Delayed protection of the heart against ischaemia

Role of Nitric Oxide in Cardiovascular Adaptation to Intermittent Hypoxia

Hypoxia is one of the most frequently encountered stresses in health and disease. The duration, frequency, and severity of hypoxic episodes are critical factors determining whether hypoxia is beneficial or harmful. Adaptation to intermittent hypoxia has been demonstrated to confer cardiovascular protection against more severe and sustained hypoxia, and, moreover, to protect against other stresses, including ischemia. Thus, the direct and cross protective effects of adaptation to intermittent hypoxia have been used for treatment and prevention of a variety of diseases and to increase efficiency of exercise training. Evidence is mounting that nitric oxide (NO) plays a central role in these adaptive mechanisms. NO-dependent protective mechanisms activated by intermittent hypoxia include stimulation of NO synthesis as well as restriction of NO overproduction. In addition, alternative, nonenzymic sources of NO and negative feedback of NO synthesis are important factors in optimizing NO concentrations. The adaptive enhancement of NO synthesis and/or availability activates or increases expression of other protective factors, including heat shock proteins, antioxidants and prostaglandins, making the protection more robust and sustained. Understanding the role of NO in mechanisms of adaptation to hypoxia will support development of therapies to prevent and treat hypoxic or ischemic damage to organs and cells and to increase adaptive capabilities of the organism.

Regional variations of vasomotion to G-protein coupled receptor agonists following heat stress in rats

Objectives

This study was designed to compare vascular contractile and relaxing responses to G-protein coupled receptor agonists among the different regions of arteries following heat stress in rats.

Methods

Heat exposure was performed by increasing the internal temperature of the rats to 42°C for 15 min. After heat stress for 48 h, a myograph system was used to monitor the contractile responses in rat renal, femoral and mesenteric arteries to agonists of endothelin type B (ETB) receptor, endothelin type A (ETA) receptor, serotonin receptor and α-adrenoceptor, respectively. In addition, calcitonin gene-related peptide (CGRP)-induced vasodilation was studied.

Key findings

The results showed that heat stress induced decreased contractions mediated by α-adrenoceptors and serotonin receptors (at lower concentration), while it increased contraction mediated by endothelin ETB receptors and enhanced relaxation mediated by CGRP receptors in the renal artery. Heat stress increased contractions mediated by endothelin ETB receptors, endothelin ETA receptors and α-adrenoceptors in the femoral artery. In the mesenteric artery, heat stress increased contractions mediated by endothelin ETB and serotonin receptors and relaxation mediated by CGRP receptors.

Conclusions

The vasomotor responses to the G-protein coupled receptor agonists with altered vascular contractions and relaxations were different in rat renal, femoral and mesenteric arteries after heat stress. This might have contributed to the redistribution of blood flow and aids understanding of the preconditioning phenomenon.

Glibenclamide attenuates the antiarrhythmic effect of endotoxin with a mechanism not involving K(ATP) channels

The role of K(ATP) channels in the antiarrhythmic effect of Escherichia coli endotoxin-induced nitric oxide synthase (iNOS) was examined in an anesthetised rat model of myocardial ischemia and reperfusion arrhythmia by using glibenclamide (1 mg kg(-1)), nateglinide (10 mg kg(-1)) and repaglinide (0.5 mg kg(-1)). Endotoxin (1 mg kg(-1)) was administered intraperitoneally 4 h before the occlusion of the left coronary artery and glibenclamide, nateglinide or repaglinide was administered 30 min before coronary artery occlusion. We also evaluated the effects of K(ATP) channel blockers and nonselective K(+) channel blocker tetraethylammonium (TEA) on cardiac action potential configuration in the atria obtained from endotoxemic rats. The mean arterial blood pressure of rats receiving endotoxin was lower during both the occlusion and reperfusion periods. Endotoxin significantly reduced the total number of ectopic beats and the duration of ventricular tachycardia. Glibenclamide, but not nateglinide and repaglinide, prevented the hypotension and antiarrhythmic effects of endotoxin. Atria obtained from endotoxin-treated rats had prolonged action potential duration. This effect was abolished with pretreatment of iNOS inhibitors, l-canavanine and dexamethasone and perfusion of glibenclamide, but not with TEA and non-sulfonylurea drug, nateglinide. We demonstrated that glibenclamide inhibits the antiarrhythmic effect of endotoxin and this effect does not appear to involve K(ATP) channels.

Drug-induced delayed cardiac protection against the effects of myocardial ischemia

Drug-induced delayed cardiac protection (DCP) against the effects of acute myocardial ischemia was first described 22 years ago by the author and his coworkers. It can be initiated by noninjurious pharmacological doses of prostacyclin (PGI2), its stable analogues, and by catecholamines. DCP protects against many consequences of ischemia, attenuating early morphological changes, limiting infarct size and suppressing arrhythmias, and can also protect against ouabain intoxication. DCP operates under a variety of pathological conditions (atherosclerosis, hypercholesterolaemia, and diabetes). DCP can also be evoked by transient myocardial ischemia and by exercise and is known in this context as "ischemic preconditioning", specifically the "second window of protection"; transient ischemia also evokes an immediate but short-lived protection known as "classical preconditioning". DCP is fundamentally different in concept to conventional drug therapy because the process appears to depend on the duration of the trigger and be related in a bell-shaped manner to the strength of the trigger. The exact mechanism is uncertain. Prolongation of the effective refractory period (ERP) and of the action potential duration (APD) may contribute to DCP suppression of arrhythmias. The protection is time and dose dependent, with optimal effects 24 to 48 hr after treatment. It can be sustained by intermittent administration of low maintenance doses. Stimulation of the adenylate-cyclase/cyclic adenosine monophosphate (cAMP) system appears to be a common feature of DCP. Responses to beta-adrenergic stimuli are also diminished. Cardiac cAMP triggers the induction of phosphodiesterase (PDE) 1 and 4 isoforms and of Na/K-ATPase. Increased amount and activity of PDE isoforms subsequently reduces excess myocardial cAMP production. Changes in Na/K-ATPase moderate ischemic myocardial potassium loss, sodium, and calcium accumulation, as well as the toxicity of ouabain. The future therapeutic challenge is to identify new drugs that can mimic DCP.

Effect of estrogen replacement treatment on ischemic preconditioning in isolated rat hearts

In the present study, we tested the effects of long-term estrogen replacement treatment on myocardial ischemia-reperfusion injury and on the cardioprotection of ischemic preconditioning in isolated hearts from ovariectomized rats. Ovariectomized rats were treated with 17β-estradiol (30 µg/kg/d, s.c.) for 12 weeks. Isolated rat hearts were perfused in the Langendorff mode. Heart rate, coronary flow, left ventricular pressure and its first derivative (±LVdp/dtmax) were recorded. Fifteen-min global ischemia and 30-min reperfusion caused a significant decrease of cardiac mechanical function, which were not affected by ovariectomy or estrogen replacement treatment. The isolated hearts in all groups could be preconditioned, and the cardioprotection afforded by preconditioning in the sham-operated rats was greater compared with ovariectomized rats with or without estrogen treatment. These results suggest that long-term estrogen replacement treatment exerts no effect on the inhibition of mechanical function after ischemia-reperfusion, and this study also suggests that estrogen does not affect ischemic preconditioning in isolated hearts of ovariectomized rats.Key words: ERT (estrogen replacement treatment), ischemia-reperfusion, ischemic preconditioning, heart, rat.

Hibernation, Stunning, and Preconditioning: Historical Perspective, Current Concepts, Clinical Applications, and Future Implications

Despite considerable advances, coronary artery disease is the leading cause of morbidity and mortality in the Western world. The development of effective therapeutic strategies for protecting the myocardium from ischemia would have major impact on patients with coronary artery disease. It is now accepted that patients with coronary artery disease can experience prolonged regional ischemic dysfunction that does not necessarily arise from irreversible tissue damage, and to some extent, can be reversed by restoration of blood flow. The initial stages of dysfunction are probably caused by chronic stunning that can be reversed after revascularization, resulting in rapid and complete functional recovery. On the other hand, the more advanced stages of dysfunction likely correspond to chronic hibernation. After revascularization, functional recovery will probably be quite delayed and mostly incomplete. Over the past decade, the possibility that an innate mechanism of myocardial protection might be inducible in the human heart has generated considerable excitement. In the last two decades, there was phenomenal growth in the understanding of the mechanism known as ischemic preconditioning that is responsible for the innate myocardial protection. Continued research and progress in this area may soon lead to the availability of preconditioning-mimetic treatments. The current concepts, mechanisms, and potential clinical applications of myocardial hibernation, stunning, and ischemic preconditioning are reviewed.

A histological analysis of the protective effect of ischemic preconditioning in the rat retina

PURPOSE

Ischemic preconditioning (IP) protects the retina from the damaging effect of subsequent ischemia in vivo. We aimed to investigate the histological alterations induced by the protective effect of IP to the retina.

METHODS

The eyes of the rats were rendered ischemic by intra-ocular pressure (IOP) elevation. IP procedure consisted of producing ischemia for 5 minutes. Sham operation was similar to IP procedure except the pressure elevation. The operational eyes of sham and IP group underwent 60 minutes of ischemia 24 hours after the first procedure. The eyes contralateral to the experimental eyes made up the control group. The eyes were histologically analysed one week after the ischemia.

RESULTS

The total retinal thickness of the sham group was significantly less than total retinal thickness of the control group (p < 0.001). There was not a significant difference between control and IP group regarding the total retinal thickness (p > 0.05). The thickness of the inner retinal layers of the sham group were significantly less than corresponding retinal layers of the control group (p < 0.001). The inner plexiform layer (IPL) and inner nuclear layer (INL) thickness values of the sham group were significantly less than same layers of the IP group (p < 0.001). Ganglion cell layer (GCL) thickness of the IP group was significantly less than GCL thickness of the control group (p < 0.001). IPL thickness of the IP group was significantly less than that of control group's (p < 0.05). The GCL and total retinal thickness of the IP group were significantly more than thickness of the corresponding layers of the sham group (p < 0.05).

CONCLUSION

IP considerably protects inner retinal layers from subsequent ischemic damage in a high IOP ischemic model. This endogenous process could further be utilized to tailor specific neuroprotective strategies for retinal cells.

Delayed cardioprotection by intestinal preconditioning is mediated by calcitonin gene-related peptide

Previous studies have shown that nitric oxide and calcitonin gene-related peptide (CGRP) are involved in mediation of the delayed cardioprotection of ischemic or pharmacological preconditioning, and nitric oxide can evoke the release of CGRP. In the present study, we examined the role of CGRP in nitric oxide-mediated delayed cardioprotection by brief intestinal ischemia in rats. The serum concentration of creatine kinase and infarct size were measured after 45-min coronary artery occlusion and 180-min reperfusion. Ischemic preconditioning was induced by six cycles of 4-min ischemia and 4-min reperfusion of the small intestine. Pretreatment with intestinal ischemic preconditioning for 24, 48, or 72 h significantly reduced infarct size and creatine kinase release, and the effects of ischemic preconditioning were completely abolished by L-nitroarginine methyl ester (L-NAME, 10 mg/kg, i.p.), an inhibitor of nitric oxide synthase, or by pretreatment with capsaicin (50 mg/kg, s.c.), which selectively depletes transmitters in capsaicin-sensitive sensory nerves. Intestinal preconditioning caused a significant increase in plasma concentrations of CGRP, and the effect was also abolished by L-NAME or capsaicin. These results suggest that the delayed cardioprotection afforded by intestinal ischemic preconditioning is mediated by endogenous CGRP via the nitric oxide pathway.

Role of nitric oxide synthases in the infarct size-reducing effect conferred by heat stress in isolated rat hearts

Nitric oxide (NO) donors are known to induce both delayed cardioprotection and myocardial heat stress protein (HSP) expression. Moreover, heat stress (HS), which also protects myocardium against ischaemic damages, is associated with a NO release. Therefore, we have investigated the implication of NO in HS-induced resistance to myocardial infarction, in the isolated rat heart model. Rats were divided in six groups (n=10 in each group), subjected or not to heat stress (42 degrees C internal temperature, 15 min) and treated or not with nitro-L-arginine-methylester (L-NAME) a non-selective inhibitor of NO synthase isoforms, or L-N(6)-(1-imino-ethyl)lysine (L-NIL), a selective inhibitor of the inducible NO synthase. Twenty-four hours after heat stress, their hearts were isolated, retrogradely perfused, and subjected to a 30-min occlusion of the left coronary artery followed by 120 min of reperfusion. Infarct-to-risk ratio was significantly reduced in HS (18.7+/-1.6%) compared to Sham (33.0+/-1.7%) hearts. This effect was abolished in L-NAME-treated (41.7+/-3.1% in HS+L-NAME vs 35.2+/-3.0% in Sham+L-NAME ) and L-NIL-treated (36.1+/-3.4% in HS+L-NIL vs 42.1+/-4.6% in Sham+L-NIL) groups. Immunohistochemical analysis of myocardial HSP 27 and 72 showed an HS-induced increase of these proteins, which was not modified by L-NAME pretreatment. We conclude that NO synthases, and in particular the inducible isoform, appear to play a role in the heat stress-induced cardioprotection, independently of HSP 27 and 72 levels. Further investigations are required to elucidate the precise role of HSPs in this adaptive response.

Ischemic preconditioning: lack of delayed protection against skeletal muscle ischemia-reperfusion

We investigated the ability of ischemic preconditioning to induce expression of heat shock protein 70 (Hsp 70) and/or to increase muscle survival after ischemia-reperfusion in the rat hind limb. Ischemic preconditioning regimens tested were; 1 x 5 min of ischemia, 4 x 5 min of ischemia interrupted by 10 min of reperfusion, 1 x 10 min of ischemia or 2 x 10 min of ischemia interrupted by 15 min of reperfusion. Western blot analysis revealed only a modest induction of Hsp 70 at 24 h after preconditioning using the latter two protocols of 1 x 10 min of ischemia or 2 x 10 min. Used at 24 h prior to prolonged ischemia, neither protocol improved muscle survival measured at 24 h after reperfusion. In conclusion, ischemic preconditioning did not produce delayed protection from ischemia-reperfusion in this model and the study suggests that ischemic preconditioning is not a useful protective strategy against skeletal muscle necrosis in the long-term.

Possible role of nitric oxide and mast cells in endotoxin-induced cardioprotection

The present study is designed to investigate the role of nitric oxide (NO) and cardiac mast cells in the cardioprotective effect of endotoxin in isolated rat heart subjected to 30 min of global ischaemia and 30 min of reperfusion. Endotoxin (2.5 mg kg(-1); i.p.) was administered 8 h before subjecting the heart to global ischaemia. Endotoxin pretreatment markedly reduced the release of lactate dehydrogenase (LDH) and creatine kinase (CK), markers of cardiac injury, in coronary effluent and the percentage incidence of ventricular premature beats (VPBs) and ventricular tachycardia/fibrillation (VT/VF) during the reperfusion phase. Endotoxin pretreatment significantly increased the release of nitrite prior to and after global ischaemia. On the other hand, endotoxin pretreatment decreased the release of mast cell peroxidase (MPO) during the reperfusion phase. The cardioprotective and antiarrhythmic effect of endotoxin pretreatment was abolished by dexamethasone (3 mg kg(-1); i.p.) or l -canavanine (20 mg kg(-1); i.p.) given 1 h before the administration of endotoxin. It is proposed that the cardioprotective and antiarrhythmic effect of the endotoxin may be ascribed to the induction of nitric oxide synthase (NOS) and subsequent increase in the release of NO. NO may stabilize cardiac mast cells and consequently decrease the release of cytotoxic mediators from these cells. Prevention of degranulation of cardiac mast cells may be responsible for the cardioprotective and antiarrhythmic effects of the endotoxin.

Müller cells in the preconditioned retinal ischemic injury rat

The role of Müller cells in the preconditioned retinal ischemic injury rat was investigated. In anesthetized Sprague Dawley rats, retinal ischemia for 5 minutes constituted the preconditioning stimulus for the left eye. After 24 hours, both eyes were clamped for 60 minutes. In 30, 60, 90, and 120, minutes and 1 day, 3 days, and 7 days after ischemia, electroretinograms were recorded, and the eyeballs were enucleated. After fixation with 4% paraformaldehyde, the avidin-biotin-peroxidase technique was applied to show glutamine synthetase (GS) and glial fibrillary acidic protein (GFAP). Furthermore, for the solubilized retinas, Western blot analysis and enzyme-linked immunosorbent assay were performed to detect GS and GFAP in the extracts. Preconditioning performed 24 hours before ischemia significantly improved the recovery of the a-, and b-waves 1 day after 60 minute ischemia. In the 30, 60, 90, and 120 minutes after ischemia, the recovery of the a-wave only was observed. There was a nonsignificant trend toward greater recovery in the first 120 minutes after 60 minute ischemia, especially in the b-wave. GS immunoreactivity had no significant difference between non-preconditioned and preconditioned groups 30, 60, 90, and 120 minutes after ischemia. In 1 day after ischemia, GS immunoreactivity decreased in both groups. In 3 and 7 days after ischemia, GS immunoreactivity recovered only in the preconditioned group. The retinas at 3 and 7 days after 1 hour of ischemia showed increased GFAP immunoreactivity in the non-preconditioned group. In the preconditioned group, only slight GFAP immunoreactivity was observed. These results suggested that the mechanism of preconditioned retinal ischemia may be related to Müller cells in the retina.

Increase in endogenous brain superoxide dismutase as a potential mechanism of lipopolysaccharide-induced brain ischemic tolerance

A low dose (0.5 mg/kg) of lipopolysaccharide (LPS), administered 72 hours before 60-minute middle cerebral artery occlusion, induced a delayed neuroprotection proven by the significant decrease (-35%) of brain infarct volume in comparison with control, whereas infarct volumes remained unchanged in rats treated 12, 24, or 168 hours before ischemia. This delayed neuroprotective effect of LPS was induced only with low doses (0.25 to 1 mg/kg), whereas this effect disappeared with a higher dose (2 mg/kg). The delayed neuroprotection of LPS was induced in the cortical part of the infarcted zone, not in the subcortical part. The beneficial effect of LPS on consequences of middle cerebral artery occlusion was suppressed by dexamethasone (3 mg/kg) and indomethacin (3 mg/ kg) administered 1 hour before LPS, whereas both drugs had no direct effect on infarct volume by themselves, suggesting that activation of inflammatory pathway is involved in the development of LPS-induced brain ischemic tolerance. Preadministration of cycloheximide, an inhibitor of protein synthesis, also blocked LPS-induced brain ischemic tolerance suggesting that a protein synthesis is also necessary as a mediating mechanism. Superoxide dismutase (SOD) could be one of the synthesized proteins because lipopolysaccharide increased SOD brain activity 72 hours, but not 12 hours, after its administration, which paralleled the development of brain ischemic tolerance. In contrast, catalase brain activity remained unchanged after LPS administration. The LPS-induced delayed increase in SOD brain content was suppressed by a previous administration of indomethacin. These data suggest that the delayed neuroprotective effect of low doses of LPS is mediated by an increased synthesis of brain SOD that could be triggered by activation of inflammatory pathway.

Signal transduction mechanisms involved in ischemic preconditioning in the rat retina in vivo

Ischemic preconditioning (IPC) protects the rat retina against the injury that ordinarily follows severe ischemia. We showed previously that release of adenosine and de novo protein synthesis were required for IPC protection. The mechanisms of IPC were studied in the rat retina by examining the signal transduction mediators responsible, in particular, those theorized to be downstream of adenosine receptors. In addition, we examined the hypothesis that nitric oxide and hydroxyl radicals were involved in the IPC protective phenomenon. Retinal ischemia was produced for 60 min in ketamine/xylazine-anesthetized Sprague-Dawley rats, and recovery was measured using electroretinography. We tested the effects on the protective effect of IPC resulting from antagonism of protein kinase C, potassium ATP channels, nitric oxide synthase, or hydroxyl radicals. The effects of the inhibition of de novo protein synthesis or of protein kinase C, and blockade of potassium ATP channels on the mimicking of IPC by adenosine receptor agonists was examined.IPC protection was strongly attenuated by inhibition of protein kinase C and by blockade of potassium ATP channels, but unaffected by the inhibition of hydroxyl radicals. Blockade of nitric oxide synthase produced a trend toward enhancement of IPC protection. Mimicking of IPC protection by adenosine receptor agonists was inhibited by blockade of protein synthesis or of protein kinase C, as well as by potassium ATP channel antagonism. These results demonstrate that protein kinase C and potassium ATP channels are mediators of the protective effect produced by IPC. In addition, the results show that stimulation of adenosine receptor subtypes A1 and A2a is responsible for IPC protection via downstream stimulation of protein kinase C, the opening of potassium ATP channels, and de novo protein synthesis.

Delayed protection against ventricular arrhythmias by monophosphoryl lipid-A in a canine model of ischaemia and reperfusion

Bacterial endotoxin reduces the severity of ventricular arrhythmias which occur when a coronary artery is occluded several hours later. We have now examined in anaesthetised dogs the effects on ischaemia and reperfusion-induced arrhythmias, of a non-toxic derivative component of the endotoxin molecule of the lipid-A (monophosphoryl lipid-A). This was given intravenously, in doses of 10 and 100 microg kg(-1), 24 h prior to coronary artery occlusion. Arrhythmia severity was markedly reduced by monophosphoryl lipid-A. During ischaemia, ventricular premature beats were reduced from 315+/-84 in the vehicle controls to 89+/-60 (with the lower dose of monophosphoryl lipid-A) and 53+/-23 (P<0.05) with the higher dose. The incidence of ventricular tachycardia was reduced from 75% to 25% (P<0.05) and 31% (P<0.05), and the number of episodes of ventricular tachycardia from 13.4+/-4.9 per dog to 1.1+/-1.1 (P<0.05) and 1. 2+/-0.9 (P<0.05) after doses of 10 and 100 microg kg(-1), respectively. The incidence of ventricular fibrillation during occlusion and reperfusion in the control group was 96% (15/16), i.e., only 6% (1/16) dogs survived the combined ischaemia-reperfusion insult. Monophosphoryl lipid-A (100 microg kg(-1)) significantly reduced the incidence of occlusion-induced ventricular fibrillation (from 50% to 7%; P<0.05), and increased survival following reperfusion to 54% (P<0.05). Monophosphoryl lipid-A also significantly reduced ischaemia severity as assessed from ST-segment elevation recorded from epicardial electrodes as well as the degree of inhomogeneity of electrical activation within the ischaemia area. There were no haemodynamic differences prior to coronary occlusion between vehicle controls and monophosphoryl lipid-A-treated dogs. These results demonstrate that monophosphoryl lipid-A reduces arrhythmia severity 24 h after administration. Although the precise mechanisms are still unclear, there is some evidence that nitric oxide and prostanoids (most likely prostacyclin) may be involved because the dual inhibition of nitric oxide synthase and cyclooxygenase enzymes by administration of aminoguanidine and meclofenamate abolished the marked antiarrhythmic protection resulted from monophosphoryl lipid-A treatment 24 h previously.

Effect of pre-exposure to vasoconstrictors on isoprenaline-induced relaxation in rat aorta: involvement of inducible nitric oxide synthase

1. The aim of this study was to determine whether a brief (30 min) episode of contractile receptor stimulation could affect the degree of a subsequent vasorelaxation. Therefore, concentration - relaxation curves of the rat aorta to isoprenaline were compared before and after exposure of the tissue to noradrenaline (100 microM) or prostaglandin F2alpha (PGF2alpha, 100 microM). 2. Exposure to noradrenaline enhanced the second maximal relaxant effect of isoprenaline (from 20 - 95% relaxation). This effect was not due to significant differences in precontraction levels and was not modified by the presence of the endothelium. Treatment with PGF2alpha mimicked the actions of noradrenaline on subsequent vasorelaxation to isoprenaline. 3. Before exposure to noradrenaline (100 microM), forskolin (10 microM) did not produce any significant relaxation of the rat aorta. After exposure to noradrenaline, forskolin caused a concentration-dependent relaxation with a maximal effect of more than 90% in rings with and without endothelium suggesting that the change in vasorelaxation to isoprenaline occurred downstream from the beta-adrenoceptor. 4. The increase in relaxation due to exposure to noradrenaline was markedly attenuated by treatment with a protein synthase inhibitor (cycloheximide), a nitric oxide (NO) synthase inhibitor (L-NG-nitroarginine methyl ester, L-NAME) and an inhibitor of the activation of soluble guanylyl cyclase (methylene blue). 5. Western blot analysis showed an increase of inducible NO synthase (iNOS) in aortic rings exposed to noradrenaline or PGF2alpha. 6. Together, these findings suggest that pretreatment of rat aorta with noradrenaline or PGF2alpha could induce vascular NOS which would in turn result in an increase in isoprenaline-induced vasorelaxation, this increase occurring downstream from receptor activation. Such a mechanism might participate in cardioprotection during preconditioning induced by noradrenaline.

Low dose of lipopolysaccharide induces a delayed enhanced nitric oxide-mediated relaxation in rat aorta

Delayed effect on vascular reactivity of isolated aorta was studied after injection of a single low dose of lipopolysaccharide (0.5 mg/kg i.p.). The maximal vascular effect was observed 72 h after lipopolysaccharide administration with an increase in maximal endothelium-dependent relaxing response to acetylcholine and parallelly a decrease in contractile response to phenylephrine. The change in contractile response was nullified by endothelium removal as well by in vitro aortic rings incubation with N(omega)-monomethyl-L-arginine but not with indomethacin. A low dose of lipopolysaccharide induces a delayed enhanced nitric oxide-mediated vascular relaxation which could contribute to its delayed anti-ischemic properties in ischemic tolerance phenomenon.

Heat stress response and myocardial protection

Prior whole-body hyperthermia is able to protect the myocardium against ischaemia-reperfusion injury by reducing cellular necrosis, preserving the ventricular function and preventing the occurrence of arrhythmias. These cardioprotective effects are associated with reduction of oxidative stress, preservation of the high-energy phosphate levels and synthesis of heat stress proteins. A better understanding of this powerful protective adaptation of the myocytes would be of interest for potential clinical application, and rational design of specific agents that activate this mechanism will hopefully follow soon.

Infarct size-reducing effect of heat stress and alpha1 adrenoceptors in rats

1. Noradrenaline (NA), which is abundantly released during heat stress (HS), is known to induce both delayed cardioprotection and heat stress protein (HSP) 72 expression by the mediation of alpha, adrenoceptors. Therefore, we have investigated the implication of alpha1 adrenoceptors in HS-induced resistance to myocardial infarction, in the isolated rat heart model. 2. Rats were pretreated with prazosin (1 mg kg(-1), i.p., Praz) or 5-methylurapidil (3 mg kg(-1), i.v., 5MU) or chloroethylclonidine (3 mg kg(-1), i.v., CEC) or vehicle (V) in order to selectively antagonize alpha1, alpha1A and alpha1B adrenoceptors. They were then either heat stressed (42 degrees C for 15 min) or sham anaesthetized. Twenty-four hours later. their hearts were isolated, retrogradely perfused, and subjected to a 30 min occlusion of the left coronary artery followed by 120 min of reperfusion. 3. Infarct-to-risk ratio was significantly reduced in HS+V (15.4+/-1.8%) compared to Sham+V (35.7+/-1.3%) hearts. This effect was abolished in Praz-treated (29.1+/-1.6% in HS+Praz vs 34.1+/-4.0% in Sham+Praz), 5MU-treated (34.5+/-2.2% in HS+5MU vs 31.2+/-2.0% in Sham+5MU) and CEC-treated (33.4+/-3.0% in HS+CEC vs 32.4+/-1.3% in Sham+CEC) groups. Western blot analysis of myocardial HSP72 showed an HS-induced increase of this protein, which was not modified by Praz, 5MU and CEC pretreatments. 4. We conclude that both alpha1A and alpha1B adrenoceptor subtypes appear to play a role in the heat stress-induced cardioprotection, independently of the HSP72 level. Further investigations are required to elucidate the precise role of HSPs in this adaptative response.

Effect of a novel tetrapeptide derivative in a model of isoproterenol induced myocardial necrosis

Isoproterenol hydrochloride (ISO), a beta adrenergic agonist, is known to cause ischemic necrosis in rats. Cardiotoxicity of three different doses of ISO were studied using physiological, biochemical and histopathological parameters. The effects of single and double dose of ISO were analysed, which illustrated that single ISO dose was more cardiotoxic than double ISO dose due to ischemic preconditioning. The tetrapeptide derivatives L-lysine-L-arginine-L-aspartic acid-L-serine (tetrapeptide A) and di-tert.butyloxycarbonyl-L-lysine-L-arginine-L-aspartic acid-tert.butyl O-tert.butyl-L-serinate (tetrapeptide B) along with acetylsalicylic acid as positive control were analysed at different time points for their cardioprotective effect. The results demonstrated that optimal protective effects were observed by pretreatment with 5 mg/kg of tetrapeptide B and this was found to be slightly better than that of acetylsalicylic acid. A lesser degree of cardioprotection was noticed when low doses of tetrapeptide B were administered. This study clearly showed that single dose of ISO (50 mg/kg, s.c.) induced myocardial necrosis could be used as a model to assess cardiovascular drugs and in this model, it was demonstrated that the tetrapeptide B could exhibit optimal cardioprotective effect.

Monophosphoryl lipid A reduces both arrhythmia severity and infarct size in a rat model of ischaemia

A non-toxic derivative of the active lipid A component of the endotoxin molecule (monophosphoryl lipid A) when given to rats in a dose of 5 mg kg(-1) by intraperitoneal injection 24 h prior to anaesthesia and coronary artery occlusion, markedly decreased the severity of ischaemia-reduced ventricular arrhythmias (ventricular fibrillation reduced from 60 to 21%; P < 0.05) and reduced myocardial infarct size (from 35.8 +/- 1.6% of the area at risk to 22.7 +/- 2.0%; P < 0.05). It did not modify blood pressure or heart rate either before or during the period of ischaemia.

Resistance to myocardial infarction induced by heat stress and the effect of ATP-sensitive potassium channel blockade in the rat isolated heart

1. Heat stress (HS) is known to protect against myocardial ischaemia-reperfusion injury by improving mechanical dysfunction and decreasing necrosis. However, the mechanisms responsible for this form of cardioprotection remain to be elucidated. ATP-sensitive potassium (K(ATP)) channels have been shown to be involved in the delayed phase of protection following ischaemic preconditioning, a phenomenon closely resembling the HS-induced cardioprotection. The aim of this study was thus to investigate the role of K(ATP) channels in HS-induced protection of the isolated rat heart. 2. Twenty four hours after whole body heat stress (at 42 degrees C for 15 min) or sham anaesthesia, isolated perfused hearts were subjected to a 15 min stabilization period followed by a 15 min infusion of either 10 microM glibenclamide (Glib), 100 microM sodium 5-hydroxydecanoate (5HD) or vehicle (0.04% DMSO). Regional ischaemia (35 min) and reperfusion (120 min) were then performed. 3. Prior heat stress significantly reduced infarct-to-risk ratio (from 42.4+/-2.4% to 19.4+/-2.9, P<0.001). This resistance to myocardial infarction was abolished in both Glib-treated (40.1+/-1.8% vs 42.3+/-1.8%) and 5HD-treated (41.2+/-1.8% vs 41.8+/-1.2%) groups. 4. The results of this study suggest that K(ATP) channel activation contributes to the cytoprotective response induced by heat stress.

17 beta-Estradiol reduces vasoconstriction in endothelium-denuded rat aortas through inducible NOS

Estrogen produces vasodilatation through the induction of nitric oxide synthase (NOS) in the endothelium, but there are many reports of endothelium-independent effects. In the present study, these processes were investigated in rat aortas isolated from ovariectomized rats. Long-term in vitro treatment with 17 beta-estradiol (10 nM for 24 h) in an organ culture system slightly reduced acetylcholine-mediated vasorelaxation in endothelium-intact aortic rings. 17 beta-Estradiol (1 and 10 nM for 24 h) also attenuated the phenylephrine-induced constriction in endothelium-denuded aortas, and this effect was inhibited by the NOS inhibitor L-N5-(1-iminoethyl)ornithine hydrochloride, as well as the estrogen receptor antagonist ICI-182,780. Furthermore, 17 beta-estradiol treatment (1 and 10 nM for 24 h) increased nitric oxide production as assessed by the conversion of [3H]arginine to [3H]citrulline in endothelium-denuded rat aortas. These effects were prevented by the protein synthesis inhibitor cycloheximide. 17 beta-Estradiol (10 nM for 24 h) treatment also induced the formation of inducible NOS (iNOS) protein in aortas. The results indicate that 17 beta-estradiol can attenuate the vasoconstrictor effect of phenylephrine by a process that involves induction of iNOS in nonendothelial cells of the aorta. We suggest that long-term estrogen therapy may induce a partial hyporesponsiveness in vascular smooth muscle via a small but sustained nitric oxide production.

Genistein, an inhibitor of tyrosine kinase, prevents the antiarrhythmic effects of preconditioning

The possible involvement of tyrosine kinase in the signal transduction processes associated with the antiarrhythmic effects of ischaemic preconditioning was assessed by the administration, prior to the preconditioning stimulus, of the tyrosine kinase inhibitor genistein (5 mg/kg i.v.) to pentobarbitone anaesthetised male rats. The hearts were then removed, perfused by the Langendorff technique and preconditioned by a single brief (3 min) coronary artery occlusion prior to the 30 min test ischaemic insult. Preconditioning reduced ventricular arrhythmias during occlusion (e.g., premature ventricular beats from 333 +/- 60 in controls to 52 +/- 5 in preconditioned hearts; P < 0.05) but this protective effect was not observed in the hearts of rats given genistein (272 +/- 37 premature beats). A similar abolition of the protective effects of preconditioning could be demonstrated in hearts perfused in vitro with genistein (100 microM). This suggests the involvement of tyrosine kinase activation in this powerful form of endogenous cardioprotection.

Bradykinin and endothelial-cardiac myocyte interactions in ischemic preconditioning

Myocardial ischemia results in the release of a variety of vasoactive substances from coronary vascular endothelial cells and/or from cardiac myocytes. Some of these substances appear to be protective and include nitric oxide and bradykinin. One hypothesis for the pronounced antiarrhythmic effects of preconditioning involves the early generation of bradykinin and, subsequently, nitric oxide. Evidence for early bradykinin release has come from clinical studies involving patients undergoing coronary angioplasty where, in 4 of 5 patients, there was evidence for elevated kinin levels in coronary sinus blood either during balloon inflation (i.e., ischemia) or deflation (reperfusion). The levels reached are sometimes considerable (increases 10-20 fold). The second piece of evidence comes from dogs subjected to a preconditioning stimulus (2 x 5 min periods of ischemia), followed 20 min later by occlusion of the same artery for a 25-min period. This preconditioning procedure markedly reduces ischemia-induced ventricular arrhythmias and, although under resting conditions there was little difference between arterial and coronary sinus bradykinin levels (125 +/- 22 and 157 +/- 41 pg/mL, respectively), there was a marked increase in coronary sinus levels in preconditioned dogs before the prolonged occlusion (637 +/- 293 pg/mL compared with 114 +/- 18 pg/mL in nonpreconditioned dogs); levels at the end of the prolonged occlusion in the preconditioned dogs were also higher (577 +/- 305 pg/mL compared with 162 +/- 34 pg/mL in control dogs). Other evidence for the involvement of bradykinin and nitric oxide comes from studies in which the generation, or effects, of these mediators have been suppressed (e.g., with the bradykinin B2 receptor blocking agent icatibant, with inhibitors of the L-arginine-nitric oxide pathway, and by methylene blue). The conclusion is that early bradykinin release is protective under conditions of ischemia, is presumably enhanced during therapy with angiotensin-converting enzyme (ACE) inhibitors and is suppressed under conditions of endothelial dysfunction.

Ischaemic preconditioning of myocardium

Myocardium has the innate potential to adapt to transient sublethal ischaemia so that it becomes more resistant to a subsequent, more severe, ischaemic insult. The response is called ischaemic preconditioning and protection of the myocardium is manifested by a slowing of adenosine triphosphate decline, limitation of ischaemic necrosis and a reduction in dysrhythmia severity. Protection conferred by preconditioning occurs in two distinct temporal phases. An early phase of protection is observed immediately but wanes within two to three hours (classic preconditioning). This is followed many hours later by a second window of protection (delayed preconditioning). The cellular mechanisms underpinning both forms of protection are currently being intensively investigated. There is evidence that human myocardium can be preconditioned ex vivo and also in situ during elective procedures such as angioplasty and coronary artery bypass grafting. Furthermore, evidence points to the possibility that preconditioning occurs naturally in some ischaemic syndromes, particularly warm-up angina and preinfarction angina. Ultimately, investigation of the mechanisms of preconditioning may contribute to the development of rational therapies for protecting the ischaemic myocardium and, perhaps more importantly, enhance our understanding of the molecular basis of ischaemic heart disease.

L-canavanine and dexamethasone attenuate endotoxin-induced suppression of ischaemia-reperfusion arrhythmias

The role of inducible nitric oxide synthase in the antiarrhythmic effects of Escherichia coli endotoxin was examined in an anaesthetised rat model of myocardial ischaemia (7 min occlusion) and reperfusion (7 min) arrhythmias by using its specific blocker L-canavanine (100 mg/kg) and dexamethasone (5 mg/kg), which inhibits its expression. Endotoxin (1 mg/kg) or its solvent saline was administered intraperitoneally 4 h before the occlusion of the left coronary artery and L-canavanine or dexamethasone was administered 1 h before endotoxin or saline injection. The mean arterial blood pressure of rats receiving endotoxin was significantly lower than that of saline-treated controls, and neither L-canavanine nor dexamethasone prevented the hypotension exerted by endotoxin. However, during both the occlusion and reperfusion periods, endotoxin significantly reduced the total number of ectopic beats (e.g., during reperfusion, saline: 1177 +/- 183, n = 11; endotoxin: 248 +/- 91, n = 9; P < 0.005) and the duration of ventricular tachycardia (e.g., during occlusion, saline: 30.9 +/- 5.7 s; endotoxin: 1.8 +/- 0.9 s; P < 0.0001) while L-canavanine or dexamethasone treatment abolished the reduction exerted by endotoxin. Therefore we conclude that endotoxin possesses significant antiarrhythmic (protectant) effects in this rat model of ischaemia-reperfusion arrhythmias, and that its mechanism appears to involve the inducible nitric oxide synthase since both L-canavanine and dexamethasone inhibited this phenomenon.

Phosphodiesterase (PDE)4 inhibitors: anti-inflammatory drugs of the future?

Phosphodiesterase type 4 (PDE4) plays a major role in modulating the activity of virtually all cells involved in the inflammatory process. Inhibitors of this enzyme family display impressive anti-inflammatory and disease-modifying effects in a variety of experimental models. In this review, Mauro Teixeira, Robert Gristwood, Nicola Cooper and Paul Hellewell examine the capacity of PDE4 inhibitors to exert anti-inflammatory actions in vivo and discuss the potential of this class of drugs to take their place as novel therapeutic agents for a variety of inflammatory diseases.

Increased cardiac workload by adrenoceptor agonists for the estimation of potential antiischemic activity in a conscious rabbit model

The antiischemic effect of drugs can be detected at a lower dose range if the cardiac workload is increased. A brief period of frequency-loading (ventricular overpacing = VOP) results in well-defined, reproducible changes in cardiac parameters in the conscious, chronically instrumented rabbit; however, rapid pacing frequently evoked ventricular tachycardia or even fatal ventricular fibrillation. Therefore, cardiac workload has been increased by i.v. administration of adrenoceptor agonists, such as isoproterenol (ISO), phenylephrine (PHE), and their combination, respectively. The doses applied (especially the combination of 2 micrograms/kg ISO and 16 micrograms/kg PHE, giving optimal changes) were sufficient to produce a marked elevation of both the ST segment in the intracavital electrogram and the left ventricular end-diastolic pressure, without evoking cardiac arrhythmias. We compared the effect of this adrenergic "test" stimulus with that of VOP on hemodynamic and electrophysiological parameters of the heart, and furthermore, on the modification of responses to both "test" stimuli by oral administration of the coronary vasodilator: Isosorbide-5-mononitrate (IS-5-N), given in a dose of 40 mg/kg. Both VOP- and ISO+PHE-induced changes were significantly attenuated by IS-5-N, and a temporal coincidence of the maximal effects was found as well. We reached the following conclusion: The combined administration of ISO and PHE not evoking fatal arrhythmias in the dose range applied can replace the more risky VOP as a "test" workload in the estimation of antiischemic action.

The case of low-dose aspirin for the prevention of myocardial infarction: but how low is low?

Although there is firm consensus that aspirin reduces the incidences (and severity) of reinfarction if given to patients after a myocardial infarction or in patients with unstable angina, there is disagreement about the optimum dose that should be used. We make the case that it could be considerably lower than the "medium dose" (75-320 mg daily) in current usage and put forward the view that a dose of 30 mg daily is sufficient. The arguments are based on the presystemic, exclusively pharmacokinetic inhibition of thromboxane synthesis in platelets by very low-dose aspirin and the importance of maintaining prostacyclin production by endothelial cells. Little attention has been in the past to the endogenous myocardial protection that results from prostacyclin generation. Such low doses have the additional advantage of not inducing gastrointestinal ulceration. We conclude that more extensive clinical trials with such "low-dose" aspirin preparations are warranted.

Delayed protection of the ischemic heart--from pathophysiology to therapeutic applications

Preconditioning the heart with brief episodes of ischemia paradoxically increases its resistance to subsequent ischemic episodes, and markedly limits infarct size. Although preconditioning is now considered as the most powerful antiischemic intervention known, its beneficial effects are short-lived since they are lost if the reperfusion period after preconditioning is extended past 2-3 h. There is, however, some evidence of a delayed phase of protection, manifest 24 h after the initial preconditioning stimulus, associated with a decrease in infarct size, a prevention of postischemic contractile dysfunction (stunning) and a reduction in endothelial injury. The delayed beneficial effects of preconditioning resemble those induced by prior heat stress, and might be related to the expression of stress proteins (heat shock proteins or HSP). Evidence for a role of HSP derives from observations showing that brief ischemia is a potent stimulus for HSP expression. Moreover, transfection of isolated cells with HSP or overexpression of HSP in transgenic mice renders the myocytes more resistant to ischemia. Once produced, HSP are believed to facilitate protein synthesis, stabilize newly formed proteins and repair denatured ones. Alternatively, delayed preconditioning may be mediated by antioxidant enzymes such as superoxide dismutase or catalase, which are also upregulated by ischemia and this could lead to a lesser production of oxygen-derived free radicals during reperfusion. Indeed, in isolated myocytes, prevention of hypoxia-induced expression of superoxide dismutase (using an antisense oligonucleotide) abolished the delayed protective effect of preconditioning. Importantly, recent in vivo evidence suggests that the delayed protection may be mediated by adenosine, through activation of A1-receptors, and by stimulation of protein kinase C. Finally, although the exact mechanisms by which preconditioning induces delayed protection are still mostly unknown, the fact that the expression of protective proteins such as HSP can be induced by many other means than ischemia suggests that it is possible to pharmacologically stimulate this expression and thus possibly mimic the endogenous protective pathway. This could lead to the development of new pharmacological interventions which induce delayed myocardial protection in clinical situations such as angioplasty, coronary bypass surgery or even in patients at high risk of infarction.

Protein kinase C-induced changes in the stoichiometry of ATP binding activate cardiac ATP-sensitive K+ channels. A possible mechanistic link to ischemic preconditioning

Activation of both ATP-sensitive K+ (KATP) channels and the enzyme protein kinase C (PKC) has been associated with the cardioprotective response of ischemic preconditioning. We recently showed that at low cytoplasmic ATP (< or = 50 mumol/L), PKC inhibits KATP channel activity. This finding is surprising, as both KATP channels and PKC are activated during preconditioning. However, PKC also altered ATP binding to the channel, changing the Hill coefficient from approximately 2 to approximately 1. This apparent change in stoichiometry would lead to a PKC-induced activation of KATP channels at more physiological (millimolar) levels of ATP. The aim of the present study was to determine whether PKC activates cardiac KATP channels at millimolar levels of ATP. The effects of PKC on single KATP channels were studied at millimolar internal ATP levels using excised inside-out membrane patches from rabbit ventricular myocytes. Application of purified constitutively active PKC (20 nmol/L) to the intracellular surface of the patches produced an approximately threefold increase in the channel open probability. The specific PKC inhibitor peptide PKC(19-31) prevented this increase. Heat-inactivated PKC had no effect on KATP channel properties. KATP channel activity spontaneously returned to control levels after washout of PKC. This spontaneous reversal did not occur in the presence of 5 nmol/L okadaic acid, suggesting that the reversal of PKC's action is dependent on activity of a membrane-associated type 2A protein phosphatase (PP2A). In the presence of exogenous PP2A (7.5 nmol/L), PKC had no effect. We conclude that the PKC-induced increase in KATP channel activity at millimolar ATP results from a crossing of the ATP concentration-response curves for inhibition of the phosphorylated and nonphosphorylated forms of the channel. This identifies a mechanism by which PKC activates KATP channels at near physiological levels of ATP and thus could link these two components in a signaling pathway that induces ischemic preconditioning.

Delayed protection against ischaemia-induced ventricular arrhythmias and infarct size limitation by the prior administration of Escherichia coli endotoxin

1. Bacterial endotoxin (lipopolysaccharide derived from Escherichia coli) was injected intraperitoneally in conscious rats in doses ranging from 0.5 to 2.5 mg kg-1. At various times afterwards the animals were anaesthetized and subjected to a 30 min period of left coronary artery occlusion. 2. Under these conditions the severity of ventricular arrhythmias was markedly suppressed, in comparison with saline-injected controls, but this was particularly marked with the higher doses (1.5 and 2.5 mg kg-1); the number of ventricular premature beats was reduced from 1687 +/- 227 over the 0.5 h coronary artery occlusion period to 190 +/- 46 in those rats administered 2.5 mg kg-1 endotoxin 8 h previously (P < 0.05). The duration of ventricular tachycardia was also significantly reduced (138 +/- 26 s to 8.9 +/- 4.2 s; P < 0.01) and there was a reduction in the incidence of ventricular fibrillation (from 56% to 10%). 3. The time course of this protection was studied following the administration of a single dose of 2.5 mg kg-1 of endotoxin by anaesthetizing rats 4, 8 or 24 h later. Protection was apparent at each time but was particularly marked at 8 h. 4. No rat given the highest dose of endotoxin (32 in all) died as a result of ventricular fibrillation, or from any other cause, during an occlusion, in contrast to a 26% mortality in the controls (P < 0.01). 5. Infarct size, measured following a 30 min period of coronary artery occlusion followed by a 3 h reperfusion period, was reduced both 8 and 24 h after the administration of 2.5 mg kg-1 endotoxin (reductions of 24.3 and 23.1% respectively; P < 0.05). Endotoxin had no significant effect on the area at risk. 6. The beneficial effects of endotoxin on infarct size and on ventricular arrhythmias were markedly attenuated by the prior administration of dexamethasone, 3 mg kg-1 given 1 h prior to endotoxin administration. Dexamethasone itself reduced infarct size (P < 0.05) but had no direct effect on arrhythmia severity following coronary artery occlusion. 7. The mechanisms of this "cross-tolerance' induced by bacterial endotoxin against ischaemia-reperfusion injury remain to be elucidated but the most likely mechanisms appear to be the induction of protective enzymes or proteins (e.g. nitric oxide synthase, cyclo-oxygenase (COX) 2) probably mediated by cytokine release.