Minor involvement of nitric oxide during chronic endotoxemia in anesthetized pigs

Infusion of increasing doses of endotoxin induces progressive acute lung injury but prevents early pulmonary hypertension in pigs

Administration of a single bolus of endotoxin is a model of sepsis response in experimental animal studies. Large animal species, such as pigs and sheep, are more sensitive to endotoxin administration due to an initial excessive pulmonary hypertensive response frequently resulting in acute right heart dysfunction. We investigated whether infusion of high-dose endotoxin in pigs but administered in an increasing dose results in inflammatory response without excessive pulmonary hypertension and right heart dysfunction. Piglets of both sexes weighing 25 to 30 kg were anesthetized and mechanically ventilated. After instrumentation and baseline measurements, animals received an infusion of total 500 microg kg(-1) i.v. endotoxin (Escherichia coli LPS) over 2 h in an increasing dose of 0.5 to 12 microg kg(-1) min(-1). Hemodynamic, respiratory, and oxygenation parameters were measured every hour. At 1 and 5 h following endotoxin, plasma levels of inflammatory and organ damage parameters were measured. Endotoxin infusion induced progressive arterial hypoxemia, an increase in peak inspiratory pressure, sustained pulmonary hypertension, and systemic hypotension that persisted throughout the experiment. Endotoxin plasma levels peaked at 1 h following infusion and declined toward baseline values at 5 h thereafter. In contrast, plasma levels of nitrite/nitrate, IL-1ra (as marker of cytokine response), remained markedly increased at 5 h after endotoxin infusion as compared with baseline values. Plasma markers of organ damage were significantly increased. Our data show that the dosing of endotoxin in an increasing manner in pigs produces a reliable model of an experimental sepsis response and organ dysfunction without immediate overwhelming pulmonary hypertension resulting in cardiovascular failure.

Characterization of an animal model of severe sepsis associated with respiratory dysfunction

PURPOSE

Pathophysiological studies in humans regarding sepsis are difficult to perform due to ethical and methodological concerns. In this context, animal models of sepsis can be useful to better understand this condition and to test therapeutic strategies. The purpose of this study was to characterize a feasible and clinically relevant model of sepsis in pigs that could be useful for testing different therapeutic interventions.

METHODS

5 White Large pigs were anesthetized, arterial and pulmonary catheters were introduced, and sepsis was induced by fecal peritonitis. Several biochemical indicators of organ dysfunction and infectious parameters were measured. The pigs were monitored until death, when fragments of organs were removed for pathology. Three animals without peritonitis served as controls and were sacrificed 24 hours after surgery without developing significant changes in organ function.

RESULTS

Septic pigs survived 17 hours on average (range, 16-18 h), and Escherichia coli was recovered from blood cultures. They developed a significant decrease in left ventricular work and a nonsignificant reduction in mixed venous oxygen saturation. Respiratory dysfunction was characterized by a decrease in the PaO2/FiO2 ratio and respiratory compliance. Pathology of the lungs revealed areas of pulmonary collapse, hemorrhage, pulmonary congestion, and discrete neutrophil infiltrate.

CONCLUSIONS

Fecal peritonitis in pigs is a clinically relevant model of sepsis associated with acute lung injury without direct pulmonary insult. This model may prove to be useful for studying pathogenic aspects of secondary lung injury as well as for validating ventilatory or pharmacologic interventions.

Adenosine A1 and A2 receptor agonists reduce endotoxin-induced cellular energy depletion and oedema formation in the lung

BACKGROUND AND OBJECTIVE

Tissue depletion of adenosine during endotoxaemia has previously been described in the lung. Therapeutic approaches to prevent adenosine depletion and the role of A1 and A2 receptor agonists, however, have not been investigated until now.

METHODS

In isolated and ventilated rabbit lungs, it was tested whether pretreatment with adenosine A1 agonist 2-chloro-N6-cyclopentyladenosine (CCPA; 10(-7) mol, n = 6) or A2 receptor agonist 5'-(N-cyclopropyl)-carboxyamido adenosine (CPCA; 10(-7) mol, n = 6) prior to injection of lipopolysaccharide (LPS) (500 pg mL-1) influenced pulmonary artery pressure (PAP), pulmonary energy content and oedema formation as compared with controls, solely infused with LPS (n = 6). Release rates of adenosine and uric acid were determined by high-performance liquid chromatography. Pulmonary tissue concentrations of high-energy phosphates were measured and the adenine nucleotide pool, adenosine 5'-triphosphate (ATP)/adenosine 5'-diphosphate (ADP) ratio and adenylate energy charge of the pulmonary tissue were calculated.

RESULTS

Administration of LPS induced increases in PAP within 2 h up to 20.8 +/- 2.9 mmHg (P < 0.01). While pretreatment with the A1 agonist merely decelerated pressure increase (13.8 +/- 1.1 mmHg, P < 0.05), the A2 agonist completely suppressed the pulmonary pressure reaction (9.6 +/- 1.0 mmHg, P < 0.01). Emergence of lung oedema after exclusive injection of LPS up to 12.0 +/- 2.9 g was absent after A1 (0.6 +/- 0.5 g) and A2 (-0.3 +/- 0.2 g) agonists. These observations were paralleled by increased adenosine release rates compared with LPS controls (P < 0.05). Moreover, tissue concentrations of ADP, ATP, guanosine 5'-diphosphate, guanosine 5'-triphosphate, nicotinamide-adenine-dinucleotide and creatine phosphate were significantly reduced after LPS. Consequently, the calculated tissue adenine nucleotide pool and the adenylate energy charge increased after adenosine receptor stimulation (P = 0.001).

CONCLUSIONS

Adenosine A1- and A2-receptor agonists reduced LPS-induced vasoconstriction and oedema formation by maintenance of tissue energy content. Thus, adenosine receptor stimulation, in particular of the A2 receptor, might be beneficial during acute lung injury.

Evaluation of Liver Support Systems for Preclinical Testing by Animal Trials

In the present review, various animal models of acute liver failure are reviewed with respect to their suitability for evaluating liver support systems (LSS) according to envisaged modes of therapy. In order to increase the value of the preclinical testing of LSS, it would be advantageous to include more than one animal model in the evaluation program. It is possible to identify appropriate sets of models, which make a suitable test system for particular clinical applications. A standardization of evaluation methods between testing groups would also be beneficial to the field of liver support.

Nitric oxide synthase inhibition in sepsis? Lessons learned from large-animal studies

Nitric Oxide (NO) plays a controversial role in the pathophysiology of sepsis and septic shock. Its vasodilatory effects are well known, but it also has pro- and antiinflammatory properties, assumes crucial importance in antimicrobial host defense, may act as an oxidant as well as an antioxidant, and is said to be a "vital poison" for the immune and inflammatory network. Large amounts of NO and peroxynitrite are responsible for hypotension, vasoplegia, cellular suffocation, apoptosis, lactic acidosis, and ultimately multiorgan failure. Therefore, NO synthase (NOS) inhibitors were developed to reverse the deleterious effects of NO. Studies using these compounds have not met with uniform success however, and a trial using the nonselective NOS inhibitor N(G)-methyl-l-arginine hydrochloride was terminated prematurely because of increased mortality in the treatment arm despite improved shock resolution. Thus, the issue of NOS inhibition in sepsis remains a matter of debate. Several publications have emphasized the differences concerning clinical applicability of data obtained from unresuscitated, hypodynamic rodent models using a pretreatment approach versus resuscitated, hyperdynamic models in high-order species using posttreatment approaches. Therefore, the present review focuses on clinically relevant large-animal studies of endotoxin or living bacteria-induced, hyperdynamic models of sepsis that integrate standard day-to-day care resuscitative measures.

Heme oxygenase-1 protects human hepatocytes in vitro against warm and cold hypoxia

BACKGROUND/AIMS

Hepatic injury induced by ischemia/reperfusion following surgery, transplantation, or circulatory shock combined with resuscitation is a major clinical problem.

METHODS

In this study, hypoxic and inflammatory conditions were mimicked by exposing human hepatocytes to N(2) (at 4 and 37 degrees C) or to cytokines/endotoxin to investigate the potential protective effects of heme oxygenase-1 (HO-1). Incubation of human hepatocytes with single cytokines (IFN-gamma, IL-1beta, TNF-alpha) or LPS, as well as a combination of all four stimuli (CM, cytomix) caused a time-dependent HO-1 mRNA expression over 12h and a decline by 24 h. In parallel, we observed a time-dependent membrane leakage for LDH and AST and a maximum HO-1 protein expression between 3-24 h.

RESULTS

Warm and cold hypoxia showed similar results in HO-1 mRNA and protein expression and the release of LDH and AST. CoPP, a potent HO-1 inducer, and bilirubin, a co-product of the HO-pathway, protected human hepatocytes from warm and cold hypoxia. HO-1 enzyme activity was highest during warm hypoxia, followed by cold hypoxia and CM which was confirmed by intracellular Fe(2+) formation.

CONCLUSIONS

Taken together, we demonstrated, that HO-1 induction protected human hepatocytes against warm and cold hypoxia. Our results also suggest that HO-1 induction may have therapeutic potential against inflammatory insults.

Endotoxin-induced shock in the pig--limited effects of low and high concentrations of inhaled nitric oxide

AIM

This study was carried out to study the prophylactic effects of inhalation of nitric oxide (NO) before and during the induction of endotoxic shock.

METHODS

Eighteen anaesthetized pigs received an infusion of 10-20 microg kg(-1) endotoxin during 2 h after pre-treatment with the cortisol-synthesis inhibitor metyrapone. Three groups were tested (n = 6 each) and received 0, 0.2 or 20 ppm inhaled NO from 30 min before start of endotoxin infusion until 4 h after start of endotoxin. Both 0.2 and 20 ppm NO were able to improve blood gas values.

RESULTS

Area above curve values of arterial P2/FiO2 from 0 to 4 h were 0.83 +/- 0.09 kPa h (control), 0.78 +/- 0.22 (0.2 ppm NO, non-significant) and 0.31 +/- 0.06 (20 ppm NO, P < 0.01, Mann-Whitney U-test, compared to control). Area under curve values of PCO2 from 0 to 4 h were 3.96 +/- 0.66 kPa h (control), 1.20 +/- 0.46 (0.2 ppm NO, P < 0.05, Mann-Whitney U-test, compared to control) and 2.78 +/- 1.06 (20 ppm NO group, non-significant). The increase in pulmonary arterial pressure (PAP) was partly prevented by 20 ppm NO, but not by 0.2 ppm NO at 4 h. Inhaled NO did not affect the levels of BAL fluid total protein, tumour necrosis factor-alpha, interleukin-8 and neutrophil counts.

CONCLUSIONS

The addition of a high (20 ppm), but not a low (0.2 ppm), concentration of NO to the inhaled air during endotoxin shock improves arterial oxygen tension and reduces pulmonary artery pressure. Neither dose affects lung mechanics or inflammatory indices, in spite of being given prophylactically.

Modulation of host defense by hydrocortisone in stress doses during endotoxemia

OBJECTIVE

To investigate the effects of low-dose hydrocortisone (HC) on neutrophil respiratory burst, phagocytosis, and elimination of E. coli from blood and tissue under endotoxemic and non-endotoxemic conditions. DESIGN. Randomized, controlled trial.

SETTING

Experimental laboratory, university hospital.

SUBJECTS

Forty-eight female chinchilla rabbits ( n=8 in six groups A-F).

INTERVENTIONS

In order to quantify the bacterial clearance process, defined numbers [10(8) colony forming units (CFU)] of Escherichia coli were injected intravenously into all anesthetized rabbits. Group A did not receive further intervention. Group B received bolus administration of HC 1.4 mg/kg and group C 14 mg/kg. Endotoxin (LPS, 40 microg/kg/h) was given to groups D, E, and F. Group E received additional bolus administration of HC 1.4 mg/kg and group F 14 mg/kg. All HC groups (B, C, E, and F) were continuously infused with HC 0.18 mg/kg/h.

MEASUREMENTS

Monitored parameters were neutrophil respiratory burst and phagocytosis activity, rates of bacterial elimination from the blood, arterial blood pressure, serum lactate and LPS concentrations, as well as nitrite and nitrate levels. Tissue samples of liver, kidney, spleen, and lung were collected for bacterial counts.

MAIN RESULTS

In controls HC significantly delayed elimination of injected E. coli from the blood (P<0.01). LPS also prolonged bacterial elimination but additional HC did not further delay removal of E. coli from the blood. Under endotoxemia HC depressed respiratory burst, whereas phagocytosis functions remained unaltered. Moreover, bacterial colonization of organs was reduced after HC in the LPS groups. Significance, however, was reached only in the liver (P<0.05). Due to HC, clearance from LPS (P<0.01) and lactate (P<0.05) were improved. Levels of nitrite and nitrate did not differ among the groups.

CONCLUSION

HC demonstrated immunomodulatory effects even in stress doses. In endotoxemic states use of low-dose HC seems to be favorable, although not in non-septic conditions.

Effect of combining nicotinamide as a PARS-inhibitor with selective iNOS blockade during porcine endotoxemia

OBJECTIVE

To investigate the effects of combined selective inducible nitric oxide synthase (iNOS) inhibition using 1400 W with nicotinamide (NAD) as a PARS-inhibitor on hepato-splanchnic hemodynamics, O(2) kinetics, and energy metabolism during hyperdynamic porcine endotoxemia.

DESIGN

Prospective, randomized, controlled, interventional experiment.

SETTING

Animal research laboratory.

SUBJECTS

Seventeen domestic pigs.

INTERVENTIONS

After 12 h of continuous i.v. endotoxin (LPS) infusion 17 pigs received either no drug (CON, n=9) or 1400 W, titrated to maintain mean arterial pressure (MAP) at pre-endotoxin level, plus 10 mg.kg.h NAD ( n=8;). Measurements were obtained before, 12 h, 18 h, and 24 h after starting LPS infusion.

MEASUREMENTS AND RESULTS

In addition to systemic and pulmonary hemodynamics and gas exchange, we measured hepatic arterial and portal venous blood flow, liver and portal venous drained viscera O(2) exchange, ileal mucosal-arterial PCO(2) gap, and portal as well as hepatic venous lactate/pyruvate ratios. Expired NO and plasma nitrate levels were assessed as a parameter of NO production. Without affecting cardiac output, therapy maintained MAP and blunted the LPS-induced rise in expired NO levels, attenuated the progressive fall in liver lactate clearance, and blunted the impairment of hepato-splanchnic redox state. The rise of ileal mucosal-arterial PCO(2) gap was not influenced.

CONCLUSIONS

Combining selective iNOS inhibition with NAD as a PARS blocker may prevent circulatory failure and attenuate the detrimental consequences of LPS in intestinal and hepatocellular energy metabolism. Given the potential hepatotoxicity of high-dose NAD treatment, more potent PARS blockers with higher selectivity might further enhance the benefit of this therapeutic approach.

Measuring end products of nitric oxide in vivo

The methods we have presented are suitable for measuring end products of the NO pathway in biological fluids. In particular, the first part of this article demonstrated how standard methods could be improved to increase the sensitivity of NO measurements in biological methods. In addition, we showed two possibilities for the estimation of NO formation in higher species, as NO standard measurements in these species are often not reliable to detect changes of the NO pathway.

In vivo measurement of nitric oxide production in porcine gut, liver and muscle during hyperdynamic endotoxaemia

1. During prolonged endotoxaemia, an increase in arginine catabolism may result in limiting substrate availability for nitric oxide (NO) production. These effects were quantitated in a chronically instrumented porcine endotoxaemia model. 2. Ten days prior to the beginning of the experiments, pigs were catheterized. On day 0, pigs received a continuous infusion of endotoxin (3 microg kg(-1) h(-1)) over 24 h and were saline resuscitated. Blood was drawn from the catheters at 0 and 24 h during primed-infusion of (15)N(2)-arginine and P-aminohippurate to assess (15)N(2)-arginine to (15)N-citrulline conversion and plasma flow rates, respectively, across the portal-drained viscera, liver and hindquarter. 3. During endotoxin infusion a hyperdynamic circulation with elevated heart rate, cardiac index and decreased mean arterial pressure was achieved, characteristic of the human septic condition. 4. Endotoxin induced NO production by the portal-drained viscera and the liver. The increased NO production was quantitatively matched by an increase in arginine disposal. Nitrite/nitrate levels remained unchanged during endotoxaemia. 5. Despite an increased arginine production from the hindquarter and an increased whole-body arginine appearance rate during endotoxin infusion, the plasma arginine concentration was lower in endotoxin-treated animals than in controls. 6 On a whole-body level, the muscle was found to serve as a major arginine supplier and, considering the lowered arginine plasma levels, seems critical in providing arginine as precursor for NO synthesis in the splanchnic region.