Modulation of alpha 1-adrenoceptors in rat left ventricle by ischaemia and acyl carnitines: protection by ranolazine.

Myocardial alpha 1-adrenoceptor number has been reported to increase during ischaemia in myocytes consequent to an increase in acyl carnitine levels. We investigated whether this phenomenon occurs in vivo and whether the novel antiischaemic agent ranolazine will protect against it. Thirty-minute occlusion of the left anterior descending coronary artery (LAD) in anaesthetised rats produced an approximate doubling of the left ventricular (LV) alpha 1-adrenoceptor population. The carnitine palmitoyl transferase 1 (CPT1) inhibitor sodium 2-[5-(4-chlorophenyl)-pentylene]oxiran-2-carboxylate (POCA 100 micrograms/kg) reduced this ischaemia-induced increase when administered intraperitoneally (i.p.) 15 min before ischaemia and abolished the increase when administered intravenously (i.v.). The CPT1 inhibitor sodium 2-tetradecyl oxirane carboxylate dihydrate (TGDA) (500 micrograms/kg) inhibited the upregulation when administered i.p. and significantly decreased alpha 1-adrenoceptor density when administered i.v.; however, this agent, unlike POCA, reduced [3H]-prazosin binding directly. The alpha 1-adrenoceptor antagonist prazosin (100 micrograms/kg i.v.) did not prevent the increase. Direct addition of palmitoyl carnitine (10 microM) to membranes from nonischaemic myocardium caused a doubling in alpha 1-adrenoceptor number, and this effect was selective for heart membranes as compared with cerebral cortex; beta-adrenoceptor number was not modified. Ranolazine (500 micrograms/kg) inhibited upregulation when administered 15 min i.v. before ischaemia or after 3-day twice-daily (b.i.d.) i.p. pretreatment. This drug did not inhibit CPT1 directly.(ABSTRACT TRUNCATED AT 250 WORDS)

Prevention of myocardial enzyme release by ranolazine in a primate model of ischaemia with reperfusion.

In control anaesthetized baboons subjected to 30 min occlusion of the left anterior descending coronary artery, followed by 5.5 h reperfusion, total plasma levels for creatine kinase (CK) and lactate dehydrogenase (LDH) were markedly elevated in a time-related manner. In a second group of baboons pretreated 10 min prior to ischaemia with ranolazine [(+/-)-N-(2,6-dimethyl-phenyl)-4[2-hydroxy-3-(2-methoxyphenoxy)propyl]-1 - piperazine acetamide dihydrochloride; RS-43285-193] at 500 micrograms kg-1 i.v., followed by continuous infusion of 50 micrograms kg-1 min-1, neither enzyme was significantly elevated at any time point. Similarly, serum levels of the cardiospecific isoenzyme CK2 were 8 fold greater in the controls than in the ranolazine-treated animals at 6 h. The results indicate that ranolazine pretreatment abolished cardiac enzyme release over a 5.5 h reperfusion period, indicating a potential protective effect.

Effects of enprostil on cardiovascular and respiratory parameters in the dog, and on hematologic parameters in the rat, monkey, and human.

The effects of enprostil (+/-)-7-[(1R*,2R*,3R*)-3-hydroxy-2-[(E)-(3R*)-3-hydroxy-4-phenoxy-1- butenyl]-5-oxocyclopentyl]-4,5-heptadienoate) were studied on cardiovascular and respiratory parameters in the dog and on hematologic parameters in the rat, monkey, and human. Anesthetized dogs were instrumented to allow measurement of heart rate, systemic blood pressure, respiratory rate or tracheal pressure, and ventricular contractile force after intragastric (i.g.) or intravenous (i.v.) administration of enprostil in the presence or absence of autonomic challenges. The effects of intraduodenal (i.d.) enprostil on arterial and venous PO2, PCO2, and pH; respiratory rate, flow rate, and volume; blood pressure (b.p.); and heart rate were also examined. Enprostil i.v. (0.3-10 micrograms/kg) significantly increased tracheal pressure in a dose-dependent manner, but minimally altered b.p., heart rate, and ventricular contractile force. Enprostil i.v. (1-10 micrograms/kg) significantly inhibited pressor and depressor responses to several autonomic challenge agents at the highest dose level, indicative of a nonspecific inhibitory effect on b.p. responses. Cardiovascular effects of enprostil (1-100 micrograms/kg i.g.) were absent. Enprostil (10-3,000 micrograms/kg i.d.) had no noteworthy effects on respiratory parameters in the dog. Platelet aggregation effects of enprostil were studied in vitro using platelet rich plasma (PRP) from the rat, monkey, and human; whole blood clotting time and prothrombin time after oral enprostil were studied in the rat; and ex vivo effects on platelet activation were studied in humans.(ABSTRACT TRUNCATED AT 250 WORDS)

A comparison of the effects of a series of anti-anginal agents in a novel canine model of transient myocardial ischaemia.

1. An anaesthetized canine model of transient myocardial ischemia (TMI) has been developed in which reproducible and reversible electrocardiographic (ECG) and haemodynamic responses are exacerbated by electrical pacing. 2. The model could separate the ECG and haemodynamic effects of compounds with anti-ischaemic properties. 3. Compounds known to possess peripheral or coronary vasodilator properties did not necessarily alleviate the ECG consequences of TMI since glyceryl trinitrate was active whereas dipyridamole was not. The effects of verapamil were complicated by its adverse conduction effects while lidoflazine inhibited the ECG changes only during the ischaemic phase and the 'metabolic modulator' oxfenicine worsened the ECG response. 4. In a model considered to lack coronary reserve, improvements observed in the ischaemic ECG and global ventricular function were considered to result from a direct myocardial effect of the drugs examined rather than by a vascular influence. This was provided to the greatest degree by the Ca2+-entry blockers nifedipine and nicardipine, with flunarizine adopting an intermediate position.

Effects of the novel class Ia and class III antiarrhythmic agent RS-87337 on myocardial conduction in the anaesthetised dog.

The effects of the novel Class Ia/III antiarrhythmic compound RS-87337 on canine myocardial conduction were compared with those of the Class I antiarrhythmic disopyramide. RS-87337 had no effects on intra-atrial (I-A) or intra-ventricular (I-V) conduction parameters up to 10 mg.kg-1 i.v. (n = 6). Only one incidence of atrioventricular (A-V) block occurred at 10 mg.kg-1 at a pacing frequency of 261 beat.min-1. Disopyramide (5-10 mg.kg-1 i.v., n = 6) produced a frequency-dependent I-A conduction block and also significantly increased resting and paced A-V conduction times. Overall, disopyramide exhibited atrioselectivity while RS-87337 appeared more selective for ventricular conduction, possibly produced by a balance of its mixed Class Ia/III properties. RS-87337 was not cardiodepressant in the normal canine myocardium and produced no adverse effects on conduction parameters at doses up to 10 mg.kg-1 i.v.

The effects of the novel anti-anginal compound RS 43285 on myocardial conduction in the anaesthetized dog.

1. A pentobarbitone-anaesthetized canine model of myocardial conduction was developed to evaluate drug effects on intra-atrial (I-A), intra-ventricular (I-V) and atrioventricular (A-V) conduction parameters, both at rest and during electrical pacing of the right atrium or ventricle. Drug effects on the ability of the sino-atrial (SA) node to re-establish sinus rhythm on switching off electrical pacing were also considered. The effects of the novel anti-anginal compound RS 43285-193 ((+/-)-N-(2,6-dimethyl-phenyl)-4[2-hydroxy-3-(2-methoxyphenoxy)propyl] -1-piperazine acetamide dihydrochloride) were compared to those of the standard anti-anginal compounds nicardipine, nifedipine and verapamil. 2. In the dose range 15-7000 micrograms kg-1, RS 43285 had no significant effects on I-A, I-V or A-V conduction either at rest or during electrical pacing and did not affect the re-establishment of sinus rhythm. 3. Nicardipine had no effects on conduction parameters at resting heart rate. There were no effects on I-A or I-V conduction on electrical pacing but A-V conduction was increased at 200-500 micrograms kg-1 (with a 2:1 A-V conduction block in two out of six dogs); this was accompanied by a prolongation of the interval to reversion of sinus rhythm. 4. Nifedipine had no significant effects on I-A or I-V conduction but significantly prolonged A-V conduction at 1000 micrograms kg-1 and this dose also increased the interval to SA node recovery. 5. Verapamil did not effect I-A or I-V conduction. However, A-V conduction was affected with a significant prolongation occurring at resting heart rate at 100-400 Atg kg-' and a 2:1 A-V block in one dog at rest. During right atrial pacing verapamil significantly increased A-V conduction at 50- 400 fig kg-'. All dogs exhibited a 2:1 A-V conduction block at the highest frequency at 400 jig kg-'.

Interactions of beta-adrenoceptor antagonists and thyroid hormones in the control of heart rate in the dog.

Propranolol, sotalol and nadolol have been infused into conscious dogs, and doses at which the three drugs are equipotent as beta-adrenoceptor antagonists determined. In euthyroid dogs, sotalol was more effective at lowering heart-rate than an equivalent dose of propranolol, while an equivalent dose of nadolol was without effect. Hyperthyroidism potentiated the lowering of heart-rate by sotalol, but inhibited that by propranolol. The effect of sotalol on heart-rate was correlated with its prolongation of the Q-T interval of the ECG. That of propranolol was correlated with its prolongation of the P-R interval. Nadolol did not affect P-R interval or Q-T interval except at relatively high dosage. We conclude that the tachycardia of hyperthyroidism is not affected by blockade of beta-adrenoceptors and therefore that it is not mediated by adrenergic mechanisms. The effectiveness of propranolol and sotalol in lowering heart-rate must be due to actions peculiar to those drugs, and not to beta-adrenoceptor antagonism.