Impaired endothelium-dependent relaxation of dog coronary arteries after myocardial ischaemia and reperfusion: prevention by amlodipine, propranolol and allopurinol.

1. Anaesthetized, open-chest dogs were subjected to 60 min of left circumflex coronary artery occlusion followed by 90 min of reperfusion. Endothelium-dependent and -independent relaxant responses of the isolated coronary arterial rings were then investigated. 2. The endothelium-dependent, acetylcholine-induced relaxation of ischaemic/reperfused arterial rings was significantly attenuated in comparison to control rings (1.9 fold rightward shift, ischaemic/reperfused maximum relaxation = 57 +/- 13% of control maximum relaxation; P less than 0.05). In contrast, glyceryl trinitrate produced similar relaxant responses in control and ischaemic rings. 3. Pretreatment of dogs with either amlodipine (3 micrograms kg-1 min-1, i.v.) or propranolol (1 mg kg-1, i.v.) completely prevented the postischaemic impairment of endothelium-dependent relaxant responses (100 +/- 3% and 90 +/- 5% of control maximum relaxation, respectively). 4. Allopurinol pretreatment (25 mg kg-1, p.o. 24 h previously, plus 50 mg kg-1 i.v. 5 min before arterial occlusion) partially protected against endothelial dysfunction by preventing the ischaemia-induced rightward shift of the acetylcholine relaxation curve and increasing the maximum relaxation response (83 +/- 7% of control rings). 5. These results confirm that endothelium-dependent coronary vascular relaxation is impaired by ischaemia and reperfusion, and that the ischaemia-induced impairment is reduced by pretreatment with amlodipine, propranolol or allopurinol.     

Effects of Bordetella pertussis toxin pretreatment on the antiarrhythmic action of ischaemic preconditioning in anaesthetized rats.

1. Bordetella pertussis toxin, which catalyses the ADP-ribosylation of certain guanine nucleotide binding proteins (G proteins), thus functionally uncoupling them from associated receptors, was examined to determine whether it modified the antiarrhythmic effect of ischaemic preconditioning in anaesthetized rats. 2. Pertussis toxin (25 micrograms kg-1, i.p., 48 h prior to heart isolation) attenuated the negative chronotropic effect of acetylcholine (ACh) in rat isolated Langendorff perfused hearts. ACh (10 microM) reduced heart rate by 4% in hearts taken from pertussis toxin-treated animals, compared to a reduction of 57% in hearts taken from animals treated only with vehicle. 3. In anaesthetized rats, ischaemic preconditioning (a single 3 min occlusion of the left main coronary artery followed by 10 min reperfusion) had a pronounced antiarrhythmic effect during a subsequent 30 min period of regional myocardial ischaemia. Compared to hearts receiving only a 30 min period of left coronary occlusion, there was a reduced mortality (67% and 0% for control and preconditioned groups, respectively; P < 0.01) and decreased incidences of ventricular tachycardia (VT) and ventricular fibrillation (VF). Pretreatment with pertussis toxin (25 micrograms kg-1, i.p., 48 h previously) did not modify the arrhythmias associated with a 30 min period of regional myocardial ischaemia, neither did it modify the reduction in mortality (from 56% to 0%; P < 0.05) associated with preconditioning. Furthermore, the decrease in total ventricular premature beat count induced by preconditioning seen in controls (from 427 +/- 130 to 95 +/- 45) was also seen in pertussis toxin-treated rats (from 252 +/- 190 to 57 +/- 25).(ABSTRACT TRUNCATED AT 250 WORDS)     

Molsidomine prevents post-ischaemic ventricular fibrillation in dogs.

Forty anaesthetized dogs were subjected to left circumflex coronary artery ligation followed by reperfusion. Molsidomine was randomly administered to 20 dogs (50 micrograms kg-1 as an i.v. bolus - 15 min prior to coronary occlusion - followed by an infusion of 0.05 micrograms kg-1 min-1. Standard electrocardiographic leads 2 and 3 were continuously recorded to measure ST segment and delta R% changes and to document both the number of ventricular premature beats and the onset of ventricular fibrillation; aortic pressure and cardiac output were measured; thromboxane B2 plasma levels, platelet aggregation produced by ADP, and molsidomine plasma levels were determined before and at 10, 30 and 75 min after the start of the drug protocol. Molsidomine protected the treated animals from early (10 min) post-ischaemic ventricular fibrillation (0 of 20 vs 6 of 20, P = 0.0202), reduced the incidence of overall post-occlusion ventricular fibrillation (3 of 20 vs 10 of 20, P = 0.0407) and improved the total survival rate (P = 0.0067). In molsidomine treated dogs: mean aortic pressure and the rate-pressure product were lowered 10 min after the start of the drug; immediate post-occlusion (3 min) ST segment changes (0.82 +/- 0.52 vs 1.52 +/- 0.78 mV, P less than 0.025) and delta R% changes (37 +/- 50 vs 90 +/- 84%, P less than 0.025) were less marked; the number of ventricular premature beats was lowered and finally, a progressive decline of platelet aggregation produced by ADP was achieved after 75 min of drug infusion. These results were obtained in the presence of mean plasma levels of molsidomine ranging from 20 to 28 ng ml-1. The time-action curve of the antifibrillatory effect of molsidomine parallels those at the level of post-ischaemic electrocardiographic changes.     

The effects of timolol on arrhythmias and prostanoid release during canine myocardial ischaemia and reperfusion

Timolol (50 micrograms kg-1), administered intravenously to chloralose-anaesthetized open-chest greyhounds 30 min prior to occlusion of the left anterior descending coronary artery, reduced heart rate and mean arterial blood pressure. This dose caused a 20 fold increase in the dose of isoprenaline required to increase heart rate by 25 beats min-1. During the first 30 min of myocardial ischaemia the number of extrasystoles in the timolol-treated dogs (327 +/- 179) was less than in the control group (888 +/- 168) and none of the dogs that received timolol fibrillated. The haemodynamic changes induced by coronary artery occlusion (decreased cardiac output and stroke volume, increased peripheral vascular resistance) were similar in both control and timolol-treated dogs as were the increases in PCO2 and decreases in PO2 and pH in blood draining from the ischaemic myocardium. Timolol did not alter the release during myocardial ischaemia, of either thromboxane B2 or prostacyclin (measured as 6-keto PGF1 alpha). Reperfusion-induced ventricular fibrillation occurred in 7 out of 8 control dogs and in 5 out of 10 timolol-treated dogs. The overall survival following occlusion and reperfusion was improved by 10% to 50% by timolol.     

Cardioprotective effects of N-hydroxyguanidine PR5 in myocardial ischaemia and reperfusion in rats

1. The potential for the N-hydroxyguanidine compound PR5 (N-(3, 4-dimethoxy-2-chlorobenzylideneamino)-N'-hydroxyguanidine) as a cardioprotective agent in heart ischaemia and reperfusion injury was investigated using rat models. 2. Administration of 1-10 mg kg-1 of PR5 5 min before 10 min of left coronary artery occlusion, followed by 20 min reperfusion, strongly inhibited reperfusion burst of arrhythmias and markedly improved the survival of the animals (e.g. ventricular fibrillation incidence 93 vs 43% (P<0.05); mortality 47 vs 0% (P<0.05), for controls and for 3 mg kg-1 of PR5, respectively). 3. Administration of 3 mg kg-1 of PR5 1 min before reperfusion to rats subjected to 10 min occlusion, 20 min reperfusion was most effective in reducing arrhythmias and decreasing mortality (43 vs 0%, P<0.05), but effects were also seen when PR5 was administered 0, 1 and 5 min after start of reperfusion. 4. Coronary occlusion/reperfusion (10 - 20 min) increased malondialdehyde (MDA) of rat hearts (0.88+/-0.13 for sham vs 1.45+/-0.12 nmol mg-1 protein for ischaemic; P<0.05). In rats where 3 mg kg-1 PR5 were administered 1 min before reperfusion the increase was attenuated (MDA being 1.04+/-0.12; P<0.05 vs ischaemic). 5. PR5 caused a substantial reduction of the infarction size in rats subjected to 180 min left coronary artery occlusion, followed by 120 min of reperfusion; the necrotic zone being 326+/-32 mg for controls vs 137+/-21 mg for animals treated with 3x3 mg kg-1 of PR5 (P<0.01). 6. PR5 reduced the elevation of the ST-segment of the ECGs, as well as caused pronounced attenuation of the rapid blood pressure drop seen at the start of reperfusion following coronary artery occlusion. 7 We conclude that the N-hydroxyguanidine PR5 provides remarkable protection against ischaemia and reperfusion induced myocardial necrosis and life-threatening arrhythmias. These effects of PR5 are discussed in relation to a recently discovered ability of N-hydroxyguanidines to function as electron acceptors at the xanthine oxidase enzyme.     

Comparison of desulfatohirudin (REVASC) and heparin as adjuncts to thrombolytic therapy with reteplase in a canine model of coronary thrombosis.

1. We compared the direct thrombin inhibitor, desulfatohirudin (REVASC) and the indirect thrombin inhibitor, heparin, as adjuncts to thrombolytic therapy with reteplase in a canine model of coronary artery thrombosis. 2. Reteplase (BM 06.022) is a recombinant unglycosylated variant of human tissue-type plasminogen activator. Thrombus formation in anaesthetized open chest dogs was induced by electrical injury. Left circumflex coronary artery blood flow was monitored for 210 min with an electromagnetic flow probe. Twenty eight dogs were randomized to receive i.v. heparin (120 iu kg-1 bolus plus 80 iu kg-1 per h) or i.v. hirudin (2.0 mg kg-1 bolus plus 2.0 mg kg-1 per h) 10 min before thrombolysis preceded by i.v. acetylsalicyclic acid (20 mg kg-1) 5 min prior to anticoagulation. Every dog received an i.v. double bolus injection of 0.14 + 0.14 u kg-1 ( = 0.24 + 0.24 mg kg-1) reteplase, 30 min apart, 1 h after thrombus formation. 3. At comparable reperfusion rates (12 out of 12 vs. 15 out of 16 dogs), hirudin enhanced time to reperfusion (14.3 +/- 1.4 vs. 23.2 +/- 3.4 min; P < 0.05) and completely prevented reocclusion after reperfusion in contrast to heparin (0 out of 11 vs. 7 out of 11 dogs; P < 0.05). Coronary blood flow quality was improved by hirudin as shown by a higher maximum blood flow after reperfusion (130 +/- 14.3 vs. 83 +/- 9.3% of baseline; P < 0.05), a higher blood flow level at 20, 30, 40, and 50 min after onset of thrombolysis (P < 0.05) and a longer cumulative patency time (195 +/- 1.7 vs. 166 +/- 12 min; P < 0.05). Activated partial thromboplastin time and buccal mucosa bleeding time were prolonged (P < 0.05) by either anticoagulant, but did not differ significantly between groups. 4. The direct thrombin inhibitor, desulfatohirudin, enhanced thrombolysis, prevented reocclusion and increased blood flow as compared with the indirect thrombin inhibitor, heparin, when investigated at one dose level each and used in conjunction with reteplase.     

Cardioprotective and endothelial protective effects of [Ala-IL8]77 in a rabbit model of myocardial ischaemia and reperfusion.

1 We studied the effects of a form of interleukin-8 (i.e., [Ala-IL8]77) on endothelial dysfunction and myocardial injury in rabbits. Pentobarbitone-anaesthetized rabbits were subjected to 1.5 h occlusion of the marginal coronary artery and 3.5 h reperfusion. [Ala-IL8]77 (50 micrograms or its vehicle) was given i.v. as a bolus 10 min prior to reperfusion. [Ala-IL8]77 was also studied in isolated perfused hearts of rabbits. 2 Myocardial ischaemia plus reperfusion in untreated rabbits produced severe endothelial dysfunction and myocardial injury, including marked myocardial necrosis, elevated cardiac myeloperoxidase (MPO) activity in ischaemic cardiac tissue, and loss of response of marginal coronary rings to the endothelium-dependent vasodilators, acetylcholine (ACh) and A23187. 3 Administration of [Ala-IL8]77 10 min prior to reperfusion resulted in significant protective effects in post-ischaemic reperfusion. Compared with untreated rabbits, [Ala-IL8]77 caused a reduced necrotic zone (P less than 0.01), lower MPO activity in the necrotic zone (P less than 0.05), and significantly preserved vasorelaxant responses of marginal coronary artery rings to endothelium-dependent vasodilators, ACh (P less than 0.001) and A23187 (P less than 0.001). 4 These results indicate that myocardial ischaemia and reperfusion result in a severe endothelial dysfunction and myocardial injury which involved the interaction of neutrophils and endothelial cells. However, [Ala-IL8]77 did not appear to exert a direct endothelial protective effect in the absence of neutrophils in rabbit isolated perfused hearts. 5 Inhibition of neutrophil accumulation in the myocardium, perhaps by prevention of endothelial dysfunction resulting from [Ala-IL8]77, leads to significant protective effects in ischaemia and reperfusion in rabbits.     

EFFECT OF NICORANDIL ON POST-ISCHAEMIC CONTRACTILE DYSFUNCTION IN THE HEART: ROLES OF ITS ATP-SENSITIVE K' CHANNEL OPENING PROPERTY AND NITRATE PROPERTY

1. This study aimed to characterize the effect of nicorandil (NC) on myocardial stunning and the role of ATP-sensitive K⁺ (K_atp) channel opening property in its cardioprotective action. 2. In open-chest anaesthetized rabbits, myocardial stunning was induced by 10 min of coronary occlusion followed by 30 min of reperfusion. As an index of regional contractile function, systolic thickening fraction (TF) was measured by an epicardial Doppler sensor. The doses of NC (10 μg/kg per min) and nitroglycerin (TNG) (1 μg/kg per min) were selected not to lower the systemic blood pressure significantly. 3. In the untreated controls, TF at 30 min after reperfusion was 46.4 ± 2.9% of the baseline value, indicating myocardial stunning. Both NC and TNG significantly improved post-ischaemic recovery of TF when administered during the pre-ischaemic and post-ischaemic periods (TF = 68.2 ± 6.4%, 64.7 ± 2.3%, respectively). However, when their infusion was restricted to a pre-ischaemic 10 min period, TF recovery was improved by NC, but not by TNG (63.4 ± 7.9%, 40.9 ± 6.2%, respectively). 4. Pretreatment with glibenclamide (GL; 0.3 mg/kg) did not influence the recovery of TF after the 10 rnin ischaemia (TF = 52.4 ± 3.9% at 30 min after reperfusion). However, after the GL injection, a cardioprotective effect from nicorandil pretreatment was not detected (TF = 51.3 ± 1.7%). 5. These results suggest that nicorandil protects the myocardium against stunning by opening the K_atp channel when it is given before ischaemia, and that the nitrate property of nicorandil may also play a role during the reperfusion period in attenuation of post-ischaemic contractile dysfunction.     

Acetylcholine induces constriction of epicardial coronary arteries in anesthetized dogs after removal of endothelium

The acetylcholine-induced relaxation of isolated coronary arteries is reversed to contraction in the absence of endothelium. The importance of endothelium for the regulation of coronary blood flow remains unclear. We thus tested the effects of acetylcholine on epicardial arteries and on coronary resistance vessels in situ in 8 anesthetized dogs. The left circumflex coronary artery was perfused at constant pressure. Epicardial vasomotion was evaluated by sonomicrometry, the vasomotion of coronary resistance vessels by calculated end-diastolic resistance. Acetylcholine (1 microgram/kg/min i.c.) decreased epicardial resistance by 8.6 +/- 1.6% and end-diastolic resistance by 65.8 +/- 6.3%. The epicardial coronary segment was perfused with distilled water for 65 +/- 5 s to denude it of endothelium. After removal of epicardial endothelium, the decrease in end-diastolic resistance caused by acetylcholine was unchanged (59.6 +/- 1.2%); however, epicardial resistance was increased by 7.7 +/- 1.7%. Application of glyceryl trinitrate (5 micrograms/kg/min i.c.) induced a similar decrease of epicardial resistance before and after removal of endothelium:7.9 +/- 1.4 and 6.2 +/- 1.9%, respectively. We conclude that acetylcholine-induced dilation of epicardial coronary arteries is endothelium-dependent in vivo. However, the constriction of epicardial coronary arteries in the absence of endothelium is insufficient to reduce blood flow and to induce myocardial ischemia.     

Regional and cardiac haemodynamic responses to glyceryl trinitrate, acetylcholine, bradykinin and endothelin-1 in conscious rats: effects of NG-nitro-L-arginine methyl ester.

1. Conscious Long Evans rats, chronically instrumented for cardiovascular measurements, were challenged with i.v. bolus doses of glyceryl trinitrate (40 nmol kg-1), acetylcholine (1.2 nmol kg-1), bradykinin (3.2 nmol kg-1), or endothelin-1 (0.25 nmol kg-1). Under control conditions these doses produced similar falls in mean arterial blood pressure (glyceryl trinitrate, -20 +/- 3 mmHg; acetylcholine, -24 +/- 2 mmHg: bradykinin, -21 +/- 3 mmHg; endothelin-1, -25 +/- 3 mmHg), associated with renal, mesenteric and hindquarters vasodilatations (except for endothelin-1 which caused mesenteric vasoconstriction). 2. In the presence of NG-nitro-L-arginine methyl ester (L-NAME, 10 mgkg-1), a potent inhibitor of nitric oxide biosynthesis and endothelium-dependent vasorelaxation in vitro, the hypotensive responses to glyceryl trinitrate, acetylcholine, and endothelin-1 were increased, although that to bradykinin was not. However, comparing the differences between the response to glyceryl trinitrate and that to any other agonist in the absence and presence of L-NAME showed that there were relative attenuations of the hypotensive responses to bradykinin and endothelin-1, but not to acetylcholine, in the presence of L-NAME. 3. This comparative analysis showed that the renal and hindquarters vasodilator responses to bradykinin and endothelin-1 were attenuated in the presence of L-NAME, but the renal, mesenteric and hindquarters vasodilator responses to acetylcholine were not. However, when L-NAME was administered in the presence of pentolinium, captopril and the vasopressin V1-receptor antagonist, d(CH2)5[Tyr-(Et)]DAVP, (to abolish baroreflex and neurohumoral mechanisms), there was attenuation of the renal and mesenteric vasodilator effects of acetylcholine relative to those seen with glyceryl trinitrate.(ABSTRACT TRUNCATED AT 250 WORDS)     

Sulprostone-induced reduction of myocardial infarct size in the rabbit by activation of ATP-sensitive potassium channels.

1. This study examined whether (i) a 1 h pretreatment with or (ii) a continuous infusion of sulprostone reduces myocardial infarct size arising from coronary artery occlusion (60 min) and reperfusion (120 min) in the anaesthetized rabbit. In addition, we investigated whether the observed cardioprotective effect of this selective agonist of prostanoid EP1/EP3 receptors were due to the activation of ATP-sensitive potassium (KATP) channels. 2. In anaesthetized rabbits pretreated with vehicle (5% ethanol in 0.9% saline; 0.05 ml min-1, i.v.) infarct size (expressed as a percentage of the area at risk) after 60 min of coronary artery occlusion followed by 120 min of reperfusion was 59 +/- 4% (n = 10). Pretreatment of rabbits with sulprostone (1.0 microgram kg-1 min-1 for 1 h, discontinued immediately prior to coronary artery occlusion) did not reduce infarct size (60 +/- 4%; n = 4). In contrast, a continuous infusion of sulprostone (1.0 microgram kg-1 min-1) starting 10 min prior to the onset of LAL occlusion and continued throughout the experiment, significantly reduced infarct size (41 +/- 5%, n = 6) when compared to the respective vehicle-treated controls (57 +/- 4%, n = 10; P < 0.05). Sulprostone (pretreatment or continuous infusion) had no effect on any of the haemodynamic parameters measured. 3. The reduction in infarct size afforded by continuous infusion of sulprostone was abolished by pretreatment of rabbits with the KATP channel blocker 5-hydroxydecanoate (5-HD 5 micrograms kg-1; 63 +/- 4%; n = 6). When administered alone, 5-HD had no effect on infarct size when compared to control (52 +/- 6, n = 10). 4. We propose that a continuous infusion of the selective EP1/EP3 prostanoid receptor agonist, sulprostone, reduces infarct size in the anaesthetized rabbit by a mechanism that involves the opening of KATP channels.     

The role of mitochondrial K(ATP) channels in antiarrhythmic effects of ischaemic preconditioning in dogs

1. In the canine a single brief (5 min) coronary artery occlusion protects the myocardium against the severe ventricular arrhythmias and reduces the ischaemic changes that result from a subsequent, more prolonged (25 min) occlusion. The main purpose of the present study was to examine whether mitochondrial K(ATP) channels are involved in this protection. 2. In chloralose-urethane anaesthetized dogs, preconditioning (PC) was induced by a single 5 min period occlusion of the left anterior descending (LAD) coronary artery, 20 min prior to a 25 min occlusion of the same artery. In some of these PC dogs 5-hydroxydecanoate (5-HD; 150 micro g kg(-1) min(-1) by intracoronary infusion) was given over a period of 30 min either before, or after PC. In other dogs the mitochondrial K(ATP) channel opener diazoxide (1 mg kg(-1); i.c.) was given, either alone or in the presence of 5-HD. Control dogs (infused with saline) were simply subjected to a 25 min occlusion and reperfusion. 3. Compared to controls, both PC and diazoxide significantly reduced the number of ventricular premature beats (VPBs; 295+/-67 to 89+/-28 and 19+/-11, respectively; P<0.05), the number of episodes of ventricular tachycardia (VT; 8.3+/-4.2 to 1.6+/-0.9 and 0.2+/-0.1; P<0.05) and the incidences of VT (100 to 43 and 33%; P<0.05) and ventricular fibrilation (VF; 60 to 0 and 17%; P<0.05) during the 25 min occlusion of the LAD. Further, 43% of the PC dogs and 58% of the diazoxide treated dogs survived the combined ischaemia-reperfusion insult (cp. 0% in the controls; P<0.05). The protection afforded by PC and diazoxide was abolished by 5-HD, especially when it was given prior to the PC occlusion. In the presence of 5-HD, three out of 10 dogs fibrillated during the PC occlusion and another three dogs died following reperfusion. Furthermore, there were no survivors in this group from the prolonged ischaemia/reperfusion insult. 5-HD given after PC only attenuated the antiarrhythmic protection. 4. Opening of mitoK(ATP) channels prior to ischaemia by preconditioning and diazoxide protects the myocardium against ischaemia and reperfusion-induced arrhythmias. This protection is abolished if the opening of these channels is prevented by the prior administration of 5-HD but only attenuated if 5-HD is given after preconditioning. The results indicate that opening of mitoK(ATP) channels prior to ischaemia is mandatory for protection against ischaemia and reperfusion-induced arrhythmias.     

Cyclic nucleotides and contractility of isolated soleus muscle.

The effects of the thromboxane antagonist AH23848 (1 mg kg-1 i.v.) were examined in anaesthetized greyhounds prepared for occlusion of the left anterior descending coronary artery with subsequent reperfusion after 40 min of myocardial ischaemia. Pretreatment with AH23848 30 min before coronary artery occlusion reduced the number of ischaemia-induced extrasystoles to 339 +/- 111 compared with 736 +/- 153 in the control group. The incidence of ventricular fibrillation following reperfusion was also markedly reduced; 25% compared with 88% in the controls. Late intervention with AH23848, 25 min after the onset of myocardial ischaemia did not significantly alter the incidence of reperfusion-induced ventricular fibrillation; 70% of the control group died and 60% of those that received AH23848. The ischaemia-induced release of thromboxane A2 and prostacyclin was not altered by pretreatment with AH23848. The results suggest that blockade of thromboxane receptors before myocardial ischaemia is an effective antiarrhythmic strategy in this model.     

Enhanced adenosine A<Subscript>2B</Subscript> mediated coronary response in reserpinised rat heart

In this study, we investigated the effect of noradrenaline depletion on contractile recovery in rat isolated heart following myocardial ischaemia. Groups tested included control tissues and hearts from reserpinised rats. Reserpine 1 mg/kg s.c. was injected into rats 18 to 24 h prior to experiments. Hearts underwent 15 min global normothermic ischaemia followed by 30 min reperfusion.Functional data (end diastolic pressure (EDP), heart rate (HR), left ventricular developed pressure (LVDP), dP/dt(max), dP/dt(min)) showed that contractile function following ischaemia-reperfusion is unaffected by reserpinisation. However, pre- and post-ischaemic coronary flow rates (CFR) were increased by 16 to 38% in hearts from reserpinised rats versus control hearts. Pre-ischaemic CFRs in control hearts (11.17+/-0.67 ml/in(-1) x g tissue(-1), n=9) were significantly lower then CFRs derived from reserpinised rat hearts (14.57+/-0.72 ml/min(-1)/g tissue(-1), n=10). Post-ischaemic reactive hyperaemia was evident in all groups. CFRs in reserpinised hearts remained elevated when compared to pre-ischaemic values through reperfusion (P<0.05). Reserpine treatment did not significantly alter pre- or post-ischaemic adenosine efflux. The A(2B) adenosine receptor antagonist alloxazine (10 microM) attenuated pre- and post-ischaemic CFRs in both control and reserpinised hearts (P<0.05) without altering the hyperaemic response while the A(2A) adenosine receptor antagonist 8-(3-chlorostyryl) caffeine (1 microM) did not alter CFRs in both groups. The A(3) adenosine receptor antagonist MRS1191 (0.1 microM) increased CFR in control and reserpinised hearts (P<0.05).Catecholamine depletion with reserpinisation enhances the responsiveness of the coronary resistance vessels to endogenous adenosine through activation of the A(2B) adenosine receptor.     

Role of endogenous endothelin in myocardial and coronary endothelial injury after ischaemia and reperfusion in rats: studies with bosentan, a mixed ETA-ETB antagonist.

1. Previous studies suggested that endothelin-1 (ET-1) may play a role in myocardial ischaemia and reperfusion. This study was designed to test the effect of a new nonpeptide antagonist of endothelin ETA and ETB receptors, bosentan, on myocardial infarct size, ventricular arrhythmias, and coronary endothelial dysfunction after ischaemia and reperfusion. 2. Anaesthetized male Wistar rats were subjected to 20 min ischaemia (left coronary artery occlusion) followed by 1 h (for the evaluation of coronary endothelial dysfunction) or 2 h (for the evaluation of infarct size) reperfusion, or 5 min ischaemia followed by 15 min reperfusion (for the evaluation of reperfusion arrhythmias). Vascular studies were performed on 1.5-2 mm coronary segments (internal diameter 250-300 microns) removed distal to the site of occlusion and mounted in wire myographs for isometric tension recording. Area at risk and infarct size were determined by Indian ink injection and triphenyl tetrazolium staining, using computerized analysis of enlarged sections after colour video acquisition. 3. Bosentan, administered at a dose which virtually abolished the pressor response to big ET-1 (3 mg kg-1, i.v. before ischaemia) did not affect heart rate, arterial pressure or the rate pressure product before ischaemia, during ischaemia and during reperfusion. Bosentan did not affect the incidence of reperfusion-induced ventricular fibrillation (controls: 86%, n = 14; bosentan: 93%, n = 15), and did not modify infarct size (% of area at risk: controls: 63 +/- 4, n = 10; bosentan: 60 +/- 6, n = 8).(ABSTRACT TRUNCATED AT 250 WORDS)     

Effects of nisoldipine on recovery of coronary blood flow, sarcoplasmic reticulum function and other biochemical parameters in post-ischaemic porcine myocardium

The effects of nisoldipine (0.1 micrograms/kg/min; n = 9) or its solvent (n = 9) were studied in pigs, in which left anterior descending coronary artery (LADCA) blood flow in both groups was reduced to 20% of baseline for 60 min and reperfused for 2 hr. Infusions were started at 30 min of ischaemia and lasted throughout reperfusion. In both groups, flow reduction abolished regional contractile function and caused similar decreases in the level of creatine phosphate (CP; by 70%) and the energy charge (from 0.91 to 0.69), mean arterial blood pressure (by 25%), LVdP/dtmax (by 30%) and cardiac output (by 30%). During ischaemia LADCA blood flow slightly increased (from 14 +/- 8 to 24 +/- 6 mL/min/100 g; P less than 0.05) in the nisoldipine-treated animals, resulting in an increase in CP to 91 +/- 24% of baseline and preventing further decreases in energy charge, as observed in the solvent-treated animals. After 2 hr of reperfusion in neither group return of contractile function of the post-ischaemic myocardium was observed. Post-ischaemic blood flow in the nisoldipine-treated pigs increased from 24 +/- 6 mL/min/100 g to 76 +/- 14 mL/min/100 g and from 19 +/- 6 mL/min/100 g to 41 +/- 6 ml/min/100 g in the solvent-treated animals (P less than 0.05) after 2 hr of reperfusion. Myocardial work was significantly higher in the nisoldipine-treated animals (111 +/- 15 mmHg.L/min as compared to 69 +/- 14 mmHg.L/min in the solvent-treated pigs after 2 hr of ischaemia). The energy charge of the post-ischaemic myocardium was similar for both groups (0.84 +/- 0.02 for the nisoldipine-treated and 0.83 +/- 0.03 for the solvent-treated animals). The rate of sarcoplasmic reticular Ca2+ uptake of the non-ischaemic segment of the nisoldipine-treated animals was 61% higher (P less than 0.05) than that of the solvent-treated animals. In the post-ischaemic myocardium similar rates of Ca2+ uptake were found in both groups, but the activities were markedly lower as compared to the non-ischaemic myocardium. It is concluded that nisoldipine increases blood flow during reperfusion, which may have been caused by coronary vasodilatation. However, attenuation of the "no-reflow" phenomenon also contributed, since more rapid rephosphorylation of ADP leading to an increase in CP during ischaemia may have preserved jeopardized cells. Moreover, nisoldipine increases the sarcoplasmic reticular Ca2+ pump activity independent of ischaemia, which may have contributed in reducing the Ca2+ overload.     

Comparisons of the effects of nicorandil, pinacidil, nicardipine and nitroglycerin on coronary vessels in the conscious dog: role of the endothelium.

1. The vasodilator properties of nicorandil on large and small coronary arteries were compared to those of nicardipine, pinacidil, nitroglycerin and acetylcholine in six conscious dogs. 2. Intravenous bolus injections of acetylcholine (0.1 micrograms kg-1), nitroglycerin (0.3-3 micrograms kg-1), pinacidil (10-100 micrograms kg-1), nicardipine (3-30 micrograms kg-1) and nicorandil (10-100 micrograms kg-1) dose-dependently increased circumflex coronary artery diameter and decreased coronary vascular resistance, indicating vasodilator effects on both conduit and resistance coronary arteries. 3. Three days after removal of the endothelium of the circumflex coronary artery (balloon angioplasty), pinacidil- and nicardipine-induced dilation of large coronary arteries was greatly reduced (both -76%, P < 0.01) whereas that produced by nitroglycerin and nicorandil was decreased only slightly and to a similar extent for both drugs (-19%, P < 0.01 and -28%, P < 0.05, respectively). 4. Thus in conscious dogs, nicardipine- and pinacidil-induced dilatation of large coronary arteries is endothelium-dependent. In contrast, the vasodilator effects of nitroglycerin and nicorandil on conduit vessels are endothelium-independent. 5. Finally, our results demonstrate that nicorandil dilates the large coronary arteries through its nitrate-like action and that the ATP-potassium channel opening properties of the drug are not involved in this effect in the conscious dog.     

Preservation of mitochondrial function may contribute to cardioprotective effects of Na+/Ca2+ exchanger inhibitors in ischaemic/reperfused rat hearts

BACKGROUND AND PURPOSE: Na+/Ca2+ exchanger (NCX) inhibitors are known to attenuate myocardial reperfusion injury. However, the exact mechanisms for the cardioprotection remain unclear. The present study was undertaken to examine the mechanism underlying the cardioprotection by NCX inhibitors against ischaemia/reperfusion injury. EXPERIMENTAL APPROACH: Isolated rat hearts were subjected to 35-min ischaemia/60-min reperfusion or 20-min ischaemia/60-min reperfusion. NCX inhibitors (3-30 microM KB-R7943 (KBR) or 0.3-1 microM SEA0400 (SEA)) were given for 5 min prior to ischaemia (pre-ischaemic treatment) or for 10 min after the onset of reperfusion (post-ischaemic treatment). KEY RESULTS: With 35-min ischaemia/60-min reperfusion, pre- or post-ischaemic treatment with KBR or SEA neither enhanced post-ischaemic contractile recovery nor attenuated ischaemia- or reperfusion-induced Na+ accumulation and damage to mitochondrial respiratory function. With the milder model (20-min ischaemia/reperfusion), pre- or post-ischaemic treatment with 10 microM KBR or 1 microM SEA significantly enhanced the post-ischaemic contractile recovery, associated with reductions in reperfusion-induced Ca2+ accumulation, damage to mitochondrial function, and decrease in myocardial high-energy phosphates. Furthermore, Na+ influx to mitochondria in vitro was enhanced by increased concentrations of NaCl. KBR (10 microM) and 1 microM SEA partially decreased the Na+ influx. CONCLUSIONS AND IMPLICATIONS: The NCX inhibitors exerted cardioprotective effects during relatively mild ischaemia. The mechanism may be attributable to prevention of mitochondrial damage, possibly mediated by attenuation of Na+ overload in cardiac mitochondria during ischaemia and/or Ca2+ overload via the reverse mode of NCX during reperfusion.     

Failure of allopurinol and a spin trapping agent N-t-butyl-alpha-phenyl nitrone to modify significantly ischaemia and reperfusion-induced arrhythmias.

The possible role of free radicals in the genesis of occlusion and reperfusion-induced arrhythmias was studied by determining the effects of the xanthine oxidase inhibitor allopurinol (400 mg p.o. 24 h before experimentation +25 mg kg-1 i.v.) and the free radical scavenger N-t-butyl-alpha-phenyl nitrone (PBN; 50 mg kg-1 i.v.) on these arrhythmias in chloralose anaesthetized greyhounds. Neither of the drugs had any major effects on haemodynamic variables, although allopurinol caused a significant increase in heart rate. The mean number of extrasystoles observed during ischaemia in dogs given allopurinol or PBN was not significantly different from those seen in controls. Further, the incidence of ventricular fibrillation during either occlusion or reperfusion was unchanged by either drug and there was thus no improvement in survival. These results suggest that, in this model of myocardial ischaemia and reperfusion, free radicals may not play a major role in the genesis of life-threatening arrhythmias.     

N-nitro L-arginine causes coronary vasoconstriction and inhibits endothelium-dependent vasodilatation in anaesthetized greyhounds.

1. The effect of N-nitro-L-arginine (L-NNA), an inhibitor of nitric oxide biosynthesis, on large coronary artery diameter and coronary blood flow was examined in anaesthetized greyhounds. The effects of L-NNA on the coronary vascular responses to acetylcholine (ACh), glyceryl trinitrate (GTN) and 5-hydroxytryptamine (5-HT) were also assessed. 2. L-NNA (5 mg kg-1), infused into the left circumflex coronary artery, increased systemic mean arterial pressure and decreased the external diameter of the artery. Infusion of L-NNA decreased coronary blood flow in 5 of the 7 dogs tested and increased mean coronary resistance but neither of these effects was statistically significant. There was no change in heart rate. 3. Intra-arterial injection of both ACh (0.01-0.05 micrograms kg-1) and GTN (0.1-0.5 micrograms kg-1) increased large coronary artery diameter and coronary blood flow. Coronary vascular responses to the endothelium-dependent vasodilator ACh were significantly reduced by L-NNA, whereas the responses to the endothelium-independent vasodilator GTN were not significantly affected. 4. 5-HT (0.1 microgram kg-1, injected into the left circumflex coronary artery) decreased coronary artery diameter but increased coronary blood flow. After the administration of L-NNA the 5-HT-induced dilatation of the coronary resistance vessels was significantly attenuated whereas the constriction of the circumflex coronary artery was increased in 3 out of 3 dogs in which diameter could be measured, although the latter effect was not statistically significant. 5. These data indicate that L-NNA causes coronary and systemic vasoconstriction and selectively inhibits endothelium-dependent vasodilatation in the coronary circulation of the anaesthetized greyhound.(ABSTRACT TRUNCATED AT 250 WORDS)