Effects of the angiotensin II subtype 1 receptor antagonist losartan on functional recovery of isolated rat hearts undergoing global myocardial ischemia-reperfusion

STUDY OBJECTIVE: To investigate the effects of the angiotensin II subtype 1 receptor (AT1R) antagonist losartan on functional recovery of isolated rat hearts undergoing global myocardial ischemia-reperfusion compared with myocardial protective effects of ischemic preconditioning. DESIGN: Ex vivo experiment using isolated perfused rat heart. SETTING: Academic laboratory. INTERVENTION: Hearts from Sprague-Dawley rats were perfused with oxygenated Krebs-Henseleit buffer and randomized to one of four groups: time control, vehicle, ischemic preconditioning, or losartan. MEASUREMENTS AND MAIN RESULTS: After randomization, hearts underwent 30 minutes of global ischemia followed by 30 minutes of reperfusion. Changes in end-diastolic pressure (EDP), left ventricular developed pressure (LVDP), and infarct size were examined between treatment groups by two-way analysis of variance with repeated measures. Cardiac angiotensin II receptor (ATR) density and infarct size were measured in control hearts and in a subgroup of hearts exposed to ischemia-reperfusion injury. Total ATR density and percentage of myocardial AT1R were increased in hearts exposed to ischemia-reperfusion. Myocardial ischemia-reperfusion injury resulted in a 56% reduction in LVDP from baseline in hearts randomized to vehicle. However, it declined by only 22% and 28% in hearts randomized to ischemic preconditioning and losartan, respectively. Compared with vehicle, both ischemic preconditioning and losartan decreased EDP (ischemic preconditioning 39 +/- 3 mm Hg, losartan 54 +/- 5 mm Hg, vs vehicle 78 +/- 8 mm Hg), and reduced infarct size (ischemic preconditioning 9%, losartan 12%, vs vehicle 36%). CONCLUSION: Treatment of isolated rat hearts with losartan before ischemia-reperfusion injury resulted in significant cardioprotection similar to that observed with ischemic preconditioning.     

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)     

Effects of the insurmountable angiotensin AT1 receptor antagonist candesartan and the surmountable antagonist losartan on ischemia/reperfusion injury in rat hearts.

Two angiotensin AT1 receptor antagonists with different receptor binding characteristics, candesartan (insurmountable antagonism) and losartan (surmountable antagonism), were compared as regards their effects on angiotensin II-induced vasoconstriction and on myocardial ischemia/reperfusion injury. In isolated rat hearts perfused under constant flow, it was found that at equipotent concentrations candesartan (10 nM) and losartan (3 microM) almost completely inhibited the angiotensin II-induced increase in coronary perfusion pressure. However, if a washout period was introduced before the angiotensin II challenge, the effect of losartan quickly vanished, while that of candesartan remained. In hearts subjected to 25 min of global ischemia and 45 min of reperfusion, pre-treatment with candesartan (10 nM) or losartan (3 microM) immediately prior to ischemia improved the recovery of left ventricular developed pressure as compared to the effect of vehicle (69 +/- 3.2 and 64 +/- 2.3 vs. 44 +/- 6.2%, respectively; mean +/- S.E.M, P < 0.05). When ischemia was initiated following 30 min of washout after drug administration, the recovery of left ventricular developed pressure was higher in the candesartan group (73 +/- 3.2%, P < 0.05), but not in the losartan group (63 +/- 2.8%), than in the vehicle group (58 +/- 4.8%). The cumulative creatine kinase release during the first 30 min of reperfusion in the washout experiments was lower in the candesartan group (28.5 +/- 2.30 U, P < 0.05), but not in the losartan (40.8 +/- 6.73 U) group, than in the vehicle group (48.1 +/- 4.35 U). No significant difference between groups in left ventricular end-diastolic pressure and coronary perfusion pressure was found. The present results demonstrate that angiotensin AT1 receptor antagonists at equipotent concentrations could differ in their cardioprotective effects in hearts subjected to ischemia/reperfusion. It is suggested that the insurmountable AT1 receptor characteristics of candesartan could provide more persistent cardioprotection than the surmountable receptor characteristics of losartan.     

Effects of captopril and losartan on myocardial ischemia-reperfusion induced arrhythmias and necrosis in rats.

Angiotensin-converting enzyme inhibitors (ACEIs) and angiotensin II type 1 (AT (1)) receptor blockers improve ischemia-reperfusion induced arrhythmias and infarct size in several animal models. However, the effects of pretreatment with ACEIs or AT (1) receptor blockers on acute myocardial infarct size and arrhythmias are controversial. Thus, we sought to assess the comparative effects of pretreatment with ACEI captopril and AT (1)-receptor blocker losartan on myocardial infarct size and arrhythmias in a rat model of ischemia-reperfusion. We randomly assigned 92 male Wistar rats for arrhythmias ( n= 60) and necrosis ( n= 32) experiments. To produce arrhythmia, the left main coronary artery was occluded for 7 min, followed by 7 min of reperfusion and to produce necrosis, the the left main coronary artery was occluded for 30 min, followed by 120 min of reperfusion. Captopril (3 mg kg (-1)) and losartan (0.2 and 2 mg kg (-1)) were given intravenously 10 min before occlusion. Captopril reduced the incidences of ventricular fibrillation (VF) and mortality associated with irreversible VF, whereas the studied doses of losartan did not. Captopril also decreased the number of ventricular beats on reperfusion. Losartan 2 mg kg (-1) reduced both the number of ventricular premature beats and the incidence of ventricular tachycardia (VT) on reperfusion, while losartan at dose of 0.2 mg kg (-1) had no effect on these arrhythmias. Compared to the control group, both captopril and losartan reduced myocardial infarct size in the rat model of ischemia-reperfusion, but this was statistically significant for captopril only. In this experimental model, although captopril did not reduce the incidence of reperfusion-induced VT, it was more effective than the AT (1)-receptor blocker losartan at preventing mortality associated with irreversible VF and to reduce myocardial infarct size in rat model of ischemia-reperfusion.     

Differential effects of eprosartan and losartan at prejunctional angiotensin II receptors

A comparison was made of the influence of losartan and eprosartan on angiotensin II effects at pre- and postjunctional receptors of the canine pulmonary artery and at prejunctional receptors of the rat left ventricle. To study postjunctional contractile responses to angiotensin II, non-cumulative concentration-response curves were determined; to study prejunctional effects of angiotensin II, the tissues were preincubated with [3H]noradrenaline and then superfused and electrically stimulated (1 Hz, 2 ms, 50 mA, 5 min). Postjunctionally, both losartan and eprosartan caused a parallel shift of the concentration-response curve of angiotensin II to the right (pKd of 8.15 and 8.28, respectively). At the prejunctional level, while eprosartan, in concentrations similar to those which were effective postjunctionally (30-100 nM), antagonized the facilitatory effect on noradrenaline release in both the dog pulmonary artery and the rat ventricle, losartan was ineffective in concentrations up to 1 microM. It is concluded that pre-junctional receptors for angiotensin II in the canine pulmonary artery and in the rat left ventricle are different from postjunctional receptors of the canine pulmonary artery. It is proposed that the prejunctional receptors of these tissues are atypical AT1 or "AT1B-like" receptors.     

Efficacy of ischaemic preconditioning in the eNOS overexpressed working mouse heart model

We recently demonstrated that exogenous nitric oxide (NO) acts as a trigger for preconditioning in the isolated rat heart model. There is however little data concerning the effects of elevated cardiac endothelial nitric oxide synthase (eNOS) expression on myocardial tolerance to ischaemia. Similarly, the effects of gender and eNOS overexpression on ischaemic preconditioning is unknown. We hypothesized that: 1) eNOS overexpression increases myocardial tolerance to ischaemia, and, 2) eNOS overexpressed hearts cannot be preconditioned, since the hearts are already maximally protected. Male and female wild-type and transgenic mice that overexpress eNOS exclusively in cardiac myocytes were perfused in the working heart mode with a modified Krebs-Henseleit buffer at a pre-load of 12.5 mm Hg and afterload of 50 mm Hg. Cardiac output, coronary flow, peak aortic systolic pressure and total work were determined before hearts were preconditioned by 4x5 min cycles of ischaemia/reperfusion, and then subjected to 20 min total global ischaemia, followed by reperfusion. Reperfusion function and myocardial infarct size were used as endpoints. Pre-ischaemic mechanical function (rate pressure product and cardiac output) was similar for wild-type and transgenic mice of both sexes. The eNOS overexpressed hearts had smaller infarcts than the hearts from their wild-type littermates (26.9+/-1.4% vs. 37.0+/-2.1% for controls, P<0.05). Preconditioning the eNOS overexpressed hearts resulted in infarct sizes comparable with control non-preconditioned hearts (27.5+/-2.0% vs. 26.9+/-1.4% for controls). Myocardial cGMP levels were elevated during sustained ischaemia in the transgenic hearts when compared with wild-type hearts (22.43+/-1.63 pmol/g ww vs 16.54+/-1.48 pmol/g ww, P<0.05). Preconditioning also elevated myocardial cGMP levels during sustained ischaemia in the wild-type hearts (26.77+/-2.81 pmol/g ww, P<0.05). We conclude that: 1) basal mechanical function is similar for both wild-type and transgenic mice of both sexes, 2) reperfusion function and infarct size was also similar for both sexes under both control conditions and after preconditioning, 3) the transgenic mice are more tolerant of ischaemia as reflected by their smaller myocardial infarcts, and, 4) the eNOS overexpressed mouse heart cannot be preconditioned regardless of whether mechanical function or infarct size is used as an end-point. These hearts may be maximally protected against ischaemia/reperfusion injury by their elevated endogenous NO levels.     

Involvement of protein kinase C in the delayed cytoprotection following sublethal ischaemia in rabbit myocardium.

Rabbit hearts were preconditioned with four 5 min coronary artery occlusions 24 h before 30 min coronary occlusion with 120 min reperfusion. Preconditioning significantly reduced the percentage of myocardium infarcting within the risk zone from 49.1 +/- 4.3% to 31.8 +/- 3.5% (P < 0.05). When the protein kinase C (PKC) inhibitor, chelerythrine, was administered just before preconditioning, the delayed protection against infarction 24 h later was abolished. We conclude that the delayed cytoprotective response associated with ischaemic preconditioning of myocardium is likely to involve the early activation of one or more PKC subtypes.     

Angiotensin II subtype AT<Subscript>1</Subscript> receptor blockade prevents hypertension and renal insufficiency induced by chronic NO-synthase inhibition in rats

Aim of the present study was to investigate the influence of the angiotensin II (ANG II) subtype 1 (AT(1)) receptor blockers fonsartan and losartan on blood pressure, cardiac -dynamics and -metabolism as well as functional and morphological changes in the kidney of rats after long-term inhibition of the nitric oxide (NO) synthase by N(G)-nitro-L-arginine methyl ester (L-NAME).Oral chronic treatment with L-NAME in a dose of 25 mg/kg/d over 6 weeks caused a significant increase in systolic blood pressure (198+/-13 mmHg) when compared to untreated rats (144+/-4 mmHg). Animals receiving simultaneously L-NAME and fonsartan (10 mg/kg/d) or losartan (30 mg/kg/d) were protected against blood pressure increase. L-NAME treatment caused a significant decrease in glomerular filtration rate (GFR) from 4.52+/-0.81 to 1.34+/-0.26 ml/kg(-1)/min(-1) and renal plasma flow (RPF) from 10.52+/-1.29 ml/kg(-1)/min(-1) to 5.66+/-1.06 ml/kg(-1)/min(-1). Co-treatment with fonsartan and losartan prevented L-NAME-induced reduction in GFR and RPF. There was no difference in urine, sodium and potassium excretion in groups under investigation. Plasma renin activity (PRA) was further stimulated by fonsartan and losartan treatment. L-NAME produced a significant elevation in urinary protein excretion which was antagonised by both AT(1) blockers. Isolated hearts from animals treated with L-NAME showed a significant prolongation in the duration of ventricular fibrillation and a significant decrease in coronary flow as compared to control hearts. Treatment with fonsartan and losartan significantly decreased the duration of ventricular fibrillation as compared to L-NAME group. In addition, both AT(1) blockers given alone significantly reduced the duration of ventricular fibrillation as compared to hearts from untreated controls. During ischemia the cytosolic enzymes lactate dehydrogenase and creatine kinase as well as lactate in the coronary effluent were significantly increased in the L-NAME group. Myocardial tissue values of glycogen, ATP, and creatine phosphate were decreased, whereas lactate was increased. Fonsartan and losartan treatment totally abolished these effects. Histological examination of kidneys revealed that simultaneous administration of fonsartan and losartan with L-NAME abolished L-NAME-induced arteriolar hyalinosis, segmental sclerosis of glomerular capillaries and focal tubular atrophies.In conclusion, long-term blockade of ANG II subtype AT(1) receptors by fonsartan and losartan prevented L-NAME-induced hypertension, renal insufficiency, as well as cardio-dynamic, cardio-metabolic, and morphological deteriorations.     

Renal hemodynamic responses to intrarenal infusion of ligands for the putative angiotensin IV receptor in anesthetized rats.

Angiotensin IV, a hexapeptide fragment (3-8) of angiotensin II metabolism, has been reported to produce vasodilatation within the renal vasculature by activation of the putative AT4 receptor. However, there are conflicting findings, with previous in vivo studies providing evidence for and against a renal vasodilator action of angiotensin IV. In this study, the renal hemodynamic responses to activation of the putative AT4 receptor were studied in anesthetized rats by left renal arterial infusion of two endogenous ligands, angiotensin IV and LVV-hemorphin-7. Angiotensin IV (10, 100, and 1,000 pmol/min) infusion caused dose-dependent reductions in blood flow to the infused kidney, which were abolished by pretreatment with losartan. In respect to this effect, angiotensin IV was approximately 300-fold less potent than angiotensin II. There were no significant effects of angiotensin IV on mean arterial pressure, heart rate, or blood flow to the noninfused kidney. Intrarenal infusion of LVV-hemorphin-7 (10, 100, and 1,000 pmol/min) had no significant effect on renal blood flow in the infused and noninfused kidneys, or on mean arterial pressure or heart rate. These results provide no evidence for a renal vasodilatory action of angiotensin IV or LVV-hemorphin-7. On the contrary, intrarenal angiotensin IV infusion produced vasoconstriction of the renal vasculature, mediated by activation of AT1 receptors. These observations provide evidence against a vasodilatory role of putative AT4 receptors in the rat kidney.     

Chronic hypotensive effects of losartan are not dependent on the actions of angiotensin II at AT 2 receptors.

We have previously demonstrated the chronic hypotensive effects of the AT1 antagonist, losartan, in normotensive, salt-replete rats. One explanation for this response is a reduction in vascular resistance due to blockade of AT1 receptors. Another explanation is that increases in angiotensin II levels during losartan administration can bind to AT2 receptors. Studies suggest that binding of angiotensin II at AT2 receptors lowers arterial pressure by vasodilation. We hypothesized that the chronic effects of losartan are mediated by activation of angiotensin II effects at AT2 receptors. We tested this hypothesis by infusing the AT2 receptor antagonist, PD123319 (74 mg/kg/day), in conjunction with losartan (10 mg/kg/day) for 10 days in rats and compared the hypotensive response in rats treated with losartan alone. After 6 days of treatment, arterial pressure decreased similarly in losartan (-21 +/- 2 mm Hg) and losartan+PD123319 (-23 +/- 2 mm Hg) treated rats. However, by day 10 of the infusion, arterial pressure had decreased to a greater extent (p < 0.05) in rats treated with losartan and PD123319 (-31 +/- 2 mm Hg) compared with rats treated with losartan alone (-22 +/- 2 mm Hg). We conclude that the hypotensive effects of losartan are not dependent on the actions of angiotensin II at AT 2 receptors in normotensive, salt-replete rats.     

Improvement of stunned myocardium in dogs with olmesartan, an angiotensin II type 1 receptor antagonist, is independent of type 2 receptors

Olmesartan is a selective angiotensin II type 1 receptor (AT1) antagonist. In pentobarbital-anesthetized open-chest dogs, ischemia/reperfusion was induced by ligating the left anterior descending coronary artery for 20 min and releasing it for 60 min, respectively. The myocardial contraction in the ischemic area decreased and returned towards its pre-ischemic level during reperfusion but incompletely. Olmesartan improved the recovery of myocardial contraction during reperfusion associated with restoration of myocardial ATP. Angiotensin II repelled by AT1 receptors occupied by olmesartan can reach and stimulate the angiotensin II type 2 (AT2) receptors, resulting in some beneficial effects on the ischemic myocardium. In fact, AT2 receptor mRNA was found in the adult dog myocardium. In addition, the plasma level of angiotensin II was significantly increased by olmesartan. PD123319, a selective AT2 receptor antagonist, however, did not modify the effect of olmesartan on the cardiac contraction. The hypertensive response to exogenous angiotensin II was completely inhibited by olmesartan, whereas PD123319 did not abolish the effect of olmesartan. In conclusion, olmesartan protects the ischemic/reperfused heart against ischemic injury through inhibition of AT1 receptors but not indirect activation of AT2 receptors.     

Preconditioning of the ischaemic myocardium; involvement of the L-arginine nitric oxide pathway.

1. Short periods of coronary artery occlusion protect the heart against the effects of a subsequent prolonged period of ischaemia. This phenomenon is known as preconditioning of the ischaemic myocardium. 2. In mongrel, chloralose-urethane anaesthetized open-chest dogs, within a restricted body weight range, two 5 min periods of occlusion of the anterior descending branch of the left coronary artery markedly reduced the severity of the early ischaemic arrhythmias resulting from a prolonged (25 min) occlusion of the same coronary artery starting 20 min later. Thus, the number of ventricular premature beats (VPBs) was reduced from 528 +/- 140 in controls to 78 +/- 27 in preconditioned dogs, the incidence of ventricular fibrillation (VF) was reduced from 47% to 0% and the incidence of ventricular tachycardia (VT) from 100% to 20%. ST-segment elevation recorded from electrodes within the ischaemic area, and the degree of inhomogeneity of conduction within the ischaemic area were markedly reduced in these preconditioned dogs. 3. The incidence of VF following reperfusion of the ischaemic myocardium at the end of the 25 min occlusion period was reduced in the preconditioned dogs from 100% to 60%; there was thus a 40% survival from the combined ischaemia-reperfusion insult compared with 0% in the controls. 4. NG-nitro-L-arginine methyl ester (L-NAME) an inhibitor of the L-arginine nitric oxide pathway, given in a dose of 10 mg kg-1 intravenously on two occasions, both before the initial preconditioning occlusion and then again before the prolonged occlusion, partially attenuated the protective effects of preconditioning.(ABSTRACT TRUNCATED AT 250 WORDS)     

Laser Doppler flowmetry for assessment of myocardial microperfusion in the beating rat heart

Although laser Doppler flowmetry (LDF) is widely used for measuring microperfusion, it is rarely used to measure coronary microcirculation. The present in vivo study investigated the use of LDF to measure myocardial microperfusion in beating rat hearts. Ascending aortic flow and other hemodynamic parameters were simultaneously recorded. A needle probe with a holder was adhered to the epicardium of the left ventricular myocardium close to the left anterior descending coronary artery in an anaesthetized open-chest rat. Myocardial microperfusion was measured in response to bolus intravenous administration of both two representative vasodilators (captopril and nifedipine) and a vasoconstrictor (pituitrin). Myocardial microperfusion was found to be predominately diastolic, and in an opposing phase to the ascending aortic flow. Captopril (5 or 10 mg/kg) increased the initial myocardial microperfusion phase. Nifedipine at 75 microg/kg caused a sustained myocardial microperfusion elevation with a peak increase of 7.1+/-1.1%, but this was not observed using 150 microg/kg nifedipine. Both drugs caused an increase in the cardiac index. In contrast, myocardial microperfusion decreased (28.7+/-0.1% maximum decrease) in response to 1 IU/kg pituitrin. In conclusion, LDF provided a means of assessing myocardial microperfusion in beating rat hearts, and can be applied to evaluate the coronary microcirculation response to drugs.     

Cardioprotective properties of humoral factors released from rat hearts subject to ischemic preconditioning

Myocardial protection can be achieved by transfer of coronary effluent from ischemically preconditioned to non-preconditioned hearts. This study was designed to test the hypothesis that preconditioned effluent from rat hearts purified by Sep-Pak C-18 cartridges could induce remote cardioprotection against ischemia/reperfusion (I/R) injury through the activation of protein kinase C signaling pathway. Buffer-perfused rat hearts were subject to 30 min ischemia and 60 min reperfusion. The myocardial I/R injury was assessed by postischemic contractile function recovery and infarct size. The protective effect of coronary effluent collected during ischemic preconditioning (IPC) was tested in non-preconditioned hearts in presence or absence of a PKC inhibitor, chelerythrine. Infarct size was 17 +/- 2% in preconditioned versus 37 +/- 1% in control hearts (P < 0.001). Hearts perfused with fresh preconditioned effluent had infarct sizes of 16 +/- 3% versus 36 +/- 1% in hearts treated with non-preconditioned effluent. The cardioprotective effect was lost when the effluent was left at room temperature during 24 h (infarct size, 40 +/- 3%) or heated to 70 degrees C (26 +/- 4%, P < 0.05) or 100 degrees C (39 +/- 1%, P < 0.001). The lyophilized effluent was stable for 30 days, and its purification in a Sep-Pak C-18 column resulted in a hydrophobic fraction that reduced the infarct size to 17 +/- 2% versus 38 +/- 2% for the hydrophilic fraction. Chelerythrine (100 microM) inhibited the reduction of infarct size induced by IPC (35 +/- 4%) or hydrophobic fraction (37 +/- 3%). Recovery of the contractile function at reperfusion was higher in preconditioned group (74 +/- 6% versus 17 +/- 7% in control, P < 0.001) and hydrophobic fraction (66 +/- 7% versus 8 +/- 4% in hydrophilic fraction, P < 0.001). Similarly, chelerythrine was able to abrogate the contractile function recovery (12 +/- 6%, P < 0.001 versus preconditioned group and 19 +/- 7%, P < 0.001 versus hydrophobic fraction). In conclusion, the cardioprotective factors released in the coronary effluent by IPC are thermolabile hydrophobic substances with molecular weights higher than 3.5 kDa and acting through PKC activation.     

San酮对大鼠缺血再灌注损伤心肌的保护作用

AIM: To investigate the protective effect of xanthones against myocardial ischemia-reperfusion injury in rats. METHODS: Ischemia-reperfusion injury was induced by 20 min of global ischemia and 40 min of reperfusion in isolated rat hearts or 60-min coronary artery occlusion and 180-min reperfusion in vivo, respectively. Heart rate, coronary flow, left ventricular pressure (LVP), and its first derivative (+/- dp/dtmax) were recorded, and the activity of creatine kinase in coronary effluent and malondialdehyde contents in myocardial tissues were measured in vitro. The activity of serum creatine kinase and myocardium infarct size were measured in vivo. RESULTS: Xanthones (90 or 300 microg/L) caused a significant improvement of cardiac function (LVP and +/- dp/dtmax) and a decrease in the release of creatine kinase in coronary effluent as well as the level of malondialdehyde in myocardial tissues. Xanthones (0.5 or 1.0 mg/kg) also markedly decreased infarct size and the release of creatine kinase in vivo. CONCLUSION: Xanthones protect the myocardium against the damages induced by ischemia-reperfusion in rats, and the effect of xanthones may be related to the inhibition of lipid peroxidation.     

Evidence for the involvement of peroxynitrite in ischaemic preconditioning in rat isolated hearts

1. The aim of this study was to investigate the involvement of peroxynitrite, reactive metabolite originating from nitric oxide and superoxide, in preconditioning of the ischaemic myocardium in rat isolated hearts. 2. Isolated hearts perfused with Krebs-Henseleit solution were preconditioned either by 3 min of coronary artery occlusion (CAO) or by peroxynitrite administration at three different concentrations (0.1, 1, 10 microM) for 3 min, followed by 10 min reperfusion and 30 min of CAO. Peroxynitrite, at 1 microM concentration, decreased the incidence of VT from 100% (n=14) to 62% (n=13) and abolished the occurrence of VF (50% in the control group). 3. N-2-mercaptopropionylglycine (MPG, 1 microM - 10 mM) produced a concentration-dependent inhibition of peroxynitrite signals in luminol chemiluminescence and 67+/-1% inhibition was observed at 100 microM (n=7). MPG (at 300 microM, n=7) added to the perfusate 10 min prior to ischaemic preconditioning or peroxynitrite infusion and maintained until CAO, significantly reversed the beneficial effects of the ischaemic and peroxynitrite-treated groups. MPG administration in the peroxynitrite-treated group increased the incidence of VT from 62% (n=13) to 100% (n=10) and total VF from 0% (n=0) to 67% (n=10). Similarly, MPG elevated the incidence of VT from 50% (n=10) to 100% (n=8) in the ischaemic preconditioned group. On its own, MPG did not affect the severity of cardiac arrhythmias. 4. These results suggest that endogenously produced peroxynitrite plays a significant role in the antiarrhythmic effect of ischaemic preconditioning in the rat isolated hearts.     

Contribution of peroxynitrite to the beneficial effects of preconditioning on ischaemia-induced arrhythmias in rat isolated hearts

We studied the effects of urate, a peroxynitrite scavenger, on ischaemia- and peroxynitrite-induced preconditioning in rat isolated hearts. Isolated hearts perfused with Krebs-Henseleit solution were preconditioned either by 3 min of coronary artery occlusion or by peroxynitrite administration (1 microM) for 3 min, followed by 10 min of reperfusion and 30 min of coronary artery occlusion. Both ischaemia and peroxynitrite produced a marked reduction in arrhythmias. Urate (1 mM) added to the perfusate 10 min prior to ischaemic preconditioning or peroxynitrite infusion and maintained until coronary artery occlusion, markedly reversed the beneficial effects in the ischaemic and peroxynitrite-treated groups. Urate administration in the peroxynitrite-treated group increased the incidence of ventricular tachycardia from 57% (n = 11) to 100% (n = 6) and total ventricular fibrillation from 0% (n=0) to 44% (n=4). Similarly, urate augmented the incidence of ventricular tachycardia from 47% (n=8) to 85% (n = 6) in the ischaemic preconditioning group. On its own, urate did not affect the severity of cardiac arrhythmias. Peroxynitrite infusion caused a marked increase in the effluent nitrate levels, from 0.05 +/- 0.1 microM (n = 5) to 0.4 +/- 0.2 microM (n = 6), and urate significantly decreased these levels to 0.08 +/- 0.03 microM (n = 9). These results suggest that peroxynitrite at low concentrations contributes to the beneficial effects of preconditioning on ischaemia-induced arrhythmias in rat isolated hearts.     

Pretreatment with Ischaemia Attenuates Acute Epirubicin-Induced Cardiotoxicity in Isolated Rat Hearts

Abstract: We investigated whether a brief ischaemic episode (ischaemic pretreatment) preconditioning might attenuate the acute cardiotoxicity of the anthracycline, epirubicin. Isolated rat hearts perfused at a constant flow rate of 10 ml/min. were preconditioned with 5 min. of global ischaemia and 10 min. of reperfusion (preconditoned hearts), or were perfused for 15 min. (control hearts). The hearts were then subjected to 20 min. of infusion with epirubicin (2 mg/ml) or vehicle by a side arm of the perfusion system at a rate of 0.1 ml/min. (0.2 mg epirubicin/min.). Attenuation of cardiotoxicity of a total dose of 4 mg of epirubicin was assessed by functional and metabolic parameters during infusion and during the following 30 min. recovery period. Cardiotoxic effects were reduced in preconditioned hearts compared to control hearts. Thus left ventricular developed pressure and heart rate product after 20 min. of epirubicin infusion was depressed to 27±7% (mean±S.D.) and 40±4% (mean±S.D.) of baseline values in the control group and the preconditioned group, respectively (P<0.05). Furthermore, we observed less contracture during epirubicin infusion and more effective reversal of contracture during the recovery period in the preconditioned hearts. Improvement in cardiac function was associated with a significantly lower (P<0.05) myocardial content of epirubicin in the preconditioned group at the end of the infusion period. We conclude that ischaemic preconditioning attenuates the acute cardiotoxicity of epirubicin, probably by reducing the myocardial accumulation of the anthracycline.     

羟苯氨酮保护大鼠心脏对抗心肌缺血-再灌注损伤

目的研究强心扩血管新药羟苯氨酮(oxyphenamone)对心肌缺血-再灌注损伤的保护作用。方法对离体或在体大鼠心脏，阻断冠脉前降支10 min后行再灌注15 min(离体)或30 min(在体)，形成心肌缺血-再灌注损伤模型，从心电图、心肌酶学与心肌超微结构等方面观察药效。结果羟苯氨酮(离体心脏灌流1-10 μmol·L^-1，或静脉注射0.1-1.0 mg·kg^-1)剂量依赖性地明显减少再灌注时的心律失常，对抗损伤所致心肌CPK，LDH与MDA的变化，减轻心肌超微结构损伤。结论羟苯氨酮明显保护离体和在体大鼠心肌对抗冠脉阻断所致缺血-再灌注损伤。     

Roles of tyrosine kinase and protein kinase C in infarct size limitation by repetitive ischemic preconditioning in the rat

In this study, we examined the possibility that infarct-size limitation by repetitive preconditioning (PC) is achieved by activation of both protein kinase C (PKC) and tyrosine kinase. In addition, we assessed whether such kinase activation is triggered by angiotensin II type 1 (AT1) and alpha1-adrenergic receptors and whether sarcolemmal and mitochondrial adenosine triphosphate (ATP)-sensitive potassium (K(ATP)) channels play roles as effectors of cardioprotection in the rat. Under pentobarbital anesthesia, myocardial infarction was induced by 20-min coronary occlusion and 3-h reperfusion in the rat. Infarct size was determined by tetrazolium and expressed as a percentage of area at risk (%IS/AR). PC with one cycle of 5-min ischemia/5-min reperfusion before 20-min ischemia significantly reduced %IS/AR from the control value of 49.4 +/- 2.0 to 35.4 +/- 2.8, and repetitive PC with two cycles of 5-min ischemia/5-min reperfusion further limited %IS/AR to 3.2 +/-0.9. Infarct-size limitation by single-cycle PC was completely abolished by a PKC inhibitor, staurosporine (100 microg/kg; %IS/ AR, 45.7 +/- 5.0). In contrast, the cardioprotection by repetitive PC was only partially blocked by staurosporine (%IS/AR, 19.8 +/- 2.4), another PKC inhibitor, polymyxin B (5 mg/kg; %IS/AR, 16.2 +/- 3.1), or a tyrosine kinase inhibitor, genistein (5 mg/kg; %IS/AR, 21.8 +/- 1.4). However, a combined injection of genistein and staurosporine additively inhibited protection of repetitive PC (%IS/AR, 36.4 +/- 1.7). Staurosporine, polymyxin B, or genistein alone did not modify %IS/AR in nonpreconditioned rat hearts. Infarct-size limitation by repetitive PC was not attenuated by pretreatment with a selective AT1-receptor blocker (CV11974, 10 mg/kg), prazosin (0.6 mg/kg; %IS/AR, 6.4 +/- 3.2 and 1.6 +/- 0.5, respectively). A selective blocker of mitochondrial K(ATP) channels, 5-hydroxydecanoate (3 mg/kg), completely abolished the cardioprotective effect (%IS/AR, 50.8 +/-3.5), but HMR1883 (3 mg/kg), a selective blocker of sarcolemmal K(ATP) channels, failed to inhibit the preconditioning effect (%IS/AR, 4.4 +/- 0.7). These findings suggest that repetition of PC provokes activation of both PKC and tyrosine kinase, leading to enhanced antiinfarct tolerance by opening of mitochondrial but not sarcolemmal K(ATP) channels. It is unlikely that activation of either AT1 or alpha1-adrenergic receptor alone is crucial to trigger preconditioning. Key Words: Tyrosine kinase-Genistein-Angiotensin II-alpha1-Adrenergic receptor-Sarcolemmal K(ATP) channel-Mitochondrial K(ATP) channel.