Abstract
To determine whether hepatic urea production is limited at low hepatic O2 delivery (Do 2) by O2 itself or by the availability of substrate for urea synthesis, we isolated livers from normal rats and perfused them with Krebs-Henseleit bicarbonate (KHB) buffer, KHB + 5 mM NH4Cl, or KHB + 5 mM glutamine (Gln) as an NH3 donor. The pump flow was lowered in stages, and we determined at each flow rate inflow and outflow O2 content and urea levels in the outflow perfusate. Urea production in Gln-perfused livers remained constant at high Do 2 and declined in direct proportion to Do 2 below a critical oxygen delivery (Do 2crit, the point below which the hepatic O2 consumption [V˙ o 2] becomes limited by the hepatic Do 2). The Do 2crit calculated from the urea release-Do 2 relationship (147 ± 32 μ l/min/ dry g) was similar to the Do 2crit calculated from the V˙ o 2-Do 2 relationship (158 ± 26 μ l/min/dry g). When Gln concentration and flow rate were maintained constant while decreasing Po 2 in the inflow perfusate (as well as hepatic Do 2), urea production declined below the Do 2crit. Furthermore, when Gln concentration in the perfusate was gradually reduced while keeping hepatic Do 2 constant, urea production decreased proportionally with Gln concentrations in the perfusate. Consequently, urea production is dependent on Gln and O2 availability and becomes limited at the same Do 2crit determined by the V˙ o 2-Do 2 relationship.
In critically ill patients, the biphasic oxygen uptake-oxygen delivery model (V˙o 2-Do 2 model) has been used to assess oxygen deficiency, which has been suspected to cause organ dysfunction (1). The pathologic dependence of V˙o 2 on Do 2 at low flows suggests that Do 2 should be increased to supranormal levels in critically ill patients to decrease organ failure (2, 3). In the V˙o 2-Do 2 model, V˙o 2 remains constant as Do 2 varies over a wide range, extracting only as much O2 from the blood as needed to maintain vital metabolism: this is known as O2 supply independence. Stable V˙o 2 during O2 supply independence is thought to signify tissue wellness and maintained oxidative metabolism despite variation in Do 2. When Do 2 declines to a critical threshold value (critical Do 2 or Do 2crit), V˙o 2 can no longer be maintained constant and V˙o 2 declines in direct proportion to Do 2: a state known as O2 supply dependence. During this O2 supply dependence, the decrease in V˙o 2 can represent an adaptive reduction in O2 demand or a manifestation of tissue hypoxia, with an O2 supply that is inadequate to support O2 demand. The most important assumption in this model is that O2 demand is constant at all Do 2 values, so that the covariation of V˙o 2 and Do 2 represents inadequate Do 2 to support metabolism and not O2 demand dependence. Indeed, when O2 demand is permitted to vary, the increased V˙o 2 is normally not supported by an increase of O2 extraction but rather by an increase in Do 2, and this covariation of V˙o 2 and Do 2 is not indicative of tissue hypoxia (4).
In the liver, the O2 supply-uptake relationship has already been studied in various conditions. Schlichtig and colleagues (5) found that hepatic V˙o 2 remains relatively constant as Do 2 progressively decreases, until a Do 2crit is reached below which hepatic V˙o 2 also decreases. By evaluating hepatic mitochondrial NAD redox state during O2 supply independence and dependence, they found that the decreased V˙o 2 during oxygen supply dependence represents hepatic dysoxia. Furthermore, by lowering the hepatic blood flow in a model of stagnant hypoxia. Samsel and colleagues (6) also correlated the hepatic oxygen supply-uptake dependence with a switch from lactate consumption to lactate production.
Few studies have sought to determine the relationship between hepatic flow and hepatic functions. Studying galactose elimination as a metabolic function of the liver, Keiding and colleagues (7) showed that galactose elimination and V˙o 2 are independent of Do 2 at high Do 2, whereas at low Do 2 both parameters decrease in parallel, but they did not determine and compare the Do 2crit in the V˙o 2-Do 2 and the galactose elimination-Do 2 relationships. In the present study, we sought to determine whether hepatic urea production is limited at low hepatic Do 2, and if so, whether the deficit in O2 or the deficit in NH3 donor induced by low flow rates limits the urea production. To answer this question, we isolated and perfused livers from normal rats and measured urea production (an important and well known function of the liver) at various hepatic flow rates (or hepatic Do 2).
Thirty-five male Sprague-Dawley rats weighing 240 to 410 g were fasted for 24 h prior to the experiment, with free access to water, and anesthetized intraperitoneally with sodium pentobarbital (Nembutal, 50 mg/kg) before the liver perfusion.
Livers were perfused in situ following the method of Hems and colleagues (8). Briefly, the abdominal cavity was opened and the portal vein was cannulated and secured. The ligature was placed around the inferior vena cava above the left renal vein. The infrarenal vena cava was transected and the perfusate was pumped into the portal vein. The flow rate was slowly increased over 1 min (> 3 ml/min/g wet weight). The chest was opened and a second cannula was inserted through the right atrium into the inferior vena cava and secured with a ligature. Finally, the ligature around the lower part of the inferior vena cava was tied, thus closing the circuit. The animal preparation lasted less than 10 min, and the liver was placed in a Plexiglas® box maintained at steady temperature (37° C). The temperature close to the liver was measured using a temperature probe (Yellow Springs Instrument Co., Yellow Springs, OH). Perfusion pressure was monitored manometrically from tubing attached proximally to the inflow cannula. The entire perfusion system consisted of reservoir, pump (Cole Palmer Instrument Co., Chicago, IL), bubble trap, filter, and oxygenator (9). The livers were perfused with a Krebs-Henseleit- bicarbonate (KHB) buffer (118 mM NaCl, 1.2 mM MgSO4, 1.2 mM KH2PO4, 4.7 mM KCl, 26 mM NaHCO3, and 2.5 mM CaCl2). The perfusate was oxygenated with a mixture of 95% O2–5% CO2, and the pH was maintained at 7.40. After the surgical procedure, the livers were allowed to recover over 30 min using a nonrecirculation perfusion. Livers were perfused with KHB buffer, KHB + 5 mM glutamine (Gln), or KHB + 5 mM NH4Cl.
After the recovery period, the pump flow was lowered in stages, reducing gradually the hepatic Do 2 or the hepatic substrate delivery. Ischemic injury was avoided by limiting the stages (six or seven stages for each liver). After the 5-min wait at each new flow, we measured portal pressure, inflow and outflow blood gases, and lactate and urea levels in the outflow perfusate.
Because hepatic Gln delivery can be modified by changing either the hepatic flow rate or the Gln concentrations in the perfusate while keeping hepatic Do 2 constant, eight livers were perfused with a constant flow rate during each experiment while reducing Gln concentrations in the perfusate (5, 2.5, 1, and 0.5 mM).
Additionally, because the hepatic Do 2 can be modified by changing the hepatic flow rate at a constant Po 2 or by changing Po 2 at a constant flow rate, we perfused eight livers with a constant flow rate (and a constant Gln concentration; 5 mM) while gradually reducing Po 2 in the perfusate. In the oxygenator, N2 was progressively increased while decreasing O2. By varying the percentage of CO2 in the oxygenator, the Pco 2 in the inflow perfusate was maintained constant.
Samples were obtained from the inflow and outflow tubings kept in ice, and assayed for Po 2 within 30 min, using an ABL2 blood gas analyzer (Radiometer, Copenhagen, Denmark) that was calibrated hourly. Urea and lactate were determined by Sigma kits (Sigma Chemicals, St. Louis, MO).
The viability of perfused livers was assessed by measuring potassium and lactate dehydrogenase (LDH) release in the perfusate at the end of each experiment. Potassium was measured using an electrolyte analyzer (Nova Biomedical, Waltham, MA), and LDH was determined with a Technicon RA500 automatic analyzer (Technicon Instruments, Tarrytown, NY). To determine the degree of swelling during the perfusion, the livers were weighed and freeze-dried over 48 h at the end of each experiment, and the wet/dry weight ratio of each liver was calculated.
Hepatic V˙o 2 (μl/min/dry g) was calculated from the difference in O2 contents between the inflow and outflow perfusates. The same solubility coefficient (24 μl of O2 per ml buffer) was used in the various buffers. Urea and lactate release were determined as (urea or lactate) × flow/liver dry weight. Results were expressed in nmol/min/dry g.
Hepatic Do 2crit was calculated in each liver from a plot of Do 2 (x-axis) and V˙o 2 (y-axis) as the point of intersection of the two best-fit linear regression lines (10). The data were sorted in each liver as Do 2-V˙o 2 pairs with decreasing Do 2. A linear regression line was then calculated for the lowest Do 2 points (O2 supply dependent), and another regression line was determined for the remaining points (O2 supply independent). A point from the supply-independent portion was then moved to the supply-dependent portion, and new regression lines were calculated. This process was repeated until the two regression lines with the lowest sum of squared residuals were calculated. The Do 2crit was determined as the Do 2 at the point of intersection of these two lines. O2ERcrit was calculated at the ratio of V˙o 2 to Do 2 at the point of intersection. The maximal oxygen extraction ratio (O2ERmax) was chosen among the calculated ratios during the final stage of each experiment (lower Do 2). Do 2crit and critical urea release (UreaRcrit) were also determined from a plot of urea release versus Do 2, using the same computing method as described above.
Data are given as means ± SD. F values were computed by analysis of variance with a Scheffe test to identify differences between groups; p < 0.05 was considered significant.
Hepatic V˙o 2 (Figure 1A) was independent of hepatic Do 2 at high Do 2 and became dependent on hepatic Do 2 below Do 2crit, as previously described in vivo (5, 6). For each liver, data adequately fitted the dual-line model. The Do 2crit for this group was 116.0 ± 42.7 μl/min/dry g (Table 1). V˙o 2max was 79.1 ± 16.3 μl/ml/dry g with a slope in the high Do 2 region averaging 0.0015 ± 0.0032. The hepatic O2ER (Figure 1B) increased in a nearly hyperbolic manner and continued to increase even after Do 2 fell below 116.0 μl/min/dry g (Table ). Lactate release (Figure 1C) appeared when hepatic Do 2 fell below this critical point.
Fig. 1. Relationship between oxygen delivery and oxygen consumption (A), oxygen extraction ratio (B), and lactate release (C ) in KHB-perfused livers (n = 6). KHB = Krebs-Henseleit-bicarbonate buffer.
[More] [Minimize]| KHB (n = 6 ) | 5 mM NH4Cl (n = 6 ) | 5 mM Gln (n = 7 ) | ||||
|---|---|---|---|---|---|---|
| Do 2crit, μl/min/dry g | 116.0 ± 42.7 | 102.7 ± 24.6 | 157.6 ± 26.4† | |||
| V˙ o 2crit, μl/min/dry g | 68.7 ± 20.3 | 65.3 ± 12.4 | 126.9 ± 21.2† | |||
| OERcrit, % | 62.3 ± 15.3 | 64.3 ± 6.3 | 81.1 ± 9.6† | |||
| Vo 2max, μl/min/dry g | 79.1 ± 16.3 | 83.9 ± 14.9 | 161.4 ± 26.7† | |||
| OERmax, % | 82.5 ± 2.3 | 82.5 ± 7.7 | 89.0 ± 1.5 |
As shown in Figure 2A, the shape of the relationship between Do 2 and V˙o 2 in this group was similar to the previous description. However, perfusion with 5 mM Gln significantly increased Do 2crit, V˙o 2crit, and O2ERcrit compared with KHB perfusion (Table ). V˙o 2max was 161.4 ± 26.7 μl/ml/dry g, with a slope in the high Do 2 region averaging 0.0088 ± 0.0051. Particularly noteworthy in this group is the high V˙o 2crit and V˙o 2max resulting from Gln addition to the perfusate.
Fig. 2. Relationship between oxygen delivery and oxygen consumption (A), oxygen extraction ratio (B), and lactate release (C ) in KHB + 5 mM Gln-perfused livers (n = 7). KHB = Krebs-Henseleit-bicarbonate buffer; Gln = glutamine.
[More] [Minimize]In order to further investigate the high V˙o 2 observed with 5 mM Gln perfusion, additional livers were perfused with 5 mM NH4Cl, as a direct NH3 donor (Figure 3A–C). Unexpectedly, all calculated parameters were similar to the KHB-perfused livers, as shown in Table .
Fig. 3. Relationship between oxygen delivery and oxygen consumption (A), oxygen extraction ratio (B), and lactate release (C ) in KHB + 5 mM NH4Cl-perfused livers (n = 6). KHB = Krebs-Henseleit-bicarbonate buffer.
[More] [Minimize]Both relationships between urea release and Do 2 and between urea release and Gln delivery were similar to the previous descriptions (Figure 4). Urea release was independent of Do 2 or Gln delivery in the high Do 2 region, and became O2 supply- dependent in the low Do 2 region. The Do 2crit (146.5 ± 32.0 μl/min/dry g) calculated from the Do 2-urea release relationship was similar to the value obtained from the Do 2-V˙o 2 relationship (157.6 ± 26.4 μl/min/dry g). At this point, urea release or UreaRcrit was 1,173 ± 263 nmol/min/dry g. UreaRcrit determined from the relationship between Gln delivery and urea release (Figure 4B) was similar (1,166 ± 280 nmol/min/ dry g). Urea release in livers perfused with NH4Cl was only detectable at low Do 2 values, and consequently the Do 2-urea release relationship could not be obtained in this group as in the KHB-perfused livers.
Fig. 4. Relationship between urea release and hepatic oxygen delivery (A) and urea release and glutamine (Gln) delivery (B) in livers perfused with Krebs-Henseleit-bicarbonate (KHB) + 5 mM Gln (n = 7). In each experiment, Gln concentration in the perfusate was kept constant and hepatic flow rate was gradually reduced.
[More] [Minimize]Because hepatic Gln delivery can be modified by changing either the hepatic flow rate (and the hepatic Do 2) or the Gln concentration in the perfusate while keeping hepatic Do 2 constant, we performed additional experiments, keeping the same flow rate during each experiment but reducing gradually Gln concentrations in the perfusate (5, 2.5, 1, and 0.5 mM). In this group, hepatic V˙o 2 remained steady at various ranges of Gln delivery (Figure 5A), but urea release decreased with the decrease of hepatic Gln delivery (or Gln concentration in the perfusate) (Figure 5B).
Fig. 5. Relationship between hepatic oxygen consumption and Gln delivery (A) and urea release and Gln delivery (B) in livers perfused with Krebs-Henseleit-bicarbonate (KHB) and various Gln concentrations (5, 2.5, 1, and 0.5 mM) (n = 8). In each experiment, hepatic flow was kept constant.
[More] [Minimize]Because the hepatic Do 2 can be modified by changing the hepatic flow rate at a constant Po 2 or by changing Po 2 at a constant flow rate, we perfused eight livers with a constant flow rate (and a constant Gln concentration; 5 mM) while gradually reducing Po 2 in the perfusate (Figure 6). The shape of the relationship between Do 2 and V˙o 2 and between Do 2 and urea release was similar to the one described in Figure 2A and in Figure 4A. The Do 2crit calculated from the two relationships was similar: 117.4 ± 7.8 μl/min/dry g (Do 2-V˙o 2 relationship) and 98.3 ± 7.7 μl/min/dry g (Do 2-urea release relationship).
Fig. 6. Relationship between hepatic oxygen consumption and hepatic oxygen delivery (A) and urea release and hepatic oxygen delivery (B) in livers perfused with Krebs-Henseleit-bicarbonate (KHB) + 5 mM Gln at constant flow rate (n = 8). Hepatic oxygen delivery was modified by decreasing Po 2 in the inflow perfusate.
[More] [Minimize]As shown in Table 2, wet/dry weight ratio was higher in livers perfused with KHB + Gln than in livers perfused with KHB alone or KHB + NH4Cl. Liver weight/body weight ratio was similar to the ratio observed in the literature. No LDH release was detected in the perfusate at the end of the experiment, and potassium release remained steady over the experimental periods.
| KHB (n = 6 ) | 5 mM NH4Cl (n = 6 ) | 5 mM Gln (n = 7 ) | ||||
|---|---|---|---|---|---|---|
| Wet weight/dry weight | 3.52 ± 0.13 | 3.42 ± 0.15 | 4.10 ± 0.25† | |||
| (Liver weight/body weight) × 100 | 2.72 ± 0.36 | 2.72 ± 0.23 | 3.18 ± 0.59 |
As observed in the biphasic V˙o 2-Do 2 relationship, when livers are perfused with Gln, urea production remains constant at high Do 2 and declines in direct proportion to Do 2 below the Do 2crit. Furthermore, Do 2crit is similar when calculated from both the V˙o 2-Do 2 model and the urea release-Do 2 relationship. Therefore, hepatic urea production becomes O2 supply-dependent at the same Do 2crit as determined by the V˙o 2-Do 2 relationship. When Gln concentration in the perfusate was gradually reduced while keeping hepatic Do 2 constant, urea production decreased proportionally with Gln concentrations in the perfusate. Furthermore, when Gln concentration and flow rate were maintained constant while decreasing Po 2 in the inflow perfusate (as well as hepatic Do 2), urea production declined below the Do 2crit. Consequently, urea production is dependent on Gln and O2 availability and becomes limited at the same Do 2crit determined by the V˙o 2-Do 2 relationship.
Numerous studies have emphasized the importance of determining the systemic Do 2-V˙o 2 relationship to improve the mortality rate of critically ill patients. Besides the systemic Do 2-V˙o 2 relationship, several investigators also investigated the relationship in various organs (11-14) and found that O2 supply dependence in organs commences at values slightly different from whole-body Do 2crit values (15-17). Nelson and colleagues (17) demonstrated that the gut Do 2crit was reached at a stage when nongut V˙o 2 was still independent of O2 supply. Similarly, supply limitation of V˙o 2 occurs at a higher systemic Do 2 in the contracting diaphragm, suggesting that in diseases associated with increased work of breathing and decreased Do 2, the diaphragm may become metabolically impaired before limitation of systemic V˙o 2 is observed (15). These studies have demonstrated that undetectable hypoxia may occur in organs without changes in whole-body parameters of oxygenation, emphasizing the importance of studying the relationship in individual organs. However, besides parameters of oxygenation, few data exist about the relationship between functions and flow in these models, and the isolated perfused livers appear to be a useful tool in determining such relationships in steady experimental conditions.
In the liver, the Do 2-V˙o 2 relationship has been studied previously. In these studies, the decrease in Do 2 was induced by progressive hemorrhage (5), selective decrease of hepatic blood flow (6), or partial occlusion of hepatic vessels (13). All these in vivo studies found a biphasic relationship between Do 2 and V˙o 2. In our model, we reached similar conclusions, V˙o 2 was independent of Do 2 at high Do 2 and became O2 supply-dependent at low Do 2. Above the Do 2crit, V˙o 2 continued to increase with a slope in the high Do 2 region, averaging 0.0015 ± 0.0032, which is similar to results reported by Samsel and colleagues (6).
Whether hepatic function becomes O2 supply-dependent at the Do 2crit determined by the V˙o 2-Do 2 model remains unclear. Schlichtig and colleagues (5) showed that below the Do 2crit, β-hydroxybutyrate-to-acetoacetate ratio increased with the fall in Do 2, whereas Samsel and colleagues (6) demonstrated that below the Do 2crit, livers change from lactate consumption to lactate production. These findings further support the hypothesis that a decrease in V˙o 2 accompanying O2 supply dependence represents hepatic hypoxia. In our study, the shape of the relationship between urea release and Do 2 was similar to the V˙o 2-Do 2 relationship. Do 2crit was similar when calculated from these two relationships. When Gln and flow rate were kept constant while hepatic Do 2 decreased by lowering Po 2 in the perfusate (Figure 6), urea release also decreased below the Do 2crit. Thus, urea production becomes O2 supply-dependent below the Do 2crit. When hepatic Gln delivery was modified by changing Gln concentrations in the perfusate at constant flow rate (and constant Do 2), we found a direct relationship between urea release and Gln delivery (or Gln concentrations in the perfusate), demonstrating that urea production is also dependent on Gln availability.
In KHB- and KHB + 5 mM NH4Cl-perfused livers, lactate release appeared below the Do 2crit, supporting the hypothesis that Do 2-V˙o 2 dependence represents hepatic hypoxia. O2 supply dependence of urea production also supports this hypothesis. However, absence of cell damage in our model did not exclude an adaptative modification of hepatic metabolism away from urea production toward preservation of cell morphology. The duration of low Do 2 needed to produce cell damage is highly variable among organs, and the liver is thought to be well preserved after several hours of ischemia. This high tissue resistance might explain the absence of cell damage despite hepatic hypoxia in our model.
This study also emphasized the high potency of Gln as a NH3 donor for urea synthesis, whereas NH4Cl was less efficient. In NH4Cl-perfused livers, urea production was undetectable at high Do 2 and we were unable to describe the urea-Do 2 relationship in this group. The high potency of Gln as a NH3 donor may be explained by the high intracellular concentrations of Gln (30 to 35 mM) obtained when livers from normal rats are perfused with 5 mM concentrations (18). When hepatic O2 demand was increased by adding Gln to the perfusate, Do 2crit also increased, suggesting that Do 2crit depends on V˙o 2. Similarly, Nelson and colleagues (19) showed that endotoxemia increases both systemic Do 2crit and V˙o 2max.
The fact that hepatic V˙o 2 doubled when Gln-perfused livers produced high amounts of urea is also surprising. However, when livers were perfused with low concentrations of Gln in the perfusate, urea release significantly decreased, whereas V˙o 2 remained constant. Thus, urea production did not account for the high O2 demand observed in Gln-perfused livers. Gln may be used in other pathways in hepatocytes and other hepatic cells such as Kupffer and endothelial cells (20). In a similar experimental model, when Gln was added to the perfusate, liver mass rapidly increased, with a net uptake of potassium followed by a marked release of potassium when the liver mass reached its new steady state (18). Additionally, transport of Gln in hepatocytes is associated with sodium entry, subsequent depolarization of the cell membrane, and activation of the Na+/K+-ATPase pump. Changes in the ion conductance through the membrane may explain the high V˙o 2 observed in these livers.
Limitations in our model must be pointed out. Livers are denervated and perfused only through the portal vein with a hemoglobin-free buffer. Thus, the high Po 2 might exaggerate the O2 gradient between vessels and hepatic cells. Hepatic V˙o 2 was in the range previously reported in livers perfused with KHB buffer in the absence of energy substrate (21). The liver weight/body weight ratio was similar to previous studies and the increased wet/dry weight ratio in Gln-perfused livers confirms previous reports.
In summary, our results demonstrate that hepatic urea production is dependent on Gln and O2 availability and becomes O2 supply-dependent at the same Do 2crit as determined by the V˙o 2-Do 2 relationship. Besides parameters of oxygenation, this finding points out the importance of studying the relationship existing between organ functions and flow. Whether the decrease in urea production at low Do 2 values represents an adaptative modification of hepatic metabolism away from urea production toward preservation of cell morphology remains to be determined. Study of additional hepatic functions should help to further understand the relationship between functions and flow, especially at low Do 2 values.
Supported by Grant GM-44100 from the National Institutes of Health and by Grant 3200-045985.95/1 from the Fond National Suisse de la Recherche Scientifique.
| 1. | Russel J. A., Phang P. T.The oxygen delivery/consumption controversy: approaches to management of the critically ill. Am. J. Respir. Crit. Care Med1491994533537 |
| 2. | Body O., Grounds R. M., Bennett E. D.A randomized clinical trial of the effect of deliberate perioperative increase of oxygen delivery on mortality in high-risk surgical patients. J.A.M.A.270199326992707 |
| 3. | Tuchschmidt J., Fried J., Astiz M., Rackow E.Elevation of cardiac output and oxygen delivery improves outcome in septic shock. Chest1021992216220 |
| 4. | Routsi C., Vincent J.-L., Bakker J., De Backer D., Lejeune P., d'Hollander A., Le Clerc J.-L.Relation between oxygen consumption and oxygen delivery in patients after cardiac surgery. Anesth. Analg77199311041110 |
| 5. | Schlichtig R., Klions H. A., Kramer D. J., Nemoto E. M.Hepatic dysoxia commences during O2 supply dependence. J. Appl. Physiol72199214991505 |
| 6. | Samsel R. W., Cherqui D., Pietrabissa A., Sanders W. M., Roncella M., Emond J. C., Schumacker P. T.Hepatic oxygen and lactate extraction during stagnant hypoxia. J. Appl. Physiol701991186193 |
| 7. | Keiding S., Vilstrup H., Hansen L.Importance of flow and haematocrit for metabolic function of perfused liver. Scand. J. Clin. Lab. Invest401980355359 |
| 8. | Hems R., Ross B. D., Berry M. N., Krebs H. A.Gluconeogenesis in the perfused rat liver. Biochem. J1011966284292 |
| 9. | Hamilton R. L., Berry M. N., Williams M. C., Severinghaus E. M.A simple and inexpensive membrane “lung” for small organ perfusion. J. Lipid. Res151974182186 |
| 10. | Samsel R. W., Schumacker P. T.Determination of the critical O2 delivery from experimental data: sensitivity to error. J. Appl. Physiol64198820742082 |
| 11. | Cain S. M., Curtis S. E.Experimental models of pathologic oxygen supply dependency. Crit. Care Med191991603612 |
| 12. | Frink E. J., Morgan S. E., Coetzee A., Conzen P. F., Brown B. R.The effects of sevoflurane, halothane, enflurane, and isoflurane on hepatic blood flow and oxygenation in chronically implanted greyhound dogs. Anesthesiology7619928590 |
| 13. | Nagano K., Gelman S., Parks D. A., Bradley E. L.Hepatic oxygen supply-uptake relationship and metabolism during anesthesia in miniature pigs. Anesthesiology721990902910 |
| 14. | Pinsky M. R., Schlichtig R.Regional oxygen delivery in oxygen supply-dependent states. Intensive Care Med161990S169S171 |
| 15. | Ward M. E., Chang H., Erice F., Hussain S. N. A.Systemic and diaphragmatic oxygen delivery-consumption relationships during hemorrhage. J. Appl. Physiol771994653659 |
| 16. | Schlichtig R., Kramer D. J., Pinsky M. R.Flow distribution during progressive hemorrhage is a determinant of critical O2 delivery. J. Appl. Physiol701991169178 |
| 17. | Nelson D. P., King C. E., Dodd S. L., Schumacker P. T., Cain S. M.Systemic and intestinal limits of O2 extraction in the dog. J. Appl. Physiol631987387394 |
| 18. | Häussinger D., Lang F., Bauers K., Gerok W.Interactions between glutamine metabolism and cell-volume regulation in perfused rat liver. Eur. J. Biochem1881990689695 |
| 19. | Nelson D. P., Samsel R. W., Wood L. D. H., Schumacker P. T.Pathological supply dependence of systemic and intestinal O2 uptake during endotoxemia. J. Appl. Physiol64198824102419 |
| 20. | Spolarics Z., Lang C. H., Bagby G. J., Spitzer J. J.Glutamine and fatty acid oxidation are the main sources of energy for Kupffer and endothelial cells. Am. J. Physiol2611991G185G190 |
| 21. | Guillem J. G., Clemens M. G., Chaudry I. H., McDermott P. H., Baue A. E.Hepatic gluconeogenic capability in sepsis is depressed before changes in oxidative capability. J. Trauma221982723729 |
