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Ann Thorac Surg 1999;68:844-849
© 1999 The Society of Thoracic Surgeons
a Division of Cardiothoracic Surgery, University Hospital at Stony Brook, State University of New York, Stony Brook, New York, USA
Address reprint requests to Dr Krukenkamp, Division of Cardiothoracic Surgery, University Hospital at Stony Brook, T19-080 Health Sciences Center, Stony Brook, NY 11794-8191
e-mail: ibkmd{at}hotmail.com
Presented at the Poster Session of the Thirty-Fifth Annual Meeting of The Society of Thoracic Surgeons, San Antonio, TX, Jan 2527, 1999.
| Abstract |
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Methods. Isolated, crystalloid-perfused rabbit hearts underwent 1 hour of left anterior descending coronary artery ischemia followed by 1 hour of reperfusion. Before ischemia, six hearts received nipradilol, six received the nitric oxide donor L-arginine, four hearts received the nitric oxide synthase inhibitor NG-nitro-L-arginine methyl ester before L-arginine, nine underwent ischemic preconditioning, and six received ß-blockade by esmolol before ischemic preconditioning. Seven hearts received no pretreatment (control). Action potential duration and ventricular pressure were measured. Infarct size was determined at the end of reperfusion.
Results. Both L-arginine and ischemic preconditioning prolonged action potential duration significantly at 60 minutes of reperfusion. Compared with control, infarct size was reduced by ischemic preconditioning (26% ± 4% versus 49% ± 3%, IPC versus control; p < 0.01), L-arginine (24% ± 2%; p < 0.01 versus control), and nipradilol (24% ± 2%; p < 0.01 versus control). Only nipradilol preserved peak developed pressure during reperfusion.
Conclusions. Despite its properties as a ß-adrenergic blocking agent, nipradilol was able to precondition the heart, probably as a result of its ability to produce nitric oxide.
| Introduction |
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-adrenergic agent phenylephrine [5, 7], and adenosine [5, 6, 8]. Recently, nitric oxide (NO) has been implicated in the mechanism of preconditioning [9]. We have shown that NO is a requisite for cofactor in the preconditioning response generated by the administration of adenosine triphosphate-sensitive potassium-channel openers [10]. Because many patients who undergo coronary operations are prescribed ß-blockers, and because we have previously shown that ß-adrenergic blockade alone prevents preconditioning, we evaluated the NO-generating ß-blocker nipradilol as a sole preconditioning stimulus.
| Material and methods |
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Thirty-eight adult male New Zealand rabbits, weighing between 2.8 and 3.0 kg, were anesthetized with sodium pentobarbital (30 mg/kg) and anticoagulated with 1000 U sodium heparin by ear vein. Once the corneal reflex was abolished, the rabbits were placed in the supine position, and the chest was entered through a bilateral thoracotomy ("clam-shell" incision). The heart was rapidly excised and placed in an iced bath of Krebs-Henseleit solution. The aorta was cannulated with a stainless steel 8F cannula, and the heart was suspended from the cannula within a heated glass chamber (Radnotti, Monrovia, CA). The aorta was then perfused with oxygenated (95% O2/5% CO2) Krebs solution at 37°C and 75 mm Hg root pressure.
The heart was permitted to equilibrate for 30 minutes. During that period, both atria were excised, and a small balloon was placed through the mitral valve into the left ventricle. Balloon pressure was monitored continuously with an indwelling catheter probe (Millar, Houston, TX); the initial end-diastolic pressure (EDP) was set to approximately 5 mm Hg by water inflation, and the volume remained constant throughout the experiment. End-systolic pressure and EDP were measured directly from the balloon pressure tracings, and peak developed pressure was calculated as the difference between end-systolic pressure and EDP for each beat. The heart was paced at 150 beats per minute with an asynchronous pacemaker (Medtronics, Minneapolis, MN). Monophasic action potentials were recorded from the left ventricular epicardium within the distribution of the left anterior descending coronary artery (LAD) with an 8F spring-loaded probe (EP Technologies, Mountainview, CA). Coronary flow was measured directly by timed collection of Krebs effluent.
Ischemia was induced by encircling the LAD close to its origin with a 3-0 silk suture and snaring the suture. At the end of 1 hour of ischemia, reperfusion was achieved by releasing the ligature and briefly massaging the LAD with a moistened cotton swab. After 1 hour of reperfusion, the heart was removed from the perfusion apparatus, the ligature resnared, and 2 mL of monastryl blue was infused through the aortic cannula. The right ventricle was excised, and then the left ventricle was sectioned horizontally at 1- to 2-mm intervals into 5 to 7 slices. The unstained area was scanned (Sigma Scan, Jandel Scientific, San Rafael, CA) into an IBM-compatible computer (Dell, Austin, TX) and labeled the area at risk. The slices were then incubated in triphenyl tetrazolium chloride for 15 minutes, and the unstained (white) area was scanned and labeled the infarcted region. Infarct area was expressed as the percentage of area at risk. Overall infarct area was computed as a weighted average of the slices, by weight.
Seven hearts served as controls without treatment before LAD ischemia and reperfusion (Fig 1). Six hearts received the NO donor L-arginine [11] (1 mmol/L) before LAD ischemia, four hearts received NO synthetase inhibitor, NG-L-nitro-L-arginine methyl ester (L-NAME, 500 µmol/L) before infusion of L-arginine, and six hearts received nipradilol (5 µmol/L). Nine hearts underwent global ischemic preconditioning (IPC) before LAD ischemia and reperfusion. Ischemic preconditioning was accomplished with two episodes of 5 minutes of complete aortic occlusion followed by 5 minutes of reperfusion. Six hearts received ß-adrenergic blocker esmolol (100 µmol/L) before IPC.
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| Comment |
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Role of nitric oxide in myocardial response to preconditioning
In the present study, infusion of both the NO donor L-arginine and the NO-generating ß-blocker nipradilol before prolonged ischemia offered myoprotective effects in terms of infarct size reduction and electrophysiologic preservation against ischemiareperfusion insult. When the NO synthase inhibitor L-NAME was administered before L-arginine infusion, these protective effects were abolished. Although the concentration of NO was not measured in this study, these results imply that the generation of NO in some way contributes to the preconditioning seen with both L-arginine and nipradilol.
The specific mechanism underlying the protective effect of NO, however, remains unclear. Several energy-sparing actions have been proposed, including antagonizing ß-adrenergic activation [14], inhibiting calcium influx into myocytes [15], decreasing myocardial contractility, and decreasing mitochondrial respiration [14].
Effect of ß-adrenergic manipulation on ischemic preconditioning
Although many drugs have been shown to promote or block the infarct size-reducing effects of IPC [38], none has been studied from either the mechanical or electrophysiologic point of view. One subcellular mechanism of particular interest as regards this study proposes that ß-adrenergic stimulation increases the concentration of intracellular cyclic adenosine monophosphate by a G protein-dependent adenylate cyclase. The rise in cyclic adenosine monophosphate activates protein kinase C, which phosphorylates intracellular enzymes and transmembrane ion channels such as the adenosine triphosphate-sensitive potassium channel. Activation of the adenosine triphosphate-sensitive potassium channel increases transmembrane conductance during the repolarization phase and shortens the action potential. Therefore, ß-adrenergic blockade can be expected to interfere with IPC [16]. Because many patients who undergo coronary operations are already prescribed ß-blockers, it is not clear what effect this will have, if any, on attempts at pharmacologic preconditioning.
Effect of nipradilol on ischemic preconditioning
In this study, nipradilol, a NO-generating ß-blocker, preserved postischemic cardiac function and reduced infarct size. With regard to mechanical function, nipradilol may have its effect by increasing coronary flow after reperfusion. It remains unclear, however, why this change was not evident immediately after infusion of nipradilol, but rather apparent only during the reperfusion period. Perhaps this effect was contributed by either NO production alone, or by both NO and ß-adrenergic modulation.
Study limitations
We have demonstrated some myoprotective effects of the NO-generating ß-blocker nipradilol in the isolated, crystalloid-perfused rabbit heart. However, NO may have many more complex effects in vivo, as it has been shown to inhibit leukocyte adherence to vascular endothelium and to diminish neutrophil and platelet aggregation [17].
Although our findings suggest that NO may protect against ischemiareperfusion injury, the peroxynitrate radical, derived from the reaction between NO and superoxide, may damage the myocardium [18]. In addition, hemoglobin is believed to be a potent inactivator of NO [17]; thus, NO might not be protective in the intact human. Furthermore, the effects of NO on other organs remain to be completely elucidated, and NO may even sometimes play a detrimental role [19]. Therefore, the effects of NO on other organs must be carefully investigated before the administration of nipradilol is considered in clinical practice.
Another limitation of this study is the short-term application of nipradilol. Clinically, this drug is likely to be given over much longer periods of time. This model of preconditioning, along with all other models, uses a short-term administration of the pharmacologic stimulus. Whether long-term administration of preconditioning stimuli reduces the protective response of the heart is unknown and will be addressed in future studies.
| References |
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