INTRODUCTION
Increased intestinal permeability is a key manifestation of intestinal dysfunction caused by I/R of the gut (1-3) and is associated with an increased risk of multiple organ failure (4-9). Short periods of ischemia (5-15 min) induce protection against tissue injury (10-14). However, it is not clear whether a more prolonged period of intestinal ischemia, as seen initially in hypovolemic and septic shock, may influence changes in intestinal permeability caused by later ischemic events. Development of reliable methods for monitoring intestinal barrier dysfunction is essential to improve the care of patients with conditions associated with I/R challenge of the gut. Microdialysis allows monitoring of extracellular substances in gut submucosa and lumen, also in the clinical setting (15). We have recently shown that microdialysis of the biomarker glycerol released to the gut lumen provide information about tissue injury after intestinal ischemia (16), whereas release of lactate to lumen may be a measure of permeability (17).
The aim of the present study was (1) to investigate the influence of a clinically relevant initial intestinal ischemic insult on transmucosal permeability after a subsequent ischemic event and (2) to investigate whether tonometry and microdialysis of biomarkers released to the gut lumen is able to reflect changes in intestinal permeability in this experimental model.
MATERIALS AND METHODS
The Norwegian State Commission for Animal Experimentation approved this study. All procedures were consistent with the National Institutes of Health Guide for the Care and Use of Laboratory Animals (NIH publication 85-23, revised 1996).
Anesthesia and surgical preparation
The superior mesenteric artery (SMA) was cross-clamped for 60 min followed by 4 h of reperfusion in 16 juvenile male pigs (bodyweight, 19-27 kg; Norwegian land swine). They were then randomized into two groups: nine pigs had a second cross-clamp of 60 min and 3 h of reperfusion (double-clamp group), whereas seven pigs were only observed for a further 4 h of reperfusion (single-clamp group). After i.m. premedication with 10 mg diazepam (Stesolid; Dumex-Alpharma, Copenhagen, Denmark), 400 mg azaperone (Stresnil; Janssen-Cilag, Wien, Austria), and 1 mg atropine (Atropin; Nycomed Pharma, Oslo, Norway), anesthesia was induced with 5 to 10 mg kg−1 thiopental sodium (Pentothal-Natrium; Abbott Scandinavia AB, Solna, Sweden) and 10 mg kg−1 ketamine (Ketalar; Parke-Davis, Solna, Sweden). A tracheotomy was performed, and the animals were mechanically ventilated with a Servo 900 B ventilator (Siemens-Elema, Solna, Sweden). FiO2 was kept at 0.3, tidal volume at 10 mL kg−1, and minute ventilation was adjusted to maintain PaCO2 of 34 to 41 mmHg (4.5-5.5 kPa) and kept unchanged during the experimental period. Anesthesia was maintained with 0.5% isoflurane (Forene; Abbot Scandinavia) and a continuous infusion of 7.5 µg kg−1 h−1 fentanyl (Alpharma AS, Oslo, Norway) and 0.5 mg kg−1 h−1 midazolam (Dormicum; Roche, Basel, Switzerland). The femoral artery was cannulated for heart rate, MAP, and blood gas measurements (ABL 330 radiometer, Copenhagen, Denmark). Another catheter was inserted into the inferior cava vein for blood samples. A 4-FG catheter was introduced via the right common carotid artery into the left cardiac ventricle for infusion of colored microspheres. The animals received Ringer acetate at an infusion rate of 10 to 15 mL kg−1 h−1. A heating blanket and warmed fluids were used to maintain a constant core temperature of 38.5°C. A midline laparotomy was performed, and a catheter was inserted into the urinary bladder. An ultrasonic transit-time flow probe (6 mm) was placed around the SMA for continuous blood flow measurements. A vessel loop for cross-clamping was placed proximal to the flow probe on the SMA. After surgical preparation, the animals were allowed to stabilize for 60 min; thereafter, baseline measurements were collected for 45 min.
Regional blood flow
Intestinal tissue blood flow and cardiac output were determined by the distribution of colored microspheres during baseline, 10 min after the first cross-clamping, and 10 min after the first and second declamping (18).
Permeability
Intestinal permeability was determined as described previously (19). Briefly, a 30-cm length of jejunum was ligated at both ends, and 100 mL (10.0 μCi) 14C polyethylene glycol (PEG-4000) (Amersham Bioscience, Buckinghamshire, UK) was injected into the lumen. Venous blood samples for determination of the concentration of PEG-4000 in plasma were taken at 30-min intervals, and urine samples were taken hourly.
Microdialysis
A microdialysis catheter (CMA 70; membrane length, 20 mm; 20 kDa; CMA Microdialysis AB, Stockholm, Sweden) was introduced into the jejunal lumen 40 cm distal to the ligament of Treitz, as recently reported from our laboratory (17). A microdialysis catheter (CMA 70) was also introduced into the subclavian artery. The microdialysis catheters were perfused at a flow rate of 1 µL min−1 with an isotonic perfusion fluid (CMA perfusion fluid T1 [Na+, 147 mM; K+, 4 mM; Ca2+, 2.3 mM; Cl−, 156 mM) and a microdialysis pump (CMA 107; CMA Microdialysis). The catheters were perfused in situ for at least 75 min before baseline measurements. Samples were collected over 30 min and were analyzed immediately on site for glycerol, lactate, pyruvate, and glucose concentrations by enzymatic fluorometric assays (CMA 600 Microdialysis analyzer) using peroxidase methodology. In vitro recovery with a flow rate of 1µL min−1 was 65% (41%-70%) (mean [range]) for lactate, 22% (17%-33%) for pyruvate, 36% (30%-44%) for glycerol, and 33% (25%-43%) for glucose.
Tonometry
A tonometry catheter (16F; Tonometrics, Datex-Ohmeda, Helsinki, Finland) was introduced into the jejunal lumen 60 cm distal to the ligament of Treitz through an antimesenterial incision and calibrated in situ. Gut mucosal PCO2 (PiCO2) was measured using an automated air tonometry system (Tonocap, TC-200; Datex-Ohmeda). Gut mucosal PiCO2 and arterial blood gas measurements (ABL 330 radiometer) were simultaneously measured every 30 min, and the difference between PiCO2 and arterial PCO2 (PaCO2), the CO2-gap, was calculated at each time point (regional CO2 gap).
Statistics and calculations
All values are expressed as medians and ranges where not otherwise mentioned. For comparisons between the double- and single-clamp groups at various points, we used the Mann-Whitney U test. To assess changes within the groups over time, the Friedman test was used. The Wilcoxon signed-rank test was applied for paired samples to evaluate differences between specific points or areas under the curve (AUCs) within the groups. Linear regression was performed to investigate and plot the association between arterial and luminal values within each pig for specific time intervals. SPSS for Windows version 13 (SPSS, Inc., Chicago, Ill) was used for the statistical analyses.
Formation and elimination kinetics of lactate, glycerol, and PEG-4000 were calculated as follows: maximum concentrations (Cmax) and the times to achieve maximum serum concentrations (tmax) were obtained directly form the measured values. Other parameters were calculated by means of the Kinetica program package, version 4.3 (InnaPhase Corporation, Philadelphia, Pa) using a noncompartment model. Areas under the curve from the start of the first and of second clamping to 240 min later (AUC0-240) were calculated using the trapezoidal rule. The parameter estimates describing the elimination phase of the log-concentrations (λz) were calculated using the best-fit regression lines, taking the degree of log-linearity into account. The elimination half-lives (t½) were calculated as ln2/λz.
Before the start of the second cross-clamping, baseline levels of lactate and glycerol were significantly higher than before the first cross-clamping. To improve the comparability between the first and the second clamps, we subtracted mean values at the corresponding time points from the single clamp group during the last episode of I/R.
RESULTS
There were no significant differences in the measured parameters at baseline between the groups except for heart rate (Table 1). All animals survived throughout the experiment.
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Table 1:
Hemodynamic variables
Hemodynamic variables
The hemodynamic variables during the experiment are presented in Table 1. MAP was elevated from baseline only during cross-clamping in both groups. Heart rate increased during the first clamp in both groups and remained elevated for the rest of the experiment. Cardiac output did not change during the experiment. The blood flow of the SMA was 0 during cross-clamping and returned to baseline during reperfusion.
Blood flow measured both in the whole jejunal wall and in the serosa fell to 0 during cross-clamping of the SMA and returned to baseline levels during reperfusion after both ischemic episodes. Jejunal mucosal blood flow also fell to 0 after clamping of the SMA. After the first ischemic episode, blood flow returned to baseline, whereas after the second ischemic episode, blood flow was higher than baseline (P = 0.027).
Intestinal permeability
14C-Polyethylene glycol at baseline and during the first clamping was not detectable in venous blood or urine. During reperfusion, the mean AUC0-240 of PEG-4000 in venous blood was two times higher after the first than after the second ischemia (44,780 [13,441-82,723] vs. 22,298 [12,213-49,698] counts min mL−1, P = 0.026) (Fig. 1A). The same pattern was seen for Cmax (mean, 394 [117-632] vs. 140 [105-330] counts mL−1, P = 0.035). The PEG-4000 levels declined log-linearly during both the first and second reperfusion with mean elimination half-lives of 133 (67-224) and 117 (76-149) min, respectively (P = 0.434). The excretion of PEG-4000 in urine largely mirrored the venous concentration of this marker molecule, although the peak excretion after declamping of the SMA occurred approximately 1 h later than the peak concentration of venous PEG-4000 (Fig. 1B).
Fig. 1:
Permeability across the intestinal mucosa. A, Mean concentrations of PEG-4000 in venous blood as an index of permeability across the intestinal mucosa in pigs subjected to cross-clamping of the SMA for 60 min twice (double-clamp group, closed squares) or once (single-clamp group, open circles), followed by reperfusion. The shaded areas represent cross-clamping of the SMA. * indicates a difference in the mean AUC during reperfusion after the first compared with after the second ischemia (P = 0.026). B, 14C-Polyethylene glycol in urine (expressed as percent of PEG-4000 instilled into the jejunum) as an index of permeability across the intestinal mucosa in pigs subjected to cross-clamping of the SMA for 60 min twice (double-clamp group, closed squares) or once (single-clamp group, open circles), followed by reperfusion. The shaded areas represent cross-clamping of the SMA. * indicates a difference in the mean AUC during reperfusion after the first compared with after the second ischemia (P = 0.03).
Intestinal lumen
Lactate
The response to the first cross-clamping of the SMA was similar in both groups with increased lactate levels within 30 min (P < 0.05) (Fig. 2A). The mean Cmax was 8.5 (5.0-14.9) and 7.8 (5.8-11.5) mM in the first clamp in the double-clamp and single-clamp groups, respectively (P = 0.408), and was reached after a median of 30 (0-180) min of reperfusion. The lactate levels declined log-linearly during reperfusion with a mean elimination half-life of 90 (26-165) min (small figure in Fig. 2A), and baseline levels were reached in the single-clamp group after 390 min of reperfusion. The response to the second cross-clamping was less pronounced compared with the first occlusion, with a mean AUC0-240 of 797 (412-1,700) vs. 1,151 (880-1,969) mmol min L−1 (P = 0.02) and a mean Cmax of 4.8 (2.7-9.4) vs. 8.5 (5.0-14.9) mM (P = 0.01).
Fig. 2:
Gut luminal concentrations of lactate and glycerol. Mean intestinal luminal concentrations of lactate (A) and glycerol (B) detected by microdialysis in pigs subjected to cross-clamping of the SMA for 60 min twice (double-clamp group, closed squares) or once (single-clamp group, open circles), followed by reperfusion. On the abscissa, 0 denotes baseline, whereas the shaded areas represents cross-clamping of the SMA. * indicates a difference in the mean AUC during reperfusion after the first compared with after the second ischemia (P = 0.02). The small figures depict the same graphs as in the main figures, but with logarithmic scales on the ordinates to illustrate the degree of log-linear elimination.
Glycerol
The first cross-clamp of the SMA caused an increase in gut luminal glycerol in both groups after 30 min (P = 0.04) (Fig. 2B). Mean Cmax for glycerol was similar in the double-clamp and the single-clamp groups (971 [735-1,538] vs. 826 [619-1,351] µM, P = 0.142) and was reached after a mean of 90 (30-180) min of reperfusion. The glycerol levels declined in a non-log-linear way during reperfusion (small figure in Fig. 2B), and baseline levels were reached in the early clamp group after 360 min of reperfusion.
The response to the second clamp differed from the first. Mean Cmax of luminal glycerol was higher in the first clamp than in the second (971 [735-1,538] vs. 404 [49-1,195] µM, P = 0.02). There was also a trend toward the same regarding AUC (131,214 [71,253-215,517] vs. 45,281 [1,581-213,489] µmol min L−1, P = 0.07). The tmax values were the same in the first and second clamp (median, 150 [90-210] vs. 120 [30-210] min, P = 0.174).
Glucose in the gut lumen was only detected during reperfusion, and the mean AUC was higher during the first than during the second clamp (43 [0-197] vs. 3 [0-28] mmol min L−1, respectively, P = 0.008).
Regional CO2 gap
The regional CO2 gap increased 4-fold during the first occlusion in both groups (P = 0.008) (Fig. 3) but returned to baseline levels after 240 min of reperfusion. During the second ischemia, the values both for the increase in and the maximal level of the CO2 gap were identical to the corresponding values during the first ischemic episode.
Fig. 3:
Intestinal mucosal - arterial CO2-gap. Mean intestinal mucosal - arterial CO2-gap in pigs subjected to cross-clamping of the SMA for 60 min twice (double clamp group, closed squares) or once (single clamp group, open circles), followed by reperfusion. On the abscissa, 0 denotes baseline, whereas the shaded areas represent cross-clamping of the SMA.
Arterial blood
Arterial blood concentrations of glucose, lactate, pyruvate, and glycerol are presented in Table 2. The arterial lactate concentration was only significantly higher than baseline at 60 min of every cross-clamp (P = 0.012). In Figure 4A, individual arterial lactate concentrations in the double-clamp group are plotted against corresponding luminal lactate concentrations during baseline and both reperfusion periods. In linear regression analyses accounting for the intraindividual and interindividual variance of the pigs, we found an association between arterial and luminal lactate levels, which was most pronounced in the first reperfusion (P < 0.01).
Table 2:
Arterial concentrations of glucose, lactate, pyruvate, and glycerol
Fig. 4:
Correlations between arterial and luminal concentrations of lactate and glycerol. A, Arterial lactate concentrations from each animal plotted against corresponding luminal concentrations during baseline and both reperfusion periods in nine pigs subjected to cross-clamping of the SMA for 60 min twice (double-clamp group) followed by reperfusion. Each symbol represents a single individual, and each line represents an individual linear regression curve (correlation coefficient, r = 0.61; regression coefficient = 1.23; P < 0.001). B, Arterial glycerol concentrations from each of the nine animal plotted against corresponding luminal concentrations during baseline and both reperfusion periods in pigs subjected to cross-clamping of the SMA for 60 min twice (double-clamp group) followed by reperfusion. Each symbol represents a single individual, and each line represents individual linear regression curve. There were no statistically significant associations.
The arterial pyruvate concentration was only significantly increased after 60 min of the first ischemia (P = 0.039) and decreased to less than baseline levels after 240 min of reperfusion in the single-clamp group (P = 0.016). Arterial lactate-pyruvate ratio was not elevated from baseline levels. The arterial glycerol concentration was only increased to more than baseline at one single time during the first clamping period (P = 0.027). The arterial glycerol concentrations are also plotted against luminal glycerol concentrations at the same three time intervals (Fig. 4B) as with lactate, but we found no significant associations between arterial and luminal glycerol levels.
DISCUSSION
In this study with two ischemic episodes, we demonstrate that mucosal permeability was less increased in response to the second ischemic insult. Gut luminal intestinal microdialysis of biomarkers, but not tonometry, closely reflect such permeability changes, supporting the assumption that microdialysis can monitor fluctuations in intestinal integrity.
The improved mucosal barrier function after the second ischemic insult is consistent with the only published study that clearly has demonstrated an effect of ischemic preconditioning on transmucosal permeability in response to a subsequent challenge by ischemia (14). The current study extends these observations by showing that even a long (60 min) ischemic event in the gut, comparable to what may be encountered before effective resuscitation after hypovolemic and septic shock, may induce the same protection against a subsequent ischemic insult.
We have found in a recent study that that there is no worsened injury of the mucosa, as judged by microscopy, throughout the reperfusion period after ischemia (16). Both in that study and in the present, hyperpermeability across the mucosa after ischemia was not sustained or increased, but instead decreased throughout the reperfusion period. Mucosal blood flow was rapidly restored to normal levels after the ischemic period, indicating absence of the no-reflow phenomenon (i.e., full return of nutritive perfusion after ischemia) in this layer of the intestinal wall. This is in accordance with a recent study from our laboratory (16). A possible explanation for this is the much lower levels of xanthine dehydrogenase/xanthine oxidase in the small intestine of juvenile pigs than in other animal species (i.e., rats) (19). Therefore, chemoattraction, rolling, and migration of neutrophil granulocytes from the small vessels of the intestine may be less pronounced in pigs than in rats. This is supported by lack of increased myeloperoxidase in response to ischemia in a previous study from our laboratory (20) and in another porcine model (19).
Qualitatively, concentrations of lactate, glucose, and glycerol in the intestinal lumen mirrored the attenuated hyperpermeability after the second ischemic insult (Figs. 1 and 2). However, the time course with regard to increase and elimination from the gut lumen indicates certain differences between lactate and glycerol. As shown in Figures 1 and 2, the maximum level of lactate and PEG-4000 occurred at exactly the same time and, importantly, the slope of the elimination curve was almost identical. In contrast, the luminal decrease of glycerol was much slower (Fig. 2). This suggests that lactate is a more precise measure of permeability than glycerol. A likely explanation for these observations may be sought by the origin of lactate and glycerol released to the gut lumen in response to ischemia. It is known that the surface epithelium of the gut is much more susceptible to ischemia than cells in the deeper part of the gut wall (21-23), and disintegration of the cell membranes induce release of glycerol into the gut lumen (16). There is a close correlation between intestinal cellular damage and glycerol levels (16); thus, the smaller total amount of glycerol in the gut lumen in the second ischemic period probably reflects less destruction of the intestinal epithelium and not altered permeability. Lactate released into the lumen is at least to a large part produced due to anaerobic metabolism in all cell types of the mucosa and muscular layer, not only the epithelium.
Arterial concentrations of the biomarkers, and particularly lactate, may also influence their release into the intestinal lumen. With an intact surface epithelial barrier, it has been shown that even high systemic levels of lactate do not influence the gut luminal concentration of lactate (24). As shown in Figure 4A, there was a correlation between systemic and luminal lactate in the reperfusion period coinciding with increased permeability. This suggests that systemic lactate may contribute to luminal release of lactate. However, considering the gradient between lumen and blood, the local production in the intestinal wall is probably the most important source of luminal lactate. There was no correlation between systemic and luminal release of glycerol. These findings further support the conclusion put forward above that luminal lactate better reflects permeability than glycerol.
The transport of lactate during ischemia is rather complex. Lactate crosses cell membranes by interaction with specific proteins-the monocarboxylate transporters, which are transmembrane proteins facilitating cotransport of a monocarboxylate ion with a proton (25). Acidosis increases both paracellular and transcellular permeability to hydrophilic (macro)molecules such as fluorescein disulfonic acid (molecular weight, 478 Da, and fluorescein isothiocyanate-labeled dextran (average molecular weight, 4 kDa) in human intestinal epithelial Caco-2BBe cells grown as monolayers (26). Metabolic acidosis due to ischemia may thus influence on the lactate transport from the cells into the intestinal lumen as with larger molecules such as PEG-4000.
On the contrary, CO2 freely diffuses out of cells and into the interstitial fluid. Increases in tissue CO2 are primarily a function of changes in regional blood flow, independent of the degree of tissue dysoxia (27). This is in accordance with our findings with the same blood flow and CO2 alterations in both ischemic episodes. It is therefore not surprising that tonometry failed to reflect permeability changes across the mucosa.
In summary, the present study provides evidence for the conclusion that even a prolonged ischemic insult of the intestine confers protection for reduced hyperpermeability against further ischemia. Microdialysis of biomarkers mirrors the permeability changes associated with this type of protection. Lactate reflects permeability across the intestinal mucosa more precisely than glycerol.
ACKNOWLEDGMENT
The authors thank Oddveig Lyng, M.Sc., for her contribution to this work.
REFERENCES
1. Wattanasirichaigoon S, Menconi MJ, Delude RL, Fink MP: Effect of mesenteric ischemia and reperfusion or hemorrhagic shock on intestinal mucosal permeability and ATP content in rats.
Shock 12:127-133, 1999.
2. Wattanasirichaigoon S, Menconi MJ, Fink MP: Lisofylline ameliorates intestinal and hepatic injury induced by hemorrhage and resuscitation in rats.
Crit Care Med 28:1540-1549, 2000.
3. Fink MP, Antonsson JB, Wang HL, Rothschild HR: Increased intestinal permeability in endotoxic pigs. Mesenteric hypoperfusion as an etiologic factor.
Arch Surg 26:211-218, 1991.
4. Ziegler TR, Smith RJ, O'Dwyer ST, Demling RH, Wilmore DW: Increased intestinal permeability associated with infection in burn patients.
Arch Surg 123:1313-1319, 1988.
5. De-Souza DA, Greene LJ: Intestinal permeability and systemic infections in critically ill patients: effect of glutamine.
Crit Care Med 33:1125-1135, 2005.
6. Ammori BJ, Leeder PC, King RF, Barclay GR, Martin IG, Larvin M, McMahon MJ: Early increase in intestinal permeability in patients with severe acute pancreatitis: correlation with endotoxemia, organ failure, and mortality.
J Gastrointest Surg 3:252-262, 1999.
7. Faries PL, Simon RJ, Martella AT, Lee MJ, Machiedo GW: Intestinal permeability correlates with severity of injury in trauma patients.
J Trauma 44:1031-1035, 1998.
8. Doig CJ, Sutherland LR, Sandham JD, Fick GH, Verhoef M, Meddings JB: Increased intestinal permeability is associated with the development of multiple organ dysfunction syndrome in critically ill ICU patients.
Am J Respir Crit Care Med 158:444-451, 1998.
9. Swank GM, Deitch EA: Role of the gut in multiple organ failure: bacterial translocation and permeability changes.
World J Surg 20:411-417, 1996.
10. Murry CE, Jennings RB, Reimer KA: Preconditioning with ischemia: a delay of lethal cell injury in ischemic myocardium.
Circulation 74:1124-1136, 1986.
11. Hotter G, Closa D, Prados M, Fernandez-Cruz L, Prats N, Gelpi E, Rosello-Catafau J: Intestinal preconditioning is mediated by a transient increase in nitric oxide.
Biochem Biophys Res Commun 222:27-32, 1996.
12. Mallick IH, Yang W, Winslet MC, Seifalian AM: Ischemia-reperfusion injury of the intestine and protective strategies against injury.
Dig Dis Sci 49:1359-1377, 2004.
13. Pasupathy S, Homer-Vanniasinkam S: Ischaemic preconditioning protects against ischaemia/reperfusion injury: emerging concepts.
Eur J Vasc Endovasc Surg 29:106-115, 2005.
14. McCallion K, Wattanasirichaigoon S, Gardiner KR, Fink MP: Ischemic preconditioning ameliorates ischemia- and reperfusion-induced intestinal epithelial hyperpermeability in rats.
Shock 14:429-434, 2000.
15. Solligård E, Wahba A, Skogvoll E, Stenseth R, Grønbech JE, Aadahl P: Endoluminal microdialysis shows increased rectal lactate in routine coronary surgery.
Anaesthesia 62:250-258, 2007.
16. Solligård E, Juel IS, Bakkelund K, Jynge P, Tvedt KE, Johnsen H, Aadahl P, Gronbech JE: Gut luminal microdialysis of glycerol as a marker of intestinal ischemic injury and recovery.
Crit Care Med 33:2278-2285, 2005.
17. Solligard E, Juel IS, Bakkelund K, Johnsen H, Saether OD, Gronbech JE, Aadahl P: Gut barrier dysfunction as detected by intestinal luminal microdialysis.
Int Care Med 30:1188-1194, 2004.
18. Yavuz Y, Rønning K, Lyng O, Marvik R, Gronbech JE: Effect of increased intraabdominal pressure on cardiac output and tissue blood flow assessed by color-labeled microspheres in the pig.
Surg Endosc 15:149-155, 2001.
19. Blikslager AT, Roberts MC, Rhoads JM, Argenzio RA: Is reperfusion injury an important cause of mucosal damage after porcine intestinal ischemia?
Surgery 121:526-534, 1997.
20. Juel IS, Solligard E, Lyng O, Stromholm T, Tvedt KE, Johnsen H, Jynge P, Saether OD, Aadahl P, Gronbech JE: Intestinal injury after thoracic aortic cross-clamping in the pig.
J Surg Res 117:283-295, 2004.
21. Takala J: Determinants of splanchnic blood flow.
Br J Anaesth 77:50-58, 1996.
22. Granger DN, Richardson PD, Kvietys PR, Mortillaro NA: Intestinal blood flow.
Gastroenterology 78:837-863, 1980.
23. Lundgren O, Haglund U: The pathophysiology of the intestinal countercurrent exchanger.
Life Sci 23:1411-1422, 1978.
24. Tenhunen JJ, Kosunen H, Alhava E, Tuomisto L, Takala JA: Intestinal luminal microdialysis: a new approach to assess gut mucosal ischemia.
Anesthesiology 91:1807-1815, 1999.
25. Poole RC, Halestrap AW: Transport of lactate and other monocarboxylates across mammalian plasma membranes.
Am J Physiol 264:C761-C782, 1993.
26. Menconi MJ, Salzman AL, Unno N, Ezzell RM, Casey DM, Brown DA, Tsuji Y, Fink MP: Acidosis induces hyperpermeability in Caco-2BBe cultured intestinal epithelial monolayers.
Am J Physiol 272:G1007-G1021, 1997.
27. Gutierrez G: A mathematical model of tissue-blood carbon dioxide exchange during hypoxia.
Am J Respir Crit Care Med 169:525-533, 2004.
