantly Ca(2+) overloaded, V(os) and fast discharge are induced through an oscillatory release of Ca(2+) in diastole from the SR; if the cytoplasm is Ca(2+) overloaded, the non-oscillatory V(ex) tail is induced at negative potentials. The decrease in contractile force by Ca(2+) overload appears to be associated with a decrease in high energy phosphates, since it is enhanced by metabolic inhibitors and reduced by metabolic substrates. The ionic currents I(os) and I(ex) underlie V(os) and V(ex), respectively, both being due to an electrogenic extrusion of Ca(2+) through the Na(+)-Ca(2+) exchange. I(os) is an oscillatory current due to an oscillatory release of Ca(2+) in early diastole from the Ca(2+)-overloaded SR, and I(ex) is a non-oscillatory current due to the extrusion of Ca(2+) from the Ca(2+)-overloaded cytoplasm. I(os) and I(ex) can be present singly or simultaneously. An increase in [Ca(2+)](i) appears to be involved in the short- and long-term compensatory mechanisms that tend to maintain cardiac output in physiological and pathological conditions. Eventually, [Ca(2+)](i) may increase to overload levels and contribute to cardiac failure. Experimental evidence suggests that clinical concentrations of digitalis increase force in Ca(2+)-overloaded cardiac cells by decreasing the inhibition of the Na(+)-K(+) pump by Ca(2+), thereby leading to a reduction in Ca(2+) overload and to an increase in force of contraction.

Possible underlying mechanism for hydrogen peroxide-induced electromechanical suppression in human atrial myocardium

Hydrogen peroxide (H(2)O(2)) and its metabolites have been shown to exert complex effects on the cardiac muscle during cardiac ischemia/reperfusion. The aim of the present study, by perfusing H(2)O(2) or/and different scavengers of oxygen free radicals (OFRs) into the human atrium, is to characterize the electropharmacological effects of H(2)O(2) and explore its possible underlying mechanism. Atrial tissues obtained from the heart of 19 patients undergoing corrective cardiac surgery were used. Transmembrane action potentials were recorded using the conventional microelectrode technique, and contraction of atrial fibers was evaluated in normal [K](o) (4 mM) in the absence and presence of tested agents. H(2)O(2) (30 micro M-3 mM) had a biphasic effect on the contractile force (an increase, followed by a decrease), reduced the 0-phase depolarizing slope (dV/dt), and prolonged the action potential duration (APD) in a concentration-dependent manner. However, even at a concentration as high as 3 mM, H(2)O(2) did not influence diastolic membrane potential (DMP). Pretreatment with N-(mercaptopropionyl)-glycine (N-MPG), a specific scavenger of the. OH free radical, significantly blocked the 3 mM H(2)O(2)-induced electromechanical changes, while the pretreatment with L-methionine (L-M), a specific scavenger of HOCl free radical, did not. Our data suggests that the toxic effects of H(2)O(2) are caused mainly through the generation of. OH, which is attributed to the electropharmacological inhibitory effects seen in the human atrium.

Functional evidence for intracellular acid extruders in human ventricular myocardium

Intracellular pH (pH(i)) is a major homeostatic system within the cell. Changes in pH(i) exert great influence on cardiac contractility and rhythm. Both the housekeeping Na+ - H+ exchanger (NHE) and the Na+ - HCO3- symporter (NHS) have been confirmed as major transporters for the active acid extrusion mechanism in animal cardiomyocytes. However, whether the NHE and NHS functionally coexist in human ventricular cardiomyocytes remains unclear. We therefore examined the mechanism of pH(i) recovery following an NH4Cl-induced intracellular acidosis in the human ventricular myocardium. The pH(i) was monitored by microspectrofluorimetry by the use of intracellular 2',7'-bis(2-carboxyethyl)-5(6)-carboxy-fluorescein (BCECF)-fluorescence. HOE 694 (30 microM), a specific NHE inhibitor could block pH(i) recovery from induced intracellular acidosis completely in nominally HCO3- -free HEPES Tyrode solution, but it only partially inhibited the pH(i) recovery in 5% CO2/HCO3- Tyrode solution. In 5% CO2/HCO3- Tyrode solution, the addition of HOE 694 together with DIDS (an NHS inhibitor) or the removal of [Na+](o) could entirely inhibit the acid extrusion. We conclude for the first time that two different acid extruders, HCO3- -independent and -dependent, were most likely the NHE and NHS, respectively, that functionally coexisted in the human ventricular cardiomyocytes.

Hydrogen peroxide-induced intracellular acidosis and electromechanical inhibition in the diseased human ventricular myocardium

Accumulation of oxygen free radicals is an important mediator of post-ischemia/reperfusion cardiac dysfunction. However, oxidative injury has not been well characterized in human cardiac tissues. In the present study, we superfused hydrogen peroxide (H(2)O(2)) into the diseased human ventricle in order to assess the effects of oxygen free radicals on the electromechanical parameters and the intracellular pH (pH(i)), and to test the ability of certain potential cardioprotective agents, including scavengers of hydrogen peroxide (dibenzamidostilbene disulfonic acid; DBDS), the.OH free radical (N-(mercaptopropionyl)-glycine; N-MPG), and the HOCl free radical (L-methionine), to protect against oxidative injury. Disease human ventricular tissues were obtained from patients undergoing heart transplantation. Electrophysiological experiments were performed using a traditional micropipette, while the pH(i) was measured by microspectrofluorimetry. We found that (a) H(2)O(2) (30 microM-3 mM) induced a significant dose-dependent intracellular acidosis, (b) H(2)O(2) (30 microM-3 mM) had a notable dose-dependent biphasic effect on the contractile force (an increase, followed by a decrease), while moderate concentrations of H(2)O(2) also inhibited the generation of action potential and increased the diastolic resting force significantly, and (c) N-MPG caused significant block of both the intracellular acidosis and the electromechanical inhibition induced by 3 mM H(2)O(2), whereas L-methionine and DBDS did not. Our data suggest that the toxic effects of H(2)O(2) are caused mainly through the generation of.OH, which is attributed to the intracellular acidosis seen in the diseased human ventricle.

Intracellular pH regulatory mechanism in human atrial myocardium: functional evidence for Na(+)/H(+) exchanger and Na(+)/HCO(3)(-) symporter

Intracellular pH (pH(i)) exerts considerable influence on cardiac contractility and rhythm. Over the last few years, extensive progress has been made in understanding the system that controls pH(i) in animal cardiomyocytes. In addition to the housekeeping Na(+)-H(+) exchanger (NHE), the Na(+)-HCO(3)(-) symporter (NHS) has been demonstrated in animal cardiomyocytes as another acid extruder. However, whether the NHE and NHS functions exist in human atrial cardiomyocytes remains unclear. We therefore investigated the mechanism of pH(i) recovery from intracellular acidosis (induced by NH(4)Cl prepulse) using intracellular 2',7'-bis(2-carboxethyl)-5(6)-carboxy-fluorescein fluorescence in human atrial myocardium. In HEPES (nominally HCO(3)(-)-free) Tyrode solution, pH(i) recovery from induced intracellular acidosis could be blocked completely by 30 microM 3-methylsulfonyl-4-piperidinobenzoyl, guanidine hydrochloride (HOE 694), a specific NHE inhibitor, or by removing extracellular Na(+). In 3% CO(2)-HCO(3)(-) Tyrode solution, HOE 694 only slowed the pH(i) recovery, while addition of HOE 694 together with 4,4'-diisothiocyanatostilbene-2,2'-disulphonic acid (an NHS inhibitor) or removal of extracellular Na(+) inhibited the acid extrusion entirely. Therefore, in the present study, we provided evidence that two acid extruders involved in acid extrusion in human atrial myocytes, one which is HCO(3)(-) independent and one which is HCO(3)(-) dependent, are mostly likely NHE and NHS, respectively. When we checked the percentage of contribution of these two carriers to pH(i) recovery following induced acidosis, we found that the activity of NHE increased steeply in the acid direction, while that of NHS did not change. Our present data indicate for the first time that two acid extruders, NHE and NHS, exist functionally and pH(i) dependently in human atrial cardiomyocytes.

Electromechanical effects of caffeine in failing human ventricular myocardium

We studied, using standard microelectrode technique, the effects of caffeine alone and in conjunction with epinephrine in ventricular myocardial fibers obtained from the failing hearts of 12 recipients of heart transplant. Results revealed that caffeine (1-3 mM) prolonged slightly the duration of fast response action potential near final repolarization and the twitch curve but slightly increased, or even decreased, the twitch force. Epinephrine (3 microM) induced a submaximal positive inotropic effect in myocardial fibers obtained from the failing hearts. Caffeine at 1 mM significantly potentiated the force of contraction and accelerated the rate of twitch relaxation increased by epinephrine. Increasing concentration of caffeine to 3 mM in the presence of epinephrine did not enhance further the twitch force but rather induced the delayed after-depolarization in two of eight experiments. In a preparation from one patient with dilated cardiomyopathy, the combination of caffeine and epinephrine induced repetitive triggered rhythms. The present findings suggest that, in human ventricular myocardium obtained from certain patients transplanted for congestive heart failure, caffeine could induce positive inotropy and triggered automaticity through a potentiation of the actions of catecholamines.

Depressant Effects of Prostacyclin in Human Atrial Fibers and Cardiomyocytes

The aim of this study was to characterize the electropharmacological effects of prostacyclin (PGI(2)) in human atrial fibers and cardiomyocytes. Atrial tissues obtained from the hearts of 28 patients undergoing corrective cardiac surgery were used. Transmembrane action potentials were recorded using a conventional microelectrode technique, and twitch force by a transducer. Effects of PGI(2) (1 nM-10 &mgr;M) on action potential characteristics and contraction of atrial fibers were evaluated in normal [K](o) (4 mM) and high [K](o) (27 mM) in the absence and presence of cardiotonic agents. In addition, atrial and ventricular myocytes were isolated enzymatically from atrial tissues and hearts of 4 patients undergoing cardiac transplant. The effects of PGI(2) on Na- and Ca-dependent inward currents (I(Na) and I(Ca)) of cardiomyocytes were tested. In 9 human atrial fibers showing fast-response action potentials (mean dV/dt(max) = 101 +/- 15 Vs(-1)) in 4 mM [K](o), PGI(2) did not influence dV/dt(max) of phase 0 depolarization even at 1 &mgr;M. However, at a concentration as low as 10 nM, PGI(2) depressed spontaneous rhythms or slow-response action potentials in high-K-depolarized fibers. PGI(2) also depressed delayed afterdepolarizations and aftercontractions induced by cardiotonic agents. In isolated cardiomyocytes, PGI(2) reduced I(Ca) but not I(Na). The present findings show that, in human atrial fibers and cardiomyocytes, PGI(2) induces greater depressant effects on the slow-response action potential, I(Ca) and triggered activity than on the fast-response action potential. It is suggested that PGI(2) may act through a selective reduction of transmembrane Ca influx. Copyright 1994 S. Karger AG, Basel

Ionic mechanisms responsible for the antiarrhythmic action of dehydroevodiamine in guinea-pig isolated cardiomyocytes

1. Dehydroevodiamine alkaloid (DeHE), an active ingredient of a Chinese herbal medicine Wu-Chu-Yu (Evodiae frutus), has been shown to decrease aterial blood pressure in experimental animals and prolong action potential duration in cardiac cells. The aim of the present study was to explore the ionic basis of its possible antiarrhythmic effects. 2. Guinea-pig atrial and ventricular myocytes were isolated enzymatically and the ionic currents were recorded under whole-cell patch-clamp with single suction pipettes. 3. DeHE at a concentration of 0.1 microM inhibited reversibly the time-dependent outward K current (delayed rectifier, Ik) and the Na-dependent inward current (INa). 4. In low-K (1 mM) and high-Ca (9 mM) solution, DeHE also depressed the delayed afterdepolarizations (DAD) and the transient inward current (Iti) induced by 2 microM strophanthidin. On the other hand, DeHE occasionally induced early afterdepolarizations and slow response action potentials at a depolarized level. 5. At higher concentrations (1 microM and above), the L-type Ca current (ICa,L) was moderately inhibited. 6. The present findings indicate that DeHE may depress triggered arrhythmias in Ca-overloaded guinea-pig cardiac myocytes through its inhibitory actions on INa, Iti and, to a smaller extent, ICa. DeHE may also exert class III antiarrhythmic effect through a reduction of outward K currents (Ik) across the sarcolemma.

Electropharmacological effects of sandostatin in human atrial fibers

We studied the actions of sandostatin (0.1-1000 nM), an analogue of somatostatin, on human atrial tissues obtained from hearts of 20 patients undergoing corrective cardiac surgery. In 3 preparations showing fast response action potential in normal [K]0 Tyrode solution, sandostatin induced little effect, even at the highest concentration (1 microM). In 10 preparations showing a slow rate of phase-0 depolarization when atrial fibers were depolarized (maximum diastolic potential near -40 mV) in high [K]0 (27 mM), sandostatin at concentrations as low as 1 nM decreased significantly the velocity of the upstroke, and the amplitude of slow response of the action potential as well as the force of contraction. In 6 experiments on spontaneously active atrial fibers (maximum diastolic potential = -53.8 +/- 2.7 mV), sandostatin increased the spontaneous cycle length in a fashion dependent upon concentration. The decrease in spontaneous rate of firing was associated with an inhibition of the late diastolic slope, a change also induced by somatostatin. A longer period of washout, however, (30 min or longer) was required for complete recovery from the depressant effects. Sandostatin (0.1-100 nM) also depressed triggered activity induced by cardiotonic agents. The present findings indicate that sandostatin induces a prolonged action in human atrial cells. Sandostatin may depress abnormal automatic rhythms through an inhibition of transmembrane influx of calcium.

Pacemaker activity is modulated by tissue levels of cyclic adenosine 3',5'-monophosphate in human atrial fibers

We studied the role of tissue cyclic AMP levels in the chronotropic effects of theophylline on automatic human atrial fibers obtained from the hearts of 17 patients undergoing corrective open-heart surgery. Atrial fibers were perfused with Tyrode solution and transmembrane action potentials were recorded with a conventional microelectrode technique. In normal Tyrode solution, theophylline (0.1-1 mM) often decreased the late diastolic slope and the spontaneous rate. In the presence of 0.3-1 microM epinephrine, however, theophylline dose-dependently increased the diastolic slope, the rate of spontaneous discharges and the force of contraction. The increase in tissue level of cyclic AMP (+288 +/- 69%) induced by 0.3 mM theophylline in the presence of epinephrine was much greater than the increase (+73 +/- 19%) in the absence of epinephrine. It is concluded that pacemaker activity in human atrial fibers is modulated by tissue levels of cyclic AMP and theophylline may induce atrial tachycardia through an increase in the diastolic slope and the rate of discharges of automatic atrial fibers.

Arrhythmogenic effects of theophylline in human atrial tissue

We studied the effects of theophylline on the transmembrane action potential and the contractile force of human atrial fibers obtained from the hearts of 15 patients, undergoing corrective open-heart surgery. Atrial fibers were perfused with Tyrode solution and driven electrically at a constant rate of 60 beats per min. Theophylline (0.1-1 mM) steepened the diastolic depolarization, increased the amplitude of oscillatory potential during diastole and facilitated the development of spontaneous slow response action potentials. These arrhythmogenic effects of theophylline were suppressed after diltiazem (0.1-0.3 microM) pretreatment. The present findings provide the electrophysiologic evidence that abnormal atrial automaticity as a result of triggered activity may be the underlying cause for atrial ectopic activity and multifocal atrial tachycardia in patients taking theophylline.