Endotoxin Induces a Second Window of Protection in the Rat Heart as Determined by Using p -Nitro-Blue Tetrazolium Staining, Cardiac Troponin T Release, and Histology

Abstract

Abstract-Pretreatment of rats with small doses of lipopolysaccharide (LPS), eg, for 24 hours, attenuates the cardiac dysfunction caused by subsequent period of myocardial ischemia. This phenomenon of enhanced tolerance to an ischemic insult has been termed "second window of protection." Although the cardioprotective effects of LPS were first reported in 1989, it is still unclear whether the observed attenuation by LPS of the ischemia-induced cardiac dysfunction is indeed secondary to the protection of cardiac myocytes against ischemic cell injury and death. This study was designed to investigate the effects of "preconditioning" with LPS on cell injury caused by regional myocardial ischemia and reperfusion in the anesthetized rat. Thirty-five Wistar rats were subjected to 25 minutes occlusion of the left anterior descending coronary artery followed by 2 hours of reperfusion. Hemodynamic parameters were continuously recorded, and at the end of the experiments, infarct size (using p-nitro-blue tetrazolium staining), cardiac troponin T release, and histological markers of cell injury and death were determined. In rats pretreated with a bolus of saline (vehicle for LPS) 2 or 24 hours before left anterior descending coronary artery occlusion and reperfusion, the infarct size was 59Ϯ4% (2 hours saline-control, nϭ6) and 61Ϯ3% (24 hours saline-control, nϭ6), respectively. Pretreatment of animals with a bolus of LPS (1 mg/kg IP) 24 hours before the onset of myocardial ischemia and reperfusion reduced both infarct size (to 18Ϯ7%; PϽ0.05, nϭ6) as well as histological signs of cell injury. Pretreatment (24 hours, as above) of rats with LPS also reduced the release of cardiac troponin T from 58Ϯ13 ng/mL (saline-control) to 16Ϯ9 ng/mL. In contrast, pretreatment of rats with LPS (2 hours, as above) did not affect infarct size (56Ϯ8%, nϭ6), cardiac troponin T release, or the histological parameters of cell injury. These data provide the first conclusive evidence that pretreatment of rats with a bolus of LPS 24 hours before intervention reduces the cell injury and death caused by a subsequent period of myocardial ischemia and reperfusion. (Arterioscler Thromb Vasc Biol. 1999;19:2276-2280.)Key Words: LPS Ⅲ myocardial infarct size Ⅲ myocardial ischemia Ⅲ reperfusion Ⅲ delayed preconditioning T he second window of protection (SWOP) was originally described as enhanced tolerance to myocardial ischemia after exposure of the rabbit 1 or canine 2 heart to brief periods of myocardial ischemia (ischemic preconditioning, IPC). This SWOP appears to occur at 24 hours after IPC and is maintained for 72 hours. The SWOP against myocardial infarction has been observed in many species, eg, in the rabbit, 3 dog, 2 pig, 4 and rat. 5 Several triggers are known to induce a SWOP, eg, brief repetitive cycles of coronary occlusion, 1 ventricular rapid pacing, 6 stimulation of adenosine A 1 receptors, 3 and administration of lipopolysaccharide (LPS) 7 or monophosphoryl lipid A (MLA). 8 Previously, the methods used to investigate SWOP involved the measure...

Toll-like receptor signaling: a critical modulator of cell survival and ischemic injury in the heart

“…For example, hearts isolated from rats pretreated with a low dose of LPS (0.5 mg/kg) 24 h before had a better preserved myocardial function after I/R compared with the saline-treated control hearts (18,105). Similar cardiac protection in LPS-treated animals was observed in vivo and in different animal models of I/R injury, such as rabbit (13,99), rat (18,88,106,138,155,166,167), and mice (48). The cardioprotective effect of LPS usually occurs between 12-24 h after the administration of LPS and is abolished by cycloheximide (106), suggesting a mechanism involving the de novo synthesis of cardioprotective proteins.…”

“…In animal models of I/R injury (13,18,48,88,89,99,106,138,155,166,167), in both in vivo (13,48,99,138,155,166,167) and ex vivo (18,88,106), a prior systemic administration of a sublethal dose of LPS reduces subsequent MI and improved cardiac functions. For example, hearts isolated from rats pretreated with a low dose of LPS (0.5 mg/kg) 24 h before had a better preserved myocardial function after I/R compared with the saline-treated control hearts (18,105).…”

“…In animal models of ischemic cardiac injury, the role of TLRs is incompletely defined. For example, the systemic administration of a sublethal dose of LPS, which signals through TLR4, reduced the subsequent ischemic MI and improved cardiac functions both in vivo and in isolated hearts (13,18,89,99,106,138,155,166,167). The activation of TLR4-myeloid differentiation primary-response gene 88 (MyD88) signaling also protects cardiomyocytes against apoptosis (26,170).…”

Clinical Implications of Circulating Microbiome in Cardiovascular Disease Patients

“…Subclinical endotoxemia has been reported to result in a three-fold increased risk of incident atherosclerosis and CVD [22,99]. Circulating endotoxins also influence cardiac troponin T levels [100]. Circulating LPS and sCD14 were associated with hypertension in HIV-infected individuals [101].…”

Cardioprotection via adaptation to hypoxia: expanding the timeline and targets?

“…Although this observation was, at the time, considered remarkable and viewed with some skepticism, the phenomenon of cardioprotection via adaptation to stress has been corroborated in a host of studies and expanded from the initial paradigm of ischemic preconditioning to encompass multiple triggers including remote ischemia or 'preconditioning at a distance' [18,27], mild hypothermia and hyperthermia [12,23,25,29], endotoxin and endotoxin analogs [19,30,31], endoplasmic reticulum stress [17] and many others. There are, however, two important caveats to the reduction of infarct size achieved with ischemic preconditioning and other stressors: the protective triggers are considered to delay (rather than preclude) cardiomyocyte death, and efficacy is achieved when the triggers are applied as a pretreatment (before ischemia) or, in some instances, during the initial secondsminutes of reperfusion.…”