ure, whereas cooling could not (16). A multicenter randomized clinical trial involving ALF patients is currently under way in France. The results will greatly help in judging the potential role of albumin dialysis in this specific group of patients. Steffen R. Mitzner Division of Nephrology Department of Medicine University of Rostock Rostock, Germany
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6.
7.
REFERENCES 1. Vaquero J, Chung C, Cahill ME, et al: Pathogenesis of hepatic encephalopathy in acute liver failure. Semin Liver Dis 2003; 23: 259 –269 2. Nau R: Osmotherapy for elevated intracranial pressure: A critical reappraisal. Clin Pharmacokinet 2000; 38:23– 40 3. Murphy N, Auzinger G, Bernel W, et al: The effect of hypertonic sodium chloride on intracranial pressure in patients with acute liver failure. Hepatology 2004; 39:464 – 470 4. Tofteng F, Larsen FS: The effect of indomethacin on intracranial pressure, cerebral perfusion and extracellular lactate and glutamate concentrations in patients with ful-
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minant hepatic failure. J Cereb Blood Flow Metab 2004; 24:798 – 804 Jalan R, Olde Damink SW, Deutz NE, et al: Moderate hypothermia in patients with acute liver failure and uncontrolled intracranial hypertension. Gastroenterology 2004; 127: 1338 –1346 Sen S, Rose C, Ytrebo LM, et al: Effect of albumin dialysis on intracranial pressure increase in pigs with acute liver failure: A randomized study. Crit Care Med 2006; 34: 158 –164 Mitzner SR, Stange J, Klammt S, et al: Extracorporeal detoxification using the molecular adsorbent recirculating system for critically ill patients with liver failure. J Am Soc Nephrol 2001; 12(Suppl 17):S75–S82 Mitzner S, Loock J, Peszynski P, et al: Improvement in central nervous system functions during treatment of liver failure with albumin dialysis MARS: A review of clinical, biochemical, and electrophysiological data. Metab Brain Dis 2002; 17:463– 475 Hassanein T, Tofteng F, Brown RS, et al: Efficacy of albumin dialysis (MARS) in patients with cirrhosis and advanced grades of hepatic encephalopathy: A prospective, controlled, randomized multicenter trial. Hepatology 2004; 40(Suppl 1):726A–727A Heemann U, Treichel U, Loock J, et al: Albumin dialysis in cirrhosis with superimposed
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acute liver injury: A prospective, controlled study. Hepatology 2002; 36:949 –958 Awad SS, Swaniker F, Magee J, et al: Results of a phase I trial evaluating a liver support device utilizing albumin dialysis. Surgery 2001; 130:354 –362 Sorkine P, Ben Abraham R, Szold O, et al: Role of the molecular adsorbent recycling system (MARS) in the treatment of patients with acute exacerbation of chronic liver failure. Crit Care Med 2001; 29:1332–1336 Cordoba J, Blei AT, Mujais S: Determinants of ammonia clearance by hemodialysis. Artif Organs 1996; 20:800 – 803 Schmidt LE, Svendsen LB, Sorensen VR, et al: Cerebral blood flow velocity increases during a single treatment with the molecular adsorbents recirculating system in patients with acute on chronic liver failure. Liver Transpl 2001; 7:709 –712 Novelli G, Rossi M, Pretagostini R, et al: A 3-year experience with Molecular Adsorbent Recirculating System (MARS): Our results on 63 patients with hepatic failure and color Doppler US evaluation of cerebral perfusion. Liver Int 2003; 23(Suppl 3):10 –15 Schmidt LE, Wang LP, Hansen BA, et al: Systemic hemodynamic effects of treatment with the molecular adsorbents recirculating system in patients with hyperacute liver failure: A prospective controlled trial. Liver Transpl 2003; 9:290 –297
The role of nitric oxide signaling in sepsis-induced myocardial dysfunction*
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ardiac myocyte contraction and relaxation are controlled by the ion-channel-mediated calcium cycle. Multiple calcium channels are involved: The L-type calcium channels enable the transsarcolemmal calcium entry following membrane depolarization. The ryanodine receptors (calcium release channels) determine the subsequent release of calcium from the sarcoplasmic reticulum (calcium-induced calcium release), which activates myofilament contraction. Relaxation requires removal of calcium from the cytoplasm and is obtained by the
*See also p. 173. Key Words: nitric oxide; sarcoplasmic calcium release channels; endotoxemia The author has no financial interests to disclose. Copyright © 2005 by the Society of Critical Care Medicine and Lippincott Williams & Wilkins DOI: 10.1097/01.CCM.0000196086.78942.6F
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sarcoplasmic reticulum calcium adenosine triphosphatase, which recharges the sarcoplasmic reticulum, and the sarcolemmal sodium-calcium exchanger, which further restores calcium homeostasis. There is clear evidence that nitric oxide influences myocardial function by modulation of the calcium channels and pores (1, 2). Nitric oxide is formed from the substrate L-arginine catalyzed by the enzyme nitric oxide synthase (NOS). Two constitutive isoforms of NOS, neuronal (nNOS or NOS1) and endothelial (eNOS or NOS3), are present in cardiac muscle. A third isoform, inducible NOS (iNOS or NOS2), is expressed under pathophysiologic conditions (3). At least two mechanisms are involved in nitric oxide signaling in cardiac muscle. First, nitric oxide activates the soluble guanylyl cyclase, which leads to the production of guanosine 3',5'-cyclic monophosphate (cGMP). Second, nitric oxide
may react with sulfhydryl (thiol) groups on various proteins, and this nitrosylation may activate proteins involved in the regulation of myocardial function (4, 5). This enables highly variable effects of nitric oxide on myocardial function. For example, whereas nitric oxide-mediated increases in cGMP inhibit L-type calcium channels and reduce contractility (6), nitrosylation-induced activation of the sarcoplasmic calcium release channels may increase contractile force (2). Furthermore, recent studies indicate that variable effects of nitric oxide may be obtained by coordinated actions of different NOS isoforms with specific subcellular localizations (7, 8): NOS1 is expressed in the sarcoplasmic reticulum and NOS3 in the sarcolemma and T-tubule membranes (9, 10). Thus, spatial confinement of different NOS isoforms with effector molecules and ion channels further allows nitric oxide signals to have indepen255
dent, and even opposite, effects on myocardial function. However, many questions and controversies exist regarding these highly complex and versatile regulatory roles of physiologic nitric oxide signaling (11). In sepsis and septic shock, pathophysiologic generation of nitric oxide contributes to cardiovascular dysfunction. Most prominently, endotoxins and cytokines increase the expression of iNOS in vascular endothelial and smooth muscle cells, resulting in the release of large amounts of nitric oxide followed by profound vasodilation manifest as systemic hypotension, hyporesponsive to pressure agents. In addition, nitric oxide plays a role in the well-established sepsis-induced myocardial depression (12, 13). Early onset effects may involve constitutive NOS activation, whereas generation of iNOS, which requires a minimum of several hours (14), may be responsible for late onset myocardial depression. In this issue of Critical Care Medicine, Dr. Cohen and colleagues (15) present an elegant study that examines the interaction between nitric oxide and the sarcoplasmic calcium release channels to help explain myocardial dysfunction during endotoxemia. The authors used rats and induced endotoxemia by intraperitoneal injection of lipopolysaccharide (LPS), a standard animal model of sepsis. The animals were killed at baseline or 4, 24, or 96 hrs after the LPS challenge for biochemical assessments and measurements of papillary muscle contractility. To examine the signaling pathways, selective inhibitors for iNOS and cGMP were used. At the 4-hr time point, the LPS-challenged rats showed unchanged myocardial contractility, whereas myocardial nitric oxide was approximately doubled and the calcium release channel activity was unchanged. In contrast, in the LPSchallenged rats that received the iNOS inhibitor, contractility and calcium release channel activity were depressed, suggesting a protective role for nitric oxide at this stage. At 24 hrs, the situation was quite opposite; the LPS-challenged rats showed an eight-fold increase in myocardial nitric oxide content and markedly reduced contractility and calcium release channel activity, whereas the iNOS-inhibited rats showed unchanged nitric oxide, normal contractility, and normal calcium release channel activity. At 96 hrs all values returned to baseline in all groups (note, however, 256
that there was a 10% mortality rate in the animals that received LPS). These findings indicate that myocardial depression during endotoxemia is indeed related to nitric oxide-induced reduction in calcium release channel activity. The authors also investigated calcium release channel protein content and found no changes following LPS administration (in contrast to some previous studies) (16); thus calcium release channel activity appears to be mainly altered posttranslationally. The authors extended their measurements with groups of LPS-challenged animals that received a cGMP inhibitor. Interestingly, at 24 hrs the cGMP-inhibited animals showed a contractility and calcium release channel activity at levels in between the severely depressed untreated LPS-challenged rats, on the one hand, and the unaffected iNOS-inhibited animals, on the other hand. This suggests that the negative effects of nitric oxide on the calcium release channel act through both cGMP-dependent and cGMP-independent pathways. Clearly, this study only examined one particular aspect of sepsis-induced myocardial dysfunction. With regard to myocardial dysfunction, previous studies have shown that virtually all steps in the excitation-contraction coupling sequence are altered during sepsis (16 –18) and they are all potentially affected by changes in nitric oxide. In addition, this study focused on systolic function, but nitric oxide is known to improve diastolic relaxation, which could be an important adaptive mechanism in sepsis (19). Obviously, the LPS-challenged rodent model without fluid resuscitation does not fully mimic the typical human situation of septic shock, and treatment (iNOS inhibition) before development of symptoms may not be clinically relevant (20). However, carefully performed, detailed studies such as this one that reveal the basic mechanisms may be the only way to further our understanding of the highly complex processes involved in sepsisinduced myocardial dysfunction. The fact that Dr. Cohen and colleagues focused on an isolated aspect should not be regarded as a limitation but rather as an advantage. Paul Steendijk, PhD Department of Cardiology Leiden University Medical Center Leiden, The Netherlands
REFERENCES 1. Campbell DL, Stamler JS, Strauss HC: Redox modulation of L-type calcium channels in ferret ventricular myocytes. Dual mechanism regulation by nitric oxide and Snitrosothiols. J Gen Physiol 1996; 108: 277–293 2. Xu L, Eu JP, Meissner G, et al: Activation of the cardiac calcium release channel (ryanodine receptor) by poly-S-nitrosylation. Science 1998; 279:234 –237 3. Balligand JL, Ungureanu-Longrois D, Simmons WW, et al: Cytokine-inducible nitric oxide synthase (iNOS) expression in cardiac myocytes. Characterization and regulation of iNOS expression and detection of iNOS activity in single cardiac myocytes in vitro. J Biol Chem 1994; 269:27580 – 27588 4. Stamler JS, Lamas S, Fang FC: Nitrosylation. The prototypic redox-based signaling mechanism. Cell 2001; 106:675– 683 5. Paolocci N, Ekelund UE, Isoda T, et al: cGMP-independent inotropic effects of nitric oxide and peroxynitrite donors: Potential role for nitrosylation. Am J Physiol Heart Circ Physiol 2000; 279:H1982–H1988 6. Mery PF, Pavoine C, Belhassen L, et al: Nitric oxide regulates cardiac Ca2⫹ current. Involvement of cGMP-inhibited and cGMPstimulated phosphodiesterases through guanylyl cyclase activation. J Biol Chem 1993; 268:26286 –26295 7. Barouch LA, Harrison RW, Skaf MW, et al: Nitric oxide regulates the heart by spatial confinement of nitric oxide synthase isoforms. Nature 2002; 416:337–339 8. Khan SA, Skaf MW, Harrison RW, et al: Nitric oxide regulation of myocardial contractility and calcium cycling: Independent impact of neuronal and endothelial nitric oxide synthases. Circ Res 2003; 92:1322–1329 9. Feron O, Dessy C, Opel DJ, et al: Modulation of the endothelial nitric-oxide synthasecaveolin interaction in cardiac myocytes. Implications for the autonomic regulation of heart rate. J Biol Chem 1998; 273: 30249 –30254 10. Xu KY, Huso DL, Dawson TM, et al: Nitric oxide synthase in cardiac sarcoplasmic reticulum. Proc Natl Acad Sci U S A 1999; 96: 657– 662 11. Ashley EA, Sears CE, Bryant SM, et al: Cardiac nitric oxide synthase 1 regulates basal and beta-adrenergic contractility in murine ventricular myocytes. Circulation 2002; 105: 3011–3016 12. Parrillo JE, Burch C, Shelhamer JH, et al: A circulating myocardial depressant substance in humans with septic shock. Septic shock patients with a reduced ejection fraction have a circulating factor that depresses in vitro myocardial cell performance. J Clin Invest 1985; 76:1539 –1553
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13. Parrillo JE, Parker MM, Natanson C, et al: Septic shock in humans. Advances in the understanding of pathogenesis, cardiovascular dysfunction, and therapy. Ann Intern Med 1990; 113:227–242 14. Bateson AN, Jakiwczyk OM, Schulz R: Rapid increase in inducible nitric oxide synthase gene expression in the heart during endotoxemia. Eur J Pharmacol 1996; 303:141–144 15. Cohen RI, Wilson D, Liu SF: Nitric oxide modifies the sarcoplasmic reticular calcium release channel in endotoxemia by both
guanosine-3',5' (cyclic) phosphate-dependent and independent pathways. Crit Care Med 2006; 34:173–181 16. Dong LW, Wu LL, Ji Y, et al: Impairment of the ryanodine-sensitive calcium release channels in the cardiac sarcoplasmic reticulum and its underlying mechanism during the hypodynamic phase of sepsis. Shock 2001; 16:33–39 17. Wu LL, Tang C, Dong LW, et al: Altered phospholamban-calcium ATPase interaction in cardiac sarcoplasmic reticulum during the
progression of sepsis. Shock 2002; 17: 389 –393 18. Zhong J, Hwang TC, Adams HR, et al: Reduced L-type calcium current in ventricular myocytes from endotoxemic guinea pigs. Am J Physiol 1997; 273:H2312–H2324 19. Court O, Kumar A, Parrillo JE, et al: Clinical review: Myocardial depression in sepsis and septic shock. Crit Care 2002; 6:500 –508 20. Vincent JL, Zhang H, Szabo C, et al: Effects of nitric oxide in septic shock. Am J Respir Crit Care Med 2000; 161:1781–1785
Linking gut-associated lymphoid tissue to multiple organ dysfunction syndrome and infection*
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linical intensive care faces the problem of secondary complications after the primary insult. Trauma, hemorrhagic shock, major surgery, cardiac shock, or septic shock as primary insults have in common the interruption of normal microvascular flow. Resuscitation with fluids and inotropes aims to restore microcirculatory flow and to limit the length of time with interruption of perfusion and oxygen supply. However, the development of an overwhelming inflammatory response and multiple organ dysfunction syndrome (MODS) often cannot be prevented. In addition, infection as a secondary insult is a frequent complication. Infection occurs after pathologic colonization and overgrowth in a situation of immune suppression following the initial inflammatory response. The recognition of this sequence of events led to the concept of the gut as the motor of organ failure. Lethal amounts of endotoxin in the gut lumen and overgrowth with potentially pathogenic microorganisms during critical illness would be able, by translocation, to enter the systemic circulation and induce both cell and endothelial dysfunction with organ failure and secondary infection. Indeed, most steps of this concept were confirmed in studies in the late 1980s and
*See also p. 182. Key Words: gut-associated lymphoid tissue; lymph; ischemia-reperfusion; multiple organ dysfunction syndrome; infection Copyright © 2005 by the Society of Critical Care Medicine and Lippincott Williams & Wilkins DOI: 10.1097/01.CCM.0000196087.29912.A0
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early 1990s. Ischemia and reperfusion lead to a decrease in gut barrier function, studied as an increased intestinal permeability (1), and bacterial translocation was observed (2). However, the results of many studies, especially in nonlethal insults, did not fit in the concept as it was. For instance, portal blood was not found to contain bacteria, endotoxin, or mediators of inflammation (3). Therefore, it became increasingly evident that the concept had to be changed. Mesenteric lymph was identified as another potential way to deliver endotoxins and bacteria into the systemic circulation and subsequently to the organs (4). However, mesenteric lymph drained from patients and animals did not contain bacteria or endotoxin (5, 6). On the other hand, ligation of the lymph duct during ischemia and reperfusion prevented morbidity and mortality (7). Consequently, gut-derived humoral factors from the gut-associated lymphoid tissue (GALT) present in mesenteric lymph gained attention (8). Indeed, mesenteric lymph and not portal blood leads to polymorphoneutrophil (PMN) activation and endothelial damage (7). Mesenteric lymph also activates endothelial cells, leading to increased PMN-endothelial interaction and endothelial apoptosis (9). Factors greater than 100 kDa might be responsible for these toxic effects (6). Moreover, lymph contains activated immune cells (10). These cells originate from the gut lymphatic system, which contains cells in the epithelium, the lamina propria, and the Peyer patches (11). In the current issue of Critical Care Medicine, Dr. Fukatsu and colleagues provide research data concerning the rel-
evance of these cells in the sequence from ischemia-reperfusion to multiple organ failure and secondary infection (12). They observed, in a murine model of intestinal ischemia and reperfusion, that the number of B- and T-lymphocytes decreased to 20% to 60% of the numbers in shamoperated mice in epithelium, lamina propria, and Peyer patches. The reduction was maximal on day 2 and more prolonged in the Peyer patches (induction site) than in the epithelial and lamina propria (effector) sites. In addition, the number of certain cell phenotypes changed. In that situation, the normal process of antigen-presenting by M cells to the Peyer patches and the T cell response to luminal bacteria are likely to be disturbed. On the other hand, the normal IgA levels in the gut suggest that the plasma cell response to antigen presented by the dendritic cells from intestinal bacteria remains intact. The normal commensal flora has antiinflammatory epithelial effects (11). In contrast, pathologic colonization, which is usually found in critically ill patients, is able to exaggerate the inflammatory response to ischemia and reperfusion (13). Mesenteric lymph from rats with bacterial overgrowth but without gut injury is able to increase the permeability of an endothelial monolayer (14). In addition, selective intestinal decontamination reduces systemic monocyte activation and improves endothelial function (15). The exact role in this process of the presently shown reduced mass of immune cells in the GALT and the relation to secondary infectious complications in critically ill patients should be investigated in upcom257