Alteration of glycogen and glucose metabolism in ischaemic and post-ischaemic working rat hearts by adenosine A1 receptor stimulation.

1. Cardioprotection by adenosine A1 receptor activation limits infarct size and improves post-ischaemic mechanical function. The mechanisms responsible are unclear but may involve alterations in myocardial glucose metabolism. 2. Since glycogen is an important source of glucose during ischaemia, we examined the effects of N6-cyclohexyladenosine (CHA), an A1 receptor agonist, on glycogen and glucose metabolism during ischaemia as well as reperfusion. 3. Isolated working rat hearts were perfused with Krebs-Henseleit solution containing dual-labelled 5-3H and 14C glucose and palmitate as energy substrates. Rates of glycolysis and glucose oxidation were measured directly from the production of 3H2O and 14CO2. Glycogen turnover was measured from the rate of change of [5-3H and 14C]glucosyl units in total myocardial glycogen. 4. Following low-flow (0.5 ml min-1) ischaemia (60 min) and reperfusion (30 min), left ventricular minute work (LV work) recovered to 22% of pre-ischaemic values. CHA (0.5 microM) improved the recovery of LV work 2 fold. 5. CHA altered glycogen turnover in post-ischaemic hearts by stimulating glycogen synthesis while having no effects on glycogen degradation. CHA also partially inhibited glycolysis. These changes accelerated the recovery of glycogen in CHA-treated hearts and reduced proton production. 6. During ischaemia, CHA had no measurable effect on glycogen turnover or glucose metabolism. Glycogen phosphorylase activity, which was elevated after ischaemia, was inhibited by CHA, possibly in response to CHA-induced inhibition of AMP-activated protein kinase activity. 7. These results indicate that CHA-induced cardioprotection is associated with alterations of glycogen turnover during reperfusion as well as improved metabolic coupling of glycolysis to glucose oxidation.     

Preconditioning of rat hearts by adenosine A1 or A3 receptor activation

A reduction in airway beta-adrenoceptor density has been reported in cystic fibrosis lung but the mechanism underlying this defect remains unclear. In this study, we have investigated whether the decrease in beta2-adrenoceptor associates with altered G protein-coupled receptor kinase (GRK) levels. We assessed GRK activity by rhodopsin phosphorylation, and beta2-adrenoceptor and GRK at the mRNA and protein levels by Northern and Western blotting in peripheral lung samples from normal donors and patients with cystic fibrosis. GRK activity was significantly increased in peripheral cystic fibrosis lung with parallel increases in GRK2/5 mRNAs and protein expression. Functionally, isoproterenol-stimulated adenylyl cyclase activity was also diminished by 65% in cystic fibrosis lung homogenates. These data suggest that the increase in GRK activity may be one of the mechanisms underlying alterations in the coupling between beta2-adrenoceptor and adenylyl cyclase via G-protein and may thus contribute to the downregulation of beta2-adrenoceptor in cystic fibrosis lung.     

Cicletanine improves myocardial function deteriorated by ischemia/reperfusion in isolated working rat hearts.

The effect of cicletanine, a novel furopyridine antihypertensive drug was compared with that of nitrendipine, a dihydropyridine slow calcium channel blocker, on cardiac function and reperfusion-induced ventricular arrhythmias in isolated working rat hearts subjected to 10-min ischemia induced by ligation of the left main coronary artery followed by 10-min reperfusion. Before ischemia, cicletanine and nitrendipine, perfused at concentrations of 3 x 10(-5), 6 x 10(-5), 10(-4), and 2 x 10(-4) or 10(-8) M, respectively, did not influence heart rate (HR), LV developed pressure (LVDP), its first derivative (LVdP/dtmax), and LV end-diastolic pressure (LVEDP), whereas aortic flow (AF) was decreased by 2 x 10(-4) M cicletanine only. Coronary flow (CF) remained unchanged by various cicletanine concentrations but was slightly increased by nitrendipine. In the concentration range of 3 x 10(-5)-10(-4) M, cicletanine improved AF either in ischemia or during reperfusion, whereas 2 x 10(-4) M had no such effect. Nitrendipine slightly attenuated ischemia/reperfusion-induced decrease in AF. Cicletanine and nitrendipine enhanced LVDP during ischemia. Ischemia-induced deterioration of LVdP/dtmax was reduced by cicletanine, during reperfusion, but this parameter was reduced by nitrendipine and the highest cicletanine concentration. Cicletanine decreased LVEDP significantly during ischemia and reperfusion, but nitrendipine had no such effect. All cicletanine concentrations reduced the incidence of irreversible ventricular fibrillation (VF) during reperfusion, an effect roughly concentration dependent in the range of 3 x 10(-5)-10(-4) M, whereas nitrendipine had no influence on arrhythmias.     

Polydeoxyribonucleotides and nitric oxide release from guinea-pig hearts during ischaemia and reperfusion.

1. Two polydeoxyribonucleotides, produced by the controlled hydrolysis of DNA of mammalian lung (defibrotide and its lower molecular weight fraction, P.O. 085 DV), were studied for their ability to modify the release of nitrite and the coronary flow in perfusates collected from isolated, normally perfused hearts of guinea-pigs and from hearts subjected to regional ischaemia and reperfusion. 2. In guinea-pig normally perfused hearts, both defibrotide (DFT) and its fraction, P.O. 085 DV, increase the amount of nitrite appearing in perfusates in a concentration-dependent fashion. At the highest concentration studied (10(-6) M), P.O. 085 DV was more effective than DFT. A concomitant increase in the coronary flow was observed. 3. The increase in nitrite in perfusates and the increase in coronary flow induced by both DFT and P.O. 085 DV were significantly reduced by NG-monomethyl-L-arginine (L-NMMA, 10(-4) M), an inhibitor of nitric oxide synthase (NOS). 4. The endothelium-dependent vasodilator, acetylcholine (ACh), enhances the formation of nitrite and the coronary flow. Both the increase in coronary flow and in the formation of nitrite were significantly reduced by L-NMMA (10(-4) M). 5. In guinea-pig hearts subjected to ischaemia and reperfusion, the effect of both compounds in increasing the amount of nitrite in perfusates was more evident and more pronounced with P.O. 085 DV. 6. Reperfusion-induced arrhythmias were significantly reduced by both compounds to the extent of complete protection afforded by compound P.O. 085 DV. 7. The cardioprotective and antiarrhythmic effects of DFT and P.O. 085 DV are discussed.     

An antiarrhythmic effect of adenosine during myocardial ischaemia and reperfusion

Adenosine (10 micrograms kg-1 min-1, infused into the lumen of the left ventricle) and dipyridamole (0.25 mg kg-1 intravenously, a dose that potentiated markedly the fall in arterial pressure in response to bolus doses of adenosine) each reduced the number of extrasystoles which occurred during the first 30 minutes following coronary artery occlusion in anaesthetised greyhound dogs (from 786 +/- 115 in control dogs to 156 +/- 44 in those treated with adenosine and to 388 +/- 167 with dipyridamole). Intracoronary adenosine (1 microgram kg-1 min-1, infused into the ischaemic area) however appeared to increase the number of extrasystoles to 1230 +/- 214. Left ventricular infusion of adenosine reduced the incidence of ventricular fibrillation (from 88 to 43%) when the ischaemic myocardium was perfused at the end of a 40 min occlusion period. In the dose used in this study (10 micrograms kg-1 min-1) adenosine caused a sustained fall in blood pressure but did not alter blood gases. In control dogs the levels of purine derivatives in blood draining the myocardium rendered ischaemic by coronary artery occlusion (local coronary venous samples) increased gradually during the ischaemic period (from 9 +/- 3 microM pre-occlusion to 25 +/- 7 microM post-occlusion). Dipyridamole increased the resting plasma concentration of purines prior to occlusion by approximately 90%; these remained raised for the occlusion period. These results support the suggestion that adenosine may act as a locally produced endogenous antiarrhythmic agent.     

Influence of beta-adrenoceptor tone on the cardioprotective efficacy of adenosine A(1) receptor activation in isolated working rat hearts

This study investigated the role of beta-adrenoceptors in the cardioprotective and metabolic actions of adenosine A(1) receptor stimulation. Isolated paced (300 beats min(-1)) working rat hearts were perfused with Krebs-Henseleit solution containing 1.2 mM palmitate. Left ventricular minute work (LV work), O(2) consumption and rates of glycolysis and glucose oxidation were measured during reperfusion (30 min) following global ischaemia (30 min) as well as during aerobic conditions. Relative to untreated hearts, N(6)-cyclohexyladenosine (CHA, 0.5 microM) improved post-ischaemic LV work (158%) and reduced glycolysis and proton production (53 and 42%, respectively). CHA+propranolol (1 microM) had similar beneficial effects, while propranolol alone did not affect post-ischaemic LV work or glucose metabolism. Isoprenaline (10 nM) impaired post-ischaemic function and after 25 min ischaemia recovery was comparable with 30 min ischaemia in untreated hearts (41 and 53%, respectively). Relative to isoprenaline alone, CHA+isoprenaline improved recovery of LV work (181%) and reduced glycolysis and proton production (64 and 60%, respectively). In aerobic hearts, CHA, propranolol or CHA+propranolol had no effect on LV work or glucose oxidation. Glycolysis was inhibited by CHA, propranolol and CHA+propranolol (50, 53 and 52%, respectively). Isoprenaline-induced increases in heart rate, glycolysis and proton production were attenuated by CHA (85, 57 and 53%, respectively). The cardioprotective efficacy of CHA was unaffected by antagonism or activation of beta-adrenoceptors. Thus, the mechanism of protection by adenosine A(1) receptor activation does not involve functional antagonism of beta-adrenoceptors.     

Inhibition of gastric acid secretion by adenosine receptor stimulation in the rat

The effects of adenosine and its metabolically stable derivative L-phenylisopropyladenosine (L-PIA), acting mainly on A1 receptors, on gastric acid secretion were studied in the rat. Although inactive by intraduodenal route, subcutaneous adenosine significantly inhibited acid secretion. This inhibition, however, was not dose-dependent. On the contrary, L-PIA was able to decrease dose-dependently acid output by both subcutaneous and intraduodenal route, its ED50 being 0.11 mg/kg subcutaneously and 0.24 mg/kg intraduodenally. The inhibitory effect of L-PIA was reduced by prior administration of theophylline. These results suggest that activation of A1-receptors inhibits acid secretion in the rat.     

L-carnitine and coenzyme Q10 protective action against ischaemia and reperfusion of working rat heart.

The protective effect of L-carnitine, coenzyme Q10 and their combination on haemodynamic and metabolic variables has been investigated in isolated perfused working rat hearts after 10 min of global normothermic ischaemia followed by 60 min of reperfusion. In untreated rats or in rats treated only with L-carnitine or with coenzyme Q10, this experimental condition did not induce any irreversible myocardial injury as measured by leakage of cardiac enzymes; however, it decreased some haemodynamic parameters such as cardiac output and minute work, as well as the ATP concentration and the total adenine nucleotide pool. No variations in haemodynamic and metabolic parameters were observed in the rats treated with L-carnitine plus coenzyme Q10. In the perfusate of the hearts of the rats treated with both compounds, a lower purine release (a good index of myocardial energy balance) was also obtained. Although the molecular mechanisms remain to be defined, it appears that the association of L-carnitine and coenzyme Q10 is more effective than using these compounds separately. The complementary and synergic actions of L-carnitine and coenzyme Q10 on metabolism and against peroxidation by oxygen reaction species may explain the efficacy of their association.     

Inhibition of post-ischaemic inflammation as a therapeutic approach to myocardial ischaemia reperfusion injury

Reperfusion injury of the heart remains an untreated condition even though reperfusion strategies have been successfully implemented in the clinical standard. Accordingly, low or no reflow phenomena with subsequent myocardial infarction, myocardial and endothelial stunning as well as arrhythmias remain aspects of reperfusion injury of the heart. Since leukocyte recruitment has been linked to these phenomena, inhibition of leukocyte adhesion and of the subsequent post-ischaemic inflammation of the heart has emerged as a therapeutic goal. Around 40 patents from 1997 to the first quarter of 2000 are implicitly or explicitly aimed at myocardial reperfusion injury.     

Effect of A2A adenosine receptor stimulation and antagonism on synaptic depression induced by in vitro ischaemia in rat hippocampal slices

1. In the present study we investigated the role of A2A adenosine receptors in hippocampal synaptic transmission under in vitro ischaemia-like conditions. 2. The effects of adenosine, of the selective A2A receptor agonist, CGS 21680 (2-[p-(2-carboxyethyl)-phenethylamino]-5'-N-ethylcarboxamidoade nos ine ), and of selective A2A receptor antagonists, ZM 241385 (4-(2-[7-amino-2-(2-furyl)-?1,2,4?-triazolo?2,3-a??1,3, 5?triazin-5-ylamino]ethyl)phenol) and SCH 58261 (7-(2-phenylethyl)-5-amino-2-(2-furyl)-pyrazolo-[4,3-e]-1,2, 4-triazolo[1,5-c]pyrimidine), have been evaluated on the depression of field e.p.s.ps induced by an in vitro ischaemic episode. 3. The application of 2 min of in vitro ischaemia brought about a rapid and reversible depression of field e.p.s.ps, which was completely prevented in the presence of the A1 receptor antagonist DPCPX (1, 3-dipropyl-8-cyclopentylxanthine) (100 nM). On the other hand both A2A receptor antagonists, ZM 241385 and SCH 58261, by themselves did not modify the field e.p.s.ps depression induced by in vitro ischaemia. 4. A prolonged application of either adenosine (100 micronM) or CGS 21680 (30, 100 nM) before the in vitro ischaemic episode, significantly reduced the synaptic depression. These effects were antagonized in the presence of ZM 241385 (100 nM). 5. SCH 58261 (1 and 50 nM) did not antagonize the effect of 30 nM CGS 21680 on the ischaemia-induced depression. 6. These results indicate that in the CA1 area of the hippocampus the stimulation of A2A adenosine receptors attenuates the A1-mediated depression of synaptic transmission induced by in vitro ischaemia.     

Selective IK blockade as an antiarrhythmic mechanism: effects of UK66,914 on ischaemia and reperfusion arrhythmias in rat and rabbit hearts.

1. UK66,914 is a specific and selective blocker of the delayed rectifying potassium current (IK). The effectiveness of IK block as a mechanism for prevention of ischaemia- and reperfusion-induced arrhythmias was tested by use of UK66,914: its actions in rat, a species deficient in cardiac IK were compared with its actions in rabbit, a species possessing functional cardiac IK. Antiarrhythmic actions in rabbit but none in rat is the only outcome possible if selective IK blockade is responsible for the antiarrhythmic actions of the drug during ischaemia and/or reperfusion. 2. During 30 min regional ischaemia, 0.3 and 1 microM UK66,914 had no influence on the incidence of ventricular fibrillation (VF) in rat (n = 9/group), values being 78% in controls, 100% in 0.3 microM-treated hearts and 78% in 1.0 microM-treated hearts (NS). UK66,914 also had no effect on reperfusion-induced VF incidence (100% in each group), nor on the latency to onset of ischaemia- or reperfusion-induced arrhythmias. In contrast, in rabbit (n = 13/group), similar concentrations of drug reduced the incidence of reperfusion-induced VF from 77% in controls, to 38% and 31% (P < 0.05) respectively. The incidence of ischaemia-induced arrhythmias was too low in controls to permit detection of an antiarrhythmic effect in rabbit; however no drug-induced proarrhythmia was seen.(ABSTRACT TRUNCATED AT 250 WORDS)     

Hemodynamic and mitochondrial parameters during hypoxia and reoxygenation in working rat hearts.

Hypoxia and reoxygenation in working rat hearts were investigated in this study. Cardiac hemodynamic parameters which decline immediately under hypoxic conditions, recover during reoxygenation. Biochemical and ultrastructural alterations exhibit a more complicated pattern. There is a primary phase in hypoxic perfusion up to 15 min with a steep increase of ADP contents and ATPase activities, and a severe fall of ATP/ADP ratios in mitochondria, as well as in tissue. High CAT (carboxyatractyloside) sensitivity of the ATPase is observed at 5 min of hypoxia. Furthermore, the number of ATPase particles visible at the inner mitochondrial membrane decreases. During the ensuing second phase of hypoxic perfusion (from 30 min on) the damage of mitochondrial ultrastructure becomes more evident. The amount of ATPase particles visible at the inner mitochondrial membrane further decreases. ATPase activities fluctuate, however, they remain connected with the membrane during hypoxia. ATP/ADP ratios attain values of almost 1. During reoxygenation (after 30 min of hypoxia) the levels of mitochondrial adenine nucleotides, oxidative phosphorylation rate and respiratory control index increase within 20 min and then slightly decline again. The ATP/ADP ratio is diminished in the course of reoxygenation. ATPase activity also decreases within 20 min of reoxygenation and the ADP/O ratio reaches control values. The ATPase activity gains its highest sensitivity towards CAT at 10 min of reoxygenation attaining a value similar to that of 5 min of hypoxic perfusion. It is suggested that hypoxia and reoxygenation under our conditions result in reversible derangement of ATPase and mitochondrial membrane structure.     

Salicylate reduces ventricular dysfunction and arrhythmias during reperfusion in isolated rat hearts.

Recent studies have shown the ability of salicylic acid (SA) to trap hydroxyl radicals (OH.) generated during reperfusion in ischemic myocardium. Since OH. is implicated in the pathogenesis of reperfusion injury, we examined the effect of SA on reperfusion-induced arrhythmias and postischemic ventricular dysfunction. Isolated rat hearts perfused by the Langendorff technique were preperfused with Krebs-Henseleit buffer containing SA for 10 min. Hearts were then made ischemic for 30 min, followed by 30 min of reperfusion. In a separate group, SA was administered only at the onset of reperfusion. The left ventricular contractile functions, left ventricular developed pressure (LVDP) and its first derivative (LV dP/dt), coronary flow (CF), and creatine kinase (CK) release were determined before and after ischemia. Epicardial electrocardiogram (ECG) was also recorded to analyze the incidence of ventricular tachycardia (VT) and ventricular fibrillation (VF). SA improved LVDP, LV dp/dt, and CF recovery and reduced CK release compared to the control group. The incidence of VT and VF during reperfusion was also significantly reduced by SA. Analysis of tissue thiobarbituric acid-reactive products indicates that SA decreased oxidative stress during reperfusion. In conclusion, these results suggest that SA reduces myocardial reperfusion injury and attenuates ventricular arrhythmias by trapping OH. radicals upon reperfusion in isolated rat hearts.     

Prevention of postischemic reperfusion damage on isolated working rat hearts by bopindolol and propranolol

Reperfusion after 30-min regional or global ischemia of the isolated working rat hearts did not restore the cardiac functions (as measured by cardiac output and power production), but exacerbated the existing damages. The lipid peroxidation product malondialdehyde (MDA) in the regional and global ischemic-reperfused myocardium increased by 37.6 and 45.2%, respectively. Bopindolol 0.1 mumol/L and propranolol 10 mumols/L protected the myocardium against the postischemic reperfusion damages, accelerated the recovery of cardiac functions during reperfusion and decreased the MDA content in the ischemic-reperfused myocardium. It is postulated that the prevention of cardiac cells from lipid peroxidation injury is related to the protection afforded by the drugs to the ischemic-reperfused hearts.     

Inhibition of nigrostriatal release of dopamine in the rat by adenosine receptor agonists: A1 receptor mediation

The stable analogues of adenosine, N-ethylcarboxamidoadenosine (NECA), R-phenylisopropyladenosine (R-PIA) and cyclohexyladenosine (CHA), dose-dependently decreased levels of 3-methoxytyramine (3-MT) in the striatum and antagonized pargyline-dependent accumulation of 3-methoxytyramine. These agents were equipotent with ED25 values of approximately 1 mg/kg, (p.o.) in inhibiting pargyline-dependent accumulation of 3-methoxytyramine. Since CHA and R-PIA are relatively selective for A1 receptors and NECA is almost equipotent at A1 and A2 sites, the data of undifferentiated potency for these 3 agents on release of dopamine (levels of 3-MT) would argue in favor of mediation of A1 receptors in this phenomenon. This conclusion was further supported by experiments with the A1-selective antagonist, 8-cyclopentyl-1,3-dipropylxanthine (CPDX), which antagonized the actions of CHA. Similar antagonism of CHA-dependent decreases in levels of cyclic GMP in the cerebellum, an action known to be mediated by A1 receptors, was also observed. These data support previous studies which indicated an adenosine receptor-mediated modulation of nigrostriatal release of dopamine. In addition, the present data indicate that this is an action on A1 receptors.     

Effects of iloprost (ZK 36374) on glutathione status during ischaemia and reperfusion of rabbit isolated hearts.

1. Reperfusion of rabbit isolated hearts after 60 min of ischaemia resulted in poor recovery of mechanical function, release of creatine phosphokinase (CPK) and of reduced (GSH) and oxidized (GSSG) glutathione, reduction of mitochondrial superoxide dismutase (Mn SOD) activity and of tissue GSH/GSSG ratio with a shift of cellular thiol redox state toward oxidation, suggesting the occurrence of oxidative stress. 2. Pretreatment of the isolated heart with the stable prostacyclin analogue (iloprost) at 27 or 270 nM, but not at 2.7 nM, improved the functional recovery of the myocardium, reduced CPK, GSH and GSSG release, maintained Mn SOD activity and attenuated the occurrence of oxidative stress. 3. This effect of iloprost cannot be explained by a decreased demand or an enhanced delivery of oxygen during ischaemia or by a direct effect on glutathione peroxidase and reductase activity.     

Caffeine-induced behavioral stimulation is dose-dependent and associated with A1 adenosine receptor occupancy.

Caffeine's psychomotor stimulant effects may relate to its blockade of central adenosine receptors. We examined acute caffeine effects on motor activity, adenosine receptor occupancy in vivo, and receptor affinity and density ex vivo. Acute doses of caffeine-sodium benzoate (0, 20, 40, and 60 mg/kg, intraperitoneally [0, 0.10, 0.21, 0.31 mu mol/kg]) were given to CD-1 mice and their activity was measured in an animal activity monitor over a 1-hour period. Adenosine receptor occupancy in vivo was quantified in mice 1 hour postdosage, using the high-affinity, A1 receptor selective adenosine antagonist [3H]-8-cyclopentyl-1,3-dipropylxanthine. Adenosine receptor binding affinities and densities were determined from analyses of binding studies in cortical, hippocampal, and brainstem membranes from treated mice (0 and 40 mg/kg caffeine). Caffeine doses of 20 and 40 mg/kg, corresponding to mean brain concentrations of 5 and 17 micrograms/g, increased all horizontal and vertical motor activity measures and stereotypy counts, as compared to doses of 0 and 60 mg/kg. Additionally, all acute caffeine doses significantly altered specific A1 binding in vivo (decreasing binding between 55% and 73% versus vehicle), presumably as it occupied A1 receptors. Therefore, at doses of 20 and 40 mg/kg, caffeine stimulated motor activity as it occupied A1 receptors; at a dose of 60 mg/kg (mean brain concentration of 26 micrograms/g) caffeine had no stimulant effect even though it appeared to occupy A1 receptors. Acute caffeine dosage did not alter ex vivo adenosine receptor binding affinity or density in any brain regions.(ABSTRACT TRUNCATED AT 250 WORDS)     

Sustained cardioprotection afforded by A2A adenosine receptor stimulation after 72 hours of myocardial reperfusion

This study was designed to determine whether cardioprotection afforded by A2A adenosine receptor stimulation can be sustained and to determine the effect of an A2A adenosine receptor agonist on Akt and cAMP response element binding protein (CREB) activation, as well as Hsp27 and Hsp70 protein expression in such events. The left anterior descending coronary artery was occluded for 40 minutes in anesthetized rats followed by 72 hours of reperfusion. A2A agonist (CGS21680 at 0.2 microg/kg/min) was administered for 120 minutes, starting either 5 minutes before (early) or after (late) the beginning of reperfusion. Infarct size was reduced significantly in the early compared with the control group (35.2 +/- 1.9% and 52.5 +/- 3.4%, respectively; P < 0.05), whereas no difference was observed with the late group (44.5 +/- 7.1%). After 72 hours of reperfusion, drug administration was accompanied by Akt activation (early, 121.8 +/- 17.6%; late, 118.1 +/- 16.4%; P < 0.05), as well as elevated Hsp27 expression (early, 197.2 +/- 27.7%; late, 203.8 +/- 36.8%; P < 0.05); CREB activation and Hsp70 expression were not altered. In another set of experiments in which reperfusion was limited to 15 minutes, Akt was activated only in the early group (121.8 +/- 17.6%; P < 0.05). Moreover, CREB was activated in both the early and late groups (98.4 +/- 8.3% and 107.0 +/- 6.5%, respectively; P < 0.05), whereas Hsp27 and Hsp70 expression were not altered. These results demonstrate that A2A adenosine receptor activation induces a sustained cardioprotection only if the therapy is instituted before reperfusion. This myocardial protection is associated by an early prosurvival Akt activation. CREB activation and Hsp27 content do not seem to be associated with cardioprotection because they are enhanced in both treated groups, suggesting indirect A2A agonist and pathology-related effects.     

Effects of the cardioselective KATP channel blocker HMR 1098 on cardiac function in isolated perfused working rat hearts and in anesthetized rats during ischemia and reperfusion

It has been argued that activation of KATP channels in the sarcolemmal membrane of heart muscle cells during ischemia provides an endogenous cardioprotective mechanism. In order to test whether the novel cardioselective KATP channel blocker HMR 1098 affects cardiac function during ischemia, experiments were performed in rat hearts during ischemia and reperfusion. Isolated perfused working rat hearts were subjected to 30 min of low-flow ischemia in which the coronary flow was reduced to 10% of its control value, followed by 30-min reperfusion. In the first set of experiments the hearts were electrically paced at 5 Hz throughout the entire protocol. At the end of the 30-min ischemic period the aortic flow had fallen to 44 +/- 2% (n=8) of its nonischemic value in vehicle-treated hearts, whereas in the presence of 0.3 micromol/l and 3 micromol/l HMR 1098 it had fallen to 29 +/- 7% (n=5, not significant) and 8 +/- 2% (n=12, P<0.05), respectively. Glibenclamide (3 micromol/l) reduced the aortic flow to 9.5 +/- 7% (n=4, P<0.05). In control hearts the QT interval in the electrocardiogram shortened from 63 +/- 6 ms to 36 +/- 4 ms (n=10, P<0.05) within 4-6 min of low-flow ischemia. This shortening was completely prevented by 3 micromol/l HMR 1098 (60 +/- 5 ms before ischemia, 67 +/- 6 ms during ischemia, n=9, not significant). When rat hearts were not paced, the heart rate fell spontaneously during ischemia, and HMR 1,098 (3 micromol/l) caused only a slight, statistically non-significant reduction in aortic flow during the ischemic period. In order to investigate whether HMR 1098 shows cardiodepressant effects in a more pathophysiological model, the left descending coronary artery was occluded for 30 min followed by reperfusion for 60 min in anesthetized rats. Treatment with HMR 1098 (10 mg/kg i.v.) had no statistically significant effects on mean arterial blood pressure and heart rate during the control, ischemia and reperfusion periods. At the end of the reperfusion period, aortic blood flow was slightly reduced by HMR 1098, without reaching statistical significance (two-way analysis of ANOVA, P=0.15). Myocardial infarct size as a percentage of area at risk was not affected by HMR 1098 (vehicle: 75 +/- 3%, HMR 1098: 72 +/- 2%, n=7 in each group). In conclusion, cardiodepressant effects of HMR 1098 were observed only in isolated perfused working rat hearts which were continuously paced during global low-flow ischemia. In the model of anesthetized rats subjected to regional ischemia, HMR 1098 had no significant effect on cardiac function or infarct size.