Comparison of cold storage and perfusion of dog livers on function of tissue slices

Poloxamer 188 and hydroxyethyl starch have a cryoprotective synergic effect improving post-thawing quality and fertility of rooster spermatozoa

Poloxamer and hydroxyethyl starch have cytoprotective effects. In the present study, effectiveness was evaluated of these compounds as a cryoprotectant for rooster semen. In Experiment 1 (E1), poloxamer 188 (1%, P188), poloxamer 407 (1%, P407), and control groups were compared after sperm cryopreservation. Experiment 2 (E2) was conducted with 3%, 5%, and 7% of hydroxyethyl starch (H3, H5, H7), also combined with P188 (H3P188, H5P188, H7P188), based on results from E1. Sperm motility was assessed using CASA, abnormal forms and hypo-osmotic swelling (HOS) were evaluated using microscopy, and viability, apoptotic-like changes, and mitochondrial activity were determined using flow cytometry. In E2, there were assessments of fertility and hatching capacity. Results from E1 indicated total and progressive motility, velocity, membrane functionality, viability, and mitochondrial activity were greater with inclusion of P188 in semen extender, with less apoptotic-like changes (P < 0.05). In E2, HES inclusion in semen extender improved total motility, membrane functionality, and mitochondrial activity (P < 0.05), especially H5, which also markedly increased sperm viability and decreased apoptotic-like changes. The combination of P188 with HES increased sperm quality overall, with inclusion of H5P188 resulting in increases of progressive motility and VSL (P < 0.05). The H5 inclusion also increased proportion of fertilized eggs (P < 0.05). Furthermore, the combination of HES and P188 increased proportions of fertilized and hatched eggs compared with the control, with inclusion of H5P188 having the greatest effects. In conclusion, supplementation of semen extender with H5P188 increases post-thawing quality and fertility of rooster sperm, being a safe and effective method for the poultry industry.

Adding the oxygen carrier M101 to a cold-storage solution could be an alternative to HOPE for liver graft preservation

Background & Aims

Hypothermic oxygenated machine perfusion (HOPE) is a promising technique for providing oxygen to the liver during graft preservation; however, because of associated logistical constraints, addition of an oxygen transporter to static cold-storage solutions (SCS) might be easier. M101 is marine worm haemoglobin that has been shown to improve kidney preservation in the clinic when added to SCS. This study evaluated the effects of the addition of M101 to SCS on the quality of pig liver graft preservation.

Methods

Pig liver grafts were preserved using SCS, HOPE, or SCS+M101, and the liver functions were compared during cold preservation and after orthotopic allotransplantation (OLT) in pigs.

Results

During preservation of the liver grafts, mitochondrial function, ATP synthesis, antioxidant capacities, and hepatocyte architecture were better preserved, and free radical production, antioxidant activities, and inflammatory mediators were lower, with HOPE or SCS+M101 than with SCS alone. However, after 1 h of preservation, liver functions with HOPE were superior to those with SCS+M101. After 6 h of preservation and OLT, blood levels of aspartate and alanine aminotransferases and lactate dehydrogenase increased with a peak effect at Day 1 post-transplant; values were similar with HOPE and SCS+M101, and were significantly lower than those in the SCS group. At Days 1 and 3, tumor necrosis factor α levels remained lower with HOPE and SCS+M101 vs. SCS. At Day 7, liver cell necrosis and inflammation were less marked in both oxygenated groups.

Conclusions

When added to SCS, M101 effectively oxygenates liver grafts during preservation, preventing post-transplant injury; although graft performances are below those achieved with HOPE.

Lay summary

When transported between donors and recipients, even cold-stored liver grafts need oxygen to maintain their viability. To provide them with oxygen, we added a marine worm super haemoglobin (M101) to the cold-storage solution UWCS. Using a pig liver transplant model, we revealed that livers cold stored with UWCS+M101 showed improved oxygenation compared with simple cold-storage solutions, but did not reach the oxygenation level achieved with machine perfusion.

Development of the Isolated Dual Perfused Rat Liver Model as an Improved Reperfusion Model for Transplantation Research

The Isolated Perfused Liver (IPL) model is a widely used and appreciated in vitro method to demonstrate liver viability and metabolism. Reperfusion is performed in a controlled setting, however, via the portal vein only. To study transplant related questions concerning bile and transport of bile, the in vitro Isolated dual Perfused Liver model is revisited. The IdPL is an in vitro reperfusion model, using both portal vein and hepatic artery. Livers from 12 Wistar rats were flushed with University of Wisconsin-organ preservation solution, procured and reperfused in either the conventional IPL-model (n=6) or the new IdPL-model (n=6). Liver injury, assessed by the release of aspartate amino transferase and lactate dehydrogenase, showed similar levels during both IPL and IdPL reperfusion, only alanine amino transferase showed an improvement. Cumulative bile production showed an improvement: 176.3 ± 8.4 in the IdPL compared to 126.1 ± 12.2 μg/g-liver in the IPL (p<0.05). Clearance of phenol red (PR) and taurocholic acid (TC) remained similar. At 90 minutes reperfusion the PR clearance showed 0.11 ± 0.01 and 0.11 ± 0.02 mg/30min/g-liver and the TC clearance 1.01 ± 0.10 and 1.01 ± 0.07 μmol/ml/30min/g-liver in the IPL and IdPL, respectively. Increasing the reperfusion time beyond the normally used 90 minutes resulted in a significant increase in transaminases and LDH and a decrease in bile production, liver morphology remained intact and glycogen content was appropriate. In conclusion, the IdPL-model showed similar or better results than the IPL-model, but the liver could not endure an extended reperfusion time using the IdPL.

Cryopreservation of whole murine and porcine livers

Preservation of vascularized organs, such as the liver, is limited to 24 h before destructive processes disqualify them for transplantation. This narrow window of opportunity prevents the performance of optimal pathogen screening and matching tests and possibly results in the need for retransplantation. Numerous problems are associated with freezing and thawing a whole liver while preserving its viability upon thawing, including complicated geometry, poor heat transfer, release of latent heat, and the difficulty of generating a uniform cooling rate. On the basis of our past success with sheep ovaries, we have now applied our novel freezing technique to a larger solid organ, the liver. Whole rat and pig livers were frozen and thawed using directional solidification apparatus, and viability of these livers was tested by means of integrity and functionality in vitro and in auxiliary liver transplantation. The thawed rat and porcine livers were intact and demonstrated >80% viability. Histology revealed normal architecture. Bile production and blood flow following auxiliary transplantation were normal as well. Our encouraging results in applying this novel cryopreservation technique in rat and pig livers suggest that this method may enable better human organ donor-recipient matching in the future.

Perspectives in organ preservation

Maintaining organ viability after donation until transplantation is critically important for optimal graft function and survival. To date, static cold storage is the most widely used form of preservation in every day clinical practice. Although simple and effective, it is questionable whether this method is able to prevent deterioration of organ quality in the present era with increasing numbers of organs retrieved from older, more marginal, and even non-heart-beating donors. This review describes principles involved in effective preservation and focuses on some basic components and methods of abdominal organ preservation in clinical and experimental transplantation. Concepts and developments to reduce ischemia related injury are discussed, including hypothermic machine perfusion. Despite the fact that hypothermic machine perfusion might be superior to static cold storage preservation, organs are still exposed to hypothermia induced damage. Therefore, recently some groups have pointed at the beneficial effects of normothermic machine perfusion as a new perspective in organ preservation and transplantation.

Oxygenation during hypothermic rat liver preservation: an in vitro slice study to demonstrate beneficial or toxic oxygenation effects

Hypothermic machine perfusion (HMP) of abdominal organs is shown to be superior compared to cold storage. However, the question remains if oxygenation is required during preservation as oxygen is essential for energy resynthesis but also generates toxic reactive oxygen species (ROS). To determine if oxygenation should be used during HMP, urea-synthesis rate, adenosine triphosphate (ATP), and generation of ROS were studied in an in vitro model, modeling ischemia-reperfusion injury. Furthermore, expression of uncoupling protein-2 (UCP-2) mRNA was assessed since UCP-2 is a potentially protective protein against ROS. Rat liver slices were preserved for 0, 24, and 48 hr in University of Wisconsin machine perfusion solution (UW-MP) with 0%, 21%, or 95% oxygen at 0-4 degrees C and reperfused for 24 hours. In the 0% and 95% groups, an increase of ROS was found after cold storage in UW-MP. After slice reperfusion, only the 0% oxygen group showed higher levels. The 0% group showed a lower urea-synthesis rate as well as lower ATP levels. mRNA upregulation of UCP-2 was, in contrast to kidney mRNA studies, not observed. In conclusion, oxygenation of UW-MP gave better results. This study also shows that ROS formation occurs during hypothermic preservation and the liver is not protected by UCP-2. We conclude that saturation of UW-MP with 21% oxygen allows optimal preservation results.

Oxygenated machine perfusion mitigates surface antigen expression and improves preservation of predamaged donor livers

The aim of the present study was to evaluate the potential benefit of machine preservation with the Belzer MPS or HTK solution, compared to standard cold storage, after procurement of marginal livers from non-heart beating donors in an experimental pilot study. Livers from male Wistar rats (250-300 g bw) were harvested after 60 min of cardiac arrest, flushed via the portal vein and cold stored submerged in HTK for 24 h at 4 degrees C while other organs were subjected to oxygenated machine perfusion with HTK or Belzer's MPS at 5 ml/min at 4 degrees C. Cold perfusion of livers with the non-colloidal HTK was not compromised by the lack of oncotic agents and there was no rise in vascular resistance during the 24 h of machine preservation with HTK or the colloidal Belzer MPS. Viability of the livers was assessed after the cold preservation period by warm reperfusion in vitro. Oxygenated machine perfusion was found to significantly increase viability of the livers vs simple cold storage with respect to portal vascular resistance upon reperfusion, enzyme release as well as functional recovery of oxygen utilization or bile production. Moreover, tissue antigen expression of ICAM-1 or histocompatibility antigen class II could be markedly reduced by oxygenated perfusion preservation as compared to cold storage. It is concluded that predamaged organs should preferably be preserved by oxygenated machine perfusion thus minimizing functional alterations and immunogenicity of the graft. In this setup HTK appeared equally effective as Belzer's MPS for machine preservation.

A role for sodium in hypoxic but not in hypothermic injury to hepatocytes and LLC-PK1 cells

BACKGROUND

Hypothermia is considered to be responsible for sodium influx during cold hypoxic incubation. However, we have previously shown that hypothermia alone leads to a pronounced decrease in cellular sodium content when liver endothelial cells or hepatocytes are incubated under such conditions. In the research described here, we therefore studied the effects of hypothermia and hypoxia, alone or combined, on cellular sodium homeostasis and assessed the role sodium plays in the pathogenesis of hypoxic and hypothermic injury to cultured liver and kidney cells.

METHODS

Isolated hepatocytes and LLC-PK1 cells were incubated in Krebs-Henseleit buffer or a sodium-free modification thereof under normoxic and hypoxic conditions at 4 degrees C as well as at 37 degrees C. Cytosolic sodium concentration was determined in isolated hepatocytes under both warm and cold conditions using digital fluorescence microscopy and the Na+-sensitive dye sodium-binding benzofuran isophthalate.

RESULTS

When hepatocytes were incubated under cold normoxic conditions the cellular sodium concentration decreased. However, it increased strongly under hypoxic conditions at 4 degrees C and at 37 degrees C. When either hepatocytes or LLC-PK1 cells were incubated under hypoxic conditions at 4 degrees C or 37 degrees C, sodium-free medium provided protection. In contrast, sodium-free medium did not alleviate the hypothermic injury observed when cells were incubated under cold normoxia.

CONCLUSIONS

The sodium influx observed during cold hypoxia is triggered by hypoxia and not by hypothermia. Sodium plays a prominent role in hypoxic injury to cultured liver and kidney cells, although hypothermic injury of these cells is independent of sodium homeostasis.

Alteration in cellular calcium and mitochondrial functions in the rat liver during cold preservation

BACKGROUND

Preservation injury is multifactorial and its mechanism is still incompletely defined. Calcium may play an important role in preservation injury.

METHODS

The effects of hypothermia on cytosolic free calcium concentration ([Ca2+]I) and total cellular calcium content in isolated rat hepatocytes were investigated by using fura-2 fluorescence and atomic absorption spectroscopy. Fura-2 loaded cells were placed into a prechilled (7 degrees C) cuvette equipped with a stirrer or preserved in the University of Wisconsin (UW) solution for up to 48 hr. In some experiments, cells were pretreated with inhibitors of Ca2+ release from mitochondria (m-iodobenzylguanidine [MIBG]) and from endoplasmic reticulum (ryanodine [RYA]) for 20 min at 37 degrees C. Mitochondrial functions after preservation were evaluated by measuring ATP and respiratory rates.

RESULTS

Cooling to 7 degrees C caused a rapid increase in [Ca2+]I that was substantially blocked by MIBG and RYA pretreatment. The elevated calcium gradually leaked out of the cells into the Ca2+-free medium. In long-term storage of the cells in the UW solution, there was a marked decrease in both cytosolic free calcium and total cellular calcium. Pretreatment of the livers with MIBG before cold preservation in the UW solution resulted in a stimulation of ATP regeneration in tissue slices. MIBG pretreatment also improved mitochondrial respiratory functions after cold preservation.

CONCLUSIONS

Thus, the loss of mitochondrial function after liver preservation in the UW solution may be related to the effects of hypothermia on calcium metabolism. Approaches to help maintain calcium homeostasis during storage may improve organ preservation.

Effect of flush-perfusion on vascular endothelial and smooth muscle function

BACKGROUND

The aim of this study was to investigate how much perfusion pressure an artery can tolerate without significant loss of endothelium-dependent relaxation (EDR) and vascular contractility.

METHODS

The abdominal aortas of 396 Sprague-Dawley rats were used. One hundred twenty aortas were flush-perfused for 1 or 5 minutes with cold St. Thomas' Hospital cardioplegic (STHC) solution or with the same solution but modified by the addition of 3.5% dextran 40. Three perfusion pressures were tested: 50, 100, and 150 mm Hg. Two hundred eighty vessels were subjected to pressures of 50, 150, or 300 mm Hg using saline or STHC solution at 22 degrees C or STHC solution at 4 degrees C, for 10 or 60 seconds. The vessels were investigated in organ baths. Contractility was tested with the thromboxane analogue U-46619, acetylcholine was used to investigate EDR, and papaverine to elicit endothelium-independent relaxation.

RESULTS

Flush-perfusion with cold STHC solution for 5 minutes at a perfusion pressure of 50 or 100 mm Hg affected neither contractility nor EDR. Vessels exposed to a flush-perfusion pressure of 150 mm Hg for 1 or 5 minutes lost 39% (p < 0.001) and 53% (p < 0.001) of their contractility, respectively. Flush-perfusion at 150 mm Hg for 1 minute did not affect EDR, whereas 5 minutes' perfusion caused a reduction of 7% (p < 0.05). A repetition of these experiments using STHC solution with 3.5% dextran 40 added gave no significantly different results. The impairment in contractility and EDR seen after perfusion at 150 mm Hg for 5 minutes disappeared after transplantation and reperfusion for 7 days. The vessels could be distended with saline or STHC solution at a pressure of 150 mm Hg without affecting contractility at 22 degrees C. At 4 degrees C, however, this pressure was harmful to contractility. Distention at a pressure of 300 mm Hg almost abolished contractility and 7 days after transplantation there had not yet been any recovery of contractility, but 30 days after transplantation the grafts had regained their normal contractility.

CONCLUSIONS

Cold STHC solution, with or without dextran 40, can be used with a perfusion pressure of 100 but not 150 mm Hg without impairing EDR or vascular smooth muscle function.

Hypothermic perfusion preservation of liver: the role of phosphate in stimulating ATP synthesis studied by 31P NMR

Hypothermic perfusion of rat livers was investigated by 31phosphorus nuclear magnetic resonance (31P NMR) spectroscopy using a temperature-controlled module that allowed data acquisition at various time points during a 48-h period. The livers were perfused with an oxygenated lactobionate/raffinose-based solution containing adenosine and inorganic phosphate, and changes in tissue oedema were monitored by direct on-line measurements of liver weight changes. Liver tissue ATP concentrations, determined by fluorimetric assay, were low immediately after organ removal, probably reflecting metabolic stress during the removal period, and these increased slightly during the next 3 h. This was reflected by changes in the 31P NMR spectra. However, by 24 h ATP levels had increased significantly, and these were maintained for up to 48 h, suggesting a shift in the balance between energy production and consumption. When inorganic phosphate was replaced by another anion (citrate), ATP was maintained at a constant lower level during perfusion for 48 h. Tissue weight changes were similar in both groups, suggesting that volume control was not affected by the different ATP contents of the livers. By combining the temperature-controlled module with a separate perfusion circuit, NMR spectroscopy can provide a sensitive method for following energy metabolism in the same organ over long periods during hypothermic perfusion.

Precision-cut tissue slices: applications in pharmacology and toxicology

Almost a decade has passed since the first paper describing the isolation and maintenance of precision-cut liver slices produced using a mechanical tissue slicer was published (1). Although tissue slices of various organs have been employed as an in vitro system for several decades, the lack of reproducibility within the slices and the relatively limited viability of the tissue preparations has prevented a widespread acceptance of the technique. The production of an automated slicer, capable of reproducibly producing relatively thin slices of tissue, as well as the development of a dynamic organ culture system, overcame several of these obstacles. Since that time, significant advances in the methods to produce and culture tissue slices have been made, as well as the application of the technique to several other organs, including kidney, lung and heart. This review will i) summarize the historical use of tissue slices prior to the development of the precision-cut tissue slice system; ii) briefly analyze current methods to produce precision-cut liver, kidney, lung and heart slices; and iii) discuss the applications of this powerful in vitro system to the disciplines of pharmacology and toxicology.

Safe lung preservation for twenty-four hours with Perfadex

The function of six porcine left lung allografts was studied after pulmonary (140 mL/kg) and bronchial (50 mL/kg) artery perfusion with Perfadex (Kabi Pharmacia, Uppsala, Sweden) at room temperature, followed by 24-hour storage of the lungs in an atelectatic state in 6 degrees to 8 degrees C Perfadex, which is a low-potassium-dextran solution. Left lung transplantation was done followed by right pneumonectomy, thereby making all the animals 100% dependent for their survival on the transplanted lungs. The pigs (mean weight = 56 kg, range = 51 to 58 kg, n = 18; 6 donors, 6 recipients, and 6 sham operated) were ventilated with a volume-controlled ventilator (20 breaths/min, 500 mL tidal volume, 8 cm H2O positive end-expiratory pressure, inspired oxygen fraction = 0.5). All the transplanted animals were in good condition throughout the 24-hour observation period with arterial oxygen tensions around 25 kPa (188 mm Hg) and arterial carbon dioxide tensions around 5 kPa (38 mm Hg). The mean pulmonary arterial pressure was around 30 mm Hg, and the pulmonary vascular resistance around 500 dyn.s.cm-5; neither showed any tendency to change with time. After 24 hours the inspired oxygen fraction was increased to 1.0 and the arterial oxygen tension increased to 43.3 +/- 5 kPa (325 +/- 38 mm Hg) (mean +/- standard error of the mean; n = 6). A sham operation (bilateral thoracotomy, right pneumonectomy) was done in 6 pigs, which were followed up for 24 hours.(ABSTRACT TRUNCATED AT 250 WORDS)

Safe pulmonary preservation for 12 hours with low-potassium-dextran solution

The function of porcine left lung allografts was studied after perfusion with (150 mL/kg) and storage for 12 hours in a 4 degrees to 6 degrees C low-potassium-dextran solution (Perfadex; Kabi Pharmacia AB, Uppsala, Sweden). After a left lung transplantation, an artificial lung in the form of venoarterial extracorporeal membrane oxygenation was established. The artificial lung has a "biological" heparin-coated surface (Carmeda AB, Stockholm, Sweden), and there is no need for systemic anticoagulation. Immediately thereafter, pneumonectomy of the normal right lung was done. All the animals were weaned from the artificial lung within 1 hour after the pneumonectomy. Six animals were followed up for 24 hours. They were in good condition throughout the 24-hour observation period with arterial oxygen tensions around 200 mm Hg (inspired oxygen fraction = 0.4) and arterial carbon dioxide tensions around 40 mm Hg. This study demonstrates a reliable method for continuous evaluation of the function of a transplanted lung immediately after transplantation and over the ensuing postoperative period. Safe 12-hour lung preservation can be obtained with the low-potassium-dextran solution Perfadex.

Effect of cold storage on tissue and cellular glutathione

One of the mechanisms thought to cause injury in preserved organs is the formation of oxygen free radicals. The cell is protected from oxidative stress by many defense mechanisms. A major defense mechanism involves glutathione and glutathione-dependent enzymes. During organ preservation by simple cold storage the loss of glutathione may sensitize the organ to free radical damage after transplantation. In this study we show that glutathione is depleted from the rabbit liver, kidney, and heart cold-stored (5 degrees C) for up to 72 h in the UW solution without glutathione. In the first 24 h kidney glutathione decreased to 84 +/- 3% of control values, liver glutathione decreased to 49 +/- 3% of control values, and heart glutathione decreased to 73 +/- 3% of control values. After 48 h of storage the kidney and liver lost an additional 30 and 20%, respectively, whereas heart glutathione changed very little. By 72 h all three organs had lost more than 50% of the glutathione found in freshly obtained tissue. To determine if glutathione added to the UW solution can effectively prevent this loss of glutathione during preservation, hepatocytes were cold-stored for up to 72 h in a preservation solution with and without glutathione. We found that adding glutathione to the preservation solution slowed the rate of loss of glutathione from the cells. These data suggest that at hypothermia the cell may be permeable to GSH. Methods to suppress the loss of glutathione during preservation of organs may be an important factor in suppressing oxygen free radical injury.

Liver slices in dynamic organ culture. II. An in vitro cellular technique for the study of integrated drug metabolism using human tissue

1. Precision cut human liver slices in dynamic organ culture have been used to study the integrated metabolism of 7-ethoxycoumarin and the conjugation of 7-hydroxycoumarin. 2. The metabolism of 7-ethoxycoumarin and 7-hydroxycoumarin was monitored for 6 h. For both substrates there was a time-dependent increase in metabolites present in the incubation medium. The low levels of free 7-hydroxycoumarin found in the medium when 7-ethoxycoumarin was the substrate suggests good coupling of phase I and phase II metabolism. 3. With suitable incubation conditions, i.e. change of medium containing new substrate every 2 h, the metabolism of both 7-ethoxycoumarin and 7-hydroxycoumarin by human liver slices was found to proceed at similar rates for up to 24 h. This was demonstrated using five separate human liver preparations. 4. Human liver slices also metabolized mono-chlorobenzene and o-, m- and p-dichlorobenzene to aqueous soluble metabolites. There was a time-dependent increase in the appearance of aqueous soluble metabolites present in the incubation medium. Metabolites were not retained by the liver slices. 5. A cold-storage transit buffer has been described and used to maintain the levels of drug metabolism in both rat and human tissue for periods of up to 6 h. 6. The use of human liver slices in dynamic organ culture as a suitable method for the direct assessment of integrated hepatic drug metabolism is proposed.

Effect of drug treatment on liver-slice function following 72-hour hypothermic perfusion

The viability of hypothermically perfused dog liver was evaluated with a tissue-slice technique. After being preserved for 72 hr, slices of liver were incubated at 30 degrees C for as long as 2 hr; then water content, K+/Na+ ratio, and ATP concentration were measured. Dog livers were assigned to the following experimental groups: Group 1 (no preservation; control); Group 2 (livers preserved for 72 hr); Group 3 (donor animals pretreated with 3.5 mg/kg of chlorpromazine (CPZ) and 20 mg/kg of methylprednisolone (MP), and livers preserved for 72 hr); Group 4 (livers pretreated with 2-deoxycoformycin (2-DOC), 50 mg/liter, and preserved for 72 hr); and Group 5 (combination of Group 3 and Group 4 treatments). Livers in Groups 2, 3, and 4 lost K+ during preservation, and the mean K+/Na+ ratio significantly decreased from a control value of 4.2 +/- 0.4 to 1.5-1.9 (P less than 0.05). Group 5 livers did not lose K+; mean K+/Na+ ratio was 3.9 +/- 0.5. Fresh livers (no preservation) rapidly reaccumulated K+ when the tissue slices were incubated for 2 hr at 30 degrees C; mean K+/Na+ ratio was 3.7 +/- 0.5. Tissue slices from Group 2 livers (72 hr preservation), and livers pretreated with CPZ-MP (Group 3) or pretreated with 2-DOC (Group 4) did not significantly reaccumulate K+ at 30 degrees C; mean K+/Na+ ratio was 1.7-2.1. Only slices prepared from liver pretreated with both CPZ-MP and 2-DOC reaccumulated K+; mean K+/Na+ ratio was 4.6 +/- 1.2.(ABSTRACT TRUNCATED AT 250 WORDS)