Premedication with the opioid analgesic butorphanol raises the threshold for ischemic preconditioning in dogs

Large animal models of stroke and traumatic brain injury as translational tools

Acute central nervous system injury, encompassing traumatic brain injury (TBI) and stroke, accounts for a significant burden of morbidity and mortality worldwide. Studies in animal models have greatly enhanced our understanding of the complex pathophysiology that underlies TBI and stroke and enabled the preclinical screening of over 1,000 novel therapeutic agents. Despite this, the translation of novel therapeutics from experimental models to clinical therapies has been extremely poor. One potential explanation for this poor clinical translation is the choice of experimental model, given that the majority of preclinical TBI and ischemic stroke studies have been conducted in small animals, such as rodents, which have small lissencephalic brains. However, the use of large animal species such as nonhuman primates, sheep, and pigs, which have large gyrencephalic human-like brains, may provide an avenue to improve clinical translation due to similarities in neuroanatomical structure when compared with widely adopted rodent models. This purpose of this review is to provide an overview of large animal models of TBI and ischemic stroke, including the surgical considerations, key benefits, and limitations of each approach.

The effect of butorphanol postconditioning on myocardial ischaemia reperfusion injury in rats

OBJECTIVES

Butorphanol tartrate is a synthetic opioid partial agonist analgesic. Butorphanol targets the heart, mainly via κ-opioid receptor (κ-OR) activation. The purpose of this study was to determine the effect and mechanism underlying butorphanol postconditioning (B-Post) on myocardial ischaemia reperfusion injury in rats.

METHODS

Seventy-five male Sprague-Dawley rats were randomly divided into five groups of 15 each: Group sham; Group I/R (ischaemia/reperfusion); Group B (butorphanol postconditioning); Group B/N (butorphanol postconditioning + antagonist of κ-OR nor-binaltorphimine [Nor-BNI]); Group B/G (butorphanol postconditioning + nonselective ATP-sensitive potassium (KATP) channel blocker glibenclamide [GLI]). The left coronary anterior descending artery (LAD) was occluded for 30 min, followed by a 120-min reperfusion. Blood samples were obtained at the end of reperfusion for determination of serum tumour necrosis factor (TNF)-α and interleukin (IL)-6 concentrations. The hearts were then excised for determination of myocardial infarct size by triphenyltetrazolium chloride staining. The myocardial tissues were used for determination of the expression of myocardial superoxide dismutase (SOD), malondialdehyde (MDA) and myeloperoxidase (MPO).

RESULTS

Myocardial infarct size was significantly reduced in B (26.4 ± 1.83%), B/N (34.5 ± 1.56%) and B/G (31.5 ± 1.27%) Groups compared with Group I/R (46.8 ± 1.41%) (all P < 0. 001). The serum TNF-α and IL-6 concentrations and the MDA and MPO activities in the ischaemic area in B, B/N and B/G Groups were significantly lower than those in the I/R Group (all P < 0.001). In addition, myocardial infarct size, TNF-α and IL-6 concentrations and the MDA and MPO activities in B/N and B/G Groups were higher than those in the B Group (all P < 0.001). In contrast, SOD activity was significantly increased in B, B/N and B/G Groups, and SOD activity in B/N and B/G Groups was less than in the B Group (all P < 0.001).

CONCLUSIONS

These results suggest that postconditioning of butorphanol tartrate can provide a potent cardioprotective effect against myocardial ischaemic and reperfusion injury. Both the κ-OR and the KATP channels were involved in this effect.

Remifentanil limits infarct size but attenuates preconditioning-induced infarct limitation

OBJECTIVES

We investigated the influence of the narcotic anesthetic remifentanil on irreversible myocardial ischemic injury.

METHODS

New Zealand White rabbits were anesthetized with propofol (0.7-1.8 mg.kg.min) and then subjected to 30 min regional myocardial ischemia and 3 h reperfusion (CON). Some animals also underwent ischemic preconditioning, elicited by either one (IP1) or two (IP2) cycles of 5 min ischemia and 5 min reperfusion, and/or remifentanil, administered either as a transient infusion mimicking the preconditioning protocol (RP2, 10 microg x kg x min) or as a continuous infusion (R, 3-10 microg x kg x min). Rabbits were randomly assigned to experimental groups. Infarct size was assessed with tetrazolium. Results are reported as mean+/-SD.

RESULTS

Non-preconditioned infarct size was approximately 50% of the area-at-risk (49.6+/-20.1% CON). Both one and two cycles of ischemic preconditioning markedly reduced infarct size (49.6+/-20.1% CON versus 18.6+/-8.6% IP and versus 7.5+/-7.6% IP2; both p<0.001). Preconditioning with remifentanil modestly reduced infarct size (49.6+/-20.1% CON versus 29.3+/-8.5% RP2; p<0.01). However, sustained administration of remifentanil did not provide protection (49.6+/-20.1% CON versus 43.9+/-16.2% R), and it attenuated the protection offered by preconditioning (49.6+/-20.1% CON versus 35.6+/-20.7% R+IP1, p=NS; and versus 14.5+/-14.5% R+IP2; p<0.05).

CONCLUSION

Transient pre-ischemic administration of remifentanil modestly reduces infarct size in propofol-anesthetized rabbits, but continuous administration of remifentanil increases the threshold for ischemic preconditioning-induced infarct limitation.

Nicorandil induces late preconditioning against myocardial infarction in conscious rabbits

Nicorandil has been shown to induce an infarct-limiting effect similar to that induced by the early phase of ischemic preconditioning (PC). The goals of this study were to determine whether nicorandil induces a delayed cardioprotection that is analogous to the late phase of ischemic PC and, if so, whether nicorandil-induced late PC is associated with upregulation of cardioprotective proteins. Chronically instrumented, conscious rabbits received vehicle (intravenous normal saline; control group, n = 10), nicorandil (100 microg/kg bolus + 30 microg x kg(-1) x min(-1) i.v. for 60 min; nicorandil group, n = 10), or ischemic PC (6 cycles of 4-min coronary occlusion/4-min reperfusion; PC group, n = 8). Twenty-four hours later, rabbits underwent a 30-min coronary occlusion, followed by 3 days of reperfusion. Myocardial infarct size was significantly reduced in rabbits pretreated with nicorandil (27.5 +/- 5.3% of the risk region) or with ischemia (30.3 +/- 4.2%) versus controls (59.1 +/- 4.7%, P < 0.05 vs. both). Furthermore, the expression of cyclooxygenase-2 (COX-2) and Bcl-2 was significantly elevated (+38% and +126%, respectively; P < 0.05) in myocardium of rabbits given nicorandil 24 h earlier versus controls. We conclude that nicorandil induces delayed cardioprotection against myocardial infarction similar to that afforded by the late phase of ischemic PC, possibly by upregulating COX-2 and Bcl-2.

Ischaemic preconditioning alters the energy metabolism and protects the ischaemic myocardium in a stepwise fashion

AIMS AND METHODS

Recently it was suggested that ischaemic preconditioning (IP) protects the myocardium in a graded pattern as assessed by myocardial infarct size estimation. Using tissue biopsies we investigated the impact of the proposed graded pattern of protection on myocardial energy state in an open-chest porcine model of IP with either one (1xIP) or four (4xIP) episodes of preconditioning. Furthermore, we evaluated the relationship between interstitial energy-related metabolite levels obtained by the microdialysis technique and the degree of subsequent ischaemic insult.

RESULTS

During the long ischaemia the difference between pre-ischaemic and post-ischaemic total adenylate pools and the sum of adenylate breakdown products (adenosine, inosine and hypoxanthine) as well as tissue lactate levels appeared as follows: non-IP > 1xIP > 4xIP (P < 0.05). Moreover interstitial peak levels of lactate, hypoxanthine and taurine displayed a graded pattern analogous to the development of ischaemic damage, where non-IP > 1xIP > 4xIP.

CONCLUSIONS

We present for the first time concordant energy metabolic and morphometric data in support of IP being a stepwise phenomenon for protection of the ischaemic myocardium. Furthermore, IP resulted in proportionally higher levels of hypoxanthine (relative to inosine) in the ischaemic myocardium, suggesting a different handling of adenine nucleotide breakdown products in the IP myocardium.

Protein tyrosine kinase signaling is necessary for NO donor-induced late preconditioning against myocardial stunning

Although protein tyrosine kinases (PTKs) signaling has been implicated in the late phase of ischemic preconditioning (PC), it is unknown whether PTK signaling is necessary for the development of nitric oxide (NO) donor-induced late PC. Thus conscious rabbits underwent a sequence of six 4-min coronary occlusion (O)/4-min reperfusion (R) cycles followed by a 5-h recovery period of reperfusion for 3 consecutive days (days 1, 2, and 3). On day 0 (24 h before the 6 O/R cycles on day 1), rabbits received no treatment (control), the NO donor diethylenetriamine (DETA)/NO (DETA/NO), the PTK inhibitor 4-amino-5-(4-chlorophenyl)-7-(t-butyl)pyrazolo[3,4-d]pyrimidine (PP2), or DETA/NO plus PP2 (DETA/NO + PP2). In control rabbits (n = 6), the six O/R cycles on day 1 resulted in delayed functional recovery, indicating severe myocardial stunning. In rabbits pretreated with DETA/NO (n = 5) on day 1, myocardial stunning caused by the six O/R cycles on day 1 was markedly attenuated, with a significant reduction ( approximately 60%) in the total deficit of wall thickening (WTh) compared with controls, indicating that DETA/NO induced a late PC effect against stunning. However, in rabbits pretreated with DETA/NO + PP2 (n = 5), the total deficit of WTh was significantly greater than that in rabbits treated with DETA/NO alone and was similar to that in controls, indicating that PP2 prevented the development of DETA/NO-induced late PC. In rabbits pretreated with PP2 on day 0 (n = 4), the total deficit of WTh was similar to that in controls, indicating that PP2 does not affect myocardial stunning in itself. We conclude that a PTK-dependent signaling mechanism is necessary for the development of NO donor-induced late PC against myocardial stunning in conscious rabbits.

Delta-opioid receptor-induced late preconditioning is mediated by cyclooxygenase-2 in conscious rabbits

Although activation of delta-opioid receptors is known to induce both early and late preconditioning (PC) against myocardial infarction, the mechanisms for this salubrious effect are unclear. Furthermore, it is unknown whether delta-opioid receptors can also induce late PC against myocardial stunning. By using conscious rabbits (n = 120) in this study, we found that the delta-opioid receptor agonist (+/-)-4-[(alpha-R*)-alpha-[(2S*,5R*)-4-allyl-2,5-dimethyl-1-piperazinyl]-3-hydroxybenzyl]-N,N-diethylbenzamide (BW-373U86) induced late PC against myocardial stunning 24 h after treatment and that this effect was abolished by the selective cyclooxygenase-2 (COX-2) inhibitors N-[2-(cyclohexyloxy)4-nitrophenyl]methanesulfonamide (NS-398) and celecoxib. This protective effect was also abrogated by the selective delta(1)-opioid receptor antagonist 7-benzylidenenaltrexone, indicating that the delta(1)-opioid receptor is necessary for BW-373U86-induced late PC. BW-373U86 did not induce early PC against stunning. In addition, BW-373U86 induced late PC against infarction, which was blocked by NS-398. At 24 h after BW-373U86 administration, myocardial COX-2 protein expression and PGE(2) and 6-keto-PGF(1alpha) levels were significantly increased. These results demonstrate that activation of delta-opioid receptors induces late PC against both stunning and infarction via a COX-2-dependent mechanism.

Cardioprotection by multiple preconditioning cycles does not require mitochondrial K(ATP) channels in pigs

To test whether cardioprotection induced by ischemic preconditioning depends on the opening of mitochondrial ATP-sensitive K(+) (K(ATP)) channels, the effect of channel blockade was studied in barbital-anesthetized open-chest pigs subjected to 30 min of complete occlusion of the left anterior descending coronary artery and 3 h of reflow. Preconditioning was elicited by two cycles of 5-min occlusion plus 10-min reperfusion before the 30-min occlusion period. 5-Hydroxydecanoate (5 mg/kg iv) was injected 15 min before preconditioning or pharmacological preconditioning induced by diazoxide (3.5 mg/kg, 1 ml/min iv). Infarct size (percentage of the area at risk) after 30 min of ischemia was 35.1 +/- 9.9% (n = 7). Preconditioning markedly limited myocardial infarct size (2.7 +/- 1.6%, n = 7), and 5-hydroxydecanoate did not abolish protection (2.4 +/- 0.9%, n = 8). Diazoxide infusion also significantly limited infarct size (14.6 +/- 7.4%, n = 7), and 5-hydroxydecanoate blocked this effect (30.8 +/- 8.0%, n = 7). Thus the opening of mitochondrial K(ATP) channels is cardioprotective in pigs, but these data do not support the hypothesis that opening of mitochondrial K(ATP) channels is required for the endogenous protection afforded by preconditioning.

Opening of mitochondrial K+ channels increases ischemic ATP levels by preventing hydrolysis

Mitochondrial ATP-sensitive K+ channels (mitoK(ATP)) have been proposed to mediate protection against ischemic injury by increasing high-energy intermediate levels. This study was designed to verify if mitochondria are an important factor in the loss of cardiac ATP associated to ischemia, and determine the possible role of mitoK(ATP) in the control of ischemic ATP loss. Langendorff-perfused rat hearts subjected to ischemia were found to have significantly higher ATP contents when pretreated with oligomycin or atractyloside, indicating that mitochondrial ATP hydrolysis contributes toward ischemic ATP depletion. MitoK(ATP) opening induced by diazoxide promoted a similar protection against ATP loss. Diazoxide also inhibited ATP hydrolysis in isolated, nonrespiring mitochondria, an effect accompanied by a drop in the membrane potential and Ca2+ uptake. In hearts subjected to ischemia followed by reperfusion, myocardial injury was prevented by diazoxide, but not atractyloside or oligomycin, which, unlike diazoxide, decreased reperfusion ATP levels. Our results suggest that mitoK(ATP)-mediated protection occurs due to selective inhibition of mitochondrial ATP hydrolysis during ischemia, without affecting ATP synthesis after reperfusion.

Role of Src protein tyrosine kinases in late preconditioning against myocardial infarction

Although Src protein tyrosine kinases (PTKs) have been shown to be essential in late preconditioning (PC) against myocardial stunning, their role in triggering versus mediating late PC against myocardial infarction remains unclear. Four groups of conscious rabbits were subjected to a 30-min coronary occlusion on day 2, with or without PC ischemia on day 1. Administration of the Src PTK inhibitor lavendustin A (LD-A; 1 mg/kg iv) before the PC ischemia on day 1 (group III, n = 7) failed to block the delayed protective effect against myocardial infarction 24 h later. Late PC against infarction, however, was completely abrogated when LD-A was given 24 h after the PC ischemia, prior to the 30-min occlusion on day 2 (group IV, n = 8). We conclude that, in conscious rabbits, Src PTK activity is necessary for the mediation of late PC protection against myocardial infarction on day 2, but not for the initiation of this phenomenon on day 1. Taken together with previous studies in the setting of stunning, these findings reveal heretofore unrecognized differences in the roles of Src PTKs in late PC against stunning versus late PC against infarction.

Role of delta-opioid receptor agonists on infarct size reduction in swine

Opioids are involved in cardiac ischemic preconditioning. Important species differences in cellular signaling mechanisms, antiarrhythmic, and antistunning effects have been described. The role of the delta-opioid receptor activation in swine remains unknown. Forty minutes before a 45-min occlusion and 180-min reperfusion of the left anterior descending coronary artery, open-chest, pentobarbital-anesthetized swine received either 1) saline (controls); 2) [D-Ala(2),D-Leu(5)]enkephalin (DADLE); 3) [D-Pen(2,5)]enkephalin (DPDPE); 4) deltorphin-D, a novel delta(2)-opioid agonist; or 5) ischemic preconditioning (IP). Assessed were 1) infarct size to area at risk (IS, triphenyltetrazolium staining), 2) regional and global myocardial function (sonomicrometry, ventricular pressure catheters), and 3) arrhythmias (electrocardiogram analyses). It was found that DPDPE and deltorphin-D pretreatment reduced IS from 64.7 +/- 5 to 36.5 +/- 6% and 27.4 +/- 11% (P < 0.01), respectively, whereas DADLE had no effect (66.8 +/- 3%). Both IP and DADLE had a proarrhythmic effect (P < 0.01). However, no differences in global or regional myocardial function or arrhythmia scores were observed between groups. This suggests that delta-receptor-specific opioids provide cardioprotection in swine.

Consequences of brief ischemia: stunning, preconditioning, and their clinical implications: part 2

In experimental studies in the dog, total proximal coronary artery occlusions of up to 15 minutes result in reversible injury, meaning that the myocytes survive this insult. The 15 minutes of ischemia, however, induce numerous changes in the myocardium, including certain monuments to the brief episode of ischemia that may persist for days. One of these monuments is stunned myocardium, which represents "prolonged postischemic contractile dysfunction of myocardium salvaged by reperfusion." The mechanism of stunning involves generation of oxygen radicals as well as alteration in calcium homeostasis and possibly alteration in contractile protein structure. Stunning has been observed in several clinical scenarios, including after percutaneous transluminal coronary angioplasty, unstable angina, stress-induced ischemia, after thrombolysis, and after cardiopulmonary bypass. Oxygen radical scavengers and calcium channel blockers have been shown to enhance function of stunned myocardium in experimental studies, and in a few clinical studies, calcium channel blockers have been shown to ameliorate stunning. Although brief periods of ischemia can contribute to prolonged left ventricular dysfunction and even heart failure, they paradoxically play a cardioprotective role. Episodes of ischemia as short as 5 minutes, followed by reperfusion, protect the heart from a subsequent longer coronary artery occlusion by markedly reducing the amount of necrosis that results from the test episode of ischemia. This phenomenon, called ischemic preconditioning, has been observed in virtually every species in which it has been studied and is a powerful cardioprotective effect. The mechanism of ischemic preconditioning involves both triggers and mediators and involves complex second messenger pathways that appear to involve such components as adenosine, adenosine receptors, the epsilon isoform of protein kinase C, the ATP-dependent potassium channels, as well as others, including a paradoxical protective role of oxygen radicals. Both an early and a late phase of preconditioning have been described, and the mechanisms underlying their induction are under investigation. That preconditioning may occur in humans is suggested by the observations that repetitive balloon inflations in the coronary artery are associated with progressively less chest pain, ST-segment elevation, lactate production, the protective effects of preinfarction angina, the anginal "warm-up phenomenon," and studies performed on human cardiac biopsies that show metabolic properties suggesting preconditioning. Development of pharmacological agents that stimulate second messenger pathways thought to be involved in preconditioning, but without causing ischemia, could result in novel approaches to treating ischemia. Hence, on one hand, brief episodes of ischemia can have a negative effect on the heart: stunning; and on the other hand, they have a protective effect: preconditioning. The future challenge is how to minimize the stunning phenomenon and maximize the preconditioning phenomenon in clinical practice.

Cyclooxygenase-2 does not mediate late preconditioning induced by activation of adenosine A1 or A3 receptors

Recent studies have demonstrated that the adenosine A1 receptor agonist 2-chloro-N6-cyclopentyladenosine (CCPA) and the adenosine A3 receptor agonist N6-(3-iodobenzyl)adenosine-5'-N-methyluronamide (IB-MECA) produce a delayed phase of protection against infarction similar to the late phase of ischemic preconditioning (PC). However, the mechanism for adenosine A1 or A3 receptor-induced late PC remains unknown. The goal of this study was to determine whether the delayed cardioprotective effects of adenosine A1 or A3 receptors are mediated by cyclooxygenase-2 (COX-2), which is an obligatory mediator of ischemic PC. We found that COX-2 protein expression (Western blotting) did not increase 24 h after the administration of either CCPA (100 microg/kg iv) or IB-MECA (300 microg/kg iv) compared with controls. To probe the role of constitutive COX-2 expression, conscious rabbits were subjected to 30-min coronary occlusion followed by 72-h reperfusion. Twenty-four hours before the occlusion, the rabbits were pretreated with CCPA (100 microg/kg iv) or IB-MECA (300 microg/kg iv). Both CCPA and IB-MECA resulted in a marked (approximately 47%) reduction in infarct size vs. controls [36.2 +/- 4.0% of the risk region (n = 9), 31.2 +/- 4.7% (n = 9), and 59.5 +/- 3.8% (n = 9), respectively; P < 0.05], similar to that induced by the late phase of ischemic PC [31.8 +/- 3.2% (n = 9)]. The selective COX-2 inhibitor N-(2-[cyclohexyloxy]4-nitrophenyl)methanesulfonamide (NS-398, 5 mg/kg), which abolished the protective effect of ischemic late PC, failed to block the protection of either CCPA or IB-MECA, indicating that COX-2 does not mediate the delayed protection of either CCPA or IB-MECA [CCPA + NS-398, 29.1 +/- 3.4% (n = 7); IB-MECA + NS-398, 34.9 +/- 2.9% (n = 8)]. NS-398 in itself did not affect infarct size [54.9 +/- 3.7% (n = 9)]. Taken together, these results demonstrate that, in contrast to ischemia-induced late PC, the mechanisms of adenosine A1 or A3 receptor-induced late PC is independent of COX-2.

Biphasic response of cardiac NO synthase isoforms to ischemic preconditioning in conscious rabbits

In conscious rabbits, a sequence of six 4-min coronary occlusion/4-min reperfusion cycles, which elicits late preconditioning (PC), caused rapid activation of calcium-dependent nitric oxide (NO) synthase (NOS) [cNOS; endothelial NOS (eNOS) and/or neuronal NOS (nNOS)], whereas calcium-independent NOS [inducible NOS (iNOS)] activity remained unchanged. The enhanced cNOS activity was associated with increased myocardial levels of NO(2) and/or NO(3) (NO(x)). Twenty-four hours after ischemic PC was induced, the opposite pattern was observed, i.e., there was a pronounced increase in cytosolic iNOS activity but no change in cNOS activity. The initial burst of ischemia-induced cNOS activity was not affected by pretreatment with the antioxidant N-2-mercaptopropionyl glycine (MPG), the protein kinase C (PKC) inhibitor chelerythrine, or the tyrosine kinase inhibitor lavendustin A, indicating that it is independent of the generation of oxidant species and the activation of PKC and tyrosine kinases. In contrast, the delayed upregulation of iNOS 24 h after PC was prevented by pretreatment with N(omega)-nitro-L-arginine, MPG, or chelerythrine before the PC ischemia, indicating that it is triggered by a signaling mechanism that involves the generation of NO, the formation of oxidant species, and the activation of PKC. Taken together, these results demonstrate that, in conscious animals, ischemic PC elicits a biphasic response in cardiac NOS activity, i. e., an immediate activation of cNOS (most likely eNOS) followed 24 h later by a delayed upregulation of iNOS. To our knowledge, this is the first study to directly measure NOS activity after brief myocardial ischemia in vivo. In conjunction with previous functional studies, the data support a distinctive role of NOS isoforms in late PC, with eNOS serving as the trigger on day 1 and iNOS as the mediator on day 2.

Nitroglycerin induces late preconditioning against myocardial stunning via a PKC-dependent pathway

Previous studies have shown that administration of nitric oxide (NO) donors induces a delayed cardioprotective effect indistinguishable from the late phase of ischemic preconditioning (PC). However, the ability of clinically relevant NO donors to elicit this phenomenon has not been evaluated. In this study we tested whether an NO-releasing agent that is nitroglycerin (NTG), which is widely used clinically, can mimic the late phase of ischemic PC. Four groups of conscious rabbits underwent six cycles of 4-min occlusion (O)/4-min reperfusion (R) for 3 consecutive days (days 1, 2, and 3). The severity of myocardial stunning was assessed as the total deficit of systolic wall thickening (WTh) after the last O/R cycle. In the control group (group I, n = 6), the total deficit of WTh was reduced by 50% and 51% on days 2 and 3 vs. day 1, respectively, indicating late PC against stunning. Pretreatment with NTG (2 microg. kg(-1). min(-1) iv over 1 h) on day 0 (group II, n = 6) was as effective as ischemic PC in mitigating myocardial stunning 24 h later (day 1); on days 2 and 3, no further reduction of stunning was seen. Coadministration of the PKC inhibitor chelerythrine (5 mg/kg) with NTG (group III, n = 6) completely abrogated the NTG-induced protection. Pretreatment with chelerythrine alone (group IV, n = 5) did not alter stunning. These results demonstrate that a relatively brief infusion of NTG induces a robust protective effect against stunning 24 h later via a protein kinase C (PKC)-dependent signaling mechanism. The magnitude of NTG-induced protection is equivalent to that observed during the late phase of ischemic PC. Late PC induced by brief treatment with NTG could be a useful therapeutic strategy for myocardial protection in patients with ischemic heart disease.

Demonstration of selective protein kinase C-dependent activation of Src and Lck tyrosine kinases during ischemic preconditioning in conscious rabbits

Src tyrosine kinases have been shown to mediate cellular responses to stress in noncardiac cells. However, the effect of myocardial ischemia on Src tyrosine kinases is unknown. Furthermore, the identity of the tyrosine kinase(s) involved in the genesis of ischemic preconditioning (PC) remains obscure. Here, we present the first evidence that ischemic PC (6 cycles of 4-minute coronary occlusion and 4-minute reperfusion) induces selective activation of 2 members of the Src family of tyrosine kinases, Src and Lck, in the heart of conscious rabbits. The activation of Src in the particulate fraction was not evident at 5 minutes after ischemic PC but became apparent at 30 minutes (+119% versus control), whereas the activation of Lck in the particulate fraction was apparent both at 5 minutes (+103% versus control) and at 30 minutes (+89%) after ischemic PC. The activity of the other 5 members of the Src tyrosine kinases expressed in the rabbit heart (Fyn, Fgr, Yes, Lyn, and Blk) was not affected by ischemic PC. Ischemic PC had no effect on the activity of epidermal growth factor receptor kinases, either at 5 or at 30 minutes. The activation of Src and Lck was completely abrogated by the tyrosine kinase inhibitor lavendustin A, given at doses that have previously been shown to block the protective effect of ischemic PC in this same conscious rabbit model, suggesting that Src and Lck kinases are essential for the development of ischemic PC. The activity of the epsilon isoform of protein kinase C (PKC) in the particulate fraction increased at 5 minutes (+72%) and at 30 minutes (+67%) after ischemic PC. Pretreatment with lavendustin A had no effect on the activation of PKCepsilon, whereas pretreatment with the PKC inhibitor chelerythrine (given at doses that have previously been shown to block ischemic PC) blocked not only the activation of PKCepsilon but also that of Src and Lck, indicating that Src and Lck are downstream of PKCepsilon in the signaling cascade of ischemic PC. This study identifies a new component of the signaling mechanism of ischemic PC. The results support the concept that, in conscious rabbits, 2 specific members of the Src family of tyrosine kinases, Src and Lck, play an important role in the genesis of late PC by serving as downstream elements of PKC-mediated signal transduction.

Increased protein synthesis is necessary for the development of late preconditioning against myocardial stunning

In phase I of this study, the rate of protein synthesis was measured by the incorporation of [(3)H]leucine into the protein pool in the heart of conscious rabbits. At 2 h after ischemic preconditioning (PC) with six 4-min occlusion/4-min reperfusion (O/R) cycles (group II), the [3H]leucine content in the ischemic-reperfused region was increased by 82% compared with that in controls (group I), indicating increased protein synthesis. This increase was completely abrogated by pretreatment with cycloheximide (CH; group III). In phase II, rabbits underwent six O/R cycles for three consecutive days (days 1-3). Controls (group IV) exhibited late PC against myocardial stunning on days 2 and 3. In group V, which received CH 30 min before the 1st O/R cycle on day 1 (same dose as group III), late PC against stunning on day 2 was completely abrogated. In group VI, pretreatment with CH 24 h before the 1st sequence of O/R cycles had no effect on myocardial stunning on day 1, indicating that the absence of late PC on day 2 in group V cannot be ascribed to delayed toxicity of CH. Taken together, these results demonstrate that, in the conscious rabbit, ischemic PC causes a rapid increase in myocardial protein synthesis and that this increased protein synthesis (or at least a fraction of it) is necessary for the development of the protection against myocardial stunning 24 h later. The late phase of ischemic PC is therefore dependent on the formation of new proteins in intact animals.

Nuclear factor-kappaB plays an essential role in the late phase of ischemic preconditioning in conscious rabbits

Although it is recognized that late preconditioning (PC) results from upregulation of cardioprotective genes, the specific transcription factor(s) that govern this genetic adaptation remains unknown. The aim of this study was to test the hypothesis that the development of late PC is mediated by nuclear factor-kappaB (NF-kappaB) and to elucidate the mechanisms that control the activation of NF-kappaB after an ischemic stimulus in vivo. A total of 152 chronically instrumented, conscious rabbits were used. A sequence of six 4-minute coronary occlusion/4-minute reperfusion cycles, which elicits late PC, induced rapid activation of NF-kappaB, as evidenced by a marked increase in p65 content (+164%; Western immunoblotting) and NF-kappaB DNA binding activity (+306%; electrophoretic mobility shift assay) in nuclear extracts isolated 30 minutes after the last reperfusion. These changes were attenuated 2 hours after ischemic PC and resolved by 4 hours. Competition and supershift assays confirmed the specificity of the NF-kappaB DNA complex signals. The mobility of the NF-kappaB DNA complex was shifted by anti-p65 and anti-p50 antibodies but not by anti-c-Rel antibodies, indicating that the subunits of NF-kappaB involved in gene activation after ischemic PC consist of p65-p50 heterodimers. Pretreatment with the NF-kappaB inhibitor diethyldithiocarbamate (DDTC; 150 mg/kg IP 15 minutes before ischemic PC) completely blocked the nuclear translocation and increased DNA binding activity of NF-kappaB. The same dose of DDTC completely blocked the cardioprotective effects of late PC against both myocardial stunning and myocardial infarction, indicating that NF-kappaB activation is essential for the development of this phenomenon in vivo. The ischemic PC-induced activation of NF-kappaB was also blocked by pretreatment with Nomega-nitro-L-arginine (L-NA), a nitric oxide synthase (NOS) inhibitor, N-2-mercaptopropionyl glycine (MPG), a reactive oxygen species (ROS) scavenger, chelerythrine, a protein kinase C (PKC) inhibitor, and lavendustin A, a tyrosine kinase inhibitor (all given at doses previously shown to block late PC), indicating that ischemic PC activates NF-kappaB via formation of NO and ROS and activation of PKC- and tyrosine kinase-dependent signaling pathways. A subcellular redistribution and increased DNA binding activity of NF-kappaB quantitatively similar to those induced by ischemic PC could be reproduced pharmacologically by giving the NO donor diethylenetriamine/NO (DETA/NO) (at a dose previously shown to elicit late PC), demonstrating that NO in itself can activate NF-kappaB in the heart. Taken together, these results provide direct evidence that activation of NF-kappaB is a critical step in the signal transduction pathway that underlies the development of the late phase of ischemic PC in conscious rabbits. The finding that four different pharmacological manipulations (L-NA, MPG, chelerythrine, and lavendustin A) produced similar inhibition of NF-kappaB suggests that this transcription factor is a common downstream pathway through which multiple signals elicited by ischemic stress (NO, ROS, PKC, tyrosine kinases) act to induce gene expression. To our knowledge, this is the first demonstration that NO can promote NF-kappaB activation in the heart, a finding that identifies a new biological function of NO and may have important implications for various pathophysiological conditions in which NO is involved and for nitrate therapy.

Isoform-selective activation of protein kinase C by nitric oxide in the heart of conscious rabbits: a signaling mechanism for both nitric oxide-induced and ischemia-induced preconditioning

Although isoform-selective translocation of protein kinase C (PKC) epsilon appears to play an important role in the late phase of ischemic preconditioning (PC), the mechanism(s) responsible for such translocation remains unclear. Furthermore, the signaling pathway that leads to the development of late PC after exogenous administration of NO in the absence of ischemia (NO donor-induced late PC) is unknown. In the present study we tested the hypothesis that NO activates PKC and that this is the mechanism for the development of both ischemia-induced and NO donor-induced late PC. A total of 95 chronically instrumented, conscious rabbits were used. In rabbits subjected to ischemic PC (six 4-minute occlusion/4-minute reperfusion cycles), administration of the NO synthase inhibitor Nomega-nitro-L-arginine (group III), at doses previously shown to block the development of late PC, completely blocked the ischemic PC-induced translocation of PKCepsilon but not of PKCeta, indicating that increased formation of NO is an essential mechanism whereby brief ischemia activates the epsilon isoform of PKC. Conversely, a translocation of PKCepsilon and -eta quantitatively similar to that induced by ischemic PC could be reproduced pharmacologically with the administration of 2 structurally unrelated NO donors, diethylenetriamine/NO (DETA/NO) and S-nitroso-N-acetylpenicillamine (SNAP), at doses previously shown to elicit a late PC effect. The particulate fraction of PKCepsilon increased from 35+/-2% of total in the control group (group I) to 60+/-1% after ischemic PC (group II) (P<0.05), to 54+/-2% after SNAP (group IV) (P<0.05) and to 52+/-2% after DETA/NO (group V) (P<0.05). The particulate fraction of PKCeta rose from 66+/-5% in the control group to 86+/-3% after ischemic PC (P<0.05), to 88+/-2% after SNAP (P<0.05) and to 85+/-1% after DETA/NO (P<0.05). Neither ischemic PC nor NO donors had any appreciable effect on the subcellular distribution of PKCalpha, -beta1, -beta2, -gamma, -delta, - micro, or -iota/lambda; on total PKC activity; or on the subcellular distribution of total PKC activity. Thus, the effects of SNAP and DETA/NO on PKC closely resembled those of ischemic PC. The DETA/NO-induced translocation of PKCepsilon (but not that of PKCeta) was completely prevented by the administration of the PKC inhibitor chelerythrine at a dose of 5 mg/kg (group VI) (particulate fraction of PKCepsilon, 38+/-4% of total, P<0.05 versus group V; particulate fraction of PKCeta, 79+/-2% of total). The same dose of chelerythrine completely prevented the DETA/NO-induced late PC effect against both myocardial stunning (groups VII through X) and myocardial infarction (groups XI through XV), indicating that NO donors induce late PC by activating PKC and that among the 10 isozymes of PKC expressed in the rabbit heart, the epsilon isotype is specifically involved in the development of this form of pharmacological PC. In all groups examined (groups I through VI), the changes in the subcellular distribution of PKCepsilon protein were associated with parallel changes in PKCepsilon isoform-selective activity, whereas total PKC activity was not significantly altered. Taken together, the results provide direct evidence that isoform-selective activation of PKCepsilon is a critical step in the signaling pathway whereby NO initiates the development of a late PC effect both after an ischemic stimulus (endogenous NO) and after treatment with NO-releasing agents (exogenous NO). To our knowledge, this is also the first report that NO can activate PKC in the heart. The finding that NO can promote isoform-specific activation of PKC identifies a new biological function of this radical and a new mechanism in the signaling cascade of ischemic PC and may also have important implications for other pathophysiological conditions in which NO is involved and for nitrate therapy.

Nitric oxide synthase is the mediator of late preconditioning against myocardial infarction in conscious rabbits

BACKGROUND

Despite intense investigation, the effector of the infarct-limiting protection observed during the late phase of ischemic preconditioning (PC) remains unknown. The goal of this study was to test the hypothesis that late PC against myocardial infarction is mediated by the activity of nitric oxide synthase (NOS).

METHODS AND RESULTS

Conscious rabbits underwent a 30-minute coronary occlusion followed by 3 days of reperfusion. In group I (control group, n= 10), infarct size (tetrazolium staining) averaged 56.8+/-5.3% of the risk region, which was decreased to 27.6+/-2.5% (P<0.05) in rabbits preconditioned 24 hours earlier with a sequence of six 4-minute occlusion/4-minute reperfusion cycles (group II, n= 10). When preconditioned rabbits were given the nonselective NOS inhibitor N(omega)-nitro-L-arginine (L-NA, 13 mg/kg i.v. [group III, n=8]) or the selective iNOS inhibitor aminoguanidine (AG, 150 mg/kg SC [group V, n=7]) before the 30-minute occlusion, the protective effect of late PC was completely abrogated; that is, infarct size (59.9+/-4.5% and 65.8+/-3.3%, respectively) was similar to that measured in the control group. Measurements of systolic wall thickening (sonomicrometry) demonstrated that L-NA and AG also abolished the improved recovery of myocardial function effected by late PC in group II. When rabbits were given L-NA or AG without prior PC (group IV [n=8] and group VI [n=6], respectively), infarct size did not differ from that observed in controls (53.8+/-4.3% and 59.8+/-4.3%, respectively), demonstrating that L-NA and AG do not increase the extent of cell death in nonpreconditioned myocardium.

CONCLUSIONS

Taken together, these results indicate that in the conscious rabbit, the infarct-sparing effect of the late phase of ischemic PC is mediated by the activity of NOS and suggest that the specific isoform primarily responsible for this cardioprotective phenomenon is iNOS. Thus, NO appears to be a pivotal component of the pathophysiological cascade of late PC.

Nitric oxide donors induce late preconditioning against myocardial stunning and infarction in conscious rabbits via an antioxidant-sensitive mechanism

The goal of this study was to test the hypothesis that the cardioprotective effects of the late phase of ischemic preconditioning (PC) can be mimicked by treatment with NO donors. In phase I (studies of myocardial stunning), conscious rabbits underwent a sequence of six 4-minute coronary occlusion/4-minute reperfusion cycles for 3 consecutive days (days 1, 2, and 3). In group I (controls, n=6), the total deficit of systolic wall thickening (WTh) after the sixth reperfusion was reduced by 54% on days 2 and 3 compared with day 1 (P<0.05), indicating a late PC effect against myocardial stunning. When rabbits were given the NO donors diethylenetriamine/NO (DETA/NO, 0.1 mg/kg i.v., 4 times [group II, n=5]) or S-nitroso-N-acetylpenicillamine (SNAP, 2.5 microg x kg(-1) x min(-1) i.v. for 75 minutes [group III, n=51) 24 hours before the first sequence of occlusion/reperfusion cycles, the deficit of WTh on day 1 was 60% (group II) and 54% (group III) less than that observed in controls (P<0.05 for both). In both groups II and III, there was no further improvement in the deficit of WTh on days 2 and 3 compared with day 1. The protective effect of DETA/NO was completely abrogated when this agent was given in conjunction with the ONOO- and .OH scavenger mercaptopropionyl glycine (MPG) (group IV, n=5). In phase II (studies of myocardial infarction), conscious rabbits underwent a 30-minute coronary occlusion followed by 3 days of reperfusion. When rabbits were preconditioned 24 hours earlier with six 4-minute occlusion/4-minute reperfusion cycles, infarct size was reduced by 43% (33.2+/-2.7% versus 58.3+/-4.1% of the region at risk in controls, P<0.05), indicating a late PC effect against myocardial infarction. When rabbits were pretreated with DETA/NO (group VII, n=8) or SNAP (group IX, n=7) 24 hours before the 30-minute occlusion, infarct size was reduced by a similar degree (29.3+/-3.6% and 32.0+/-3.3% of the region at risk, respectively; P<0.05 versus controls). The degree of protection could not be increased by doubling the dose of DETA/NO (group VIII, n=5). Coadministration of MPG completely abrogated the infarct-sparing action of DETA/NO (group X, n=7). Taken together, these results demonstrate that in conscious rabbits the administration of 2 structurally unrelated NO donors induces protection 24 hours later against both reversible (stunning) and irreversible (infarction) ischemia/reperfusion injury and that the magnitude of this protection is indistinguishable from that observed during the late phase of ischemic PC. The fact that the late phase of ischemic PC can be mimicked by NO donors provides direct evidence that NO in itself is sufficient to elicit this cardioprotective mechanism. The fact that NO donor-induced late PC was abrogated by MPG indicates that the mechanism whereby NO induces this phenomenon involves the generation of oxidant species, possibly ONOO- and/or .OH. Since a relatively brief treatment with hemodynamically inactive doses of NO donors can induce long-lasting protective effects, these agents could be useful for preconditioning the heart in patients.

Direct evidence that protein kinase C plays an essential role in the development of late preconditioning against myocardial stunning in conscious rabbits and that epsilon is the isoform involved

Brief ischemic episodes confer marked protection against myocardial stunning 1-3 d later (late preconditioning [PC] against stunning). The mechanism of this powerful protective effect is poorly understood. Although protein kinase C (PKC) has been implicated in PC against infarction, it is unknown whether it triggers late PC against stunning. In addition, the entire PKC hypothesis of ischemic PC remains controversial, possibly because the effects of PKC inhibitors on PC protection have not been correlated with their effects on PKC activity and/or translocation in vivo. Thus, conscious rabbits underwent a sequence of six 4-min coronary occlusion (O)/4-min reperfusion (R) cycles for three consecutive days (days 1, 2, and 3). In the control group (group I, n = 7), the recovery of systolic wall thickening after the six O/R cycles was markedly improved on days 2 and 3 compared with day 1, indicating the development of late PC against stunning. Administration of the PKC inhibitor chelerythrine at a dose of 5 mg/kg before the first O on day 1 (group II, n = 10) abrogated the late PC effect against stunning, whereas a 10-fold lower dose (0.5 mg/kg; group III, n = 7) did not. Administration of 5 mg/kg of chelerythrine 10 min after the sixth reperfusion on day 1 (group IV, n = 6) failed to block late PC against stunning. When rabbits were given 5 mg/kg of chelerythrine in the absence of O/R (group V, n = 5), the severity of myocardial stunning 24 h later was not modified. Pretreatment with phorbol 12-myristate 13-acetate (4 microg/kg) on day 1 without ischemia (group VI, n = 11) induced late PC against stunning on day 2 and the magnitude of this effect was equivalent to that observed after ischemic PC. In vehicle-treated rabbits (group VIII, n = 5), the six O/R cycles caused translocation of PKC isoforms epsilon and eta from the cytosolic to the particulate fraction without significant changes in total PKC activity, in the subcellular distribution of total PKC activity, or in the subcellular distribution of the alpha, beta1, beta2, gamma, delta, zeta, iota, lambda, and mu isoforms. The higher dose of chelerythrine (5 mg/kg; group X, n = 5) prevented the translocation of both PKC epsilon and eta induced by ischemic PC, whereas the lower dose (0.5 mg/kg; group XI, n = 5) prevented the translocation of PKC eta but not that of epsilon, indicating that the activation of epsilon is necessary for late PC to occur whereas that of eta is not. To our knowledge, this is the first demonstration that a PKC inhibitor actually prevents the translocation of PKC induced by ischemic PC in vivo, and that this inhibition of PKC translocation results in loss of PC protection. Taken together, the results demonstrate that the mechanism of late PC against myocardial stunning in conscious rabbits involves a PKC-mediated signaling pathway, and implicate epsilon as the specific PKC isoform responsible for the development of this cardioprotective phenomenon.

The protective effect of late preconditioning against myocardial stunning in conscious rabbits is mediated by nitric oxide synthase. Evidence that nitric oxide acts both as a trigger and as a mediator of the late phase of ischemic preconditioning

Seventy-four conscious rabbits undergoing a sequence of six 4-minute coronary occlusion/4-minute reperfusion cycles for 3 consecutive days (days 1, 2, and 3) were assigned to nine groups. In group I (controls, n = 8), the recovery of systolic wall thickening (WTh) after the sixth reperfusion was markedly improved on days 2 and 3 compared with day 1, indicating late preconditioning (PC) against myocardial stunning; the total deficit of WTh after the sixth reperfusion was reduced by 56% on day 2 and 50% on day 3 compared with day 1 (P < .01). Administration on day 2 of the nonselective NO synthase (NOS) inhibitor N omega-nitro-L-arginine (L-NA) (group II, n = 8) or of the selective inducible NOS inhibitors aminoguanidine (AG) (group IV, n = 8) and S-methylisothiourea sulfate (SMT) (group VI, n = 6) completely abrogated late PC against stunning on day 2. On day 3, the expected PC effect became manifest in all groups. Administration of L-NA, AG, or SMT on day 1 (groups III [n = 7], V [n = 6], and VII [n = 5], respectively) had no discernible effect on the deficit of WTh on day 1, indicating that these agents do not augment the severity of myocardial stunning in nonpreconditioned myocardium. In group VIII (n = 7), the abrogation of late PC by SMT on day 2 was completely reversed by the concomitant administration of L-arginine (595 mg/kg IV), indicating that it was not due to nonspecific NOS-unrelated actions. Administration of L-arginine alone on day 2 (group IX [n = 5]) had no effect on the deficit of WTh. Furthermore, administration of L-NA on day 1 (group III) prevented the appearance of the PC effect on day 2, whereas AG (group V) and SMT (group VI) did not, suggesting that the development of late PC on day 1 is triggered by the endothelial (type III) isoform of NOS. This study demonstrates that three structurally different NOS inhibitors (L-NA, AG, and SMT), given 24 hours after the PC ischemia, consistently abrogate late PC against myocardial stunning in conscious rabbits, indicating that this cardioprotective effect is mediated by the activity of NOS. The results obtained with AG and SMT specifically implicate the inducible (type II) isoform as the mediator of the protection on day 2. Previous studies have shown that NO triggers the development of late PC. The present results indicate that NO plays a dual role in late PC against stunning, acting initially as the trigger and subsequently as the mediator of the protection.