Pilar Macho

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.

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.

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.

Role of endothelium-derived relaxing factor on coronary blood flow regulation in the dog

The endothelium plays a key role in the regulation of vasoreactivity. To assess its importance on coronary flow regulation, we studied the participation of endothelium-derived relaxing factor-nitric oxide (EDRF-NO) on coronary reactive hyperemia and on the hyperemia that occurs secondary to an increase in myocardial oxygen consumption. In 15 dogs, the reactive hyperemic response decreased substantially after inhibition of EDRF-NO synthesis with N-omega-nitro-L-arginine (P < 0.01). In contrast, the hyperemia secondary to an increase in myocardial oxygen consumption, characterized by a linear correlation between myocardial oxygen consumption and coronary flow, did not change significantly after inhibition of EDRF-NO production (regression analysis, P > 0.1). Thus EDRF-NO synthesis by the endothelium is an important mechanism mediating the reactive hyperemic response but it does not seem to be essential for the metabolic regulation of coronary vascular resistance during hyperemia induced by an increased metabolic demand on the myocardium.

Non-ischemic myocardial preconditioning

The reduction of infarct size produced by brief ischemic episodes prior to a sustained occlusion of a coronary artery, called ischemic preconditioning, is a well known phenomenon that occurs in several species, but its mechanism is still under investigation. Recent reports support the idea that this protection can also be obtained by non-ischemic maneuvers like distention of the left ventricle and metabolic stimulation of myocardial cells. The features of non-ischemic preconditioning (temporal limitation, second window, tolerance development, remote preconditioning and efficiency of the protection), as opposed to those of ischemic preconditioning, are still to be determined. Neither is it known if non-ischemic preconditioning occurs in humans. From a physiological point of view the protective effect of an increase in metabolic rate of the heart means a constant feed-back mechanism in the myocardial cell that counteracts the presumptive damage consequent to the increase in metabolism. Therefore, in the presence of a sudden coronary occlusion the metabolic rate of the heart immediately before the occlusion would have a dual role of increasing the degree of ischemia and of protecting against it.

Non Ischemic Myocardial Preconditioning by Tachycardia and Exercise

Myocardial preconditioning can be induced not only by ischemia but also by physiological stimuli like tachycardia and exercise. We studied in dogs the preconditioning effect of tachycardia. Five brief periods of tachycardia prior to occluding a coronary artery followed by reperfusion, reduced the infarct size by about 50%. This effect was prevented by blockade of adenosine receptors and also of mitochondrial ATP dependent potassium channels. Tachycardia did not produce ischemia and the results were not explained by changes in collateral flow to the ischemic region or in the hemodynamic variables. Since tachycardia produced preconditioning, we studied the possibility that exercise also induces preconditioning. In instrumented dogs the performance of brief periods of exercise on a treadmill a few minutes (early preconditioning) or 24 hours (late preconditioning) prior to a coronary occlusion and reperfusion, reduced the infarct size by about 77% and 46% respectively. Exercise did not produce ischemia and the results could not be explained by changes in collateral flow nor in the hemodynamic variables. This effect may partly explain the decrease in the incidence of coronary acute syndromes by regular exercise in humans. Because tachycardia and the intracoronary administration of Ca2+ induce preconditioning, we studied the changes in calcium transient induced by ischemia and the effect of tachycardia on these changes in myocardial sarcoplasmic reticulum vesicles. Ischemia increased Ca2+ release and decreased Ca2+ uptake by the sarcoplasmic reticulum. These effects were reverted in hearts preconditioned with tachycardia. These results suggest that the protective effect of tachycardia and probably that of exercise may be partly due to a decrease in the cytosolic Ca2+ overload produced by ischemia.