Raúl J Domenech

Exercise induces early and late myocardial preconditioning in dogs

OBJECTIVE We tested the hypothesis that exercise induces myocardial preconditioning in dogs. METHODS We instrumented dogs with a snare on the anterior descending coronary artery and catheters in the root of the aorta, left ventricular cavity and coronary sinus. After recovering from surgery the dogs were trained to stay in the laboratory and run on a treadmill. Subsequently, they were randomly allocated to five groups: (1) non-preconditioned dogs: under anesthesia, the anterior descending coronary artery was occluded during 1 h and then reperfused during 4.5 h. (2) Early preconditioned dogs: procedure similar to group 1 but the dogs performed exercise on a treadmill for five periods of 5 min each before the coronary occlusion. (3) Late preconditioned dogs: procedure similar to group 2 but 24 h were allowed to elapse between the preconditioning exercise and the coronary occlusion. (4) Early preconditioned dogs plus 5-hydroxydecanoate: procedure similar to group 2 but 5-hydroxydecanoate was administered prior to exercise. (5) Non-preconditioned dogs with 5-hydroxydecanoate: procedure similar to group 1 but 5-hydroxydecanoate was administered at a time equivalent to that in group 4. RESULTS Exercise did not induce myocardial ischemia and the hemodynamics during the experiments did not differ between groups. Exercise immediately before the coronary occlusion decreased the infarct size (percent of the risk region) by 78% (P<0.05), an effect that was abolished with 5-hydroxydecanoate. Exercise 24 h prior to coronary occlusion decreased infarct size by 46% (P<0.05 vs. non-preconditioned dogs, P<0.05 vs. early preconditioned dogs). 5-Hydroxydecanoate by itself did not modify infarct size. These effects could not be explained by changes in collateral flow to the ischemic region. CONCLUSIONS Exercise prior to a coronary occlusion induces early and late preconditioning of the infarct size. The early effect is mediated through mitochondrial ATP-sensitive potassium channels.

Neuropeptide Y (NPY): a coronary vasoconstrictor and potentiator of catecholamine-induces coronary constriction

The vasoactive effect of neuropeptide Y (NPY) a peptide commonly found in perivascular nerves, including those of the heart, was assessed in the coronary circulation of the isolated perfused dog heart and in superfused segments of isolated canine coronary arteries. The intracoronary administration of 0.7-23.5 nmol NPY to hearts during beta adrenergic blockade produced a dose-dependent increase in coronary vascular resistance ranging from 0.10 to 0.49 mmHg.min-1.ml-1.100 g-1 without changes in myocardial oxygen consumption. The potency of NPY as a coronary vasoconstrictor was about 250 times that of noradrenaline. Pretreating the coronary system of these hearts with NPY caused a marked potentiation of the vasocontractile effect of noradrenaline, displacing its dose-response curve to the left in a non-parallel fashion. The addition of 0.2-3.7 nmol NPY did not induce contraction in superfused helical segments of large coronary arteries but it potentiated the tension developed in response to 0.18 microM adrenaline in a concentration-dependent manner. Pretreatment of these arteries with 3.7 nmol NPY caused a significant leftward displacement of the adrenaline contractile effect. These results show that NPY is a potent coronary vasoconstrictor and a potentiator of the contractile effect of catecholamines and support the hypothesis that NPY may participate in the regulation of coronary vascular resistance.

Effect of endothelin on total and regional coronary resistence and on myocardial contractility

Endothelin is a 21-amino acid peptide produced by the endothelium and has a potent vasoconstrictor effect. Because of the importance of the endothelium on vasomotor regulation, we studied the effect of endothelin on total and regional coronary vascular resistance and on myocardial contractility in the intact heart of anesthetized dogs. Intracoronary administration of 2 to 80 pmol/kg of endothelin produced a dose-dependent increase in coronary resistance, ischaemic decrease in myocardial contractility and atrium-ventricular blockade. The increase in resistance was greater towards the outer layer of the left ventricular wall. When the coronaries were perfused at a constant rate and vasoconstriction was prevented with adenosine or nitroglycerine, endothelin did not produce inotropic changes. These results show that endothelin is a potent vasoconstrictor of the resistance coronary vessels, producing a redistribution of transmural blood flow and a decrease in myocardial contractility secondary to ischaemia.

Preconditioning A New Concept About the Benefit of Exercise

Benefits of Exercise Clinicians have learned about the beneficial effects of several factors that may prevent a myocardial infarction (MI), including avoidance of smoking; treatment of high blood pressure, diabetes, dyslipidemia, and obesity; and regular performance of exercise. This last factor is based on epidemiological observations such as a decrease in the incidence of MI in men who perform heavy work1,2; however, it is only in the last few years that the beneficial effect of exercise has obtained plausible explanations of its own, that is, apart from its effect on other risk factors. There are at least 3 distinct mechanisms for this benefit: (1) Improvement of endothelial function, thereby preventing atherosclerosis and coronary occlusion3; (2) prevention of remodeling after MI through the expression of oxidative metabolism–related genes4; and (3) delaying acute ischemic injury after a coronary occlusion by preconditioning. Since the discovery of ischemic preconditioning by Murry et al5 in 1986, studies have appeared in the literature searching for its mechanisms and for alternative ways to trigger it. The concept that 1 or 2 episodes of brief ischemia ( 5 minutes in duration each), induced a few minutes or a few hours (early preconditioning) or 24 to 72 hours (late preconditioning or second window) before a prolonged coronary occlusion, followed by reperfusion substantially decreases the speed of the ischemic injury and limits infarct size is firmly established in all animal species studied in the experimental laboratory.6 It is one of the most powerful means of protecting the myocardium with the exception of early reperfusion. Several lines of evidence in coronary patients suggest but do not prove that the human myocardium is also protected by ischemic preconditioning. For example, preinfarction angina is associated with a smaller infarct size; a lower incidence of congestive heart failure, shock, and ventricular arrhythmias; and decreased mortality.7–9 The ST-segment elevation observed during angioplasty decreases after subsequent occlusions,10 which suggests that each occlusion provides preconditioning for the ischemic effect of the next one. Protocols of ischemic preconditioning before coronary artery bypass grafting preserve ATP levels during the subsequent global ischemic period11 and decrease serum levels of troponin T, thereby suggesting a smaller infarct size.12 Finally, the progressive decrease in the magnitude of ischemia during several consecutive episodes of exercise in patients with demand angina (warm-up phenomenon) suggests the preconditioning effect of each episode.13 The protective effect of ischemic preconditioning can be reproduced by several drugs, thus avoiding the necessity of ischemic periods to induce it. Pharmacological preconditioning is potentially a strong therapeutic tool. For example, the opening of mitochondrial ATP-sensitive potassium channels appears to be an important mediator of ischemic preconditioning. The administration of a mitochondrial ATP-sensitive potassium channel opener before planned procedures that involve a potentially ischemic insult (such as coronary artery surgery or angioplasty in the presence of a non–

Coronary vascular resistance during halothane anesthesia.

To study the effect of halothane on the coronary circulation, the circumflex diastolic coronary vascular resistance was measured in the working heart and total mean coronary resistance (TCR) in the isolated nonworking heart of the dog during administration of 100 per cent oxygen and during administration of 2--3 per cent halothane in oxygen. In the working heart, when the diastolic aortic pressure was kept at a nearly control level, halothane induced decreases of 12 per cent in circumflex diastolic coronary vascular resistance and 18 per cent in left ventricular arteriovenous oxygen content difference and no significant change in diastolic coronary blood flow. This effect occurred in spite of the absence of any significant change of myocardial oxygen consumption. In the nonworking beating, arrested or fibrillating heart, halothane induced a decrease of 24 per cent in total mean coronary resistance. Since the decrease in circumflex diastolic coronary vascular resistance in the working heart connot be attributed to myocardial hypoxia and since the results in the isolated nonworking heart eliminate the influences of mechanical and neurohumoral factors on coronary resistance, it is concluded that the observed decrease in resistance is probably due to vasodilation produced by a direct action of halothane on the coronary vessels. This effect was not modified by beta-adrenergic blockade.

Mitochondrial ATP dependent potassium channels mediate non-ischemic preconditioning by tachycardia in dogs

Brief episodes of tachycardia without myocardial ischemia prior to a coronary occlusion decrease myocardial infarct size in dogs. This non-ischemic preconditioning is mediated by adenosine. Because ischemic preconditioning is mediated through ATP dependent potassium channels, particularly the mitochondrial ones, we studied whether non-ischemic preconditioning is also mediated through these channels. In anesthetized dogs heart rate was kept constant at 120 cycles/min and aortic pressure changes were damped. Myocardial infarction was induced by occlusion of the anterior descending coronary artery for 60 min and reperfusion for 270 min. In a control group the infarct size (necrotic volume/risk region volume × 100) was 15.8 ± 1.5%. Preconditioning with five periods of tachycardia, 5 min in duration each at 213 cycles/min with intervening periods of 5 min of basal heart rate at 120 cycles/min, reduced the infarct size by 45.6% (p < 0.05) with respect to the control group. This effect was completely reverted by the blockade of ATP dependent potassium channels with glibenclamide or 5 hydroxydecanoate (a specific blocker of mitochondrial ATP dependent potassium channels) prior to preconditioning. These effects were not due to differences in collateral flow, risk region size or hemodynamic variables between the groups. These results show that mitochondrial ATP dependent potassium channels mediate non-ischemic preconditioning by tachycardia in dogs.

Late cardiac preconditioning by exercise in dogs is mediated by mitochondrial potassium channels.

We previously showed that exercise induces myocardial preconditioning in dogs and that early preconditioning is mediated through mitochondrial adenosine triphosphate-sensitive potassium channels. We decided to study if late preconditioning by exercise is also mediated through these channels. Forty-eight dogs, surgically instrumented and trained to run daily, were randomly assigned to 4 groups: (1) Nonpreconditioned dogs: under anesthesia, the coronary artery was occluded during 1 hour and then reperfused during 4.5 hours. (2) Late preconditioned dogs: similar to group 1, but the dogs run on the treadmill for 5 periods of 5 minutes each, 24 hours before the coronary occlusion. (3) Late preconditioned dogs plus 5-hydroxydecanoate (5HD): similar to group 2, but 5HD was administered before the coronary occlusion. (4) Nonpreconditioned dogs plus 5HD: similar to group 1, but 5HD was administered before the coronary occlusion. Infarct size (percent of the risk region) decreased by effect of exercise by 56% (P < 0.05), and this effect was abolished with 5HD. 5HD by itself did not modify infarct size. Exercise did not induce myocardial ischemia, and the hemodynamics during ischemia-reperfusion period did not differ among groups. These effects were independent of changes in collateral flow to the ischemic region. We concluded that late cardiac preconditioning by exercise is mediated through mitochondrial adenosine triphosphate-sensitive potassium channels.

Aging differentially modifies arterial sensitivity to endothelin-1 and 5-hydroxytryptamine: Studies in dog coronary arteries and rat arterial mesenteric bed

The influence of age on vascular reactivity to endothelin-1 (ET-1) and 5-hydroxytryptamine (5-HT) was studied in coronary artery rings from dogs of 9 years of age or younger, and dogs older than 9 years. ET-1 caused concentration-dependent contractions that developed about 100% of the 70 mM KCl-induced tension in the younger dogs; those from older dogs did not generate more than 20%. In contrast, 5-HT developed only about 20% of the KCl-induced tension in rings from young dogs, whereas in the older animals, it developed up to 120% of the KCl tension. No significant difference in the tension developed by 70 mM KCl was noted between both groups of dogs. Mechanical denudation of the endothelial cell layer caused a modest, yet significant, leftward shift of the ET-1 and 5-HT concentration-response curves only in the younger dogs. N omega-Nitro-L-arginine (15 microM) shifted the ET-1 concentration-response curves to the left in rings from both groups of dogs. Rings precontracted with 20 mM KCl relaxed in a concentration-dependent fashion with acetylcholine; its sensitivity was about threefold less in the older group of dogs. To validate the changes in vascular reactivity with age, a parallel study was performed perfusing the arterial mesenteric bed of rats of 3, 7, and 30 weeks of age. In this experimental model, the efficacy of ET-1 significantly decreased with age and that of 5-HT was significantly increased. The vasomotor reactivity of noradrenaline was modestly affected by aging, whereas the acetylcholine-induced vasorelaxation was significantly reduced with age.

Preconditioning tachycardia decreases the activity of the mitochondrial permeability transition pore in the dog heart.

Cardioprotection by preconditioning is a central issue of current research on heart function. Several reports indicate that preventing the assembly and opening of the mitochondrial permeability transition pore (mPTP) protects the heart against ischemia-reperfusion injury. We have previously reported that brief episodes of tachycardia decrease the infarct size produced by subsequent prolonged occlusion of a coronary artery, indicating that controlled tachycardia is an effective preconditioning manoeuvre. The effects of preconditioning tachycardia on mPTP activity have not been reported. Therefore, in this work we investigated if preconditioning tachycardia protects against calcium-induced mitochondrial swelling, a measure of mPTP activity. We found that tachycardia decreased by 2.5-fold the rate of mitochondrial calcium-induced swelling, a factor that presumably contributes to the cardioprotective effects of tachycardia. The oxidative status of the cell increased after tachycardia, as evidenced by the decrease in the cellular and mitochondrial GSH/GSSG ratio. We also observed increased S-glutathionylation of cyclophilin-D, an essential mPTP component, after tachycardia. This reversible redox modification of cyclophilin-D may account, al least in part, for the decreased mPTP activity produced by preconditioning tachycardia.

Exercise preconditioning of myocardial infarct size in dogs is triggered by calcium.

Abstract: We showed that exercise induces early and late myocardial preconditioning in dogs and that these effects are mediated through nicotinamide adenine dinucleotide phosphate reduced form (NADPH) oxidase activation. As the intracoronary administration of calcium induces preconditioning and exercise enhances the calcium inflow to the cell, we studied if this effect of exercise triggers exercise preconditioning independently of its hemodynamic effects. We analyzed in 81 dogs the effect of blocking sarcolemmal L-type Ca2+ channels with a low dose of verapamil on early and late preconditioning by exercise, and in other 50 dogs, we studied the effect of verapamil on NADPH oxidase activation in early exercise preconditioning. Exercise reduced myocardial infarct size by 76% and 52% (early and late windows respectively; P < 0.001 both), and these effects were abolished by a single low dose of verapamil given before exercise. This dose of verapamil did not modify the effect of exercise on metabolic and hemodynamic parameters. In addition, verapamil blocked the activation of NADPH oxidase during early preconditioning. The protective effect of exercise preconditioning on myocardial infarct size is triggered, at least in part, by calcium inflow increase to the cell during exercise and, during the early window, is mediated by NADPH oxidase activation.