Electrophysiological and antiarrhythmic effects of UK 52,046-27 during ischaemia and reperfusion in the guinea-pig heart

Beneficial effects of amiodarone and dronedarone (SR 33589b), when applied during low-flow ischemia, on arrhythmia and functional parameters asssessed during reperfusion in isolated rat hearts

The effects of short-term amiodarone and dronedarone treatments on action potential characteristics and arrhythmia (ventricular tachycardia ) induced by reperfusion after global low-flow ischemia were studied in rat hearts. The actions of amiodarone and SR on recovery of coronary flow and contractile function were also determined. Isolated hearts were stabilized for 40 min and were then submitted to 25-min global low-flow ischemia (constant coronary flow, 0.3 ml/min) followed by 30 min of reperfusion at constant pressure. Drugs were applied only during ischemia: consequently, action potential duration (APD) tended to widen. During reperfusion, APD tended to recover or shorten, and the more complete the recovery, the less the arrhythmia. Despite its ability to widen APD during ischemia, amiodarone facilitated APD recovery during reperfusion. Moreover, APD shortening and ventricular tachycardia suppression exhibit a bell-shaped concentration-response relation, implying that the drugs affect ventricular tachycardia by a class III-independent action. These results point to an anti-ischemic action supported by improvement in function and inhibition of reactive hyperemia.

Effects of selective alpha 1A-adrenoceptor antagonists on reperfusion arrhythmias in isolated rat hearts

Stimulation of alpha 1-adrenoceptors (AR) during ischaemia in the rat heart by exogenous phenylephrine exacerbates reperfusion arrhythmias, an effect apparently mediated by the alpha 1A-AR subtype. We tested whether alpha 1A-AR stimulation by endogenous catecholamines, released during ischaemia, could modulate reperfusion arrhythmias, using as pharmacological tools the selective alpha 1A-AR antagonists abanoquil (UK52046) and WB4101. Isolated rat hearts (n = 12/group) were subjected to dual coronary perfusion. After 15 min of aerobic perfusion of both coronary beds, abanoquil or WB4101 was infused selectively into the left coronary bed (LCB) for 5 min. The LCB was then subjected to 10 min of zero-flow ischaemia and 5 min of reperfusion. Effects on PR interval, width of the ventricular complex (QRST90) and reperfusion arrhythmias were assessed. Abanoquil at concentrations of 0.03, 0.1 and 0.3 microM tended to reduce the incidence of reperfusion-induced ventricular fibrillation (VF) in a dose-dependent manner from 75% in controls to 58, 33 and 25%, but this effects did not achieve statistical significance. Similarly, WB4101 at 0.1, 0.3 and 1 microM also tended to reduce VF incidence from 67% in controls to 67, 42% and 33% (NS). The incidence of ventricular tachycardia (VT) was 100% in all groups and ECG parameters were not altered significantly by either drug. These results suggest that, in this denervated isolated heart preparation, alpha 1A-AR stimulation during ischaemia by endogenous catecholamines does not significantly modulate reperfusion arrhythmias.

Alpha 1-adrenergic blockade and sudden cardiac death

INTRODUCTION

The primary goal of the present study was to test whether selective pharmacologic blockade of alpha 1 receptors, and specifically of the subtype alpha 1a, could prevent ventricular fibrillation (VF) during acute myocardial ischemia.

BACKGROUND

The development of new autonomic interventions is of clinical interest in view of the failure of traditional antiarrhythmic drugs to prevent sudden death. Experimental evidence indicates that alpha 1 receptors, and in particular the subtype alpha 1a, may be involved in the genesis of malignant arrhythmias during acute myocardial ischemia and reperfusion. Despite this evidence, questions have been raised about the actual antifibrillatory efficacy of alpha-adrenergic blockade in the acutely ischemic myocardium. The effects of prazosin and of abanoquil (UK 52,046), a highly selective alpha 1a receptor blocker, were tested and compared with propranolol in a conscious animal preparation for sudden death.

METHODS AND RESULTS

Ten dogs with a 1-month-old anterior wall myocardial infarction were studied. These dogs had all developed, in control conditions, VF during a 2-minute occlusion of the circumflex coronary artery while exercising (n = 9) or lying on the table (n = 1). Afterwards, the dogs underwent additional tests with the following intravenously administered drugs: abanoquil (n = 10; 1 micrograms/kg), prazosin (n = 9; 0.1 mg/kg), and propranolol (n = 10; 1 mg/kg). Internal control analysis was used. All dogs tested had recurrence of VF with both alpha-adrenergic blockers. Propranolol significantly reduced heart rate during ischemia and prevented VF in 5 of 10 dogs tested (P < 0.05). When heart rate was kept constant by atrial pacing (n = 3), 2 of the 3 animals remained protected by propranolol. Just prior to onset of VF, heart rate was not significantly different in the control and in the abanoquil tests (237 +/- 45 and 253 +/- 34 beats/min, respectively), whereas it was higher (P < 0.05) with prazosin (288 +/- 40 beats/min).

CONCLUSIONS

Alpha 1 and alpha 1a receptor blockers do not prevent VF secondary to acute myocardial ischemia in the presence of elevated sympathetic activity and heart rate. In the same setting, beta-adrenergic blockade prevents the reflex heart rate increase due to ischemia and provides a significant protection.

Alpha-adrenoceptors

Major advances have been made in our understanding of the molecular structure and function of the alpha-adrenoceptors. Many new subtypes of the alpha-adrenoceptor have been identified recently through biochemical and pharmacological techniques and several of these receptors have been cloned and expressed in a variety of vector systems. Currently, at least seven subtypes of the alpha-adrenoceptor have been identified and the molecular structure and biochemical functions of these subtypes are beginning to be understood. The alpha-adrenoceptors belong to the super family of receptors that are coupled to guanine nucleotide regulatory proteins (G-proteins). A variety of G-proteins are involved in the coupling of the various alpha-adrenoceptor subtypes to intracellular second messenger systems, which ultimately produce the end-organ response. The mechanisms by which the alpha-adrenoceptor subtypes recognize different G-proteins, as well as the molecular interactions between receptors and G-proteins, are the topics of current research. Furthermore, the physiological and pathophysiological role that alpha-adrenoceptors play in homeostasis and in a variety of disease states is also being elucidated. These major advances made in alpha-adrenoceptor classification, molecular structure, physiologic function, second messenger systems and therapeutic relevance are the subject of this review.

Haemodynamic dose-response effects of UK-52,046 in ischaemic disease with or without impaired left ventricular function

1. The haemodynamic effects of a new cardioselective postsynaptic alpha 1-adrenoceptor antagonist UK-52,046, were evaluated in 25 patients with stable coronary disease, with or without impaired left ventricular function. At rest the haemodynamic effects to two dose-response regimens were determined. In an initial eight patients 0.125, 0.125 and 0.25 micrograms kg-1 were administered peripherally at 15 min intervals; the haemodynamic measurements were determined between 10 to 15 min after each dose. In a further 17 patients, the dose regimen was doubled yielding a cumulative dose-regimen of 0.25, 0.5 and 1.0 micrograms kg-1. The exercise effects were determined by comparison of measurements during 4 min of supine sub-maximal bicycle exercise at a fixed workload before and after drug treatment. 2. At rest, the lower dose regimen of UK-52,046 significantly reduced systemic mean arterial blood pressure (-5 mm Hg; P less than 0.05) and increased cardiac index (+0.2 l min-1 m-2, P less than 0.01). The higher dose regimen of UK-52,046 reduced systemic mean arterial blood pressure (-7 mm Hg; P less than 0.01), pulmonary artery occluded pressure (PAOP) (-2 mm Hg, P less than 0.01) and vascular resistance index (-314 dyn s cm-5 m2; P less than 0.05) with an increase in heart rate (+7%, P less than 0.05) and cardiac index (+0.2 l min-1 m-2, P less than 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)

An acute study of the electrophysiological and haemodynamic effects of intravenous UK-52,046, a novel alpha 1-adrenoceptor antagonist

The acute electrophysiological and haemodynamic effects of UK-52,046, a novel alpha 1-adrenoceptor antagonist (0.5 microgram kg-1) were studied in 10 patients. Resting and paced conduction intervals, refractory periods, Wenckebach cycle length and sinus node recovery time were measured and compared with baseline values. Haemodynamic measurements including cardiac output were measured before and after drug administration. Changes in QRS interval (83 to 105 ms) and QRS duration during sinus rhythm (83 to 105 ms) or during constant atrial pacing (85 to 109 ms), were not significant (P greater than 0.05). Myocardial refractoriness and sinus node recovery time were not altered by the drug. Slight increases in mean heart rate (72.5 to 78.8 beats min-1), mean right atrial pressure (4.5 to 5.5 mm Hg) and mean cardiac output (5.4 l min-1 to 5.7 l min-1), were not significant (P greater than 0.05). Systolic and diastolic arterial pressure, cardiac work indices, and vascular resistance remained unchanged. These results demonstrate the safety of this drug given intravenously in a non-ischaemic setting, and warrant its further investigation as an antiarrhythmic agent in myocardial ischaemia.

Function of myocardial alpha-adrenoceptors

In addition to beta-adrenoceptors (beta ARs), cardiac myocytes of animals and man possess alpha 1ARs, but not alpha 2ARs. Norepinephrine and epinephrine have a higher affinity for myocardial alpha 1ARs than for beta ARs. Unlike beta AR stimulation, myocardial alpha 1AR stimulation does not increase the slow inward current. The alpha 1AR-mediated positive inotropic effect seen in isolated heart preparations appears to involve increased Ca sensitivity of myofibrils and production of inositol triphosphate (IP3) and diacylglycerol (DAG), but the functions of IP3 and DAG are not clear. Myocardial alpha 1AR stimulation reduces rate of isolated atria and Purkinje fibers and lengthens refractory period and action potential duration. Hypoxia increases alpha 1AR density in cardiomyocytes. alpha 1AR-mediated arrhythmias occur in isolated Purkinje fibers during hypoxia, following infarction, and in the presence of Ba2+ or high Ca2+. In animals, coronary artery occlusion and/or reperfusion increase myocardial alpha 1AR density and responsiveness, and alpha AR blocking drugs attenuate arrhythmias. However, an antiarrhythmic effect of alpha AR blocking drugs mediated by action on coronary vascular alpha ARs cannot be excluded. Presently available drugs do not differentiate between myocardial and vascular alpha ARs and thus affect the coronary and systemic circulations and, indirectly, the heart. Additional myocardial alpha 1AR-mediated effects include production of cardiac hypertrophy, stimulation of glucose uptake and phosphofructokinase and cyclic AMP phosphodiesterase activity, and release of atrial natriuretic peptide.