Differential Electrophysiologic Effects of Chronically Administered Amiodarone on Canine Purkinje Fibers versus Ventricular Muscle

BACKGROUND: Acute and chronic treatment with amiodarone has been reported to cause different electrocardiographic changes in patients. The cellular electrophysiologic effects of chronic administration (50 mg/kg/day orally for 6 weeks) and acute superfusion (5 µM in the tissue bath) of amiodarone were therefore studied in dog cardiac ventricular muscle and Purkinje fibers using conventional microelectrode techniques. METHODS AND RESULTS: During stimulation at 1 Hz, chronic amiodarone treatment lengthened the ventricular muscle action potential duration (APD) from 227.8 +/- 6.3 ms (n = 20) to 262.3 +/- 5.2 ms (n = 21; P <.01), but shortened that of Purkinje fibers from 337.6 +/- 9.2 (n = 21) to 308.3 +/- 7.1 (n = 19; P <.05). Acute superfusion of 5 µM amiodarone in cardiac tissue obtained from chronically treated dogs did not change ventricular muscle APD but shortened Purkinje fiber AP from 309.7 +/- 13.6 ms to 281.9 +/- 11.9 ms (n = 8; P <.05). Neither the chronic nor the acute amiodarone exposure prevented the APD shortening in ventricular muscle evoked by 10 µM pinacidil, suggesting that amiodarone does not interfere with the ATP-dependent potassium channels. The normal difference in APD between ventricular muscle and Purkinje fibers in untreated, control preparations was 110 ms but decreased to 46 ms in fibers obtained from dogs chronically treated with amiodarone and increased to 185 ms in fibers obtained from dogs chronically treated with amiodarone and increased to 185 ms in the presence of 30 µM sotalol, a class III antiarrhythmic drug used for comparison. Amiodarone (5 µM) applied directly abolished early afterdepolarizations (EADs) (induced by 1 µM dofetilide + 20 µM BaCl(2) + 2 mM CsCl) in 5 of 6 experiments and caused strong use-dependent V(max) block with relatively fast onset kinetics (rate constant = 1.23 +/- 0.13 AP(-1), n = 5) and offset (time constant = 364 +/- 62.5 ms, n = 5). After chronic amiodarone treatment, in contrast with acute sotalol application (30 µM), no reverse use-dependent effect was observed on the APD in Purkinje fibers. CONCLUSIONS: These results provide further evidence that amiodarone differs from other recognized class III antiarrhythmic drugs (ie, it is a mixed type agent with acute fast kinetic class I [type B] and a unique class III antiarrhythmic action characterized by decreased dispersion of APDs between ventricular muscle and Purkinje fibers). Amiodarone can abolish EADs unlike other class III agents that are usually associated to induction of EADs. These features might be responsible not only for the antiarrhythmic efficacy, but also for the relative safety (low incidence of torsade de pointes) of amiodarone in clinical settings.

Cardiovascular effects of conventional sulfonylureas and glimepiride

Sulfonylureas have, in the past, been reported to have adverse cardiovascular effects. Glimepiride is a new sulfonylurea. In spite of stimulating less insulin secretion, it has, depending on the species, equal or higher blood glucose decreasing activity and according to preliminary studies less cardiovascular activity than glibenclamide. Further studies were performed to confirm the lower cardiovascular activity of glimepiride. The IC50 for inhibition of rilmakalim-activated KATP channel currents in isolated ventricular myocytes was 31.6 nM for glimepiride and 6.8 nM for glibenclamide. In endotoxin shock-rats at a dose of 1 x 2 mg/kg i.v., glibenclamide induced a significantly higher blood pressure increase than glimepiride. At two i.v. doses of 20 mg/kg 4 min apart, in normal rats, glibenclamide produced signs of ischemia in the ECG in nearly all animals, glimepiride almost none, in diabetic rats, glibenclamide produced in all animals a lethal cardiogenic shock preceeded by serious ECG changes, glimepiride only in one fifth of the animals. In open-chest dogs, on intracoronary infusion of equieffective blood glucose-lowering doses, glibenclamide, gliclazide and glimepiride all reduced coronary blood flow, increased coronary resistance, depressed the mechanical activity of the heart, enhanced myocardial O2-extraction, reduced the serum potassium level and induced a moderate endocardial ST-segment elevation, but glimepiride to a significantly less extent than glibenclamide and gliclazide. The presented data confirm that glimepiride at equivalent blood glucose decreasing doses has less cardiovascular activity than conventional sulfonylureas.

Haemodynamic and other effects of sulphonylurea drugs on the heart

Antidiabetic sulphonylureas have attracted a great deal of interest in experimental cardiology to evaluate the role of ATP-sensitive potassium channels in the cardiovascular system. It is well established that KATP channels are present in cardiac cells and also in vascular smooth muscle cells and are implicated in the regulation of myocardial and vascular function. It follows that drugs which open, or inhibit the opening of these channels, might profoundly modify cardiovascular function both under physiological and pathophysiological conditions. This paper reviews the evidence for the role of KATP channels in the cardiovascular system and discusses how the different generations of sulphonylurea drugs interfere with cardiac function. We will particularly concentrate on the haemodynamic effects of different sulphonylureas and shortly discuss how these drugs modify ischaemia-reperfusion arrhythmias.

Nicorandil suppressed ventricular arrhythmias in a canine model of myocardial ischaemia

These experiments were designed to explore the possibility that a K+ channel opener which also donates nitric oxide to the myocardium (nicorandil) may modify ischaemia-induced ventricular arrhythmias in a large animal model. In mongrel dogs anaesthetised with chloralose-urethane and thoracotomised, a side branch of the left anterior descending artery was catheterised for the local intracoronary infusion of nicorandil (2.5 micrograms kg-1 min-1 for 20 min prior to coronary artery occlusion and then continuing throughout the 25 min occlusion period). In this dose, nicorandil had no haemodynamic effects, increased coronary blood flow by up to 16% and significantly reduced the severity of ischaemia-induced arrhythmias (e.g. from nearly 500 ventricular premature beats in the controls to 160 +/- 60 in the nicorandil group). There was a significant reduction in the number of episodes of ventricular tachycardia during the ischaemic period and a reduced incidence of ventricular fibrillation following reperfusion resulting in a 42% survival from the combined ischaemia-reperfusion insult (cf. 0% in the control; P < 0.05). The marked changes that occurred in ST-segment elevation (mapped with epicardial electrodes) and in the inhomogeneity of electrical activation within the ischaemic area in control dogs was markedly reduced in those dogs administered nicorandil. We conclude that the local intracoronary administration of nicorandil reduces the severity of both ischaemia and the life-threatening arrhythmias that result from an abrupt reduction in coronary blood flow in this canine model. Possible mechanisms include an increase in coronary blood flow, a reduction in the severity of myocardial ischaemia and an ability of the compound to "donate' nitric oxide to the ischaemic area.

ATP-sensitive potassium channel modulators: both pinacidil and glibenclamide produce antiarrhythmic activity during acute myocardial infarction in conscious rats

We investigated the effects of pinacidil, an ATP-sensitive potassium channel opener, and of glibenclamide, an ATP-sensitive potassium channel inhibitor, on the incidence of arrhythmias and sudden cardiac death after coronary artery ligation in conscious rats. Occlusion of the left main coronary artery was produced by tightening a previously placed loose silk ligature. In the control group (n = 25) only 40% and 24% of the animals survived for 15 min and 16 hr after coronary artery ligation, respectively. Intravenous pretreatment with 0.1, 0.3 or 1 mg/kg pinacidil increased the survival rate to 67% (n = 15), 70% (n = 20) and 67% (n = 12) in the first 15 min and to 60%, 55% and 67% in the first 16 hr, respectively. Glibenclamide pretreatment (5.0 mg/kg i.p.) improved the survival rate at both time-points to 87% (n = 16). Both types of pretreatment significantly decreased the incidence of life-threatening arrhythmias and increased the number of animals that survived without developing any arrhythmia. In conclusion, the present findings demonstrate that in conscious rats, pretreatment with pinacidil and pretreatment with glibenclamide, although they obviously have different mechanisms of action, may result in a very similar final outcome with respect to arrhythmias and sudden cardiac death during the acute phase of experimental myocardial infarction.

Cardiac electrophysiological effects of levosimendan, a new calcium sensitizer

1. The conventional microelectrode and the patch-clamp techniques were used to study the electrophysiological effects of levosimendan, a new calcium-sensitizing cardiotonic drug, in cardiac ventricular muscle. 2. Levosimendan (5 microM) did not change the main repolarizing currents, such as the inward rectifier potassium, transient outward and the delayed rectifier outward potassium current, in rabbit ventricular myocytes. 3. In rabbit ventricular muscle, levosimendan, at relatively low concentrations (0.1-1 microM), did not change significantly the amplitude of the inward calcium current but increased the amplitude of the twitch tension. 4. In guinea pig ventricular muscle, levosimendan, at higher concentrations (1-5 microM), significantly increased the amplitude of the inward calcium current and the slow-response action potential parameters. 5. It is concluded that levosimendan, in addition to its calcium sensitizing properties characterized by "silent electrophysiology," exhibits cardiac electrophysiological effects similar to those of phosphodies-terase inhibitors.

Comparison of the chronic and acute effects of amiodarone on the calcium and potassium currents in rabbit isolated cardiac myocytes

1. The acute and chronic effects of amiodarone were studied on the transmembrane ionic currents in rabbit single ventricular myocytes at 35 degrees C by applying the whole-cell configuration of the patch-clamp technique. 2. Acute exposure to 1 and 5 microM amiodarone significantly reduced the amplitude (-53.9 +/- 3.9%, n = 5 and -64.0 +/- 2.0%, n = 3, P < 0.01), but chronic amiodarone treatment (i.p. 50 mg kg-1 day-1 for 3-4 weeks) changed neither the amplitude nor the kinetics of the inward calcium current. 3. Both acute superfusion with amiodarone (1 and 5 microM) and chronic amiodarone treatment significantly decreased the amplitude of the delayed rectifier outward potassium current (IK). 4. Acute application of amiodarone (1 and 5 microM) did not alter but chronic amiodarone treatment moderately depressed the transient outward current (Ito). 5. Neither the acute (1 and 5 microM) nor the chronic amiodarone treatment changed the magnitude of the inward rectifier potassium current (Ik1). 6. It is concluded that acute amiodarone application and chronic amiodarone treatment alter transmembrane ionic currents of ventricular myocytes differently. This may explain, at least in part, the marked differences in the cardiac electrophysiological effects observed after acute and chronic amiodarone treatment in patients.

The cellular electrophysiological effects of tedisamil in human atrial and ventricular fibers

The cellular electrophysiological effects of 1 microM tedisamil (KC 8857) were studied in human atrial and ventricular fibers. Conventional microelectrode technique was applied to record the transmembrane action potentials at stimulation frequency of 100 per min and 37 degrees C. Tedisamil lengthened action potential duration (APD) more in atrial than in ventricular muscle fibers; prolongation of APD90 was 28.9 +/- 3.3% (n = 6; p < 0.05) for atrial and 13.3 +/- 5.2% (n = 6; P < 0.05) for ventricular tissue. The maximal rate of depolarization was depressed slightly, but significantly by 1 microM tedisamil only in ventricular fibers (12.9 +/- 6.5%, n = 6, P < 0.05). From these cellular electrophysiological data it is concluded that the bradycardic/antiischemic agent tedisamil possesses marked Class III properties not only in cardiac tissues of experimental animals but also those of man.

Effect of glimepiride and glibenclamide, inhibitors of ATP-dependent K(+)-channel, on ischaemia-reperfusion induced arrhythmias in rats

Glibenclamide or glimepiride pretreatment (5 mg/kg i.p. 30 min prior to coronary ligation) significantly improved the survival rate during reperfusion after 6 min myocardial ischaemia in rats (82% or 67% vs. 17% in controls). Smaller dose of glibenclamide (0.01 mg/kg) did not influence the survival rate (31%), while glimepiride still offered a significant protection (61%). These results suggest that different KATP inhibitors may increase the chance to survive life-threatening arrhythmias during myocardial ischaemia-reperfusion.

Effect of restacorin on the early and delayed after depolarization in dog Purkinje fibres

The effect of restacorin and flecainide on the early afterdepolarization (EAD) and on the delayed afterdepolarization (DAD) was studied in dog cardiac Purkinje fibers by applying the conventional microelectrode technique. Restacorin and flecainide at 5 microM concentration partially abolished EAD. Further increase of the concentrations of the drugs to 10 microM EADs were completely abolished. The amplitude of DADs were significantly decreased by both restacorin (9.9 +/- 2.1 mV v.s. 2.9 +/- 1.8 mV, p < 0.01, n = 5) and flecainide (11.1 +/- 1.3 mV v.s. 0.6 +/- 0.6 mV, p < 0.01, n = 7). These results suggest that the well-established antiarrhythmic effect of restacorin and flecainide at least partially may be explained by their beneficial action against triggered abnormal automaticity.

Comparison of the effects of restacorin and flecainide on various cardiac transmembrane potassium currents

The effect of restacorin (5 microM) and flecainide (5 microM) on the transmembrane ionic currents was compared in dog and rabbit ventricular muscle by applying the conventional microelectrode and patch-clamp techniques. Neither restacorin nor flecainide influenced significantly the ATP-sensitive and inward rectifier potassium currents. Flecainide moderately decreased the amplitude of the transient outward current while restacorin did not change this current. The delayed rectifier potassium current was depressed markedly by flecainide and moderately by restacorin. These results suggest that although flecainide and restacorin cause similar changes in the various transmembrane ionic currents, there are some differences between the effects of the two antiarrhythmic compounds.

Effect of almokalant a specific inhibitor of IKr on myocardial ischaemia-reperfusion induced arrhythmias in rabbits

The antiarrhythmic effect of almokalant, a new type III antiarrhythmic agent, was examined by occluding and releasing the left circumflex coronary artery for 10 min, respectively, in openchest, pentobarbital-anaesthetized albino rabbits. Almokalant pretreatment increased the number of animals developing no arrythmias (5/9 vs. 1/12 in controls), and decreased the incidence of ventricular fibrillation (1/9 vs. 9/12) during reperfusion. According to our results almokalant can protect the heart against arrhythmias induced by ischaemia and reperfusion.

Long-term ischaemic preconditioning of the heart induced by repeated beta-adrenergic stress

In the present study we tested the preconditioning effect of repeated beta adrenergic stress induced by Isoproterenol in chronically instrumented, conscious rabbits. We have found that at least 5 intravenous administrations of Isoproterenol, repeated at 10 min intervals, were necessary to induce a long-term cardiac adaptation manifested by a significant reduction of the harmful ischaemic changes due to cardiac stress 24 and 48 hours after preconditioning. These results suggest that a well-defined threshold level of the preconditioning stress is needed to trigger induction of metabolic changes leading to development of delayed and long-term cardiac adaptation.

Nitric oxide activates an iberiotoxin-sensitive potassium channel in human saphenous vein

Synthetic iberiotoxin (IBTX), an inhibitor of large conductance calcium-activated potassium channel (BKCa), was used to study the possible involvement of a specific hyperpolarizing ion channel in nitric oxide (NO)-induced venodilation. Serotonin (0.125 microM)-induced contraction of isolated human saphenous vein was dose-dependently relaxed with 50-1550 nanoM exogenous NO. 30 min preincubation of venous preparations with 90 nanoM IBTX decreased vasorelaxation induced by NO. In conclusion, a hyperpolarizing potassium channel, BKCa, may be involved in the venodilator effect of endogenous NO.

Pharmacology of moxonidine, an I1-imidazoline receptor agonist

Moxonidine is a second-generation, centrally acting antihypertensive drug with a distinctive mode of action. Moxonidine activates I1-imidazoline receptors (I1-receptors) in the rostroventrolateral medulla (RVLM), thereby reducing the activity of the sympathetic nervous system. Moxonidine leads to a pronounced and long-lasting blood pressure reduction in different animal models of hypertension, e.g., spontaneously hypertensive rats, renal hypertensive rats, and renal hypertensive dogs. Blood pressure reduction with moxonidine is usually accompanied by a reduction in heart rate which, however, in most studies is of shorter duration and lesser magnitude than the fall in blood pressure. Chronic administration of moxonidine to SHRs with established hypertension causes normalization of myocardial fibrosis, capillarization, and regressive changes in myocytes, in parallel with the reduction of blood pressure. Left ventricular hypertrophy and renal glomerulosclerosis are also significantly reduced. After withdrawal of chronic moxonidine treatment, blood pressure gradually rises to pretreatment values. Direct injection of moxonidine into the vertebral artery of cats elicits a more pronounced fall in blood pressure compared with i.v. injection of an equivalent dose. This observation and others clearly indicate that moxonidine's antihypertensive activity is centrally mediated. The RVLM is the site of action within the CNS that mediates pronounced blood pressure reduction after direct administration of moxonidine into the RVLM of anesthetized SHRs. Selective I1-receptor antagonists introduced into this area abolish the action of systemic moxonidine. Receptor binding studies have shown high and selective affinity of moxonidine for I1-receptors vs. alpha(2)-adrenergic receptors. In vivo studies using a variety of selective I1 or alpha(2)-adrenergic agonists and antagonists have confirmed the primary role of I1-receptors in blood pressure regulation by moxonidine. In addition to lowering blood pressure, moxonidine possesses further properties that appear likely to be relevant in its therapeutic application in the hypertensive syndrome. Moxonidine increases urine flow rate and sodium excretion after central and direct intrarenal administration. It is active against ventricular arrhythmias in a variety of experimental settings. It lacks the respiratory depressant effect attributed to central alpha 2 activation. It exerts beneficial effects on glucose metabolism and blood lipids in genetically hypertensive obese rats. It exhibits anti-ulcer activity. And, finally, moxonidine lowers intraocular pressure, suggesting a possible benefit in glaucoma. Therefore, moxonidine, by its novel mode of action, represents a new therapeutic principle in the treatment of hypertension. Because of its unique profile, moxonidine may prove to be effective in slowing progression of the disease by providing protective effects beyond merely blood pressure reduction. Further studies are needed to verify this potential.

Effect of selective inhibition of potassium channels on vasorelaxing response to cromakalim, nitroglycerin and nitric oxide of canine coronary arteries

A comparative study was performed on the sensitivity of in-vitro vasorelaxation by nitroglycerin and cromakalim to block glibenclamide, a blocker of ATP-sensitive potassium channels, and iberiotoxin, a selective inhibitor of large-conductance calcium-activated potassium channels. In isolated canine coronary arteries preconstricted with 25 microM prostaglandin F2 alpha, nitroglycerin (0.005-1.8 microM) and cromakalim (0.15-9.6 microM) produced dose-dependent vasodilations. Glibenclamide (30 microM) had no significant effect on relaxation of the dose-response curve to nitroglycerin and almost completely abolished the relaxation by cromakalim, a known opener of ATP-sensitive potassium channels. Iberiotoxin (90 nM) decreased the maximal response to nitroglycerin and had no effect on the vasodilation induced by cromakalim. The effect of iberiotoxin on the vasorelaxing action of nitric oxide, the active metabolite of nitroglycerin, was also examined. In a low potassium chloride (14.4-20.4 mM) medium, as a contractile stimulus, iberiotoxin inhibited relaxations by exogenous nitric oxide (100-200 nM). Enhancement of potassium concentrations to 35.4-40.4 mM significantly decreased relaxation by nitric oxide and under these conditions the inhibitory action of iberiotoxin disappeared. The present study demonstrated that in canine coronary arteries, the functional role of two potassium channels can be separated by pharmacological means. Nitroglycerin-induced vasorelaxation may be mediated, at least in part, by its enzymatic breakdown product, nitric oxide that activates large-conductance calcium-activated potassium channels.

Bradykinin as an endogenous myocardial protective substance with particular reference to ischemic preconditioning--a brief review of the evidence

The present brief review summarizes the evidence for the possibility that endogenously released bradykinin plays a major role in protecting the heart against the consequences of acute myocardial injury. This evidence includes the facts that kinins are generated under myocardial ischemia; that when they are administered, they are cardioprotective (e.g., antiarrhythmic); that drugs that enhance the release of bradykinin from the ischemic heart reduce the ischemic injury and, conversely, drugs that block bradykinin receptors attenuate the reduction in ischemic injury resulting from the release of, or administration of, bradykinin. The possible mechanism of bradykinin in the cardioprotection afforded by ischemic preconditioning is summarized. Ischemic preconditioning can be defined as the marked reduction in the severity of ischemic changes that result from coronary artery occlusion when that occlusion is preceded by brief periods of myocardial ischemia, either regional or global, induced, for example, by complete or partial coronary artery occlusion or by rapid ventricular pacing. The possible mechanisms of cardioprotection elicited by bradykinin (and ischemic preconditioning) are summarized. The most likely is the generation of cyclic GMP within the ischemic myocardium following bradykinin-stimulated nitric oxide generation and release from endothelial cells.

Electrophysiological effects of tiracizin and its metabolites in dog cardiac Purkinje fibers

1. Conventional microelectrode technique was used to study the effects of tiracizin and its two main metabolites on the action potential parameters in dog cardiac Purkinje fibers. 2. Tiracizin induced a use-dependent depression of the maximal rate of depolarization during the upstroke of the action potential (EC50 = 0.323 +/- 0.059 microM, n = 5) and decreased the action potential duration (EC50 = 0.230 +/- 0.024 microM, n = 5). 3. The two main metabolites exerted similar effects, but only at concentrations about one order of magnitude higher than tiracizin. 4. It was concluded that the electrophysiological effects of tiracizin in dog Purkinje fibers correspond to those of Class I antiarrhythmic drugs also in cardiac Purkinje fibers. The main metabolites are less potent than the parent compound, therefore it is likely that their effects play a less important role in the beneficial effect of tiracizin in cardiac arrhythmias.

Effects of Levosimendan, a cardiotonic agent targeted to troponin C, on cardiac function and on phosphorylation and Ca2+ sensitivity of cardiac myofibrils and sarcoplasmic reticulum in guinea pig heart

A new cardiotonic agent, (R)-[[4-(1,4,5,6-tetrahydro-4-methyl-6-oxo-3-pyridazinyl)-phenyl] hydrazono]propanedinitrile (Levosimendan), has been developed and screened for its ability to bind to cardiac troponin C. In perfused hearts, low concentrations of 0.03 or 0.1 mumol/L Levosimendan increased +dP/dt, but did not affect the speed of relaxation and produced only a slight increase in spontaneous heart rate in the hearts perfused with 0.1 mumol/L of the drug. In these same hearts, perfusion with 0.03 mumol/L Levosimendan did not alter the 32P incorporation into troponin I or C protein, whereas a slight but significant increase was noted for phospholamban, with no detectable change in tissue cAMP levels. Administration of 0.1 or 0.3 mumol/L Levosimendan significantly increased myocardial cAMP levels as well as the phosphorylation of phospholamban, troponin I, and C protein. Levosimendan (0.03 to 10 mumol/L) reversibly increased force generated by detergent-extracted fiber bundles over a range of submaximally activating free Ca2+ concentrations with no significant effect on maximum force or on Ca2+ binding to myofilament troponin C. There was no direct effect of Levosimendan on Ca2+ uptake by vesicles of sarcoplasmic reticulum (SR). In contrast, under conditions optimal for cAMP-dependent phosphorylation, Levosimendan slightly but significantly lowered the concentration of Ca2+, yielding half-maximal uptake rates by the SR vesicles. Our results indicate that at low concentrations Levosimendan acts preferably as a Ca2+ sensitizer, whereas at higher concentrations its action as a phosphodiesterase inhibitor contributes to the positive inotropic effect.

Left ventricular dysfunction induced by occlusion of coronary arteries in conscious dogs

The aim of the present study was to investigate stable left ventricular dysfunction resulting from severe myocardial ischemia in conscious dogs, in order to evaluate the action of cardiotonic agents under pathological conditions mimicking moderate cardiac failure. Mongrel dogs with a catheter implanted in the left ventricle were trained on a treadmill and subjected to a standardized exercise before and after a Harris-type ligation of the anterior descending branch of the left coronary artery in two stages. Two weeks later the lower third of the left circumflex coronary branch was also occluded, and the exercise test repeated for at least two additional weeks to evaluate the changes in the left ventricular function indicated by left ventricular systolic pressure, end-diastolic pressure, heart rate, positive and negative dP/dtmax and dP/dt/P. Noninvasive radionuclide investigations of the left ventricular function and myocardial perfusion were done before and after the development of cardiac failure. Following occlusion of the anterior descending and circumflex coronary arteries, the baseline end-diastolic pressure increased from 7.6 +/- 2.3 mmHg to 23.3 +/- 3.0 mmHg (p < 0.05) and increased even further during exercise (to 49.2 +/- 3.5 mmHg, p < 0.05). After the development of cardiac failure, no substantial change occurred in the end-diastolic pressure, either during rest or repeated exercise tests.(ABSTRACT TRUNCATED AT 250 WORDS)

Cardiovascular effects of the calcium sensitizer, levosimendan, in heart failure induced by rapid pacing in the presence of aortic constriction

1. The haemodynamic effects of a novel cardiotonic drug, levosimendan, which has both calcium-sensitizing and phosphodiesterase III (PDE III) inhibitory properties, were studied in conscious dogs in which heart failure had been induced by prolonged cardiac pacing in the presence of aortic constriction. These effects were compared with those in sham-operated dogs with essentially normal cardiac function. 2. Eighteen mongrel dogs were instrumented for the measurement of left ventricular pressure (LVSP, LVEDP) and contractile function (dP/dt; dP/dt/P). In twelve dogs a balloon catheter, positioned in the thoracic aorta, was inflated producing an approximate 60% reduction in effective aortic diameter. Twenty min later rapid ventricular pacing (240 beats mean-1) was commenced and maintained for 48 h by means of a bipolar pacing electrode introduced into the right ventricle. This electrode served also for recording changes in the endocardial electrogram in the absence of pacing. Six of these dogs were used to evaluate the haemodynamic changes of pacing-induced heart failure; a further six of these dogs the haemodynamic changes elicited by levosimendan under these conditions. Six sham-operated dogs (group 2) served as controls. 3. In six dogs (group 1) the haemodynamic alterations were assessed after the development of heart failure. In the presence of aortic constriction, 48 h continuous rapid cardiac pacing resulted in a marked deterioration in left ventricular function which remained stable for at least 48 h after cessation of pacing. Thus, there was a marked reduction in LVSP (15%), +dP/dtmax (35%), -dP/dtmax (36%) and also in dP/dt/P (29%), whereas LVEDP was increased considerably (from 6.4 +/- 1.4 to 20.0 +/- 2.2 mmHg). A marked elevation occurred in endocardial ST-segment (138%), lasting for 20 min.4. Levosimendan was administered intravenously in doses of 0.005, 0.01 and 0.03 micromol kg-1 to 2 groups of conscious dogs. In the sham-operated dogs (group 2), only the higher dose (0.03 micromol kg-1)produced significant increases in LVSP (19%), + dP/dtmax (37%), and in dP/dt/P (32%). In dogs with heart failure (group 3) doses of 0.005, 0.01 and 0.03 micromol kg-1 levosimendan resulted in an improve mentin +dP/dtmax (26%, 38% and 49%), -dP/dtmax (20%, 25% and 38%) and in dP/dt/P (19%, 34%and 50%) and reduction in the elevated LVEDP (from 20 =/- 2.2 mmHg to 16 +/- 1.0, 10 +/- 1.3 and 9 +/- 1.0 mmHg, respectively).5. Levosimendan proved to be a potent cardiotonic drug at the doses used, and was approximately three times more effective under conditions of impaired left ventricular function than in normal hearts.