Tonometric assessment of jejunal mucosal nitric oxide formation in anaesthetized pigs

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.

Candesartan improves survival following severe hypovolemia in pigs; a role for the angiotensin II type 2 receptor?

OBJECTIVE

To investigate the involvement of intestinal angiotensin II type 2 receptors in the outcome of acute severe hypovolemia as well as systemic and regional mesenteric hemodynamics and intestinal mucosal functions in anesthetized pigs.

DESIGN AND SETTING

Prospective, interventional animal study in a university research laboratory.

SUBJECTS

53 landrace pigs, 28-35 kg.

INTERVENTIONS

30+30% or 20+20% hemorrhage of estimated total blood volume followed by retransfusion performed in untreated controls, in animals treated with the angiotensin II type 1 receptor blocker candesartan or with a combination of candesartan and the angiotensin II type 2 receptor blocker PD123319.

MEASUREMENTS AND RESULTS

Following 30+30% hemorrhage the candesartan-treated animals attained a significantly higher survival rate than controls and animals treated with PD123319 in combination with candesartan. Less pronounced hemorrhage (20+20%) resulted in no mortality and functional variables were assessed. A significantly higher output of jejunal intraluminal nitric oxide occurred during hypovolemia in the candesartan treated group than in controls and animals that received PD123319 in combination with candesartan. Jejunal transmucosal potential difference was significantly better preserved after retransfusion in candesartan-treated animals than in controls. Expression of angiotensin II type 2 receptors in intestinal tissue was significantly higher in animals surviving the 30+30% hemorrhage than in nonsurvivors.

CONCLUSIONS

Lethal circulatory failure is possibly influenced by use of angiotensin receptor ligands, and activation of intestinal angiotensin II type 2 receptors may play a significant role in improving the outcome of severe hypovolemia.

Inducible nitric oxide synthase inhibition improves intestinal microcirculatory oxygenation and CO2 balance during endotoxemia in pigs

OBJECTIVE

We examined whether selective inhibition of inducible nitric oxide synthase (iNOS) promotes intestinal microvascular oxygenation (microPO2) and CO2 off-load after endotoxic shock.

DESIGN AND SETTING

Prospective, controlled experimental study in a university animal research laboratory.

SUBJECTS

13 domestic pigs.

INTERVENTIONS

After baseline measurements shock was induced by 1 microg kg-1 h-1 endotoxin until mean arterial pressure fell below 60 mmHg. After 30 min in shock the animals were resuscitated with either fluid alone (control, n=6) or fluid and the iNOS inhibitor N-[3-(aminomethyl)benzyl]acetamidine hydrochloride (1400W, n=7). As final experimental intervention all animals received the nonselective NOS inhibitor L-NAME.

MEASUREMENTS AND RESULTS

Systemic and regional hemodynamic and oxygenation parameters were measured at baseline, during endotoxemia and shock, hourly for 3 h of 1400W therapy, and 30 min after the final L-NAME administration. microPO2 was assessed by the Pd-porphyrin phosphorescence technique, and the arterial to intestinal PCO2 gap was determined by air tonometry. Endotoxemia and shock resulted in a decrease in ileal mucosal and serosal microPO2 and a rise in PCO2 gap. The combination of 1400W and fluid resuscitation, but not fluid alone, normalized both the serosal microPO2 and the intestinal PCO2 gap. Administration of L-NAME decreased cardiac output and oxygen delivery and intestinal microPO2 and blood flow in both groups.

CONCLUSIONS

Partial blockade of NO production by 1400W increased serosal microvascular oxygenation and decreased the intestinal CO2 gap. This findings are consistent with the idea that 1400W corrects pathological flow distribution and regional dysoxia within the intestinal wall following endotoxic shock.

Acid challenge to the esophageal mucosa: effects on local nitric oxide formation and its relation to epithelial functions

To evaluate the effect of esophageal acid exposure on epithelial function, transmucosal potential, histopathological markers of acute tissue damage, and local nitric oxide production were examined in healthy volunteers treated with proton pump inhibitors (group I), patients with treated reflux disease (group II), and patients with untreated erosive reflux disease (group III). The participants were randomized to esophageal perfusion with either saline or HCl. Denominators of acute acid exposure were balloon cells in superficial layers and superficial densification. The nitric oxide concentrations in groups I to III increased from < 1, 10.0+/-10.0, and 20.6+/-19.9 ppb, respectively, to 300+/-80, 1360+/-1080, and 920+/-700 ppb after HCl infusion (P < 0.001). Inducible nitric oxide synthase was consistently expressed in the epithelium. Blood flow was lower among reflux patients but did not correlate with acid exposure or nitric oxide. Nitric oxide is formed following acid perfusion and predominantly in gastroesophageal reflux disease.

Epithelial barrier integrity and intraluminal nitric oxide production in response to acid perfusion of the ferret oesophagus

AIM

To evaluate the source and role of acid-induced intraluminal nitric oxide (NO) production in the oesophagus by studying how the exposure of the oesophagus to acid affects NO release, via the NO-producing enzyme NO synthase and its relation to changes in epithelial barrier integrity.

METHODS

Ferrets were anaesthetized and their oesophagi were divided at both ends. The test subjects were pre-treated with the intravenous NO synthase inhibitors N(G)-nitro-L-arginine-methyl ester (L-NAME, 100 mg kg(-1)) and 1400W (12 mg kg(-1)). Untreated and N(G)-nitro-D-arginine-methyl ester pre-treated (D-NAME, 100 mg kg(-1)) animals served as controls. The oesophagus was then perfused with either HCl (0.1 m) or physiological saline for 20 min. The intraluminal NO concentration was determined before and after the acid/saline infusion while the transmucosal potential difference (PD) was monitored continuously. Oesophageal biopsies were examined for expression of inducible NO synthase using immunohistochemistry.

RESULTS

The intraluminal NO concentration increased after acid exposure. This was blocked by L-NAME and 1400W, but not by D-NAME. The peak PD response was not affected by agents affecting NO synthesis, while the plateau response was attenuated by L-NAME, D-NAME and 1400W. Immunohistochemistry revealed inducible NO synthase expression in the epithelium.

CONCLUSIONS

Exposing the ferret oesophageal mucosa to acid elicited an increase in juxtamucosal NO formation through the activation of inducible NO synthase. The corresponding electrophysiological observations suggested an association between mucosal NO production and epithelial integrity.

Intestinal nitric oxide output during reduced mucosal blood flow in healthy volunteers

OBJECTIVE

Nitric oxide regulates epithelial permeability and other properties of the intestinal mucosal barrier. It previously has been shown in animals that intestinal mucosal nitric oxide production is impaired during gut hypoperfusion. The study was performed to confirm the presence of intestinal mucosal nitric oxide production in humans and to investigate the effect of gut hypoperfusion due to moderate arterial hypotension on intestinal nitric oxide concentrations.

DESIGN

Open study where each subject served as his own control.

SETTING

Clinical research laboratory.

SUBJECTS

Nine healthy volunteers were intubated with a nasogastrointestinal tube for recordings in the distal duodenum. Intestinal nitric oxide output and motility were assessed by tonometry and manometry, respectively. Laser Doppler flowmetry and plasma angiotensin II concentration were used to investigate mucosal perfusion and a vasoregulatory response.

INTERVENTIONS

Moderate hypotension was induced with lower body negative pressure over 1 hr.

MEASUREMENTS AND MAIN RESULTS

Intestinal nitric oxide production varied in parallel with the migrating motor complex. Low values were obtained during phase I and peak values during phase III. Lower body negative pressure was initiated at a well-defined point in the migrating motor complex cycle. It was followed by a 40 +/- 6% reduction of laser Doppler flow signal, a 778 +/- 138% increase in angiotensin II, and a reduction in intestinal mucosal nitric oxide production by 48 +/- 8%. After lower body negative pressure, laser Doppler signal and angiotensin II concentrations returned to baseline levels within 1 hr, whereas intestinal nitric oxide output remained decreased.

CONCLUSIONS

Intestinal tonometry in humans exhibits a considerable mucosal nitric oxide formation that varies in relation to intestinal motility. Intestinal nitric oxide production is depressed during conditions with lowered mucosal blood perfusion.

The angiotensin II receptor type 2 agonist CGP 42112A stimulates NO production in the porcine jejunal mucosa

BACKGROUND

This study was conducted to elucidate if nitric oxide is released by the porcine jejunal mucosa upon selective stimulation of AT2 receptors and the possible involvement of iNOS, and to investigate the presence of jejunal AT1 and AT2 receptors. Young landrace pigs were anaesthetized with ketamine and alpha-chloralose. Jejunal luminal NO output was assessed by intraluminal tonometry and analysed by chemiluminescense. Western blot analysis quantified mucosal iNOS and detected AT1 and AT2 receptor protein expression. AT1 and AT2 receptor RNA expression was detected by rtPCR.

RESULTS

Baseline luminal NO output correlated significantly to baseline mucosal iNOS-protein content. In animals treated with the AT2-receptor agonist CGP42112A (n = 11) luminal NO output increased significantly (at 0.1 micrograms kg(-1) min(-1) and 1.0 micrograms kg(-1) min(-1)), but not in animals simultaneously treated with the AT2-receptor antagonist PD123319 (bolus 0.3 mgkg-1, infusion 0.03 mg kg(-1) h(-1)) (n = 7). No differences in iNOS protein expression were found between groups or before/after the administration of drugs. Western blot and rtPCR recognised expression of the AT1 and AT2 receptors in jejunal tissue.

CONCLUSION

The results suggest that activation of AT2 receptors increases jejunal luminal NO output. This response was not due to an increase in the expression of the iNOS protein in the mucosa.

Inducible nitric oxide synthase mediates early epithelial repair of porcine ileum

Reports conflict regarding the effect of nitric oxide (NO) on intestinal epithelium. In chronic injury, NO appears detrimental by combining with reactive oxygen to form potent-free radicals. In contrast, inhibition of NO synthesis after acute injury exacerbates damage and inflammation. Recent studies have disclosed constitutive expression of inducible NO synthase (iNOS) by normal intestinal epithelia, yet little attention has been given to the role of iNOS in acute epithelial repair. We studied the local effects of iNOS on early epithelial repair of porcine ileal mucosa injured by deoxycholate within Ussing chambers. iNOS was constitutively expressed by the villous epithelium, and after deoxycholate injury, iNOS was expressed by injured and detaching enterocytes. Selective inhibition of iNOS abolished increases in NO synthesis and villous reepithelialization after injury. Exogenous L-arginine rescued baseline reepithelialization from NOS inhibitors but was only capable of stimulating additional repair in the presence of serum. These results demonstrate that iNOS-derived NO is a key mediator of early villous reepithelialization following acute mucosal injury.

ARDS. Monitoring tissue perfusion

A clinically feasible method for assessing regional splanchnic perfusion is still lacking. Methods used for research purposes demonstrate that the effects of current therapies on splanchnic perfusion are not predictable in intensive care patients with and without ARDS. Tonometry, laser Doppler flowmetry, and spectrophotometry have been used to assess splanchnic perfusion. Combining the available methods in different parts of the gastrointestinal tract may help assess splanchnic perfusion more accurately in the near future.