Myocardial Stunning In Dogs Research Articles

Coronary Microcirculation Changes during Myocardial Stunning in Dogs

Objective: To investigate changes in the coronary microcirculation during myocardial stunning in dogs. Methods: Male mongrel dogs underwent a 15- or 60-min occlusion of the left anterior descending coronary artery, followed by a 120-min reperfusion. Myocardial contrast echocardiography was performed before and after treatment with acetylcholine (ACH) or nitroglycerin (NG). Peak videointensity (PVI) in the myocardial zone was measured, and myocardial samples were examined using transmission electron microscopy. Results: In the 15-min group, the ratio of the PVI between the stunned and intact myocardial zone (PVIR) before and after treatment with NG (NG-PVIR) or ACH (ACH-PVIR) declined markedly in the early period of reperfusion and then returned to preligation levels. In the 60-min group, NG-PVIR was reduced in the early period of reperfusion and then returned to its preligation level. A low level of ACH-PVIR lasted during the entire 120-min reperfusion. Similar changes in the ratio of the PVIR before and after treatment with NG or ACH were observed. In the 60-min group, capillary endothelial edema and widening of intercellular linking gaps were observed. Conclusions: We observed microvascular endothelial damage and endothelium-dependent dilatation impairments in stunned myocardium, and their severity and recovery rate are affected by the duration of ischemia.

The Cornucopia of “Pleiotropic” Actions of Statins

See related article, pages 255–264 Initially described in patients with coronary artery disease undergoing coronary artery bypass graft (CABG) surgery, myocardial “hibernation” denotes a state of adaptive reduction in myocardial contractile function in response to limited energy supply caused by reduced blood flow.1,2 In this precarious equilibrium, the myocardium can remain viable for sustained periods of time (although not indefinitely) and slowly resumes contractile activity on restoration of perfusion.2,3 Hibernating myocardium resulting from a flow-limiting coronary artery stenosis is a common clinical entity that contributes to myocardial dysfunction in many patients with ischemic heart disease1,2; its clinical importance stems from the fact that is it is potentially reversible. The pathophysiological basis of this syndrome was proposed as a conceptual framework in 19924 and subsequently tested in various animal models5–8: it is likely to involve recurrent brief episodes of ischemia, which initially result in repetitive “stunning” with normal resting flow and eventually cause a persistent decrease in resting function and flow.2,4–6,8 Over time, these changes lead to myocyte loss resulting from degenerative changes and apoptosis.2,8 Because mechanical dysfunction is caused by a restriction in blood flow, revascularization has traditionally been the cornerstone of therapy for myocardial hibernation. Indeed, restoration of blood flow via CABG or percutaneous coronary interventions often improves the mechanical performance of hibernating myocardium.1–3,8,9 In those cases in which these procedures are not feasible, however, there is a need for alternative approaches. Direct transmyocardial laser revascularization10 and intramyocardial gene transfer of phVEGF16511 or fibroblast growth factor-57 have been reported to enhance perfusion and contractile reserve and improve regional function in experimental models of hibernation. Transplantation of circulating progenitor cells following revascularization has also been reported to attenuate hibernation (ie, to increase coronary flow reserve …

Effects of Rosuvastatin and Pitavastatin on Ischemia-Induced Myocardial Stunning in Dogs

Incomplete recovery of myocardial contraction after reperfusion following brief ischemia is called the “stunning phenomenon” in an animal experiment. A hydrophilic 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase inhibitor (statin) does not affect this phenomenon, but lipophilic statins further reduce the contraction during reperfusion. The effects of novel hydrophilic rosuvastatin and lipophilic pitavastatin on myocardial stunning in dogs were examined. In a preliminary experiment in vitro, pitavastatin reduced L6 cell viability at 10−6M and higher, whereas rosuvastatin and pravastatin up to 10−5M did not show such effects. An empty capsule or a capsule filled with rosuvastatin (2 mg/kg per day) or pitavastatin (0.4 mg/kg per day) was orally administered to dogs. After 3 weeks, both statins lowered the serum cholesterol level to the same extent. Under pentobarbital anesthesia, dogs were subjected to 15-min ischemia followed by 120-min reperfusion. Ischemia arrested the myocardial contraction in the ischemic area, and reperfusion recovered it but incompletely, showing the stunning phenomenon. Rosuvastatin did not modify the stunning phenomenon, while pitavastatin further deteriorated the myocardial contraction during reperfusion.

Differential effects of postconditioning on myocardial stunning and infarction: a study in conscious dogs and anesthetized rabbits

Postconditioning, i.e., brief intermittent episodes of myocardial ischemia-reperfusion performed at the onset of reperfusion, reduces infarct size after prolonged ischemia. Our goal was to determine whether postconditioning is protective against myocardial stunning. Accordingly, conscious chronically instrumented dogs (sonomicrometry, coronary balloon occluder) were subjected to a control sequence (10 min coronary artery occlusion, CAO, followed by coronary artery reperfusion, CAR) and a week apart to postconditioning with four cycles of brief CAR and CAO performed at completion of the 10 min CAO. Three postconditioning protocols were investigated, i.e., 15 s CAR/15 s CAO (n=5), 30 s CAR/30 s CAO (n=7), and 1 min CAR/1 min CAO (n=6). Left ventricular wall thickening was abolished during CAO and similarly reduced during subsequent stunning in control and postconditioning sequences (e.g., at 1 h CAR, 33+/-4 vs. 34+/-4%, 30+/-4 vs. 30+/-4%, and 33+/-4 vs. 32+/-4% for 15 s postconditioning, 30 s postconditioning, and 1 min postconditioning vs. corresponding control, respectively). We confirmed this result in anesthetized rabbits by demonstrating that shortening of left ventricular segment length was similarly depressed after 10 min CAO in control and postconditioning sequences (4 cycles of 30 s CAR/30 s CAO). In additional rabbits, the same postconditioning protocol significantly reduced infarct size after 30 min CAO and 3 h CAR (39+/-7%, n=6 vs. 56+/-4%, n=7 of the area at risk in postconditioning vs. control, respectively). Thus, contrasting to its beneficial effects on myocardial infarction, postconditioning does not protect against myocardial stunning in dogs and rabbits. Conversely, additional episodes of ischemia-reperfusion with postconditioning do not worsen myocardial stunning.

Effects of SEA0400, a novel inhibitor of the Na +/Ca 2+ exchanger, on myocardial stunning in anesthetized dogs

Activation of the Na +/Ca 2+ exchanger may contribute to Ca 2+ overload during reperfusion after transient ischemia. We examined the effects of 2-[4-[(2,5-difluorophenyl) methoxy]phenoxy]-5-ethoxyaniline (SEA0400), a selective inhibitor of Na +/Ca 2+ exchange, on a canine model of ischemia/reperfusion injury (myocardial stunning). Myocardial stunning was induced by a 15-min occlusion of the left anterior descending coronary artery followed by a 4-h reperfusion in anesthetized open-chest dogs. Reperfusion gradually restored myocardial percent segment shortening but remained depressed during a 4-h reperfusion period. A bolus intravenous injection of SEA0400 (0.3 or 1.0 mg/kg), given 1 min before reperfusion, improved significantly the recovery of percent segment shortening in the ischemic/reperfused myocardium. SEA0400 did not affect the hemodynamics and electrocardiogram parameters. In addition, SEA0400 did not affect reperfusion-induced change in coronary blood flow. These results suggest that the Na +/Ca 2+ exchanger is involved in the stunned myocardium of dogs after reperfusion, and that SEA0400 has a protective effect against myocardial stunning in dogs.

TY-12533, a novel Na +/H + exchange inhibitor, prevents myocardial stunning in dogs

Anesthetized open-chest dogs were subjected to 15-min myocardial ischemia followed by 2-h reperfusion to induce myocardial stunning. A novel Na +/H + exchange inhibitor 6,7,8,9-tetrahydro-2-methyl-5 H-cyclohepta[ b]pyridine-3-carbonylguanidine maleate (TY-12533), administered 10 min before or 10 min after start of ischemia (3 mg/kg/10 min, i.v.), did not affect reductions in regional myocardial wall thickening, blood flow and pH during ischemia, but it significantly improved recovery of the wall thickening and blood flow after reperfusion. These results indicate that TY-12533, even when administered during ischemia, could prevent myocardial stunning without affecting myocardial dysfunction or acidosis induced by brief ischemia.

Beneficial effects of the T- and L-type calcium channel antagonist, mibefradil, against exercise-induced myocardial stunning in dogs.

Abnormalities in calcium homeostasis such as calcium overload have been shown to participate in the pathogenesis of myocardial stunning. The goal of this study was to investigate the effects of mibefradil, a mixed T- and L-type calcium channels antagonist on exercise-induced ischemia (i.e., high-flow ischemia). Nine dogs were permanently instrumented to measure left ventricular wall thickening (Wth) and coronary blood flow (Doppler). Infusion of saline or mibefradil (30 and 40 microg/kg/min, i.v., for 20 min) was started 10 min before exercise (10 min, 14 km/h; slope, 13%) and stopped at its end. Circumflex coronary artery stenosis (pneumatic occluders) was set up 5 min before exercise to suppress exercise-induced increase in mean coronary blood flow without simultaneously affecting Wth at rest. Mibefradil (30 microg/kg/min) was also administered at the beginning of the recovery period in a subset of four dogs. During exercise with saline, Wth was dramatically reduced (-77 +/- 7%; p < 0.05) and recovered only after 24 h. Mibefradil at both doses significantly limited tachycardia during exercise (211 +/- 7 and 210 +/- 5 beats/min vs. 240 +/- 8 beats/min for mibefradil, 30 microg/kg/min, mibefradil, 40 microg/kg/min, and saline, respectively) but exerted no negative inotropic effects. Mibefradil at both doses significantly reduced the intensity of myocardial stunning and the time to recovery in Wth (3 h). Administration of mibefradil at the beginning of the recovery period did not protect against myocardial stunning. Administration of a mixed T- and L-type calcium channel antagonists before ischemia confers cardioprotection against exercise-induced myocardial stunning. This may potentially be related to the limitation of exercise-induced tachycardia and/or the prevention of calcium overload.

S7-4 - Effects of HMS-CcA reductase inhibitors on myocardial stunning in dogs

S7-4 - Effects of HMS-CcA reductase inhibitors on myocardial stunning in dogs

Lipophilic HMG-CoA reductase inhibitors worsen myocardial stunning in dogs

Lipophilic HMG-CoA reductase inhibitors worsen myocardial stunning in dogs

Intra-coronary administration of L-arginine aggravates myocardial stunning through production of peroxynitrite in dogs.

The aim of this study was to investigate how the enhanced nitric oxide (NO) production by intra-coronary infusion of L-arginine acts in myocardial stunning in dogs by focusing on the involvement of peroxynitrite, a reaction product of NO and superoxide anion. Dogs were divided into six groups; a control non-treated group (CON, n = 9), and NG-nitro L-arginine methyl ester (L-NAME, n = 6), 1 mM L-arginine (L-ARG, n = 8), D-arginine (D-ARG, n = 6), L-arginine plus superoxide dismutase (L-ARG + SOD, n = 6), and SOD alone (SOD, n = 6) treated groups. L-NAME, or L- or D-arginine was continuously infused into the left anterior descending coronary artery (LAD) starting just prior to reperfusion, whereas SOD was intravenously injected before occlusion. During 120 min of reperfusion after 15 min occlusion of LAD, myocardial contractile function in the ischemic region gradually recovered and reached approximately 70% of the preischemic level in CON, D-ARG and SOD, but it remained dyskinetic (-46%) in L-ARG. On the other hand, it was improved in L-NAME (90%). Tissue malondialdehyde was elevated (p < 0.005) after reperfusion, and myocardial NO metabolites measured by an intratissue-microdialyzer increased (approximately 150%, p < 0.05) in the ischemic region during reperfusion in L-ARG but not in the CON, L-NAME, D-ARG or SOD groups. In the L-ARG + SOD group, L-arginine-induced contractile dysfunction and elevation of malondialdehyde were prevented, but the increase in NO metabolites remained. These results suggest that L-arginine aggravated myocardial stunning through oxidative stress and the cytotoxicity was caused by NO derivatives but not by NO itself. The formation of nitrotyrosine, a footprint of peroxynitrite, was immunohistochemically confirmed in the ischemic region of L-ARG. Our results demonstrate for the first time in vivo that NO has a detrimental role in myocardial stunning through the production of peroxynitrite.

17-Beta estradiol regulation of myocardial glutathione and its role in protection against myocardial stunning in dogs.

We studied the effect of 2-week treatment with estradiol 17beta on myocardial glutathione concentration in dogs and isolated perfused rat heart subjected to brief coronary ischemia and reperfusion. Estradiol protected against ischemia/reperfusion-induced myocardial systolic shortening and malonylaldehyde production and increased myocardial glutathione concentration and glucose-6-phosphate dehydrogenase enzyme activity. Reduction of myocardial glutathione with buthionine sulfoximine to levels seen in the absence of estrogen reversed the protective effect of estradiol against myocardial dysfunction and lipid peroxidation associated with ischemia/reperfusion. These results suggest that the antioxidant effect of estradiol in ischemia/reperfusion may be mediated by regulation of myocardial glutathione metabolism.

Monophosphoryl lipid A attenuates myocardial stunning in dogs: role of ATP-sensitive potassium channels.

Results of previous studies indicate that monophosphoryl lipid A (MLA) reduces myocardial infarct size when administered 24 but not 1 h before a prolonged period of regional ischemia in dogs and rabbits. This cardioprotective effect of MLA could be reversed by the administration of the adenosine triphosphate (ATP)-sensitive potassium channel (K(ATP)) blockers, glibenclamide, or 5-hydroxydecanoate. MLA also was shown to attenuate myocardial stunning in dogs; however, its mechanism in this model remains unknown. Therefore the major aim of our study was to determine the dose-related effect of MLA to enhance contractile function in stunned myocardium and to determine the role of the K(ATP) channel in mediating its cardioprotective effect. To produce myocardial stunning, barbital-anesthetized dogs were subjected to five cycles of 5 min of left anterior descending (LAD) coronary artery occlusion interspersed with 10 min of reperfusion and finally followed by 2 h of reperfusion. Regional segment shortening (%SS) was determined by sonomicrometers implanted in the subendocardium of the ischemic region. Single intravenous doses of MLA in the range of 10-35 microg/kg given 24 h before ischemia resulted in an improvement in %SS over a 2-h reperfusion period. Similar to results obtained in the canine and rabbit infarct models, cardioprotection against stunning with MLA appears to require activation of K(ATP) channels during ischemia, because glibenclamide (50 microg/kg, 15 min before ischemia) completely blocked the effect of MLA to improve regional %SS during reperfusion. Cardioprotective doses of MLA were without effect on systemic hemodynamics, blood gases, and pH throughout the experiment. No treatment-related effects on regional myocardial blood flow were observed during ischemia or reperfusion. These results suggest that MLA improves %SS at doses of 10-35 microg/kg by an ATP-sensitive potassium channel-dependent process, and that MLA may mimic the antistunning effects observed during the second window of ischemic preconditioning.

Effects of adenine nucleotide analogues on myocardial dysfunction during reperfusion after ischemia in dogs.

We examined effects of adenine nucleotide on ischemic myocardial stunning in dogs. Pentobarbitalanesthetized open-chest dogs were subjected to 20-min ligation of the left anterior descending coronary artery (LAD), followed by reperfusion for 30 min. Either saline, 5 mM 8-bromo-5'-AMP (tributyryl-AMP), or 30 mM N6, 2', 3'-tributyryl-5'-AMP (tributyryl-AMP), 5 mM 5-amino-4-imidazole carboxamide riboside (AICAr) as a positive reference, was infused at 0.1 ml/kg/min in the left femoral vein throughout the experiment. The myocardial contractile function was measured by ultrasonometry. The tissue levels of high-energy phosphates in the reperfused heart were determined. Myocardial contractile function assessed by % segment shortening (%SS) in the saline-infused group decreased during ischemia and returned toward the preischemic level during reperfusion but incompletely. A significant improvement in the %SS during reperfusion was observed in the 8-bromo-AMP- and AICAr-infused groups but not in the tributyryl-AMP-infused group. The magnitude of the protective effect of the drugs on myocardial contractility during reperfusion was 8-bromo-AMP > AICAr > tributyryl-AMP = saline. Only in the 8-bromo-AMP-infused group were the levels of ATP, ADP, and total adenine nucleotides in the reperfused heart significantly higher than those in the saline-infused group. The present result indicates that 8-bromo-AMP improves the ability of the heart to recover from ischemia and reperfusion associated with a significant restoration of ATP.

Effects of a new endothelin receptor antagonist, TAK-044, on myocardial stunning in dogs

The effects of a new endothelin receptor antagonist, TAK-044, (cyclo[ d-α-aspartyl-3-[(4-phenylpiperazin-1-yl)carbonyl] l-alanyl- l-α-aspartyl- d-2-(2-thienyl)glycyl- l-leucyl- d-tryptophyl]d salt, on ischemic and post-ischemic myocardial dysfunction (stunned myocardium) were studied in anesthetized open-chest dogs. A short (15 min) occlusion of the left anterior descending coronary artery followed by 5-h reperfusion significantly reduced myocardial segment shortening during and after the ischemic period in the ischemic region. Regional myocardial blood flow was also decreased significantly 10 min after the occlusion, whereas it returned almost completely to its pre-ischemic value 5 h after reperfusion. TAK-044 (3 mg/kg,i.v.) administered 10 min before occlusion significantly improved the reduced myocardial segment shortening in the ischemic region during and after occlusion. Cardiovascular hemodynamics and regional myocardial blood flow in a TAK-044-treated group were identical to those in the control group. These results indicate that endogenous endothelin contributes to the cause of ischemic and post-ischemic myocardial dysfunction without changing either hemodynamics or regional myocardial blood flow.

Staged reperfusion attenuates myocardial stunning in dogs. Role of transient acidosis during early reperfusion.

Acidosis during early reperfusion is reported to be beneficial for myocardial stunning. We tested in 31 dogs the hypothesis that staged reperfusion is beneficial to myocardial stunning. Contractile dysfunction was observed 3 hours after the onset of reperfusion after 15 minutes of occlusion of the coronary artery. In the staged reperfusion, pH of the coronary venous blood was lower for 20 minutes and fractional shortening was significantly improved compared with the control reperfusion group. When we increased pH of the reperfused myocardium by an intracoronary infusion of sodium bicarbonate, beneficial effects of the staged reperfusion were abolished. Furthermore, an intracoronary infusion of hydrogen chloride, which mimicked the changes in pH in coronary venous blood of the staged reperfusion, attenuated myocardial stunning. These results indicate that acidosis during staged reperfusion primarily attenuates myocardial stunning. This procedure is clinically applicable for attenuation of reperfusion injury.

Beneficial effects of alpha 1-adrenoceptor activity on myocardial stunning in dogs.

This study was undertaken to elucidate whether alpha-1 adrenoceptor activity is beneficial to contractile dysfunction during reperfusion after a brief period of ischemia (stunned myocardium) in 54 open-chest dogs. Contractile dysfunction assessed by fractional shortening (FS) was observed 3 hours after the onset of reperfusion following 15 minutes of complete occlusion of the left anterior descending coronary artery. Pretreatment with prazosin (4 micrograms/kg/min i.c.) further deteriorated contractile dysfunction compared with the untreated condition (12.7 +/- 0.6% versus 6.9 +/- 0.4% with prazosin treatment, p less than 0.001). Conversely, alpha 1-adrenoceptor agonists, methoxamine (1.0 microgram/kg/min i.c.) and norepinephrine (0.24 microgram/kg/min i.c.) with rauwolscine and propranolol, significantly attenuated contractile dysfunction (FS in the methoxamine-treated group, 17.3 +/- 0.3%, p less than 0.001 versus the untreated group; FS in the norepinephrine-treated group, 18.0 +/- 0.9%, p less than 0.05 versus 13.6 +/- 1.1% in the propranolol group). Both adenosine release and hyperemic coronary flow response during the early reperfusion period were significantly attenuated in the prazosin-treated group, and both were enhanced in the alpha 1-adrenoceptor stimulation groups. These results suggest that beneficial effects of alpha 1-adrenoceptor activity may be due to the enhanced release of adenosine. To test the cause-effect relation between the extent of adenosine release and contractile dysfunction during reperfusion, 8-phenyltheophylline was infused to block adenosine receptors in the methoxamine-treated group. The treatment with 8-phenyltheophylline completely abolished (FS, 7.4 +/- 0.3%) the beneficial effect of the enhanced adenosine release by alpha 1-adrenoceptor stimulation. Furthermore, in the prazosin-treated group, adenosine (9 micrograms/kg/min) was additionally infused into the left anterior descending coronary artery 5 minutes before and 2 hours after the onset of reperfusion. Both hyperemic coronary flow and contractile dysfunction (FS, 17.3 +/- 0.3%) recovered to the levels of the alpha 1-adrenoceptor stimulation groups. However, treatment with papaverine could not prevent deleterious effects of prazosin despite the fact that comparable hyperemic flow was obtained. Instead, lactate production up to 10 minutes after the onset of reperfusion was significantly larger (p less than 0.01) despite augmented contractile function in the prazosin-treated and the 8-phenyltheophylline with methoxamine-treated groups compared with the untreated group. The electron microscopic examination revealed no irreversible myocardial injury with and without pharmacological interventions. Thus, we conclude that alpha 1-adrenoceptor activity can reduce the magnitude of myocardial stunning and that its cellular mechanism is due to enhanced adenosine release by alpha 1-adrenoceptor activity.(ABSTRACT TRUNCATED AT 400 WORDS)

Myocardial stunning in dogs: Preconditioning effect and influence of coronary collateral flow

In open-chest dogs the left anterior descending (LAD) coronary artery diagonal branch was encircled with a pneumatic occluder. Pairs of ultrasonic crystals were inserted into LAD myocardium and remote normal muscle. The coronary artery was occluded for 5 minutes, followed by 10 minutes of reperfusion. This occlusion-reperfusion cycle was repeated 12 times, and after a final 90-minute reperfusion period the hearts were removed and stained with triphenyltetrazolium chloride. No heart had evidence of necrosis. Baseline shortening normalized for end-diastolic length averaged 10.4 ± 1.0% in the LAD area and 7.4 ± 0.8% in the remote normal myocardium. When analyzed as a percentage of baseline, segment shortening in the normal myocardium was not significantly altered by LAD occlusion and reperfusion. In contrast, during occlusions the LAD myocardium paradoxically lengthened. With the initial reperfusion, shortening was significantly depressed to 28.6 ± 8.6% of baseline. Although with subsequent reperfusions the return of function progressively decreased, the rate of deterioration was markedly attenuated after the first occlusion. By the end of the protocol many LAD segments lengthened paradoxically even after reperfusion, but in five hearts in which active contraction was preserved there was no significant change in regional function after the third cycle, suggesting a protective or preconditioning effect of earlier ischemia. There was a moderately good correlation between collateral blood flow and the degree of dysfunction following the initial 10-minute reperfusion ( r = −0.73). This correlation deteriorated during subsequent reperfusion periods, implying the collateral blood flow can be a predictor of the extent of myocardial stunning only after the initial one or two reperfusion cycles. Thereafter other as yet unidentified factors make baseline collateral flow unimportant.