Arrhythmogenic potential of positive inotropic agents.

The clinical use of positive inotropic agents has been associated with increased mortality, with proarrhythmia speculated to be a contributing factor. This study compares the arrhythmogenic potentials of six positive inotropic agents representing different mechanistic classes: the beta-adrenergic agonist dobutamine, the adenylyl cyclase activator forskolin, the phosphodiesterase-III inhibitor milrinone, the cardiac glycoside ouabain, and the sodium channel agonists DPI 201-106 and BDF 9148. These agents were studied in dogs with anterior myocardial infarction using lower and higher dose i.v. regimens targeted to elicit 20-40% and 70-90% increases in LV+dP/dt, respectively. Precipitation of new ventricular arrhythmia by programmed ventricular stimulation was observed in all treatment groups. Incidences of new arrhythmia were comparable in the lower dose regimens, ranging from 16.7% (3/18 animals with BDF 9148) to 31.6% (6/19 animals with DPI 201-106), and in the higher dose regimens, ranging from 10.0% (1/10 animals with milrinone) to 27.7% (5/18 animals with DPI 201-106). The overall incidence of new ventricular arrhythmia ranged from 27.3% (3/11 animals with ouabain) to 47.4% (9/19 animals with DPI 201-106). No differences were observed in underlying infarct size or time from infarction to electrophysiologic study between subgroups of animals in which new arrhythmias were precipitated vs. those remaining non-responsive in any treatment group. The positive inotropic agents tested displayed diverse total group effects on heart rate, electrocardiographic intervals including QTc and ventricular refractoriness. Within individual treatment comparisons revealed a general but not universal pattern of greater ventricular refractory period values in newly inducible vs. non-inducible subgroups in the DPI 201-106, BDF 9148 and ouabain (low and high dose); milrinone and dobutamine (high dose) treatment groups. These findings indicate that regardless of underlying cellular mechanism of action, the six positive inotropic agents tested all displayed comparable proarrhythmic potentials unrelated to underlying infarct size and time from infarction. This observation suggests the general shared property of increased myocardial contractility, potentially adversely affecting myocardial oxygen balance, myocardial perfusion and electrical stability in the setting of previous myocardial infarction, to be a common underlying cause for arrhythmogenesis. Additionally, alterations in ventricular refractoriness and repolarization may contribute significantly to proarrhythmia with some positive inotropic interventions.

Antiarrhythmic efficacy of selective blockade of the cardiac slowly activating delayed rectifier current, I(Ks), in canine models of malignant ischemic ventricular arrhythmia.

BACKGROUND: To date, the lack of potent and selective inhibitors has hampered the physiological assessment of modulation of the cardiac slowly activating delayed rectifier current, I(Ks). The present study, using the I(Ks) blocker L-768,673, represents the first in vivo assessment of the cardiac electrophysiological and antiarrhythmic effects of selective I(Ks) blockade. METHODS AND RESULTS: In an anesthetized canine model of recent (8.5+/-0.4 days) anterior myocardial infarction, 0.003 to 0.03 mg/kg L-768,673 IV significantly suppressed electrically induced ventricular tachyarrhythmias and reduced the incidence of lethal arrhythmias precipitated by acute, thrombotically induced posterolateral myocardial ischemia. Antiarrhythmic protection afforded by L-768,673 was accompanied by modest 7% to 10% increases in noninfarct zone ventricular effective refractory period, 3% to 5% increases in infarct zone ventricular effective refractory period, and 4% to 6% increases in QTc interval. In a conscious canine model of healed (3 to 4 weeks) anterior myocardial infarction, ventricular fibrillation was provoked by transient occlusion of the left circumflex coronary artery during submaximal exercise. Pretreatment with 0.03 mg/kg L-768,673 IV elicited a modest 7% increase in QTc, prevented ventricular fibrillation in 5 of 6 animals, and suppressed arrhythmias in 2 additional animals. CONCLUSIONS: The present findings suggest that selective blockade of I(Ks) may be a potentially useful intervention for the prevention of malignant ischemic ventricular arrhythmias.

EXP3174, the AII antagonist human metabolite of losartan, but not losartan nor the angiotensin-converting enzyme inhibitor captopril, prevents the development of lethal ischemic ventricular arrhythmias in a canine model of recent myocardial infarction.

OBJECTIVES: The antiarrhythmic efficacies of the competitive angiotensin II (AII) antagonist losartan, losartan's more potent noncompetitive AII antagonist human metabolite EXP3174 and the angiotensin-converting enzyme inhibitor captopril were assessed in a canine model of recent myocardial infarction. BACKGROUND: Multiple hemodynamic and electrophysiologic effects of AII may contribute to cardiac electrical instability. In the recent Losartan Heart Failure Study, Evaluation of Losartan in the Elderly (ELITE), a 722-patient trial primarily designed to assess effects on renal function, an unexpected survival benefit was observed with losartan compared with captopril, with the lower mortality using losartan primarily confined to a reduction in sudden cardiac death. METHODS: Intravenous losartan (1 mg/kg + 0.03 mg/kg/min), EXP3174 (0.1 mg/kg + 0.01 mg/kg/min), captopril (1 mg/kg + 0.5 mg/kg/h) or vehicle were infused in anesthetized dogs with recent (8.1 +/- 0.4 days) anterior myocardial infarction. Electrolytic injury of the left circumflex coronary artery to induce thrombotic occlusion and posterolateral ischemia was initiated 1 h after the start of treatment. RESULTS: Losartan, EXP3174 and captopril elevated plasma renin activities and comparably and significantly reduced mean arterial pressure. No significant electrocardiographic or cardiac electrophysiologic effects were noted with any treatment. Incidences of acute posterolateral ischemia-induced lethal arrhythmias were: vehicle, 7/9 (77%); losartan, 6/8 (75%); EXP3174, 2/8 (25%; p < 0.05 vs. vehicle control); captopril, 7/10 (70%). There were no among-group differences in time to onset of acute posterolateral ischemia or underlying anterior infarct size. CONCLUSIONS: EXP3174, but not losartan nor captopril, reduced the incidence of lethal ischemic ventricular arrhythmia in this preparation. The antiarrhythmic efficacy of EXP3174 may be due to an attenuation of deleterious effects of local cardiac AII formed during acute myocardial ischemia or, alternatively, a non-AII-related activity specific to EXP3174. These findings suggest that in humans, metabolic conversion of losartan to EXP3174 may afford antiarrhythmic protection.

Antithrombotic efficacy of thrombin inhibitor L-374,087: intravenous activity in a primate model of venous thrombus extension and oral activity in a canine model of primary venous and coronary artery thrombosis.

The small molecule direct thrombin inhibitor L-374,087 was characterized across species in an in vitro activated partial thromboplastin clotting time (aPTT) assay and in vivo in rhesus monkey and dog thrombosis models. In vitro in rhesus, dog, and human plasma, L-374,087 concentrations eliciting 2-fold increases in aPTT were 0.25, 1.9, and 0.28 microM, respectively. In anesthetized rhesus monkeys, 300 microgram/kg bolus plus 12 microgram/kg/min and 300 microgram/kg bolus plus 30 microgram/kg/min L-374,087 i.v. infusions significantly reduced jugular vein thrombus extension, with both regimens limiting venous thrombus extension to 2-fold that of baseline thrombus mass compared with a 5-fold extension observed in the vehicle control group. Antithrombotic efficacy in the rhesus with the lower-dose regimen was achieved with 2.3- to 2.4-fold increases in aPTT and prothrombin time. In a conscious instrumented dog model of electrolytic vessel injury, the oral administration of two 10 mg/kg L-374,087 doses 12 h apart significantly reduced jugular vein thrombus mass, reduced the incidence of and delayed time to occlusive coronary artery thrombosis, and significantly reduced coronary artery thrombus mass and ensuing posterolateral myocardial infarct size. Antithrombotic efficacy in the dog was achieved with 1.6- to 2.0-fold increases in aPTT at 1 to 6 h after oral dosing with L-374,087. These results indicate significant antithrombotic efficacy against both venous and coronary arterial thrombosis with L-374,087 with only moderate elevations in aPTT or prothrombin time. The oral efficacy of L-374,087 characterizes this compound as a prototype for the further development of orally active direct thrombin inhibitors.

Nonpeptide glycoprotein IIb/IIIa inhibitors: 14: oral antithrombotic efficacy of L-738,167 in a conscious canine model of coronary artery electrolytic injury.

BACKGROUND: A conscious dog model of left circumflex coronary artery electrolytic injury was used to assess the oral antithrombotic efficacy of L-738,167, a potent nonpeptide antagonist of platelet GP IIb/IIIa. L-738,167 was administered either as a single oral pretreatment dose 2 hours before initiation of vessel injury or as two oral doses administered 24 hours apart, 12 hours before and after initiation of vessel injury. METHODS AND RESULTS: In untreated controls, electrolytic coronary injury (50 microA, 3 hours) resulted in thrombotic occlusion and myocardial ischemia in 15 of 16 dogs, with 4 developing lethal arrhythmias. Significant reductions in thrombus mass and complete prevention of myocardial ischemia and infarction were achieved with a single 100- to 300-microg/kg dose of L-738,167 pretreatment and with two 100-microg/kg doses administered 12 hours before and after initiation of vessel injury. Delays and/or reductions in incidence of ischemia, thrombus mass, and infarct sizes also were achieved with 10- to 30-microg/kg pretreatment and with two 30-microg/kg doses administered 12 hours before and after initiation of vessel injury. None of the L-738,167-treated animals developed lethal arrhythmias. A single oral 100-microg/kg dose of L-738,167 achieved >90% inhibitions of ADP (extent)- and collagen (rate)-induced ex vivo platelet aggregation and fivefold to sixfold or greater elevations in bleeding time; a single oral 30-microg/kg dose of L-738,167 achieved sustained 40% to 70% inhibitions of ADP- and collagen-induced ex vivo platelet aggregation and modest twofold to threefold elevations in bleeding time. At 12 to 24 hours after single oral 30- and 100-microg/kg doses of L-738,167, a substantially greater L-738,167 concentration was associated with platelets than free in plasma. CONCLUSIONS: These findings are indicative of potent and sustained oral antithrombotic efficacy and suggest that L-738,167 possesses potential for the oral management of chronic thrombotic occlusive disorders.

Differential efficacy of the class III agent MK-499 against programmed stimulation-induced and ischemic-induced ventricular arrhythmias in a canine model of previous myocardial infarction.

Class III activity has been proposed as a potential mechanism for the treatment of reentrant arrhythmias. The purpose of the present study was to assess the concordance in antiarrhythmic efficacy of MK-499, a selective blocker of IKr, the rapidly activating component of cardiac delayed rectifier K+ current, against programmed ventricular stimulation (PVS)-induced ventricular tachycardias and thrombotically induced lethal ischemic arrhythmias, and to characterize the electrophysiologic determinants of antiarrhythmic efficacy in the canine model of previous myocardial infarction. Single i.v. doses of 1.0, 3.0 and 10.0 micrograms/kg MK-499 were administered to anesthetized dogs with anterior myocardial infarctions. Protection (suppression + stabilization/slowing) vs. PVS-induced ventricular tachycardias occurred in 5/11 (45%) preparations at 1.0 microgram/kg, in 9/12 (75%) preparations at 3.0 micrograms/kg and in 10/11 (91%) preparations at 10.0 micrograms/kg i.v. MK-499. The incidences of lethal ventricular arrhythmias developing in response to thrombotically induced posterolateral myocardial ischemia were 34/40 (85%) in vehicle controls, 7/11 (64%) at 1.0 microgram/kg, 6/12 (50%, P < .05) at 3.0 micrograms/kg and 4/11 (36%, P < .01) at 10.0 micrograms/kg i.v. MK-499. Low-dose i.v. MK-499 prolonged ECG QT interval and increased noninfarct zone and infarct zone ventricular refractoriness. However, there was a poor concordance (56%) between response to PVS with MK-499 and response to thrombotically induced acute myocardial ischemia. Furthermore, different trends of association between site and magnitude of Class III effect and antiarrhythmic efficacy were observed for PVS- vs. ischemia-induced arrhythmias. Hence, although low-dose i.v. MK-499 provided significant protection against both electrically and ischemically triggered arrhythmias in the setting of previous myocardial infarction, protection against PVS-induced ventricular tachycardias was not highly predictive of protection against lethal ischemic arrhythmias in this preparation.

Cardiovascular profile of RWJ 29009, a new potassium channel activator, in anesthetized and conscious dogs.

RWJ-29009, (6S)-trans(-)-1-(6,7-dihydro-6-hydroxy-5,5-dimethyl-2-nitro-5 H-thieno[3,2-b]pyran-7-yl)-2-piperidinone, is a structurally novel and extremely potent potassium channel activator that may be useful for treatment of hypertension and ischemic heart disease. We assessed the cardiovascular profile of RWJ 29009 in anesthetized and conscious dogs. RWJ 29009 (0.1-2 micrograms/kg intravenously, i.v.) dose-relatedly increased coronary blood flow (CBF) and decreased arterial pressure in anesthetized dogs. Total peripheral resistance and coronary vascular resistance were concurrently reduced without significant changes in heart rate (HR) or cardiac output (CO). Left ventricular (LV) dP/dtmax and myocardial contractile force were decreased only at the highest dose of 10 micrograms/kg. Cromakalim (3-100 micrograms/kg), although much less potent, had a qualitatively similar profile. Glyburide pretreatment (5 mg/kg i.v.) shifted the dose response of RWJ 29009 for increasing CBF and decreasing arterial pressure to the right. The dose responses of cromakalim were similarly shifted to the right, whereas the effects of nifedipine on CBF and arterial pressure were not affected by glyburide. RWJ 29009 (0.3 and 1 microgram/kg) had no effect on myocardial O2 consumption (MVO2) except for a transient increase immediately after administration of 1 microgram/kg. MVO2 returned to control 15 min after dosing, although CBF remained significantly increased. In conscious dogs, RWJ 29009 (0.3-10 micrograms/kg, i.v. and orally, p.o.) produced dose-related increases in CBF and decreases in arterial pressure similar to those produced in anesthetized dogs, except that HR was increased concurrently. The i.v. and p.o. potency of RWJ 29009 were comparable, indicating high oral bioavailability. Thus, RWJ 29009 is an extremely potent coronary and peripheral vasodilator with a cardiovascular profile similar to that of other potassium channel activators. Like those of other potassium channel activators, its mechanism of action appears to involve activation of ATP-regulated potassium channels.

In vivo cardiac electrophysiologic effects of RWJ 29009, a new potassium-channel activator, in comparison to cromakalim and nicardipine.

RWJ 29009 is a new potassium channel activator with prominent coronary and peripheral vasodilating actions. Because of the potential direct cardiac electrophysiologic actions of increased potassium conductance in myocardium, we evaluated the effects of RWJ 29009 on cardiac conduction and refractoriness in comparison to its vasodilator activity in anesthetized, open-chest dogs. We assessed effects during both intrinsic sinus rhythm and during constant atrial pacing. RWJ 29009 markedly increased coronary blood flow and decreased mean arterial blood pressure (MAP) dose dependently (0.3-10 micrograms/kg intravenously, i.v.). RWJ 29009 had no effect on PR interval but decreased AV-nodal conduction time (AH) and Wenkebach cycle length slightly. RWJ 29009 decreased QT interval, left ventricular (LV) monophasic action potential duration (APD), and ventricular and atrial refractory period. These effects were consistent with shortening of cardiac repolarization. RWJ 29009 had no effect on QRS or His-Purkinje conduction time. Cromakalim had a qualitatively similar profile but was much less potent (3-300 micrograms/kg i.v.). In addition, the effects of cromakalim on repolarization parameters were somewhat less marked than those of RWJ 29009. Nicardipine also markedly increased coronary blood flow and decreased arterial pressure (10-300 micrograms/kg i.v.). Unlike the potassium channel activators, nicardipine (100-300 micrograms/kg), did not affect cardiac repolarization, but increased PR and AH interval, and Wenkebach cycle length (WENK) and reduced heart rate (HR) consistent with calcium channel blockade. These results indicate that RWJ 29009, like cromakalim, increases coronary blood flow at low doses without substantial electrophysiologic effects. Electrophysiologic effects observed at higher doses indicated a shortening of repolarization, expectedly produced by potassium channel activation in cardiac tissue.

Investigation of electrophysiologic mechanisms for the antiarrhythmic actions of R 56865 in cardiac glycoside toxicity.

R 56865 is an experimental compound that has been shown to ameliorate the effects of cardiac glycoside toxicity and myocardial ischemia. We evaluated the direct electrophysiological effects of R 56865 and its effects on the electrophysiological sequelae of ouabain toxicity in vivo and in vitro. In normal anesthetized dogs, R 56865 alone at doses of 0.04 to 0.16 mg/kg i.v. had no effect on atrial, AV nodal, or ventricular conduction times and refractoriness, but at doses of 0.64 to 2.5 mg/kg it tended to increase these parameters. In ouabain-pretreated dogs, R 56865 (0.08 to 0.32 mg/kg i.v.) dose-relatedly reduced ouabain-induced ventricular arrhythmias. In normal isolated canine Purkinje fibers, R 56865 (1-10 microM) reduced Vmax at short pacing cycle lengths and decreased the action potential duration at concentrations of 0.1 to 10 microM. R 56865 at concentrations through 10 microM had no significant effect on normal action potentials of canine ventricular muscle and slow response action potentials in guinea pig papillary muscles. In Purkinje fibers exposed to toxic concentrations of ouabain, R 56865 (1 microM) reduced the delayed after depolarization (DAD) amplitude and inhibited triggered activity. R 56865 had no effect on normal automaticity in canine Purkinje fibers at 1 microM, but 10 microM significantly slowed it. R 56865 at 10 microM did not affect isoproterenol-enhanced automaticity and only slightly reduced barium-induced abnormal automaticity that occurred at reduced membrane potentials. These results demonstrate that R 56865 reverses cardiac glycoside-induced arrhythmias in anesthetized dogs at doses that do not significantly affect conduction or refractoriness. Suppression of ouabain-induced DAD and triggered activity in isolated Purkinje fibers, at concentrations not affecting normal or abnormal automaticity, may be the mechanism of R 56865's antiarrhythmic actions in vivo. Suppression of DAD does not appear to be associated with blockade of voltage-dependent calcium channels, but R 56865 may prevent intracellular sodium overload by limiting excessive sodium entry during ouabain intoxication.

Cardiac electrophysiologic effects of McN-5691, a new calcium-channel blocking antihypertensive agent.

The purpose of this study was to evaluate the cardiac electrophysiological effects of McN-5691, a new calcium-channel blocking antihypertensive drug. In anesthetized dogs, the primary electrophysiological effect of McN-5691 was dose-related prolongation of AV-nodal conduction time and refractoriness (0.1-1.0 mg/kg i.v.), which correlated with McN-5691 plasma levels. There were no significant effects on atrial or ventricular conduction times, QTc, or ventricular monophasic action potential duration. This profile was similar to that of verapamil. McN-5691 caused concentration-related, rate-dependent reductions in Vmax and amplitude of slow-response action potentials in guinea pig papillary muscle: ED-20% for depression of Vmax was 0.72 +/- 0.32 microM. Verapamil was more potent in depressing these action potentials: ED-20% for depression of Vmax was 0.03 +/- 0.01 microM. McN-5691 also caused rate-dependent reduction in Vmax and amplitude of canine Purkinje fiber action potentials, but only at relatively high concentrations: ED-20% for depression of Vmax was 55 +/- 12 microM. McN-5691 also reduced the action potential duration (0.3-30 microM) without affecting the slope of phase 4 depolarization and the maximum diastolic potential. Verapamil also reduced Vmax in Purkinje fibers (ED-20% for depression of Vmax was 32 +/- 3 microM) and shortened the action potential duration. The results show that McN-5691 has cardiac electrophysiological effects consistent with blockade of the slow inward calcium current, and that this activity occurs at concentrations well below those having local anesthetic activity. In addition, its lower potency in comparison to verapamil in depressing slow responses suggests a lesser propensity for negative inotropic effects.