Application of ventricular tachyarrhythmia definitions of the updated Lambeth Conventions provides incompatibility with earlier results, masks antifibrillatory activity and reduces inter-observer agreement

The Lambeth Conventions (LC I), a landmark guidance document for arrhythmia research was updated and arrhythmia definitions were changed in the new Lambeth Conventions II (LC II). This study examined whether the arrhythmia definitions of LC I and LC II yield the same qualitative results and whether LC II improves inter-observer agreement. Two independent investigators performed blinded arrhythmia analysis of the electrocardiograms of isolated, Langendorff rat hearts subjected to regional ischemia and perfused with Class I antiarrhythmics with 3 or 5 mM K⁺ in the perfusate. Data obtained with arrhythmia definitions of LC I and LC II were compared within and between observers. Applying ventricular fibrillation (VF) definition of LC II significantly increased VF incidence and reduced VF onset time irrespective of treatment by detecting 'de novo' VF episodes not found by LC I. LC II reduced the number of ventricular tachycardia (VT) episodes and simultaneously increased the number of VF episodes as compared with the respective values obtained according to LC I. Using VF definition of LC II masked the significant antifibrillatory effects of flecainide and the high K⁺ concentration identified with the VF definition of LC I. When VF incidence was tested, a very strong inter-observer agreement was found according to LC I, whereas using VF definition of LC II reduced inter-observer agreement. It is concluded that LC II shifts some tachyarrhythmias from VT to VF class, and thus results obtained by arrhythmia definitions of LC I and LC II are not compatible; VF definition of LC II may change the conclusion of pharmacological, physiological and pathophysiological arrhythmia investigations and may reduce inter-observer agreement. Thus, VT and VF definitions of LC II should be amended in order to increase compatibility and inter-observer agreement.

Hyperventilation assists proarrhythmia development during delayed repolarization in clofilium-treated, anaesthetized, mechanically ventilated rabbits

Hyperventilation reduces partial pressure of CO₂ (PCO₂) in the blood, which results in hypokalaemia. Hypokalaemia helps the development of the life-threatening torsades de pointes type ventricular arrhythmia (TdP) evoked by repolarization delaying drugs. This implies that hyperventilation may assist the development of proarrhythmic events. Therefore, this study experimentally investigated the effect of hyperventilation on proarrhythmia development during delayed repolarization. Phenylephrine (an α₁-adrenoceptor agonist) and clofilium (as a representative repolarization delaying agent inhibiting the rapid component of the delayed rectifier potassium current, I_Kr) were administered intravenously to pentobarbital-anaesthetized, mechanically ventilated, open chest rabbits. ECG was recorded, and the onset times and incidences of the arrhythmias were determined. Serum K⁺, pH and PCO₂ were measured in arterial blood samples. Clofilium prolonged the rate corrected QT interval. TdP occurred in 15 animals (TdP+ group), and did not occur in 14 animals (TdP- group). We found a strong, positive, linear correlation between serum K⁺ and PCO₂. There was no relationship between the occurrence of TdP and the baseline K⁺ and PCO₂ values. However, a positive, linear correlation was found between the onset time of the first arrhythmias and the K⁺ and PCO₂ values. The regression lines describing the relationship between PCO₂ and onset time of first arrhythmias were parallel in the TdP+ and TdP- groups, but the same PCO₂ resulted in earlier arrhythmia onset in the TdP+ group than in the TdP- group. We conclude that hyperventilation and hypocapnia with the resultant hypokalaemia assist the multifactorial process of proarrhythmia development during delayed repolarization. This implies that PCO₂ and serum K⁺ should be controlled tightly during mechanical ventilation in experimental investigations and clinical settings when repolarization-delaying drugs are applied.

Analysis of the contribution of I(to) to repolarization in canine ventricular myocardium

BACKGROUND AND PURPOSE

The contribution of the transient outward potassium current (I(to)) to ventricular repolarization is controversial as it depends on the experimental conditions, the region of myocardium and the species studied. The aim of the present study was therefore to characterize I(to) and estimate its contribution to repolarization reserve in canine ventricular myocardium.

EXPERIMENTAL APPROACH

Ion currents were recorded using conventional whole-cell voltage clamp and action potential voltage clamp techniques in canine isolated ventricular cells. Action potentials were recorded from canine ventricular preparations using microelectrodes. The contribution of I(to) to repolarization was studied using 100 µM chromanol 293B in the presence of 0.5 µM HMR 1556, which fully blocks I(Ks).

KEY RESULTS

The high concentration of chromanol 293B used effectively suppressed I(to) without affecting other repolarizing K(+) currents (I(K1), I(Kr), I(p)). Action potential clamp experiments revealed a slowly inactivating and a 'late' chromanol-sensitive current component occurring during the action potential plateau. Action potentials were significantly lengthened by chromanol 293B in the presence of HMR 1556. This lengthening effect induced by I(to) inhibition was found to be reverse rate-dependent. It was significantly augmented after additional attenuation of repolarization reserve by 0.1 µM dofetilide and this caused the occurrence of early afterdepolarizations. The results were confirmed by computer simulation.

CONCLUSIONS AND IMPLICATIONS

The results indicate that I(to) is involved in regulating repolarization in canine ventricular myocardium and that it contributes significantly to the repolarization reserve. Therefore, blockade of I(to) may enhance pro-arrhythmic risk.

Relevance of anaesthesia for dofetilide-induced torsades de pointes in alpha1-adrenoceptor-stimulated rabbits

BACKGROUND AND PURPOSE

No information is available concerning the effects of anaesthetics in the most frequently used in vivo pro-arrhythmia model. Accordingly, in this study we examined the effect of pentobarbital, propofol or alpha-chloralose anaesthesia on the pro-arrhythmic activity of the class III anti-arrhythmic dofetilide in alpha(1)-adrenoceptor-stimulated rabbits.

EXPERIMENTAL APPROACH

Rabbits anaesthetized intravenously with pentobarbital, propofol or alpha-chloralose were infused simultaneously with the alpha(1)-adrenoceptor agonist phenylephrine (15 microg kg(-1) min(-1), i.v.) and dofetilide (0.04 mg kg(-1) min(-1), i.v.). The electrocardiographic QT interval, the T (peak)-T (end) interval and certain QT variability parameters were measured. The heart rate variability and the baroreflex sensitivity were utilized to assess the vagal nerve activity. The spectral power of the systolic arterial pressure was calculated in the frequency range 0.15-0.5 Hz to assess the sympathetic activity.

KEY RESULTS

Pentobarbital considerably reduced, whereas propofol did not significantly affect the incidence of dofetilide-induced torsades de pointes (TdP) as compared with the results with alpha-chloralose (40% (P=0.011) and 70% (P=0.211) vs 100%, respectively). In additional experiments, neither doubling of the rate of the dofetilide infusion nor tripling of the rate of phenylephrine infusion elevated the incidence of TdP to the level seen with alpha-chloralose. None of the repolarization-related parameters predicted TdP. The indices of the parasympathetic and sympathetic activity were significantly depressed in the alpha-chloralose and propofol anaesthesia groups.

CONCLUSIONS AND IMPLICATIONS

In rabbits, anaesthetics may affect drug-induced TdP genesis differently, which must be considered when results of different studies are compared.

Effects of citalopram and fluoxetine on the corticocerebral blood flow in conscious rabbits

Depression, which is associated with an increased incidence of vascular events, frequently occurs following stroke. Selective serotonin reuptake inhibitory drugs (SSRIs) as antidepressants, are well tolerated, and also seem to be effective in post-stroke depression. The aim of this study was to investigate the effects of the SSRIs citalopram and fluoxetine, on the corticocerebral blood flow (cCBF) in rabbits with unilateral carotid occlusion induced cerebral ischemia. The cCBF was measured by the hydrogen clearance technique. After determination of the mean baseline cCBF, the effects of individual doses (0.1, 0.3 and I mg/kg) of citalopram or fluoxetine on the cCBF were investigated. Following the induction of an impaired cCBF, the changes in cCBF after drug treatments in this condition were likewise measured. The mean arterial blood pressure (MABP) and the heart rate (HR) from the electrocardiogram (ECG) were also determined. Neither citalopram nor fluoxetine influenced the cCBF in the control group. Fluoxetine improved the cCBF only very slightly in the ischemic animals. In contrast, all the doses of citalopram exerted pronounced and dose-dependent cCBF-increasing effects in the animals with unilateral carotid occlusion (maximal mean ACBF: 10, 16 and 27 ml/min/100 g tissue). The HR was decreased in both groups. Only citalopram treatment led to a slight MABP-decreasing effect. Besides enhancement of the serotonergic transmission in the brain, the cCBF-increasing effect of citalopram under ischemic conditions may be of benefit in post-stroke and vascular depression.

Combined pharmacological block of I(Kr) and I(Ks) increases short-term QT interval variability and provokes torsades de pointes

BACKGROUND AND PURPOSE

Assessing the proarrhythmic potential of compounds during drug development is essential. However, reliable prediction of drug-induced torsades de pointes arrhythmia (TdP) remains elusive. Along with QT interval prolongation, assessment of the short-term variability of the QT interval (STV(QT)) may be a good predictor of TdP. We investigated the relative importance of I(Ks) and I(Kr) block in development of TdP together with correlations between QTc interval, QT interval variability and incidence of TdP.

EXPERIMENTAL APPROACH

ECGs were recorded from conscious dogs and from anaesthetized rabbits given the I(Kr) blocker dofetilide (DOF), the I(Ks) blocker HMR-1556 (HMR) and their combination, intravenously. PQ, RR and QT intervals were measured and QTc and short-term variability of RR and QT intervals calculated.

KEY RESULTS

DOF increased QTc interval by 20% in dogs and 8% in rabbits. HMR increased QTc in dogs by 12 and 1.9% in rabbits. Combination of DOF+HMR prolonged QTc by 33% in dogs, by 16% in rabbits. DOF or HMR given alone in dogs or HMR given alone in rabbits induced no TdP. Incidence of TdP increased after DOF+HMR combinations in dogs (63%) and following HMR+DOF (82%) and DOF+HMR combinations (71%) in rabbits. STV(QT) markedly increased only after administration of DOF+HMR combinations in both dogs and rabbits.

CONCLUSION AND IMPLICATIONS

STV(QT) was markedly increased by combined pharmacological block of I(Kr) and I(Ks) and may be a better predictor of subsequent TdP development than the measurement of QTc interval prolongation.

Effect of glibenclamide and glimepiride treatment on the development of myocardial infarction in rats

The effect of glibenclamide and glimepiride, two orally active antidiabetic sulphonylurea derivatives, on the development of myocardial infarction has been compared. Permanent coronary artery ligation was induced in rats and the development of infarction was evaluated by a computer-assisted method after nitroblue-terazolium staining. Seven-day coronary ligation produced enlargement of the left ventricular cavity, scar thinning and thickening of the non-infarcted myocardium. Glibenclamide treatment (5 mg/kg b.i.d. intraperitoneally) decreased the infarct volume (29.1 +/- 3.5% vs. 39.1 +/- 3.2% in controls), that occurred primarily as a result of more significant thinning of the scar tissue (1.6 +/- 0.04 mm vs. 2.0 +/- 0.13 mm in controls). Glibenclamide also inhibited the thickening of the non-infarcted ventricular septum (2.1 +/- 0.10 mm vs. 2.9 +/- 0.10 mm in controls). In contrast to the effects of glibenclamide, glimepiride treatment (5 mg/kg b.i.d. intraperitoneally) inhibited the enlargement of the left ventricular cavity (15.2 +/- 1.1% vs. 19.9 +/- 1.2% of the left ventricular volume in controls), it did not precipitate scar thinning and did not influence the development of hypertrophy of the non-infarcted myocardium. These results suggest that glimepiride treatment might inhibit the development of left ventricular dilatation after myocardial infarction. Glibenclamide treatment, however, producing a thinning of the scar tissue may further precipitate morphological changes that can contribute to the development of heart failure.

The slow component of the delayed rectifier potassium current in undiseased human ventricular myocytes

OBJECTIVE

The purpose of this study was to investigate the properties of the slow component of the delayed rectifier potassium current (I(Ks)) in myocytes isolated from undiseased human left ventricles.

METHODS

The whole-cell configuration of the patch-clamp technique was applied in 58 left ventricular myocytes from 15 hearts at 37 degrees C. Nisoldipine (1 microM) was used to block inward calcium current (I(Ca)) and E-4031 (1-5 microM) was applied to inhibit the rapid component of the delayed rectifier potassium current (I(Kr)).

RESULTS

In 31 myocytes, an E-4031 insensitive, but L-735,821 and chromanol 293B sensitive, tail current was identified which was attributed to the slow component of I(K) (I(Ks)). Activation of I(Ks) was slow (tau=903+/-101 ms at 50 mV, n=14), but deactivation of the current was relatively rapid (tau=122.4+/-11.7 ms at -40 mV, n=19). The activation of I(Ks) was voltage independent but its deactivation showed clear voltage dependence. The deactivation was faster at negative voltages (about 100 ms at -50 mV) and slower at depolarized potentials (about 300 ms at 0 mV). In six cells, the reversal potential was -81.6+/-2.8 mV on an average which is close to the K(+) equilibrium potential suggesting K(+) as the main charge carrier.

CONCLUSION

In undiseased human ventricular myocytes, I(Ks) exhibits slow activation and fast deactivation kinetics. Therefore, in humans I(Ks) differs from that reported in guinea pig, and it best resembles I(Ks) described in dog and rabbit ventricular myocytes.

Influence of nitrovasodilators and cyclooxygenase inhibitors on cerebral vasoreactivity in conscious rabbits

Since the nitric oxide (NO) and cyclooxygenase pathways have been suggested to have important roles in most vasodilations, our aim was to study the influence of cyclooxygenase inhibitors and nitrovasodilators on cerebrovascular reserve capacity. Corticocerebral blood flow was measured by hydrogen polarography during hypercapnia and acetazolamide stimuli in conscious rabbits. The measurements were repeated in the presence of N(omega)-nitro-L-arginine methyl ester (L-NAME) and indomethacin as nitric oxide synthase (NOS) and cyclooxygenase inhibitors. The effects of nitroglycerin and isosorbide-5-nitrate were also tested. L-NAME completely, while indomethacin markedly inhibited the hypercapnic corticocerebral blood flow response. Nitroglycerin and isosorbide-5-nitrate significantly attenuated hypercapnia elicited corticocerebral blood flow increase. The different treatments reduced only moderately the acetazolamide-induced corticocerebral blood flow response. These results lend support to the hypothesis that antithrombotic and antiinflammatory medication (cyclooxygenase inhibitors) and nitrovasodilator treatments could interfere with the measurement of cerebrovascular reactivity resulting in underestimation of the cerebrovascular reserve capacity in patients taking these drugs.

Effects of endothelin-1 on calcium and potassium currents in undiseased human ventricular myocytes

Endothelins have been reported to exert a wide range of electrophysiological effects in mammalian cardiac cells. These results are controversial and human data are not available. Our aim was to study the effects of endothelin-1 (ET-1, 8 nmol/l) on the L-type calcium current (ICa-L) and various potassium currents (rapid component of the delayed rectifier, IKr; transient outward current, Ito; and the inward rectifier K current, IK1) in isolated human ventricular cardiomyocytes. Cells were obtained from undiseased donor hearts using collagenase digestion via the segment perfusion technique. The whole-cell configuration of the patch-clamp technique was applied to measure ionic currents at 37 degrees C. ET-1 significantly decreased peak ICa-L from 10.2+/-0.6 to 6.8+/-0.8 pA/pF at +5 mV (66.7% of control, P<0.05, n=5). This reduction of peak current was accompanied by a lengthening of inactivation. The voltage dependence of steady-state activation and inactivation was not altered by ET- 1. IKr, measured as tail current amplitudes at 40 mV, decreased from 0.31+/-0.02 to 0.06+/-0.02 pA/pF (20.3% of control, P<0.05, n=4) after exposure to ET-1. ET-1 failed to change the peak amplitude of Ito, measured at +50 mV (9.3+/-4.6 and 9.0+/-4.4 pA/pF before and after ET-1, respectively), or steady-state IK1 amplitude, measured at the end of a 400-ms hyperpolarization to -100 mV (3.6+/-1.4 and 3.7+/-1.4 pA/pF, n=4). The present results indicate that in undiseased human ventricular myocytes ET-1 inhibits both ICa-L and IKr; however, the degree of suppression of the two currents is different.

Isosorbide-2-mononitrate reduces the consequences of myocardial ischaemia, including arrhythmia severity: implications for preconditioning

The effects of the intracoronary administration of isosorbide-2-mononitrate (ISMN; 3 microg kg(-1) min(-1)), a major metabolite of isosorbide dinitrate, were examined in chloralose-urethane anaesthetized dogs before and during a 25 min, acute occlusion of the left anterior descending coronary artery. The only significant haemodynamic effects of ISMN administration were a slight (-11 +/- 2 mmHg) decrease in arterial blood pressure and a decrease (< 12%) in diastolic coronary vascular resistance. Coronary occlusion in the presence of ISMN led to a markedly reduced incidence and severity of ventricular arrhythmias compared to those in control, saline-infused dogs. There were fewer ectopic beats (62 +/- 35 versus 202 +/- 72; p < 0.05), a lower incidence (25% versus 75%; p < 0.05) and number of episodes (0.7 +/- 0.4 versus 4.3 +/- 2.1; p < 0.05) of ventricular tachycardia and fewer dogs fibrillated during the ischaemic period (17% versus 82%; p < 0.05). More dogs given ISMN survived the combined ischaemia-reperfusion insult (50% versus 0%; p < 0.05). Changes in ST-segment elevation (recorded by epicardial electrodes) and in the degree of inhomogeneity of electrical activation within the ischaemic area were much less pronounced throughout the occlusion period in dogs given ISMN. These results add weight to the hypothesis that the previously reported antiarrhythmic effects of ischaemic preconditioning, and of the intracoronary administration of nicorandil, involve nitric oxide.

Atrial natriuretic peptide reduces the severe consequences of coronary artery occlusion in anaesthetized dogs

The aim of the present study was to examine the effects of atrial natriuretic peptide (ANP) on the responses to coronary artery occlusion. In chloralose-urethane anaesthetised mongrel dogs either saline (controls) or human synthetic ANP was infused intravenously (10 microg kg(-1) + 0.1 microg kg(-1) min(-1)), starting 30 min before and continuing 10 min during a 25 min occlusion of the left anterior descending coronary artery (LAD). ANP infusion resulted in a fall in mean arterial blood pressure (by 17 +/- 2 mmHg, p < 0.05), a transient (max. at 5 min) increase in coronary blood flow (by 24 +/- 5 ml min(-1), p < 0.05), and a reduction in coronary vascular resistance (by 0.27 +/- 0.05 mmHg ml(-1), p < 0.05). When the LAD coronary artery was occluded, there was a less marked elevation in left ventricular end-diastolic pressure (LVEDP) in the ANP-treated dogs than in the controls (9.0 +/- 0.9 versus 12.2 +/- 0.8 mmHg, p < 0.05). Compared to the controls, ANP reduced the number of ventricular premature beats (VPBs, 26 +/- 12 versus 416 +/- 87, p < 0.05), the number of episodes of ventricular tachycardia per dogs (VT, 0.7 +/- 0.3 versus 12.4 +/- 4.2, p < 0.05), and the incidences of VT (45% versus 100%, p < 0.05) and ventricular fibrillation (VF 18% versus 57%, p < 0.05) during occlusion. Reperfusion of the ischaemic myocardium at the end of the occlusion period led to VF in all the control dogs (survival from the combined ischaemia-reperfusion insult was therefore 0%), but VF following reperfusion was much less in the dogs given ANP (survival 64%; p < 0.05). The severity of myocardial ischaemia, as assessed from changes in the epicardial ST-segment and the degree of inhomogeneity, was significantly less marked in dogs given ANP. We conclude that ANP protects the myocardium from the consequences of myocardial ischaemia resulting from acute coronary artery occlusion and reperfusion in anaesthetized dogs.

Antiarrhythmic and electrophysiological effects of GYKI-16638, a novel N-(phenoxyalkyl)-N-phenylalkylamine, in rabbits

The effect of N-[4-[2-N-methyl-N-[1-methyl-2-(2, 6-dimethylphenoxy)ethylamino]-ethyl]-phenyl]-methanesulfonamide. hydrochloride (GYKI-16638; 0.03 and 0.1 mg/kg, i.v.), a novel antiarrhythmic compound, was assessed and compared to that of D-sotalol (1 and 3 mg/kg, i.v.) on arrhythmias induced by 10 min of coronary artery occlusion and 10 min of reperfusion in anaesthetized rabbits. Also, its cellular electrophysiological effects were studied in rabbit right ventricular papillary muscle preparations and in rabbit single isolated ventricular myocytes. In anaesthetized rabbits, intravenous administration of 0.03 and 0.1 mg/kg GYKI-16638 and 1 and 3 mg/kg D-sotalol significantly increased survival during reperfusion (GYKI-16638: 82% and 77%, D-sotalol: 75% and 83% vs. 18% in controls, P<0.05, respectively). GYKI-16638 (0.1 mg/kg) significantly increased the number of animals that did not develop arrhythmias during reperfusion (46% vs. 0% in controls, P<0.05). In isolated rabbit right ventricular papillary muscle, 2 microM GYKI-16638, at 1 Hz stimulation frequency, lengthened the action potential duration at 50% and 90% repolarization (APD(50-90)) without influencing the resting membrane potential and action potential amplitude (APA). It decreased the maximal rate of depolarization (V(max)) in a use-dependent manner. This effect was statistically significant only at stimulation cycle lengths shorter than 700 ms. The offset kinetics of this V(max) block were relatively rapid, the corresponding time constant for recovery of V(max) was 328.2+/-65.0 ms. In patch-clamp experiments, performed in rabbit ventricular myocytes, 2 microM GYKI-16638 markedly depressed the rapid component of the delayed rectifier outward and moderately decreased the inward rectifier K(+) current without significantly altering the slow component of the delayed rectifier and transient outward K(+) currents. These results suggest that in rabbits, GYKI-16638 has an in vivo antiarrhythmic effect, comparable to that of D-sotalol, which can be best explained by its combined Class I/B and Class III actions.

Improvement by phosphoramidon of damaged endothelial function in porcine coronary artery

BACKGROUND

The bradykinin (BK)-induced endothelium-dependent relaxation is impaired in the presence of elevated potassium concentration enhancing the vasospastic tendency of large coronary arteries. Inhibition of the angiotensin-converting enzyme responsible for bradykinin degradation was found to enhance the endothelium-dependent relaxation by BK. The aim of the present study was to investigate the effect of phosphoramidon, known to inhibit a BK-metabolizing neutral endopeptidase enzyme, on relaxation of porcine-isolated coronary artery in depolarizing solution.

METHODS

Endothelium intact porcine coronary artery rings were studied in organ chambers. The rings were isometrically contracted with potassium chloride (30 mmol/L) and the response to BK (1 to 1,000 nmol/L)-induced relaxation was investigated in the presence of nitric oxide synthase inhibitor Nomega-nitro-L-arginine (300 micromol/L) alone and in combination with the cyclooxygenase inhibitor indomethacin (10 micromol/L), and that of the inhibitor of calcium-dependent potassium channels tetraethylammonium (7 mmol/L). Under these conditions, phosphoramidon (10 micromol/L), an inhibitor of a neutral endopeptidase enzyme (EC.3.4.24.11.), which is responsible for the degradation of BK, was used to enhance the endothelium-dependent relaxation.

RESULTS

Phosphoramidon potentiated the maximum vasorelaxant effect of BK in Nomega-nitro-L-arginine (control 26.6%+/-10.86% versus phosphoramidon 49.05%+/-4.52%; n = 6, p < 0.05) or in Nomega-nitro-L-arginine + indomethacin-pretreated rings (control 20.7%+/-9.92% versus phosphoramidon 42.0%+/-12.26%; n = 5, p < 0.05) and this increased vasodilation was not modified by tetraethylammonium.

CONCLUSIONS

In the present study phosphoramidon potentiated the effect of BK in the absence of nitric oxide and prostaglandins in porcine-isolated coronary artery. This effect did not depend on tetraethylammonium-sensitive potassium channels. Phosphoramidon may be a useful pharmacologic tool for preserving the vasorelaxing capacity of coronary arteries after cardioplegia.

New antiarrhythmic agents: a conceptually novel approach

Synthesis and biological evaluation of novel phenoxyalkyl amines exhibiting both class Ib and class III type electrophysiological properties are described. Two compounds showed excellent antiarrhythmic effects against reperfusion induced arrhythmias.

Effects of moxonidine on corticocerebral blood flow under normal and ischemic conditions in conscious rabbits

Hypertension associated with excessive liberation of circulating and tissue catecholamines is an independent risk factor for further cardiovascular complications and an important predictor of stroke. Moxonidine is a centrally acting anti-hypertensive drug with potent action on I1-imidazoline receptors. It inhibits catecholamine release and is therefore expected to exert an antiadrenergic effect at various levels in the regulation of the cardiovascular system. The aim of this study was to investigate the effect of moxonidine (0.025-0.1 mg/kg, i.v.) on the normal and unilateral carotid occlusion-induced impaired corticocerebral blood flow (cCBF) determined by hydrogen polarography, on mean arterial blood pressure (MABP) and heart rate (HR) in conscious rabbits. Moxonidine produced a reduction of MABP and HR. On the other hand, after administration of the drug, a significant increase in the normal and impaired cCBF was observed. Because the improvement in cCBF was conspicuous in both normal and ischemic conditions, moxonidine might be beneficial not only in the treatment of hypertension but also in the management of cerebral ischemia.

The role of the delayed rectifier component IKs in dog ventricular muscle and Purkinje fibre repolarization

1. The relative contributions of the rapid and slow components of the delayed rectifier potassium current (IKr and IKs, respectively) to dog cardiac action potential configuration were compared in ventricular myocytes and in multicellular right ventricular papillary muscle and Purkinje fibre preparations. Whole-cell patch-clamp techniques, conventional microelectrode and in vivo ECG measurements were made at 37C. 2. Action potential duration (APD) was minimally increased (less than 7%) by chromanol 293B (10 microM) and L-735,821 (100 nM), selective blockers of IKs, over a range of pacing cycle lengths (300-5000 ms) in both dog right ventricular papillary muscles and Purkinje fibre strands. D-Sotalol (30 microM) and E-4031 (1 microM), selective blockers of IKr, in the same preparations markedly (20-80%) lengthened APD in a reverse frequency-dependent manner. 3. In vivo ECG recordings in intact anaesthetized dogs indicated no significant chromanol 293B (1 mg kg-1 i.v.) effect on the QTc interval (332.9 +/- 16.1 ms before versus 330.5 +/- 11.2 ms, n = 6, after chromanol 293B), while D-sotalol (1 mg kg-1 i.v.) significantly increased the QTc interval (323.9 +/- 7.3 ms before versus 346.5 +/- 6.4 ms, n = 5, after D-sotalol, P < 0.05). 4. The current density estimated during the normal ventricular muscle action potential (i.e. after a 200 ms square pulse to +30 mV or during a 250 ms long 'action potential-like' test pulse) indicates that substantially more current is conducted through IKr channels than through IKs channels. However, if the duration of the square test pulse or the 'action potential-like' test pulse was lengthened to 500 ms the relative contribution of IKs significantly increased. 5. When APD was pharmacologically prolonged in papillary muscle (1 microM E-4031 and 1 microg ml-1 veratrine), 100 nM L-735,821 and 10 microM chromanol 293B lengthened repolarization substantially by 14.4 +/- 3.4 and 18. 0 +/- 3.4% (n = 8), respectively. 6. We conclude that in this study IKs plays little role in normal dog ventricular muscle and Purkinje fibre action potential repolarization and that IKr is the major source of outward current responsible for initiation of final action potential repolarization. Thus, when APD is abnormally increased, the role of IKs in final repolarization increases to provide an important safety mechanism that reduces arrhythmia risk.

Analysis of the electrophysiological effects of ambasilide, a new antiarrhythmic agent, in canine isolated ventricular muscle and purkinje fibers

The aim of the study was to determine the in vitro rate-dependent cellular electrophysiological effects of ambasilide (10 and 20 microM/l), a new investigational antiarrhythmic agent, in canine isolated ventricular muscle and Purkinje fibers by applying the standard microelectrode technique. At the cycle length (CL) of 1000 ms, ambasilide significantly prolonged the action potential duration measured at 90% repolarization (APD(90)) in both ventricular muscle and Purkinje fibers. Ambasilide (10 microM/l) produced a more marked prolongation of APD(90) at lower stimulation frequencies in Purkinje fibers (at CL of 2000 ms = 56.0 +/- 16.1%, n = 6, versus CL of 400 ms = 15.1 +/- 3.7%, n = 6; p < 0.05), but, in 20 microM/l, this effect was considerably diminished (15.2 +/- 3.6%, n = 6, versus 7.3 +/- 5.1%, n = 6, p < 0.05). In ventricular muscle, however, both concentrations of the drug induced an almost frequency-independent lengthening of APD(90) in response to a slowing of the stimulation rate (in 20 microM/l at CL of 5000 ms = 19.0 +/- 1.5%, n = 9, versus CL of 400 ms = 16.9 +/- 1.4%, n = 9). Ambasilide induced a marked rate-dependent depression of the maximal rate of rise of the action potential upstroke (V(max)) (in 20 microM/l at CL of 300 ms = -45.1 +/- 3.9%, n = 6, versus CL of 5000 ms = -8.5 +/- 3.9%, n = 6, p < 0. 05, in ventricular muscle) and the corresponding recovery of V(max) time constant was tau = 1082.5 +/- 205.1 ms (n = 6). These data suggest that ambasilide, in addition to its Class III antiarrhythmic action, which is presumably due to its inhibitory effect on the delayed rectifier potassium current, possesses I/B type antiarrhythmic properties as a result of the inhibition of the fast sodium channels at high frequency rate with relatively fast kinetics. This latter effect may play an important role in its known less-pronounced proarrhythmic ("torsadogenic") potential.

Comparison of the vasorelaxing effect of cromakalim and the new inodilator, levosimendan, in human isolated portal vein

In the present study the vasorelaxing capacity of cromakalim, an ATP-sensitive potassium-channel (KATP channel) activator, and that of levosimendan, a new positive inotropic and vasodilating drug with calcium sensitizing and potassium-channel-activating properties, were compared in human isolated portal vein. Based on the 50% effective concentrations (EC50), levosimendan was found to be about 16-fold more potent (EC50 = 0.281+/-0.03 microM) as a relaxing agent than cromakalim (EC50 = 4.53+/-0.12 microM) in noradrenaline-precontracted portal venous preparations. Glibenclamide, the known inhibitor of KATP channels, was able to prevent the cromakalim-induced venodilation completely. Glibenclamide (15 microM) decreased the quasi-maximal effect of levosimendan (at 1.27 microm by about 60%) and also the effects of those submicromolar concentrations of the inodilator (at 0.1 microM by 23%, at 0.3 microM by 27% and at 0.7 microM by 19%, on average) which were therapeutically effective in preliminary human studies. These findings indicate that, in the human portal vein, both cromakalim and levosimendan are powerful vasorelaxants and that a considerable part of the relaxing effect induced by levosimendan is of cromakalim type.

Pharmacological modification of the dispersion of repolarization in the heart: importance of the M cells

Several in vitro and in vivo investigations have provided data supporting the existence of M cells in the deep subepicardial layers of the ventricles in a number of species. Characterized by unique electrophysiological and pharmacological features, this population of cells is regarded to have a significant role in creating dispersion of repolarization in the ventricular wall and thus contribute importantly to arrhythmogenesis, in particular to intramural reentry and triggered activity. Focusing on M cells, the authors summarize recent findings and concepts concerning the pharmacological heterogeneity of different cell and tissue types found within the ventricles and explore how these differences may contribute to electrocardiographic manifestations. On the basis of literary data and of their own results they conclude that studying the electrical and pharmacological inhomogeneity within the ventricular wall may provide a better understanding of the pathophysiological processes that give rise to cardiac rhythm disturbances and the mechanisms by which antiarrhythmic agents act to suppress and in some cases aggravate arrhythmias.

Delayed protection against ventricular arrhythmias by monophosphoryl lipid-A in a canine model of ischaemia and reperfusion

Bacterial endotoxin reduces the severity of ventricular arrhythmias which occur when a coronary artery is occluded several hours later. We have now examined in anaesthetised dogs the effects on ischaemia and reperfusion-induced arrhythmias, of a non-toxic derivative component of the endotoxin molecule of the lipid-A (monophosphoryl lipid-A). This was given intravenously, in doses of 10 and 100 microg kg(-1), 24 h prior to coronary artery occlusion. Arrhythmia severity was markedly reduced by monophosphoryl lipid-A. During ischaemia, ventricular premature beats were reduced from 315+/-84 in the vehicle controls to 89+/-60 (with the lower dose of monophosphoryl lipid-A) and 53+/-23 (P<0.05) with the higher dose. The incidence of ventricular tachycardia was reduced from 75% to 25% (P<0.05) and 31% (P<0.05), and the number of episodes of ventricular tachycardia from 13.4+/-4.9 per dog to 1.1+/-1.1 (P<0.05) and 1. 2+/-0.9 (P<0.05) after doses of 10 and 100 microg kg(-1), respectively. The incidence of ventricular fibrillation during occlusion and reperfusion in the control group was 96% (15/16), i.e., only 6% (1/16) dogs survived the combined ischaemia-reperfusion insult. Monophosphoryl lipid-A (100 microg kg(-1)) significantly reduced the incidence of occlusion-induced ventricular fibrillation (from 50% to 7%; P<0.05), and increased survival following reperfusion to 54% (P<0.05). Monophosphoryl lipid-A also significantly reduced ischaemia severity as assessed from ST-segment elevation recorded from epicardial electrodes as well as the degree of inhomogeneity of electrical activation within the ischaemia area. There were no haemodynamic differences prior to coronary occlusion between vehicle controls and monophosphoryl lipid-A-treated dogs. These results demonstrate that monophosphoryl lipid-A reduces arrhythmia severity 24 h after administration. Although the precise mechanisms are still unclear, there is some evidence that nitric oxide and prostanoids (most likely prostacyclin) may be involved because the dual inhibition of nitric oxide synthase and cyclooxygenase enzymes by administration of aminoguanidine and meclofenamate abolished the marked antiarrhythmic protection resulted from monophosphoryl lipid-A treatment 24 h previously.

Effects of pituitary adenylate cyclase-activating polypeptide on the cyclooxygenase pathway of rat platelets and on platelet aggregation

Several data suggest that pituitary adenylate cyclase-activating polypeptide (PACAP) is involved in the regulation of local circulation. One possible role of PACAP in the regulation of circulation is that, it may modify the cyclooxygenase pathway of the arachidonate cascade in platelets. Our study was designed to study the effect of PACAP on the cyclooxygenase pathway of rat platelets and on platelet aggregation. PACAP (10(-7) and 10(-6) M) significantly inhibited the cyclooxygenase pathway of platelets, mostly the thromboxane synthesis. Pretreatment with a PACAP receptor antagonist, PACAP(6-38), or with an inhibitor of protein kinase A, H-89, shows that the effects of PACAP on the cyclooxygenase pathway were diminished. In the aggregation studies, PACAP inhibited both the arachidonic acid-induced and the thrombin-induced platelet aggregation. It can be concluded that PACAP inhibits the cyclooxygenase pathway of rat platelets via a specific PACAP receptor-activated, cAMP-dependent pathway, and these effects of PACAP are involved in the inhibition of platelet aggregation.

Repeated cardiac pacing extends the time during which canine hearts are protected against ischaemia-induced arrhythmias: role of nitric oxide

Right ventricular pacing in lightly anaesthetized dogs (4x5 min periods at a pacing rate of 220 beats/min) protects against the consequences of coronary artery occlusion when this is initiated 24 h after the pacing stimulus. The main purpose of the present experiments was to determine whether repeating the pacing stimulus, at a time when protection from the initial stimulus had faded (48 h), prolonged the protection afforded against ischaemia-induced ventricular arrhythmias and other ischaemic changes (epicardial ST-segment mapping; changes in the degree of electrical inhomogeneity in the ischaemic region). Dogs were paced on two occasions, with a 48 h period between and, at different times (48, 72 and 96 h) after the second pacing stimulus, were re-anaesthetized and subjected to occlusion of the left anterior descending coronary artery. There was a marked reduction in the severity of ischaemia-induced arrhythmias 48 and 72 h after the second pacing stimulus (reduction in occlusion-induced and reperfusion-induced ventricular fibrillation, e.g. at 72 h 0/11 during occlusion and only 3/11 following reperfusion, compared to 7/21 and 10/21 respectively in the controls P<0.05). The protection had disappeared 96 h following the second pacing stimulus. Changes in ST-segment elevation and in the degree of inhomogeneity largely followed these changes in the severity of ventricular arrhythmias. The results suggest the possibility of maintaining protection against life-threatening arrhythmias following coronary occlusion by repeating a preconditioning pacing stimulus. We also demonstrate that this prolonged protection afforded by repeated cardiac pacing is mediated by nitric oxide, since the marked antiarrhythmic effect observed, e.g. 72 h after the second pacing stimulus, was abolished when S-(2-aminoethyl)-isothiourea (AEST), a particularly selective inhibitor of iNOS, had been administered before coronary artery occlusion.

Application of neural networks in structure-activity relationships

Methodology and application of artificial neural networks in structure-activity relationships are reviewed focusing on the most frequently used three-layer feedforward back-propagation procedure. Two applications of neural networks are presented and a comparison of the performance with those of CoMFA and a classical QSAR analysis is also discussed.

Vasoactive substances produced by cultured rat brain endothelial cells

The vasoactive substances synthesized by primary cultures of rat brain endothelial cells were investigated and compared to those from two, immortalized cell lines, RBE4 and GP8. The vasoactivity of endothelium-derived substances was measured on isolated canine coronary artery. Vascular tone was significantly decreased by both primary and GP8, but not by RBE4 cells. Indomethacin pretreatment of primary and GP8 cells turned vasorelaxation into contraction while N(omega)-nitro-L-arginine pretreatment decreased the vasorelaxation induced by primary, but not by GP8 cells. Eicosanoid production was determined after incubation with [14C]arachidonic acid. The predominant vasoactive eicosanoid was prostaglandin E2 in both primary and GP8 cells. RBE4 cells synthetized mainly prostaglandin E2 and thromboxane B2 and significantly less prostaglandin E2 than did either primary or GP8 cells. The capacity of cerebral endothelium to regulate vascular tone by production of dilator and constrictor substances can be preserved under certain circumstances in immortalized cell lines.

Comparison of the efficacy of glibenclamide and glimepiride in reperfusion-induced arrhythmias in rats

The effect of glibenclamide and glimepiride, two orally active antidiabetic sulphonylurea derivatives, was investigated on the development of reperfusion-induced arrhythmias and it was compared to their blood glucose lowering action. Arrhythmias were produced by reperfusion following 6 min coronary artery ligation in anaesthetised rats. Glimepiride pretreatment (0.001-0.01-0.1-5.0 mg/kg i.p., 30 min before coronary occlusion) significantly decreased the incidence of irreversible ventricular fibrillation and increased the survival rate during reperfusion (64%, 61%, 60%, and 67% vs. 27% in controls). Glibenclamide produced similar effect (81% survival) only in a dose of 5 mg/kg, while smaller doses were ineffective. The minimal hypoglycaemic dose and the dose required to inhibit significantly the oral glucose loading-induced hyperglycaemia were similar (1 and 0.1 mg/kg, respectively) after glibenclamide and glimepiride. It is concluded that although the blood glucose lowering potency of glibenclamide and glimepiride is rather similar, glimepiride appears to be more potent than glibenclamide in preventing reperfusion-induced cardiac arrhythmias.

Delayed rectifier potassium current in undiseased human ventricular myocytes

OBJECTIVE

The purpose of the study was to investigate the properties of the delayed rectifier potassium current (IK) in myocytes isolated from undiseased human left ventricles.

METHODS

The whole-cell configuration of the patch-clamp technique was applied in 28 left ventricular myocytes from 13 hearts at 35 degrees C.

RESULTS

An E-4031 sensitive tail current identified the rapid component of IK (IKr) in the myocytes, but there was no evidence for an E-4031 insensitive slow component of IK (IKs). When nifedipine (5 microM) was used to block the inward calcium current (ICa), IKr activation was fast (tau = 31.0 +/- 7.4 ms, at +30 mV, n = 5) and deactivation kinetics were biexponential and relatively slow (tau 1 = 600.0 +/- 53.9 ms and tau 2 = 6792.2 +/- 875.7 ms, at -40 mV, n = 7). Application of CdCl2 (250 microM) to block ICa altered the voltage dependence of the IKr considerably, slowing its activation (tau = 657.1 +/- 109.1 ms, at +30 mV, n = 5) and accelerating its deactivation (tau = 104.0 +/- 18.5 ms, at -40 mV, n = 8).

CONCLUSIONS

In undiseased human ventricle at 35 degrees C IKr exists having fast activation and slow deactivation kinetics; however, there was no evidence found for an expressed IKs. IKr probably plays an important role in the frequency dependent modulation of repolarization in undiseased human ventricle, and is a target for many Class III antiarrhythmic drugs.

Comparison of the cellular electrophysiological characteristics of canine left ventricular epicardium, M cells, endocardium and Purkinje fibres

Electrophysiological differences among M cells, epicardium, endocardium and Purkinje fibres of the canine ventricle were studied over a wide range of stimulation cycle lengths, and the pharmacological response of these cell types to the sodium channel blocker tetrodotoxin, calcium channel blocker nifedipine and ATP-sensitive potassium channel activator pinacidil was compared. The experiments were carried out by applying standard intracellular microelectrode technique in isolated dog left ventricular preparations. The results confirmed the existence of M cells in the canine ventricle, in addition, the distribution of the rate of rise of the action potential upstroke and action potential amplitude values reflecting probably the inhomogeneity of the fast sodium current in these cells was revealed. It was also demonstrated that M cells differ from Purkinje fibres in some aspects which were not expected from previous investigations: (1) The early portion of the action potential duration restitution curve in M cells is more similar to that of endocardial and epicardial cells than to Purkinje fibres. (2) The plateau phase of the action potentials in Purkinje fibres developed at a more negative potential range than that in the other cell types studied. (3) The pharmacological response to tetrodotoxin and pinacidil in M cells resembles to that in the endocardial and epicardial cells more than in the Purkinje fibres. Our results provide further evidence in support of the existence of M cells but also indicate that there are important electrophysiological as well as pharmacological differences between M cells and Purkinje fibres.

Differential efficacy of vasodilators in hypercholesterolaemic rabbits

The effects of hypercholesterolaemia on the endothelium-dependent and -independent vascular reactivity of the superior mesenteric artery has been examined in anaesthetized rabbits in-vivo. Rabbits were fed with either standard or cholesterol-enriched diet for 24 weeks. Plasma lipids and changes in the endothelin content of plasma and vascular tissue were measured in the superior mesenteric artery and in the thoracic aorta. The functional severity of atherosclerosis was determined by examining vascular responses in the isolated thoracic aorta. The blood flow in the superior mesenteric artery was measured by transit-time flowmetry and drugs were injected through an intra-abdominal aortic catheter. Acetylcholine (5, 10, 20 microg kg(-1)) elicited dose-dependent, mesenteric vasodilation in normocholesterolaemic rabbits. In hypercholesterolaemic animals the response to acetylcholine was completely abolished and even became a vasoconstriction. Endothelin levels in plasma and in the vascular tissue were significantly elevated in hypercholesterolaemic animals compared with controls. Cromakalim at a dose of 3 microg kg(-1), elicited similar mesenteric vasodilation in hypercholesterolaemic and normocholesterolaemic animals. These experiments show that the endothelium-dependent responses of the superior mesenteric artery to acetylcholine are functionally impaired by prolonged hypercholesterolaemia, that this altered vascular reactivity is associated with the elevation of endothelin levels in the circulation and in vascular tissues, and that in hypercholesterolaemia the mesenteric vasodilator effect of the K+-channel opener cromakalim is entirely preserved, suggesting that severe hypercholesterolaemia does not depress the function of ATP-sensitive potassium channels in mesenteric vascular smooth muscle.

Endothelium-dependent vasorelaxant and anti-aggregatory effect and mechanism of action of some antifibrinogen RGD (Arg-Gly-Asp-containing) peptides

Vasorelaxation caused by some antifibrinogen RGD (Arg-Gly-Asp-containing) peptides and their basic mechanism of action was studied on rabbit isolated thoracic aortic rings preconstricted with 0.25 microM phenylephrine. GRGDS (Gly-Arg-Gly-Asp-Ser-OH) and RGDV (Arg-Gly-Asp-Val-OH) caused dose-dependent relaxation. RGDS (Arg-Gly-Asp-Ser-OH) had a biphasic effect (a transient relaxation followed by a contraction) while GRGDS-[SE] (Gly-Arg-Gly-Asp-Ser(SO3)-OH) did not change the isometric tone of precontracted aortic preparations. GRGDS and RGDV exerted no relaxing effect on endothelium-denuded blood vessels suggesting that the vascular action of these peptides was entirely dependent on the presence of functionally intact endothelium. L-NG-Nitro-arginine (30 microM) attenuated the relaxation induced by GRGDS and abolished that induced by RGDV. All of the four RGD congeners inhibited ADP-induced aggregation of human platelets. These findings indicate that the relaxant effect of RGDV is mediated exclusively by the nitric oxide pathway, but GRGDS could cause, besides nitric oxide release, the release of another substance which is different from nitric oxide. Because the rank order of the vasorelaxant potencies of RGD peptides differed from that found for their anti-aggregatory activities, a vascular effector mechanism mediated by an RGD-recognizing structure other than the known glycoprotein IIb/IIIa-like RGD-binding site is suggested.

Comparison of the antiarrhythmic and the proarrhythmic effect of almokalant in anaesthetised rabbits

In this study the antiarrhythmic and the proarrhythmic activities of almokalant, a selective class III antiarrhythmic agent, were compared. The antiarrhythmic effect of the drug was tested in pentobarbital-anaesthetised rabbits. Arrhythmia was evoked by occluding and releasing the left circumflex coronary artery. Almokalant in a dose of 250 nmol/kg i.v., significantly decreased the incidence of reperfusion induced ventricular fibrillation (21% vs. 75% in the control group) and increased the proportion of surviving animals during reperfusion (86% vs. 42%). The proarrhythmic effect of almokalant was examined during alpha1-adrenoceptor stimulation in chloralose-anaesthetised rabbits. Almokalant (75 nmol/kg per min) triggered torsade de pointes arrhythmias in 8 animals out of 11. The dose of almokalant (mean+/-S.E.M.) required to produce this effect was 1181+/-519 nmol/kg. It is concluded that, although almokalant is an effective antiarrhythmic agent against ischaemia-reperfusion induced arrhythmias, it has marked proarrhythmic activity during alpha1-adrenoceptor stimulation.

The dopamine receptor agonist Z1046 reduces ischaemia severity in a canine model of coronary artery occlusion

Z1046, (S)-6[[6-[[2-(2-methoxyphenoxy)ethyl]amino]propyl]amino]-5,6,7,8-tetra-h ydro-1,2-naphtalenediol dihydrochloride, is an agonist at both dopamine D1 and D2 receptors. Since stimulation of dopamine D2 receptors inhibits noradrenaline release, and because cardiac noradrenaline release has been implicated in the genesis of early ischaemia-induced, life-threatening ventricular arrhythmias, the effect of Z1046 has been examined for its effects on coronary artery occlusion in chloralose urethane anaesthetised mongrel dogs. Z1046 (10 microg kg(-1) intravenously or 1 microg kg(-1) by local intracoronary injection) decreased heart rate and reduced arterial blood pressure and coronary blood flow, effects prevented by the prior administration of domperidone (40 microg kg(-1) i.v.). The ischaemic changes induced by a 25-min occlusion of the left anterior descending coronary artery (including ST-segment elevation and ventricular ectopic activity) were much less marked in those dogs administered Z1046 and survival from the combined ischaemia reperfusion insult was increased from 7% to 36% (P < 0.05). These effects of Z1046 were partly attenuated by domperidone. We conclude that the anti-ischaemic effects of Z1046 are due to inhibition of cardiac sympathetic responses. Studies using rat isolated perfused mesenteric vascular bed preparations subjected to sympathetic nerve stimulation confirmed that Z1046 inhibits synaptic transmission without modifying vascular responses to noradrenaline.

Polarographic detection of nitric oxide released from cardiovascular compounds in aqueous solutions

In order to detect the concentration of nitric oxide, known to be one of the biologically active principles of certain cardiovascular compounds, a highly selective polarographic/amperometric device was used. The nitric oxide-releasing properties of sodium nitroprusside, nitroglycerine, nicorandil, and the molsidomine metabolite, 3-morpholinosydnonimine, were compared in the following cell-free experimental solutions in vitro: in Krebs-Henseleit solution with and without a sulfhydryl donor, L-cysteine, in an acidic, reducing medium, and in Krebs-Henseleit solution with superoxide dismutase enzyme. Sodium nitroprusside released similar concentrations of nitric oxide in Krebs-Henseleit solution and in the acidic, reducing medium. L-Cysteine inhibited the release of nitric oxide at physiological pH. In the presence of nitroglycerine, nitric oxide signals were detected in the acidic, reducing environment and in L-cysteine-rich Krebs-Henseleit solution but not in the absence of the sulfhydryl donor. Amperometric signals could not be detected after adding nicorandil in all the experimental conditions used. 3-Morpholinosydnonimine released nitric oxide only in the presence of the superoxide dismutase enzyme. Our results suggest that the polarographic electrode is able to detect the release of nitric oxide from sodium nitroprusside, nitroglycerine, and 3-morpholinosydnonimine in the absence of biological material. The present observations support the importance of the chemical environment during the detection of nitric oxide from donor compounds in the common in vitro bathing systems.

Effect of disopyramide on potassium currents in rabbit ventricular myocytes

The effects of disopyramide (1-30 microM) on the 4-aminopyridine sensitive transient outward current (I(to)), on the rapid component of the delayed rectifier potassium current (I(Kr)) and on the inward rectifier potassium current (I(kl)) were studied in single rabbit ventricular myocytes at 35 degrees C by applying the whole-cell configuration of the patch clamp technique. Disopyramide significantly decreased the amplitude of I(to) (from 1510 +/- 122 pA at control to 1015 +/- 21 pA after 30 microM disopyramide at +50 mV; n = 5). This effect was not voltage- or use-dependent. Disopyramide (10 microM) influenced neither the recovery from inactivation of I(to) nor the steady-state inactivation curve. The drug dose dependently decreased the time constant of the fast component of the decay of I(to) (tau(f) = 6.41 +/- 0.25 ms, n = 24 for control; and 2.20 +/- 0.38 ms, n = 5 after 30 microM disopyramide at +50 mV). The fractional block caused by 30 microM disopyramide as a function of time was well fitted by a single exponential function with time constant of 1.48 +/- 0.18 ms (n = 5), most likely reflecting the binding kinetics of the drug to the open channel. The offset kinetics of the drug was estimated by using a double-pulse protocol and its time constant was 3.9 +/- 0.5 ms. Disopyramide (30 microM) did not influence significantly the onset of inactivation measured at -20 mV. The estimated EC50 value for the I(to) block by disopyramide was 14.1 microM. Our results are consistent with an open-channel block of I(to) by disopyramide, however, a weak, drug-induced increase of the rate of inactivation and a moderate tonic block cannot be excluded. The amplitude of the outward tail current attributed to I(Kr) was depressed dose dependently by disopyramide (after clamping the cells back to the holding potential from +30 mV, 139.5 +/- 10.9 pA for control, and 30.7 +/- 3.2 pA in the presence of 10 microM disopyramide; n = 11). The estimated EC50 was 1.8 microM. I(to) is thus less sensitive to disopyramide than I(Kr). I(kl) was not influenced significantly by disopyramide, even when applied in the highest tested concentration (30 microM). It is concluded that in rabbit ventricular myocytes disopyramide blocks not only I(Kr), but also I(to), both of which may play an important role in the well established repolarization lengthening and antiarrhythmic effects of the drug.

Suppression of erythromycin-induced early afterdepolarizations and torsade de pointes ventricular tachycardia by mexiletine

Erythromycin is a selective IKr-blocking, action potential duration (APD)-prolonging drug, which may induce early afterdepolarizations (EADs) and torsade de pointes ventricular tachycardia. The successful termination of an erythromycin-induced clinical torsades de pointes by the authors with mexiletine prompted them to investigate in vitro whether erythromycin is able to induce EADs in Purkinje fibers and, if so, whether EADs are suppressible or not by mexiletine. Electrically stimulated canine Purkinje fibers (n = 9) were superfused with erythromycin (200 mg/l) and action potentials were recorded by an intracellular microelectrode technique. Erythromycin induced a pronounced prolongation of APD and the appearance of EADs in all Purkinje preparations (9/9). After the addition of mexiletine (10 mM), a marked shortening of APD and the disappearance of EADs (7/9) were observed. Mexiletine, an inhibitor of the tetrodotoxin-sensitive window Na(+)-current, may prevent IKr-blocking drug-induced torsade de pointes ventricular tachycardia by abolishing APD prolongation and EADs.

Frequency-dependent Cardiac Electrophysiologic Effects of Tedisamil: Comparison With Quinidine and Sotalol

BACKGROUND: Tedisamil is a potent bradycardiac/antiischemic drug known to lengthen cadiac repolarization by blocking various potassium channels. Recent in vivo experiments revealed that it is an antiarrhythmic agent. It was therefore of interest to compare the cellular electrophysiologic effects of tedisamil with those of quinidine and sotalol in isolated cardiac preparations. METHODS AND RESULTS: The conventional microelectrode technique was applied in isolated dog cardiac Purkinje and ventricular muscle fibers and in rabbit left atrial muscle. Tedisamil (1 µM) and sotalol (30 µM) lengthened, while quinidine (10 µM) shortened action potential duration in dog Purkinje fibers. The phase 1 repolarization was delayed by tedisamil and quinidine and not changed by sotalol. In dog ventricular muscle and in rabbit atrial muscle, all three drugs studied lengthened repolarization. In dog Purkinje fiber, tedisamil and sotalol lengthened action potential duration more at slow than at high stimulation frequency (reverse use-dependence). In dog ventricular muscle fibers, the effect of the drugs was not clearly frequency dependent. In rabbit atrial muscle fibers, the quinidine-evoked repolarization lengthening was most pronounced at intermediate cycle lengths (500-1000 ms). Tedisamil and quinidine but not sotalol depressed the maximal rate of depolarization (V(max)), which depended on the stimulation frequency (use-dependence). The nature of the use-dependent V(max) block differed between quinidine and tedisamil. Quinidine decreased V(max) at a relatively wide range of stimulation frequencies whle tedisamil. Quinidine decreased V(max) at a relatively wide range of stimulation frequencies while tedisamil decreased V(max) largely at high rate of stimulation. Tedisamil and quiinidine prevented or decreased the pinacidil-evoked action potential shortening in dog ventricular muscle, suggesting block of the ATP-dependent potassium channels (I(KATP)), while with sotalol such effect was not observed. CONCLUSIONS: Although tedisamil, quinidine, and sotalol are known to lengthen the QT interval, their cellular electrophysiologic effects substantially differ. Tedisamil lengthens repolarization and prevents pinacidil-evoked action potential duration shortening, suggesting I(K(ATP)) blockade. Its effect on the V(max) is limited mostly to fast heart rate. These electrophysiologic effects of tedisamil resemble those of chronic amiodarone treatment.

Does the opening of ATP-sensitive K+ channels modify ischaemia-induced ventricular arrhythmias in anaesthetised dogs?

These experiments were designed to determine whether there is any change in the severity of ventricular arrhythmias resulting from coronary artery occlusion in anaesthetised mongrel dogs if ATP-sensitive potassium channels were already open at the time of coronary occlusion. To achieve this we locally infused the K(ATP) channel opener levcromakalim, in a total dose of 3 microg/kg, and given by slow infusion over a 30 min period directly into a side branch of the left anterior descending coronary artery. This dose increased blood flow in that main artery by 30% (and by 7% in the adjacent left circumflex artery). The degree of inhomogeneity of electrical activation, measured from the left ventricular wall distal to the occlusion site, was unaffected by levcromakalim administration but there was significant epicardial ST-elevation, perhaps indicating K+ egression from cells. Following coronary artery occlusion there was no marked difference in the severity of arrhythmias between control and levcromakalim-treated dogs, except for an increased number of episodes of ventricular tachycardia due entirely to effects in two of the nine treated dogs. We conclude that opening cardiac K(ATP) channels with levcromakalim, at this one dose level, and administered directly to the left ventricular wall, does not significantly modify arrhythmia severity during ischaemia. These results cannot be extrapolated to studies in which such drugs markedly reduce coronary perfusion pressure.

Bradykinin and endothelial-cardiac myocyte interactions in ischemic preconditioning

Myocardial ischemia results in the release of a variety of vasoactive substances from coronary vascular endothelial cells and/or from cardiac myocytes. Some of these substances appear to be protective and include nitric oxide and bradykinin. One hypothesis for the pronounced antiarrhythmic effects of preconditioning involves the early generation of bradykinin and, subsequently, nitric oxide. Evidence for early bradykinin release has come from clinical studies involving patients undergoing coronary angioplasty where, in 4 of 5 patients, there was evidence for elevated kinin levels in coronary sinus blood either during balloon inflation (i.e., ischemia) or deflation (reperfusion). The levels reached are sometimes considerable (increases 10-20 fold). The second piece of evidence comes from dogs subjected to a preconditioning stimulus (2 x 5 min periods of ischemia), followed 20 min later by occlusion of the same artery for a 25-min period. This preconditioning procedure markedly reduces ischemia-induced ventricular arrhythmias and, although under resting conditions there was little difference between arterial and coronary sinus bradykinin levels (125 +/- 22 and 157 +/- 41 pg/mL, respectively), there was a marked increase in coronary sinus levels in preconditioned dogs before the prolonged occlusion (637 +/- 293 pg/mL compared with 114 +/- 18 pg/mL in nonpreconditioned dogs); levels at the end of the prolonged occlusion in the preconditioned dogs were also higher (577 +/- 305 pg/mL compared with 162 +/- 34 pg/mL in control dogs). Other evidence for the involvement of bradykinin and nitric oxide comes from studies in which the generation, or effects, of these mediators have been suppressed (e.g., with the bradykinin B2 receptor blocking agent icatibant, with inhibitors of the L-arginine-nitric oxide pathway, and by methylene blue). The conclusion is that early bradykinin release is protective under conditions of ischemia, is presumably enhanced during therapy with angiotensin-converting enzyme (ACE) inhibitors and is suppressed under conditions of endothelial dysfunction.

Modification by bradykinin B2 receptor blockade of protection by pacing against ischaemia-induced arrhythmias

In dogs, rapid cardiac pacing, by way of a pacing electrode in the right ventricle, protects against ventricular arrhythmias when a coronary artery is occluded immediately after cessation of the pacing period. This represents a form of ischaemic preconditioning. The role of bradykinin in mediating the protective effects of rapid cardiac pacing in this model was investigated using a selective antagonist of bradykinin at B2 receptors (icatibant; HOE 140). In the presence of icatibant cardiac pacing (220 beats min(-1)) resulted in more severe ischaemia (as assessed by ST-segment elevation from the pacing electrode at the end of the stimulus) and to a higher incidence of ventricular arrhythmias during the pacing protocol. When the coronary artery was occluded under such conditions the antiarrhythmic protection afforded by cardiac pacing was not seen although other indices of reduced ischaemia severity (epicardial ST-segment mapping; changes in the degree of inhomogeneity of electrical activation within the ischaemic area) were not affected by icatibant treatment. These results suggest that bradykinin is an important trigger mediator involved in the protective effects of cardiac pacing. Whether this is due to the generation of endothelium-derived protective substances (such as nitric oxide and prostacyclin) or whether it results from a direct effect on B2 receptors in cardiac myocytes is unclear.

KATP channel modulators increase survival rate during coronary occlusion-reperfusion in anaesthetized rats

We investigated the effect of ATP-sensitive K+ channel (KATP) openers (pinacidil and cromakalim), and a KATP blocker (glibenclamide) on reperfusion-induced arrhythmias in pentobarbitone-anaesthetized rats. Arrhythmias were induced by reperfusion following a 6 min ligation of the left main coronary artery. Rats were pretreated with pinacidil (0.1 or 0.3 mg/kg), or cromakalim (28 or 56 micrograms/kg), or glibenclamide (5 mg/kg), or vehicle. Pinacidil and cromakalim produced dose-related reductions in blood pressure. Pinacidil (0.1 mg/kg) and cromakalim (56 micrograms/kg) significantly decreased the incidence of reperfusion-induced ventricular fibrillation and increased survival. Glibenclamide did not decrease ventricular fibrillation incidence, yet improved survival by increasing the possibility of recovery from ventricular fibrillation. The present study suggests that both opening and blocking KATP channels may increase survival during coronary occlusion and reperfusion in anaesthetized rats.

Increased cardiac workload by adrenoceptor agonists for the estimation of potential antiischemic activity in a conscious rabbit model

The antiischemic effect of drugs can be detected at a lower dose range if the cardiac workload is increased. A brief period of frequency-loading (ventricular overpacing = VOP) results in well-defined, reproducible changes in cardiac parameters in the conscious, chronically instrumented rabbit; however, rapid pacing frequently evoked ventricular tachycardia or even fatal ventricular fibrillation. Therefore, cardiac workload has been increased by i.v. administration of adrenoceptor agonists, such as isoproterenol (ISO), phenylephrine (PHE), and their combination, respectively. The doses applied (especially the combination of 2 micrograms/kg ISO and 16 micrograms/kg PHE, giving optimal changes) were sufficient to produce a marked elevation of both the ST segment in the intracavital electrogram and the left ventricular end-diastolic pressure, without evoking cardiac arrhythmias. We compared the effect of this adrenergic "test" stimulus with that of VOP on hemodynamic and electrophysiological parameters of the heart, and furthermore, on the modification of responses to both "test" stimuli by oral administration of the coronary vasodilator: Isosorbide-5-mononitrate (IS-5-N), given in a dose of 40 mg/kg. Both VOP- and ISO+PHE-induced changes were significantly attenuated by IS-5-N, and a temporal coincidence of the maximal effects was found as well. We reached the following conclusion: The combined administration of ISO and PHE not evoking fatal arrhythmias in the dose range applied can replace the more risky VOP as a "test" workload in the estimation of antiischemic action.

Influence of anesthetics on the incidence of reperfusion-induced arrhythmias and sudden death in rats

PURPOSE

We compared the influence of pentobarbital (P, 60 mg/kg), urethane (U, 1.8 g/kg), and a contribution of diazepam with ketamine hydrochloride (D+K, 10 + 100 mg/kg) anesthesia on the incidence of ischemia/ reperfusion-induced arrhythmias in rats. In anesthetized rats, myocardial ischemia was produced by a 6-min ligation of the left main coronary artery, followed by 5 min of reperfusion. The incidence of reperfusion-induced ventricular fibrillation (VF) and ventricular tachycardia (VT) was markedly decreased in the D+K-anesthetized animals compared with the P-anesthetized group (VF, 46 vs. 89%; VT, 64 vs. 94%). The mean blood pressure (MBP) before coronary ligation was significantly lower in U-anesthetized animals (72 +/- 3.5 vs. 102 +/- 4.1 and 108 +/- 5.9 mm Hg in P and D+K, respectively). The MBP recovery was the best in the D+K-anesthetized group. In experimental work, pentobarbitone anesthesia may be recommended for studying arrhythmias, while the combination of diazepam and ketamine may be useful for hemodynamic investigations.

Electrical restitution in rat ventricular muscle

The mechanism of electrical restitution was studied in isolated rat ventricular muscle using drugs that inhibit specific ion currents. The effect of transient changes in cytosolic Ca concentration and Na/Ca exchange in relation to the restitution process was also studied in single ventricular cardiomyocytes. Conventional microelectrode techniques were applied to record action potentials having gradually increasing coupling intervals, each evoked following a train of stimuli with a frequency of 1 Hz. Ion currents were recorded from enzymatically isolated cells using the whole cell patch clamp technique. Ca transients were monitored in myocytes loaded with the fluorescent dye, indo-1. The electrical restitution process in multicellular rat ventricular preparations at 37 degrees C was described as a sum of three exponential components: an early positive component, a subsequent fast negative component and a late negative component, having time constants of 21.9 +/- 1.9, 73.1 +/- 6.0 and 1053 +/- 61 ms, respectively (n = 9). Inhibition of the transient outward K current, the delayed rectifier K current, or the chloride current did not substantially alter these time constants. The early positive and fast negative components were fully abolished by nifedipine or MnCl2. In the presence of caffeine, the fast negative component was absent, while the time constant of the early positive component increased to 39.5 +/- 5.8 ms (n = 5). In single myocytes loaded with indo-1, the Ca transients decayed with a time constant of 151 +/- 12 ms at room temperature (n = 5). These Ca transients were accompanied by inward current tails, identified as a Na/Ca exchange current, having a decay time constant of 140 +/- 4.5 ms. It is concluded that electrical restitution in rat ventricular muscle is relatively little affected by recovery from voltage-dependent inactivation of ion channels, it is rather governed by transient changes in cytosolic Ca concentration possible via Ca-dependent inactivation of the L-type Ca current and activation of the Na/Ca exchange current.