Na(+)/H(+) exchange inhibition-induced cardioprotection in dogs: effects on neutrophils versus cardiomyocytes.

Numerous studies have examined the effect of Na(+)/H(+) exchanger (NHE) inhibition on the myocardium; however, the effect of NHE-1 inhibition on neutrophil function has not been adequately examined. An in vivo canine model of myocardial ischemia-reperfusion injury in which 60 min of left anterior descending coronary artery occlusion followed by 3 h of reperfusion was used to examine the effect of NHE-1 inhibition on infarct size (IS) and neutrophil function. BIIB-513, a selective inhibitor of NHE-1, was infused before ischemia. IS was expressed as a percentage of area at risk (IS/AAR). NHE-1 inhibition significantly reduced IS/AAR and reduced neutrophil accumulation in the ischemic myocardium. NHE-1 inhibition attenuated both phorbol 12-myristate 13-acetate- and platelet-activating factor-induced neutrophil respiratory burst but not CD18 upregulation. Furthermore, NHE-1 inhibition directly protected cardiomyocytes against metabolic inhibition-induced lactate dehydrogenase release and hypercontracture. This study provides evidence that the cardioprotection induced by NHE-1 inhibition is likely due to specific protection of cardiomyocytes and attenuation of neutrophil activity.

Inhibition of the Na(+)/H(+) exchanger attenuates phase 1b ischemic arrhythmias and reperfusion-induced ventricular fibrillation.

The sodium-hydrogen exchanger-isotype 1 (NHE-1) plays a critical role in myocardial ischemia-reperfusion injury. While studies employing less selective sodium-hydrogen inhibitors have demonstrated antiarrhythmic activity, only one study has examined the in vivo efficacy of selective NHE-1 inhibition in a canine model of ischemia-reperfusion-induced arrhythmia. In the present study, the antiarrhythmic activity of Benzamide, N-(aminoiminomethyl)-4-¿4-(2-furanylcarbonyl)-1-piperazinyl -3-(methy lsulfonyl), methanesulfonate (BIIB 513), a novel NHE-1 inhibitor, was examined. An in vivo canine model of myocardial ischemia-reperfusion injury in which 60 min of left anterior descending coronary artery (LAD) occlusion followed by 3 h of reperfusion was employed. BIIB 513 was infused either prior to ischemia or prior to reperfusion. Arrhythmias were quantified by single lead electrocardiogram. Infarct size, determined by triphenyltetrazolium staining, was expressed as a percent of the area-at-risk. In vivo, NHE-1 inhibition did not affect phase 1a arrhythmias, which occur within the first 10 min of occlusion, however, BIIB 513 significantly reduced the incidence of ischemia-induced phase 1b arrhythmias which occur between 10 and 30 min following occlusion and the incidence of reperfusion-induced ventricular fibrillation. Furthermore, NHE-1 inhibition significantly reduced infarct size, when the drug was administered either prior to ischemia or prior to reperfusion. NHE-1 inhibition selectively reduces both ischemia-induced phase 1b arrhythmias and reperfusion-induced ventricular fibrillation, and also markedly reduces myocardial infarct size when the drug is administered prior to ischemia or prior to reperfusion.

Inhibition of the Na(+)/H(+) exchanger confers greater cardioprotection against 90 minutes of myocardial ischemia than ischemic preconditioning in dogs.

BACKGROUND: This study compared the efficacy of ischemic preconditioning (IPC) and sodium-hydrogen exchanger (NHE)-1 inhibition to reduce infarct size (IS) induced by a 90-minute ischemic insult and examined the interaction between NHE-1 inhibition and IPC. METHODS AND RESULTS: In a canine infarct model, either IPC, produced by 1 or four 5-minute coronary artery occlusions, or the specific NHE-1 inhibitor BIIB 513, 0.75 or 3.0 mg/kg, was administered 15 minutes before either a 60- or 90-minute coronary artery occlusion followed by 3 hours of reperfusion. IS was determined by TTC staining and expressed as a percentage of the area at risk (IS/AAR). Although both IPC and BIIB 513 at 0.75 mg/kg produced comparable and significant reductions in IS/AAR in the 60-minute occlusion model, insignificant reductions in IS/AAR were observed in the 90-minute occlusion model. However, BIIB 513 at 3.0 mg/kg markedly reduced IS in both models (P<0.05). Next, to examine the interaction between NHE-1 blockade and IPC, BIIB 0.75 mg/kg was administered either before IPC or during the washout phase of IPC before 90 minutes of coronary artery occlusion. Both combinations resulted in a greater-than-additive reduction in IS/AAR (P<0.05). CONCLUSIONS: These data demonstrate that although IPC and NHE-1 inhibition provide comparable protection against 60 minutes of myocardial ischemia, NHE-1 inhibition is more efficacious than IPC at protecting against a 90-minute ischemic insult. Furthermore, the combination of NHE-1 inhibition and IPC produces a greater-than-additive reduction in IS/AAR, suggesting either that NHE activity limits the efficacy of IPC or that different mechanisms are involved in the cardioprotective effect of IPC and NHE-1 inhibition.

Muscarinic receptor subtypes mediating myocardial blood flow redistribution.

Selective M1 and M2 muscarinic receptor antagonists pirenzepine and AF-DX 116, respectively, were administered to open-chest, pentobarbital-anesthetized dogs to determine the muscarinic receptor subtype responsible for the preferential vasodilation of subendocardial blood vessels during intracoronary methacholine infusion. There were no changes in systemic hemodynamics responsible for the myocardial blood flow redistribution produced by methacholine. Selective M1 receptor blockade with pirenzepine attenuated increases in transmural perfusion and prevented the redistribution of blood flow favoring the subendocardium during methacholine infusion. M2 receptor blockade with AF-DX 116 also attenuated the increase in transmural perfusion produced by methacholine but did not selectively block increases in subendocardial blood flow. These results support the hypothesis that M1 muscarinic coronary receptors are responsible for the redistribution of blood flow to the subendocardium (increased endo/epi) during cholinergic coronary vasodilation, whereas M2 receptors mediate an increase in total coronary blood flow. In additional experiments in isolated rabbit aorta, AF-DX 116 was 0.38 times less potent than pirenzepine with respect to endothelium-dependent muscarinic vasodilation but 1.47 times more potent with respect to endothelium-independent muscarinic vasoconstriction. This suggests that differences exist in the receptors found on the vascular smooth muscle and endothelium and that the preferential distribution of blood flow to the subendocardium may be due to stimulation of M1 receptors located on the vascular endothelium.

Effects of bradycardic agents on ischaemic myocardium and perfusion in anesthetized dogs.

The effects of several new specific bradycardic agents, AQ-A39 and AQ-AH 208, were evaluated in two models of myocardial ischaemia in anesthetized dogs. In the first model, equal bradycardic doses of AQ-AH 208 and propranolol were compared for their effects on coronary collateral blood flow in dogs subjected to an acute coronary artery occlusion. AQ-AH 208 produced a significant (P less than 0.05) increase (35%) in transmural collateral perfusion whereas propranolol had no effect. Atrial pacing to control heart rate only partially negated the beneficial effect of AQ-AH 208 on collateral flow. In the second model of ischaemia, AQ-AH 208 and AQ-A39 were evaluated for their effects on subendocardial segment shortening (% SS) and regional myocardial blood flow in dogs subjected to 15 minutes of coronary artery occlusion, followed by 3 hours of reperfusion. Both compounds produced similar decreases in heart rate (24%) and increases in the endocardial/epicardial distribution of collateral blood flow. During occlusion and throughout reperfusion, AQ-AH 208 and AQ-A39 resulted in a significant improvement in % SS in the ischaemic-reperfused region. These results suggest that specific bradycardic agents may have potential for therapeutic use in certain types of myocardial ischaemia by several different mechanisms.

Effect of the new specific bradycardia agent AQ-A39 (falipamil) on coronary collateral blood flow in dogs.

The effect of two doses (0.5 and 1.0 mg/kg i.v.) of a new specific bradycardic agent, AQ-A39 (5,6-dimethoxy-2-[3- [( alpha- (3,4,-dimethoxy)phenylethyl]methyl-amino)propyl]phthalimidine hydrochloride), on three indices of collateral function--retrograde pressure, retrograde flow, and tissue blood flow (radioactive microspheres)--was studied in anesthetized dogs following acute occlusion of the left anterior descending coronary artery. AQ-A39 produced a significant (p less than 0.05) dose-related decrease in heart rate without any other hemodynamic changes. Retrograde flow and subendocardial blood flow were significantly increased by the lower dose of AQ-A39, whereas retrograde pressure, retrograde flow, and midmyocardial and subendocardial flow were increased by the higher dose. Atrial pacing to the control heart rate eliminated the beneficial effects of AQ-A39 on collateral function. These results suggest that an increase in collateral perfusion may be one mechanism by which AQ-A39 alleviates myocardial ischemia.

Beneficial effects of two specific bradycardic agents AQ-A39 (falipamil) and AQ-AH 208 on reversible myocardial reperfusion damage in anesthetized dogs.

The effects of 2 specific bradycardic agents, AQ-A39 [5,6-dimethoxy-2-[3-([alpha-(3,4-dimethoxy)phenylethyl]methyl-amino) propyl]phthalimidine hydrochloride] and AQ-AH 208 [3,4-dihydro-6,7-dimethoxy-2-]3-[(2-[3, 4-dimethoxy-phenyl)-ethyl]-aminoethyl]propyl[-1(2H)isoquinone] were evaluated for their effects on subendocardial segment shortening (% SS) as measured by sonomicrometry and regional myocardial blood flow (radioactive microspheres) in anesthetized dogs subjected to a 15 min left anterior descending coronary occlusion followed by 3 hr of reperfusion. AQ-A39 (2.5-mg/kg bolus + 100 micrograms/kg/min i.v.) and AQ-AH 208 (0.5-mg/kg bolus + 25 micrograms/kg/min i.v.) were administered 15 min before coronary occlusion, during occlusion and throughout reperfusion. Both agents produced equivalent reductions in heart rate (24%) and the heart rate-systolic pressure product (27%) without any other significant hemodynamic changes. Collateral blood flow to the ischemic area was not different between the drug-treated and control groups. During coronary occlusion and throughout reperfusion, however, % SS was significantly (P less than .05) improved by both agents in the ischemic-reperfused area as compared to a control group. Thus, the beneficial actions of AQ-A39 and AQ-AH 208 on improving the recovery of subendocardial contractile function (% SS) may be explained partially by a reduction in myocardial oxygen requirements as a result of bradycardia. These results suggest that specific bradycardic agents may have potential for treatment of certain types of myocardial ischemia.

AQ-AH 208, a new bradycardic agent, increases coronary collateral blood flow to ischemic myocardium.

The effect of AQ-AH 208 [3,4-dihydro-6,7-dimethoxy-2-(3 - ((2-(3,4-dimethoxphenyl)ethyl)-amino- methyl)propyl)-1(2H)-isoquinolinone], a new selective bradycardic agent, on coronary collateral perfusion was investigated in anesthetized open-chest dogs following acute occlusion of the left anterior descending coronary artery. AQ-AH 208 (0.3 mg/kg i.v.), propranolol (0.3 mg/kg i.v.), and N-dimethylpropranolol (DMP; 2.5 mg/kg i.v.) were equieffective in reducing heart rate approximately 15%. AQ-AH 208 increased collateral flow to the subepicardium, midmyocardium, and subendocardium by 24, 46, and 35% (p less than 0.05), respectively, while propranolol and DMP had no effect. Atrial pacing to predrug levels in the presence of AQ-AH 208 reduced the increases in collateral flow to the different myocardial layers to 16, 25, and 30%, respectively; however, these increases were still significantly greater than control. It is concluded that part of the AQ-AH 208-induced increase in collateral perfusion is due to an increase in diastolic duration. The nature of the frequency-independent component of the effect is unknown but may be explained by a selective decrease in extravascular coronary resistance in the ischemic zone or an increase in the conductance of large epicardial coronary or collateral vessels.

Effects of AQ-AH 208, a new specific bradycardic agent, on myocardial ischemia-reperfusion injury in anesthetized dogs.

The effects of a new specific bradycardic agent, AQ-AH 208--3,4-dihydro-6,7-dimethoxy-2-[3-( [ 2-(3,4-dimethoxyphenyl)-ethyl]-aminoethyl)-propyl ]-1(2H)isochinolinon--on ischemia-reperfusion-induced myocardial infarct size following a 2-h occlusion and 30-min reperfusion period of the left anterior descending coronary artery (LAD) were determined in anesthetized dogs. Regional myocardial blood flow was measured by use of radioactive microspheres (15 micron), and tissue injury was determined by use of the triphenyltetrazolium chloride histochemical stain. In two groups of dogs, vehicle (saline) or AQ-AH 208 (500-micrograms/kg bolus followed by 25 micrograms/kg/min) was administered intravenously 10 min prior to LAD occlusion and infused throughout the occlusion and reperfusion periods. In a third group, AQ-AH 208 was given 10 min after LAD occlusion and infused throughout the remainder of the experiment. AQ-AH 208 treatment resulted in a significant reduction in heart rate and heart rate-systolic pressure product during occlusion and reperfusion. AQ-AH 208 produced variable increases in collateral blood flow as compared with the saline-treated group 20 min and 2 h following LAD occlusion. These increases in collateral perfusion were primarily to the midmyocardium and subendocardium, as reflected by increases in the endocardial/epicardial blood flow ratio (endo/epi). During reperfusion, subendocardial flow was significantly elevated and subepicardial flow reduced in both AQ-AH 208-treated groups, as compared with control, and the endo/epi was markedly increased. In all three groups, left ventricular weight, area at risk, percent of the left ventricle at risk, and retrograde flow and pressure prior to drug treatment were similar.(ABSTRACT TRUNCATED AT 250 WORDS)

Pharmacological properties of an extremely selective beta 1-adrenoceptor antagonist, 2-[4-[3-(tert-Butylamino)-2-hydroxypropoxy] phenyl]-3-methyl-6-methoxy-4(3H)-quinazolinone [+/-)HX-CH 44 BS).

2-[4-[3-(tert-Butylamino)-2-hydroxypropoxy] phenyl]-3-methyl-6-methoxy-4(3H)-quinazolinone (+/-)HX-CH 44 BS) is a new beta 1-selective adrenoceptor antagonist. Affinity for beta 1-adrenoceptors was determined in isolated guinea-pig papillary muscle (pA2 = 7.4), in isoprenaline-stimulated guinea-pig cardiac adenylate cyclase (pA2 = 7.3) and in receptor binding experiments with rat heart membranes (pKi = 8.0). Due to a direct relaxing action of (+/-)HX-CH 44 in smooth muscles its beta 2-affinity could only be estimated from receptor binding experiments (pKi = 5.0). (+/-)HX-CH 44 BS shows an extremely high beta 1/beta 2-affinity ratio of about 800 and therefore appears to be much more selective than atenolol or metoprolol (beta 1/beta 2 = 10-50). In contrast to propranolol or atenolol, (+/-)HX-CH 44 hardly antagonized bronchodilation induced by i.v. isoproterenol in anaesthetized guinea-pigs in vivo. Furthermore, cardiac actions of the beta 1-selective agonist norepinephrine in cats in vivo were competitively antagonized by (+/-)HX-CH 44 whereas inotropic and chronotropic effects of the non-selective beta-agonist epinephrine were not. This phenomenon may be explained by the mixed population of beta 1-and beta 2-adrenoceptors in cat heart and by the high beta 1-selectivity of (+/-)HX-CH 44.