Therapeutic potential of complement inhibitors in myocardial ischaemia.

Under normal conditions, the complement system functions to eradicate microbes and other membrane bound pathogens. In other situations, complement activation comprises a pivotal mechanism for mediating tissue demolition in inflammatory disorders, including ischaemia/reperfusion injury. Complement-mediated tissue damage has long been recognised as a significant contributor to myocardial reperfusion injury. However, clinical use of complement inhibitors to reduce the extent of irreversible tissue injury related to reperfusion, remains in the early stages of development. Activation of the complement system generates anaphylatoxins, opsonins and the lytic moiety known as the membrane attack complex (MAC). In addition, fragments of the complement cascade proteins (e.g., C3a and C5a) secondarily initiate processes deleterious to myocytes by recruiting and stimulating inflammatory cells, such as neutrophils and macrophages, within the area of reperfusion. Damaged tissue itself, is capable of upregulating the genes that encode the formation of complement proteins leading to assembly of the MAC, which in turn further advances tissue injury. All of these factors contribute to the development of myocardial infarction subsequent to ischaemia and reperfusion. This paper provides an overview of how the complement system operates and examines the various inhibitors, both endogenous and exogenous, that regulate the complement cascade. Activation and inhibition of the complement system will be discussed primarily in the context of myocardial ischaemia and reperfusion injury.

Sublytic complement attack reduces infarct size in rabbit isolated hearts: evidence for C5a-mediated cardioprotection.

Sublytic complement attack can elicit protective cellular responses without precipitating cell death. Our investigation examined the effects of non-lethal complement activation in isolated hearts. New Zealand white rabbit hearts were subjected to 30 min of ischemia followed by 1 h of reperfusion. Prior to ischemia, hearts were perfused for 20 min with 0.5% normal human plasma (NHP). Hearts treated with NHP developed significantly (p<0.05) smaller infarcts compared with controls, expressed as percent of area at risk (AAR) (25.3+/-4.0% vs. 40.9+/-4.3%, respectively). Heat-inactivation, soluble complement receptor 1 (sCR1; 20 nM), and anti-C5a antibody reversed the protective effect of NHP (39.0+/-3.1%, 41.7+/-5.1% and 38.4+/-2.3% AAR, respectively). Hearts treated with 3 nM C5a exhibited infarct sizes similar to those exposed to NHP (27.6+/-5.0% AAR). sCR1 alone did not affect infarct size (37.9+/-4.5% AAR). The results suggest that non-lethal complement activation attenuates reperfusion injury through formation of C5a.

Preconditioning reduces myocardial complement gene expression in vivo.

This investigation examined the effect of preconditioning in an in vivo model of ischemia-reperfusion injury. Anesthetized New Zealand White rabbits underwent 30 min of regional myocardial ischemia followed by 2 h of reperfusion. Hearts preconditioned with two cycles of 5 min ischemia-10 min reperfusion (IPC) or with the ATP-sensitive K (K(ATP)) channel opener, diazoxide (10 mg/kg), exhibited significantly (P < 0.05) smaller infarcts compared with control. These treatments also significantly (P < 0.001 to P < 0.05) reduced C1q, C1r, C3, C8, and C9 mRNA in the areas at risk (AAR). The K(ATP) channel blocker 5-hydroxydecanoate (5-HD; 10 mg/kg) attenuated infarct size reduction elicited by IPC and diazoxide treatment. 5-HD partially reversed the decrease in complement expression caused by IPC but not diazoxide. There were no significant differences in complement gene expression in the nonrisk regions and livers of all groups. Western blot analysis revealed that IPC also reduced membrane attack complex expression in the AAR. The data demonstrate that preconditioning significantly decreases reperfusion-induced myocardial complement expression in vivo.

Free radicals upregulate complement expression in rabbit isolated heart.

Both free radicals and complement activation can injure tissue. Our study determined whether free radicals alter complement production by the myocardium. Isolated hearts from New Zealand White rabbits were perfused on a Langendorff apparatus and exposed to xanthine (X; 100 microM) plus xanthine oxidase (XO; 8 mU/ml) (X/XO). The free radical-generating system significantly (P < 0.05) increased C1q and also increased C1r, C3, C8, and C9 transcription compared with controls. Immunohistological examination revealed augmented membrane attack complex deposition on X/XO-treated tissue. X/XO-treated hearts also exhibited significant (P < 0.05) increases in coronary perfusion pressure and left ventricular end-diastolic pressure and a decrease in left-ventricular developed pressure. N-(2-mercaptopropionyl)-glycine (3 mM), in conjunction with the superoxide dismutase mimetic SC-52608 (100 microM), significantly (P < 0.05) reduced the upregulation of C1q, C1r, C3, C8, and C9 mRNA expression elicited by X/XO. The antioxidants also ameliorated the deterioration in function caused by X/XO. Local complement activation may represent a mechanism by which free radicals mediate tissue injury.

Acute cocaine exposure up-regulates complement expression in rabbit heart.

The exact mechanism of the cardiotoxic actions of cocaine remains unclear. The finding that the heart may be a source of injurious complement components led us to investigate whether cocaine promotes myocardial expression of complement. Rabbit isolated hearts were perfused for 70 min with either cocaine hydrochloride (1 or 10 microM), the synthetic isomer (+)-cocaine (10 microM), or procaine hydrochloride (10 or 30 microM). Compared with controls perfused with drug-free buffer, both cocaine and procaine significantly (P <. 05) increased myocardial C1q, C1r, C8, and C9 mRNA expression, whereas 10 microM (+)-cocaine had no effect on complement mRNA expression. Cocaine also significantly increased (P <.05) C3 mRNA transcription. In addition, in vivo administration of cocaine (1 mg/kg) for three consecutive days significantly increased myocardial complement mRNA expression. Cocaine and procaine also increased membrane attack complex (MAC) formation in the myocardium. The antioxidant 2-N-mercaptopropionyl glycine, attenuated the increases in complement mRNA expression induced by 1 microM cocaine and 10 microM procaine. In vivo heparin administration (300 U/kg i.v.), 2 h before removal of the heart and exposure to 1 microM cocaine, did not inhibit C1q, C1r, C3, and C8 mRNA transcription, but decreased MAC expression. It was determined previously that heparin reduces myocardial ischemia/reperfusion injury. Our results suggest that cocaine may cause myocardial injury by up-regulating local complement expression, possibly via the production of reactive oxygen species. Furthermore, the glycosaminoglycan heparin may modulate the cytotoxic effects of cocaine by impeding formation of the MAC.

Preconditioning reduces tissue complement gene expression in the rabbit isolated heart.

Both preconditioning and inhibition of complement activation have been shown to ameliorate myocardial ischemia-reperfusion injury. The recent demonstration that myocardial tissue expresses complement components led us to investigate whether preconditioning affects complement expression in the isolated heart. Hearts from New Zealand White rabbits were exposed to either two rounds of 5 min global ischemia followed by 10 min reperfusion (ischemic preconditioning) or 10 microM of the ATP-dependent K+ (KATP) channel opener pinacidil for 30 min (chemical preconditioning) before induction of 30 min global ischemia followed by 60 min of reperfusion. Both ischemic and chemical preconditioning significantly (P < 0.05) reduced myocardial C1q, C1r, C3, C8, and C9 mRNA levels. Western blot and immunohistochemistry demonstrated a similar reduction in C3 and membrane attack complex protein expression. The K(ATP) channel blocker glyburide (10 microM) reversed the depression of C1q, C1r, C3, C8, and C9 mRNA expression observed in the pinacidil-treated hearts. The results suggest that reduction of local tissue complement production may be one means by which preconditioning protects the ischemic myocardium.

Ex vivo reversal of heparin-mediated cardioprotection by heparinase after ischemia and reperfusion.

Glycosaminoglycans, including heparin, have been demonstrated both in vitro and in vivo to protect the ischemic myocardium against reperfusion injury. In the present study, we sought to determine whether the cardioprotective effects of heparin administration could be reversed by the heparin-degrading enzyme heparinase. New Zealand white rabbits were pretreated with heparin (300 U/kg i.v.) or vehicle (saline). Two hours after treatment, hearts were removed, perfused on a Langendorff apparatus, and subjected to 25 min of global ischemia, followed by 45 min of reperfusion. Hemodynamic variables were obtained before ischemia (baseline) and every 10 min throughout the reperfusion period. Compared with vehicle-treated rabbits, the left ventricular end-diastolic and left ventricular developed pressures were improved significantly (p <.05) in the heparin-treated group. Ex vivo administration of heparinase (5 U/ml) immediately before the onset of global ischemia was associated with a reversal of the heparin-mediated cardioprotection. The uptake of a radiolabeled antibody to the intracellular protein myosin and creatine kinase release were used to determine membrane integrity and discriminate between viable and nonviable myocardial tissue. The uptake of radiolabeled antimyosin antibody and release of creatine kinase after reperfusion were increased in heparin-pretreated hearts exposed to heparinase, indicating a loss of membrane integrity and increased myocyte injury. These results demonstrate that neutralization of heparin by heparinase promotes increased myocardial injury after reperfusion of the ischemic myocardium.

Reduction of myocardial infarct size after ischemia and reperfusion by the glycosaminoglycan pentosan polysulfate.

Activation of the complement system contributes to the tissue destruction associated with myocardial ischemia/reperfusion. Pentosan polysulfate (PPS), a negatively charged sulfated glycosaminoglycan (GAG) and an effective inhibitor of complement activation, was studied for its potential to decrease infarct size in an experimental model of myocardial ischemia/reperfusion injury. Open-chest rabbits were subjected to 30-min occlusion of the left coronary artery followed by 5 h of reperfusion. Vehicle (saline) or PPS (30 mg/kg/h) was administered intravenously immediately before the onset of reperfusion and every hour during the reperfusion period. Treatment with PPS significantly (p < 0.05) reduced infarct size as compared with vehicle-treated animals (27.5+/-2.9% vs. 13.34+/-2.6%). Analysis of tissue demonstrated decreased deposition of membrane-attack complex and neutrophil accumulation in the area at risk. The results indicate that, like heparin and related GAGs, PPS possesses the ability to decrease infarct size after an acute period of myocardial ischemia and reperfusion. The observations are consistent with the suggestion that PPS may mediate its cytoprotective effect through modulation of the complement cascade.

Endotoxin disrupts the estradiol-induced luteinizing hormone surge: interference with estradiol signal reading, not surge release.

Three experiments were conducted to investigate whether the immune/inflammatory stimulus endotoxin disrupts the estradiol-induced LH surge of the ewe. Ovariectomized sheep were set up in an artificial follicular phase model in which luteolysis is simulated by progesterone withdrawal and the follicular phase estradiol rise is reproduced experimentally. In the first experiment, we tested the hypothesis that endotoxin interferes with the estradiol-induced LH surge. Ewes were either infused with endotoxin (300 ng/kg/h, i.v.) for 30 h beginning at onset of a 48-h estradiol stimulus or sham infused as a control. Endotoxin significantly delayed the time to the LH surge (P < 0.01), but did not alter surge amplitude, duration, or incidence. The second experiment tested the hypothesis that the delaying effects of endotoxin on the LH surge depend on when endotoxin is introduced relative to the onset of the estradiol signal. Previous work in the ewe has shown that a 14-h estradiol signal is adequate to generate GnRH and LH surges, which begin 6-8 h later. Thus, we again infused endotoxin for 30 h, but began it 14 h after the onset of the estradiol signal. In contrast to the first experiment, endotoxin given later had no effect on any parameter of the LH surge. In the third experiment, we tested the hypothesis that endotoxin acts during the first 14 h to disrupt the initial activating effects of estradiol. Estradiol was delivered for just 14 h, and endotoxin was infused only during this time. Under these conditions, endotoxin blocked the LH surge in five of eight ewes. In a similar follow-up study, endotoxin again blocked the LH surge in six of seven ewes. We conclude that endotoxin can disrupt the estradiol-induced LH surge by interfering with the early activating effects of the estradiol signal during the first 14 h (reading of the signal). In contrast, endotoxin does not disrupt later stages of signal processing (i.e. events during the interval between estradiol signal delivery and surge onset), nor does it prevent actual hormonal surge output. Thus, endotoxin appears to disrupt estrogen action per se rather than the release of GnRH or LH at the time of the surge.

The semisynthetic polysaccharide pentosan polysulfate prevents complement-mediated myocardial injury in the rabbit perfused heart.

Pentosan polysulfate (PPS) is a highly sulfated semisynthetic polysaccharide possessing a higher negative charge density and degree of sulfation than heparin. Like other glycosaminoglycans, the structural and chemical properties of PPS promote binding of the drug to the endothelium. Glycosaminoglycans, including heparin, inhibit complement activation independent of an action on the coagulation system. This ability provides a compelling argument for the implementation of this class of compounds in experimental models of cellular injury mediated by complement. The objective of this study was to examine whether PPS could reduce myocardial injury resulting from activation of the complement system. We used the rabbit isolated heart perfused with 4% normal human plasma as a source of complement. Hemodynamic variables were obtained before addition of PPS (0.03 01 mg/ml) and every 10 min after the addition of human plasma. Compared with vehicle-treated hearts, left ventricular end-diastolic pressure was improved at the conclusion of the 60-min protocol in hearts treated with PPS (58.9 +/- 13.6 vs. 15. 2 +/- 4.8 mm Hg). Further evidence as to the protective effects of PPS was demonstrated by decreased creatine kinase release compared with vehicle (86.5 +/- 28.5 U/l vs. 631.0 +/- 124.8 U/l). An enzyme-linked immunosorbent assay for the presence of the membrane attack complex in lymph and tissue samples demonstrated decreased membrane attack complex formation in PPS-treated hearts, which suggests inhibition of complement activation. This conclusion was supported further by the ability of PPS to inhibit complement-mediated red blood cell lysis in vitro. The results of this study indicate that PPS can reduce tissue injury and preserve organ function that otherwise would be compromised during activation of the human complement cascade.

Attenuation of interleukin-8 expression in C6-deficient rabbits after myocardial ischemia/reperfusion.

Neutrophil accumulation and activation of the complement system with subsequent deposition of the cytolytic membrane attack complex (MAC) have been implicated in the pathogenesis of myocardial ischemia/reperfusion injury. The MAC, when present in high concentrations, promotes target cell lysis. However, relatively little is known about the potential modulatory role of sublytic concentrations of the MAC on nucleated cell function in vivo. In vitro studies demonstrated that the MAC regulates cell function by promoting the expression of pro-inflammatory mediators, including adhesion molecules and pro-inflammatory cytokines. We examined, using C6-deficient and C6-sufficient rabbits, the regulatory role of the MAC in mediating IL-8 expression and subsequent neutrophil recruitment in the setting of myocardial ischemia/reperfusion injury. C6-deficient and C6-sufficient rabbits were subjected to 30 min of regional myocardial ischemia followed by a period of reperfusion. In addition to a significant reduction in myocardial infarct size in C6-deficient animals, analysis of myocardial tissue demonstrated a decrease in neutrophil influx into the infarcted region. The reduction in neutrophil influx correlated with the decreased expression of the neutrophil chemotactic cytokine IL-8, as determined by ELISA and immunohistochemical analysis. The results derived from this study provide evidence that the MAC has an important function in mediating the recruitment of neutrophils to the reperfused myocardium through the local induction of IL-8.

Reduction of myocardial infarct size in vivo by carbohydrate-based glycomimetics.

One of the foremost mechanisms involved in the pathogenesis of myocardial reperfusion injury is the adhesion of neutrophils within the myocardium. The initial neutrophil-endothelial cell interactions are mediated by the selectin family of adhesion molecules. Blockade of this group of adhesion molecules, through the use of synthetic carbohydrate analogs to the selectin ligand sialyl Lewisx and glycomimetics, has been beneficial in reducing neutrophil influx and infarct size. In the present study, glycyrrhizin (GM1292), a natural structural glycomimetic, was analyzed for the ability to decrease myocardial infarct size after regional myocardial ischemia/reperfusion. To determine the structural requirements for optimal cardioprotective activity, two additional compounds related to glycyrrhizin, GM3290 and GM1658 (glycyrrhetinic acid), were studied. The molecular structures of the latter two compounds differ in the number of glucuronic acid residues in their respective molecules. Open-chest, anesthetized rabbits were subjected to 30 min occlusion of the left coronary artery followed by 5 hr of reperfusion. Vehicle or glycomimetic (10 mg/kg/hr) was administered intravenously immediately before the onset of reperfusion and every hour during the reperfusion period. Myocardial infarct size in rabbits treated with GM1292 (two glucuronic acid residues) and GM3290 (one glucuronic acid residue) was reduced significantly when compared with vehicle-treated animals (P < .05). GM1658, which lacks glucuronic acid residues, did not provide a protective effect in vivo. The data suggest that GM1292 and GM3290, which contain carbohydrate moieties, are effective in reducing the degree of myocardial injury after an acute period of ischemia/reperfusion.

Reviparin-sodium prevents complement-mediated myocardial injury in the isolated rabbit heart.

The cytoprotective action of reviparin-sodium (LU-47311: Clivarin), a low-molecular-weight heparin, was examined in an ex vivo model of complement-mediated myocardial injury. The effective concentration of reviparin was determined by using an in vitro rabbit erythrocyte-lysis assay using 4% normal human plasma. Reviparin (0.01-2.73 mg/ml) reduced erythrocyte lysis in a concentration-dependent manner. Subsequently, 0.91 mg/ml of reviparin was evaluated in an ex vivo rabbit isolated-heart model of human complement-mediated injury. Hearts perfused in the presence of 0.91 mg/ml of reviparin (n = 10) demonstrated significant preservation of ventricular function compared with vehicle-treated hearts (n = 10), as evidenced by coronary artery perfusion pressure, left ventricular developed pressure, and left ventricular end-diastolic pressure. A reduction in myocyte creatine kinase release was observed in reviparin-treated hearts compared with controls. Myocardial injury in vehicle-treated hearts was associated with an increased assembly of the membrane-attack complex, as determined by immunohistochemical localization of C5b-9 neoantigen. Reviparin decreased fluid-phase Bb formation detected in the lymphatic drainage of plasma-perfused hearts. The results of this study demonstrate that reviparin inhibits complement-mediated myocardial injury as assessed in an ex vivo experimental model of complement activation.

Effects of tedisamil (KC-8857) on cardiac electrophysiology and ventricular fibrillation in the rabbit isolated heart.

1. The direct cardiac electrophysiological and antifibrillatory actions of tedisamil (KC-8857) were studied in rabbit isolated hearts. 2. Tedisamil (1, 3, and 10 microM), prolonged the ventricular effective refractory period (VRP) from 120 +/- 18 ms (baseline) to 155 +/- 19, 171 +/- 20, and 205 +/- 14 ms, respectively. Three groups of isolated hearts (n = 6 each) were used to test the antifibrillatory action of tedisamil. Hearts were perfused with 1.25 microM pinacidil, a KATP channel activator. Hearts were subjected to hypoxia for 12 min followed by 40 min of reoxygenation. Ventricular fibrillation (VF) developed during hypoxia and reoxygenation in both the control and 1 microM tedisamil-treated groups (5/6 and 4/6, respectively). Tedisamil (3 microM) reduced the incidence of VF (0/6, P = 0.007 vs. control). 3. In a separate group of hearts, VF was initiated by electrical stimulation. The administration of 0.3 ml of 10 mM tedisamil, via the aortic cannula, terminated VF in all hearts, converting them to normal sinus rhythm. 4. Tedisamil (3 microM) reversed pinacidil-induced negative inotropic effects in rabbit isolated atrial muscle which were equilibrated under normoxia, as well as in atrial muscle subjected to hypoxia and reoxygenation. 5. The results demonstrate a direct antifibrillatory action of tedisamil in vitro. The mechanism responsible for the observed effects may involve modulation by tedisamil of the cardiac ATP-regulated potassium channel, in addition to its antagonism of IK and Ito.