Small intestinal function following syngeneic transplantation in the rat

Paracellular nutrient absorption in a gum-feeding new world primate, the common marmoset Callithrix jacchus

The common marmoset is one of the few callitrichid species that is not threatened or endangered in the wild, and is widely used in biomedical research, yet relatively little is understood about its digestive physiology. Dietary specialization on plant exudates has lead to relatively reduced small intestines, yet the common marmoset has exceptional dietary breadth, allowing it to successfully utilize a variety of habitats. We predicted that passive, paracellular nutrient absorption would be used by the common marmoset to a greater extent than in other non-flying mammals. We measured the bioavailability and rates of absorption of two metabolically inert carbohydrates not transported by mediated pathways (L-rhamnose and cellobiose, molecular masses of 164 and 342, respectively) to measure paracellular uptake, and of a non-metabolized D-glucose analog (3-O-methyl-D-glucose) to measure total uptake by both mediated and paracellular pathways. We found high bioavailability of 3-O-methyl-D-glucose (83+/-5%), and much higher bioavailability of the paracellular probes than in similarly sized non-flying mammals (30+/-3% and 19+/-2% for L-rhamnose and cellobiose, respectively). Passive, paracellular nutrient absorption accounts for around 30% of total glucose absorption in common marmosets and intestinal permeability is significantly higher than in humans, the only other species of primate measured to date. This may allow the common marmoset to maintain high digestive efficiency when feeding on higher quality foods (fruit, arthropods, gums with higher proportions of simple sugars), in spite of relatively reduced small intestines correlated with adaptations for fermentative digestion of plant gums. We find no evidence to support, in primates, the hypothesis that reliance on paracellular nutrient absorption should increase with body size in mammals, but suggest instead that it may be associated with small body size and/or taxon-specific adaptations to diet.

Paracellular absorption: a bat breaks the mammal paradigm

Bats tend to have less intestinal tissue than comparably sized nonflying mammals. The corresponding reduction in intestinal volume and hence mass of digesta carried is advantageous because the costs of flight increase with load carried and because take-off and maneuverability are diminished at heavier masses. Water soluble compounds, such as glucose and amino acids, are absorbed in the small intestine mainly via two pathways, the transporter-mediated transcellular and the passive, paracellular pathways. Using the microchiropteran bat Artibeus literatus (mean mass 80.6±3.7 g), we tested the predictions that absorption of water-soluble compounds that are not actively transported would be extensive as a compensatory mechanism for relatively less intestinal tissue, and would decline with increasing molecular mass in accord with sieve-like paracellular absorption. Using a standard pharmacokinetic technique, we fed, or injected intraperitonealy the metabolically inert carbohydrates L-rhamnose (molecular mass = 164 Da) and cellobiose (molecular mass = 342 Da) which are absorbed only by paracellular transport, and 3-O-methyl-D-glucose (3OMD-glucose) which is absorbed via both mediated (active) and paracellular transport. As predicted, the bioavailability of paracellular probes declined with increasing molecular mass (rhamnose, 90±11%; cellobiose, 10±3%, n = 8) and was significantly higher in bats than has been reported for laboratory rats and other mammals. In addition, absorption of 3OMD-glucose was high (96±11%). We estimated that the bats rely on passive, paracellular absorption for more than 70% of their total glucose absorption, much more than in non-flying mammals. Although possibly compensating for less intestinal tissue, a high intestinal permeability that permits passive absorption might be less selective than a carrier-mediated system for nutrient absorption and might permit toxins to be absorbed from plant and animal material in the intestinal lumen.

The digestive adaptation of flying vertebrates: high intestinal paracellular absorption compensates for smaller guts

Anecdotal evidence suggests that birds have smaller intestines than mammals. In the present analysis, we show that small birds and bats have significantly shorter small intestines and less small intestine nominal (smooth bore tube) surface area than similarly sized nonflying mammals. The corresponding >50% reduction in intestinal volume and hence mass of digesta carried is advantageous because the energetic costs of flight increase with load carried. But, a central dilemma is how birds and bats satisfy relatively high energy needs with less absorptive surface area. Here, we further show that an enhanced paracellular pathway for intestinal absorption of water-soluble nutrients such as glucose and amino acids may compensate for reduced small intestines in volant vertebrates. The evidence is that l-rhamnose and other similarly sized, metabolically inert, nonactively transported monosaccharides are absorbed significantly more in small birds and bats than in nonflying mammals. To broaden our comparison and test the veracity of our finding we surveyed the literature for other similar studies of paracellular absorption. The patterns found in our focal species held up when we included other species surveyed in our analysis. Significantly greater amplification of digestive surface area by villi in small birds, also uncovered by our analysis, may provide one mechanistic explanation for the observation of higher paracellular absorption relative to nonflying mammals. It appears that reduced intestinal size and relatively enhanced intestinal paracellular absorption can be added to the suite of adaptations that have evolved in actively flying vertebrates.

Acute denervation alters the epithelial response to adrenoceptor activation through an increase in alpha1-adrenoceptor expression on villus enterocytes

Loss of sympathetic input due to intestinal denervation results in hypersensitivity and increased intestinal secretion. It is unknown whether denervation-induced alterations in intestinal epithelial physiology are the result of changes in adrenoceptors on enterocytes (ENTs). The purpose of this study was to examine adrenoceptor distribution and pharmacology on small intestinal ENTs following acute intestinal denervation. Lewis rats underwent small bowel transplantation (SBT) or sham operation and proximal small intestinal segments were harvested 1, 2 and 4 weeks postoperatively. Intestinal electrolyte movement was assessed using short-circuit current (Isc) measurements of stripped epithelial sheets following stimulation with phenylephrine (PE), an alpha(1)-adrenoceptor agonist. The presence of adrenoceptor subtypes on separated villus and crypt ENTs was assessed using flow cytometry. Alpha(1)-adrenoceptors were found on approximately 27% of jejunal villus ENTs, but not crypt ENTs, following acute extrinsic denervation. ENTs from the Lewis rat have few beta-adrenoceptors. Alpha(1)-adrenoceptor stimulation of acutely denervated intestinal epithelial sheets decreased Isc by -13.45%. This effect was mediated by a reduction in chloride (Cl(-)) secretion; the absence of Cl(-) reversed the Isc to +13.79%. In conclusion, loss of sympathetic innervation to the gastrointestinal epithelium causes acute upregulation of alpha(1)-adrenoceptors on villus ENTs, leading to inhibition of Cl(-) secretion at the villus tip. The increase in adrenoceptors may reflect a compensatory mechanism to combat the increased secretory state of the bowel due to the loss of the sympathetic innervation and tonic control over intestinal secretion.

Effect of orthotopic small bowel transplantation on mineral metabolism in an experimental model

BACKGROUND

Numerous experimental models have been described for investigation of short bowel syndrome. The aim of this study was to examine the effect of orthotopic small bowel transplantation (OSBT) on universal metabolism in an inbred rat model, with particular emphasis on mineral metabolism.

METHODS

Jejunoileal resection and syngeneic OSBT was performed in 12-week-old male Lewis rats. Metabolic studies were performed over the following 16 weeks. Bones were analysed by physicochemical methods, dual X-ray absorptiometry, biomechanical procedures and histomorphometry. Biochemical markers of bone turnover were also measured.

RESULTS

Jejunoileal resection induced severe short bowel syndrome with profoundly reduced food efficiency, bone size, fracturing energy and bone mineral content, but no cancellous bone osteopenia. After OSBT rats showed normal growth; bones were of normal size, and bone mineral content and fracturing energy were similar to those in sham-operated controls. However, tibial, but not vertebral, cancellous bone osteopenia was found after transplantation.

CONCLUSION

OSBT with portal venous drainage achieves almost optimal mineral and bone metabolism. In the absence of immunosuppressive therapy, OSBT does not appear to have major untoward side-effects on bone in rats.

Intestinal transplantation

Advances in immunosuppressive treatment as well as better monitoring and control of acute rejection have brought intestinal transplantation (ITx) into the realm of standard treatment for permanent intestinal failure. The results from the intestinal Transplant International Registry (www.intestinaltransplant.org) indicate that ITx is currently an acceptable clinical modality for selected patients with permanent intestinal failure. The goal of this short review is to deal with indications, clinical results and complications of ITx. Although it has been used in humans for the past two decades, very few data are available regarding graft function and its monitoring.

Modified techniques of heterotopic total small intestinal transplantation in rats

AIM

To establish a successful model of heterotopic total small intestinal transplantation (SIT) in rats in order to reduce the complications and increase the survival rate.

METHODS

A total of 196 Wistar rats underwent heterotopic SIT with microsurgical technique. Technical modifications included shortening fasting time and supplying energy before surgery, administering optimal volume of crystalloid fluid to the donor and recipient during surgical procedures, reducing mechanical and ischemic injuries to donor intestine, revascularizing small intestinal graft with a combination of conventional aorta to aorta anastomosis and a cuffed portal vein to left renal vein anastomosis which resulted in an acceptably short warm ischemic time, and also an adequate blood supply and drainage of the graft.

RESULTS

The average time for the donor surgery was 86 min +/- 20 min, the mean operative time for the recipient was 115 min +/-20 min and warm ischemia time was shortened to 40 min +/- 5 min. There was a shorter revascularizing time of the graft, the abdominal aorta (AA) to AA anastomosis being 21 min +/- 10 min, and the cuffed portal vein (PV) to the renal vein anastomosis being 5 min +/- 5 min. The one-week survival rate of 98 rats with SIT was 88.78% (87/98), without thrombosis and stenosis of anastomosis. The longest survival time of recipient rats was more than 389 days after SIT, the rats were maintaining normal weight, with perfect intestinal function and intact intestinal histology.

CONCLUSION

These modified techniques for SIT would remarkably reduce the complications and improve survival rate in rats, which provided a potentially more consistent and practical model for experimental and clinical studies.

Expression of peptide transporter following intestinal transplantation in the rat

BACKGROUND

The absorptive function of the intestinal graft is one of the most important factors for successful intestinal transplantation. To clarify whether the intestinal H(+)/peptide cotransporter (PEPT1) was expressed in the transplanted intestine, we examined the expression of PEPT1 in an experimental model of rat small intestinal transplantation in comparison with expression of Na(+)/glucose cotransporter (SGLT1).

MATERIALS AND METHODS

Heterotopic intestinal transplantation was performed in allogeneic and syngeneic rat strain combinations. An additional group of allogeneic recipients was treated with tacrolimus (1 mg/kg) prior to transplantation, then daily for 7 days. Intestinal grafts were examined for histopathology and PEPT1 and SGLT1 expression.

RESULTS

In the isografts, the levels of messenger RNA (mRNA) encoding both transporters were not changed, while the amount of SGLT1 protein was decreased and that of PEPT1 protein was increased. In the allografts, mRNA level and protein amount of both transporters and the amount of villin protein were decreased, and microscopic examination revealed histopathological features of rejection on day 7. Tacrolimus treatment ameliorated the histopathological features and prevented the decrease in villin protein expression. However, the decreases in PEPT1 and SGLT1 expression (both mRNA and protein) were partially prevented by tacrolimus treatment.

CONCLUSION

This study indicated that the expression of transporters should be determined to evaluate intestinal graft function in addition to histopathological examination of the mucosa and that the levels of mRNA encoding intestinal nutrient transporters in biopsy specimens may be useful for evaluating the intestinal graft function for intestinal transplant patients.

Metabolic consequences of orthotopic pancreaticoduodenal transplantation with preservation of near normal physiology

BACKGROUND

Several case reports suggested the use of pancreaticoduodenal allotransplantation alone or in combination with multivisceral transplants to treat exocrine and endocrine deficiency after pancreatectomy for chronic pancreatitis, upper abdominal malignancies, and cystic fibrosis. Our objective was to establish the metabolic consequences of this technique.

METHODS

Inbred rats, which either underwent pancreaticoduodenectomy before receiving an orthotopic duodenopancreas transplant (Tx, n= 18) or laparotomy (sham, n=18), were subjected 3 months postoperatively to oral and "isoglycemic" i.v. glucose tolerance tests with arterial blood sampling (n=12) or oral glucose tolerance test with additional portal blood sampling (n=6). Fecal fat and chymotrypsin were evaluated in the 11th postoperative week as indicators of pancreatic exocrine function in eight animals of each group.

RESULTS

The incremental arterial plasma glucose integrated over a 90-min period was similar after oral and i.v. glucose in the respective groups, but was significantly lower in Tx versus sham rats after oral glucose. Incremental portal glucose was also lower after oral glucose, while hepatic glucose extraction remained unchanged. The incremental response of arterial glucose-dependent insulinotropic peptide, and of arterial and portal insulin, was comparable in Tx and sham rats; also in both groups the arterial response was significantly greater with oral versus i.v. glucose, and the incretin effect for insulin was intact after transplantation. Fecal fat and chymotrypsin levels did not differ between the two groups.

CONCLUSIONS

1) In the Tx rat lower incremental plasma glucose after oral glucose intake likely results from decreased intestinal glucose uptake; 2) preservation of a normal entero-insular axis of insulin together with the absence of intestinal malabsorption of lipids suggest that orthotopic transplantation of a duodeno-pancreas preserved endocrine and exocrine pancreatic function and therefore qualifies as treatment modality for the above named indications.

Intestinal transplantation: indications, results and strategy

The term 'intestinal failure' is now often used to describe gastrointestinal function insufficient to satisfy body nutrient and fluid requirements. The first recognized condition of intestinal failure was short bowel syndrome. Severe motility disorders such as chronic intestinal pseudo-obstruction syndrome in children as well as congenital intractable intestinal mucosa disorders are also forms of intestinal failure, because no curative treatment for these diseases is yet available. Parenteral nutrition and home parenteral nutrition remain the mainstay of therapy for intestinal failure, whether it is partial or total, provisional or permanent. However, some patients develop complications while receiving standard therapy for intestinal failure and are considered for intestinal transplantation. Indeed, recent advances in immunosuppressive treatment and the better monitoring and control of acute rejection have brought intestinal transplantation into the realm of standard treatment for intestinal failure. Although it has been used in humans for the past two decades, this procedure has had a slow learning curve. According to the current results, this challenging procedure may be performed in children or adults, only under certain conditions.

Disaccharidase activities and fat assimilation in pediatric patients after intestinal transplantation

BACKGROUND

Intestinal transplantation has become an accepted therapy for short bowel syndrome and other types of intestinal failure. In order to assess digestive capabilities and feeding practices in a group of 22 pediatric patients after intestinal transplantation, we assessed mucosal disaccharidase activities and assimilation of total dietary lipid and vitamin E. Twelve of the patients had undergone contemporaneous liver transplantation.

METHODS

Mucosal biopsies were assayed for disaccharidase activities between 15 and 412 days after transplantation in 7 of the 22 when all were receiving some enteral nutrition and were free of rejection. Coefficients of lipid absorption were determined in those patients receiving total enteral feeding (two-thirds polymeric/one-third elemental) between 43 and 1032 days after transplantation; oral vitamin E tolerance tests were done at about the same time.

RESULTS

Activities of lactase, sucrase, maltase, and palatinase consistently exceeded reference ranges (P<0.05). Mean coefficient of lipid absorption equaled 86+/-12% and was not influenced by duration of time after transplantation. No patient required dietary lipid restriction. No significant absorption of vitamin E was demonstrated until 160 days after transplantation. Vitamin E absorption did correlate with length of time elapsed after surgery (r=0.64, P<0.0011).

CONCLUSIONS

The results of this investigation show that, in the absence of histologic or clinical indications of allograft rejection, pediatric intestinal transplant recipients do not have primary disaccharidase deficiencies. Similarly, absorption of usual dietary lipid content is adequate once weaning from parenteral nutrition is complete. In contrast, early assimilation of vitamin E is poor. Vitamin E absorption subsequently improves, but the mechanism is obscure.

Hormonal therapy for short bowel syndrome

PURPOSE

Treatment of short bowel syndrome (SBS) can be difficult; this study examines the effect of parental administration of different peptide hormones in a rat model of SBS.

METHODS

Juvenile male Lewis rats (220 to 240 g) underwent resection of the proximal 90% of small bowel and were assigned randomly to treatment groups: growth hormone (GH), insulinlike growth factor-1 (IGF-1), glucagonlike peptide-2 (GLP-2; given as ALX-0600, a potent protease resistant analogue of the human GLP-2), control-resected (Con-R), or control-transected (Con-T). Drugs were delivered by continuous subcutaneous infusion via Alzet mini-pumps: controls received equivalent volumes of drug vehicle. Animals were pair-fed (23 g chow per day) and followed up for 14 days monitoring weight gain. Animals were killed and active transport, hormone profiles, and intestinal morphology were assessed.

RESULTS

Hormonal treatments significantly increased weight gain in all groups (GH, 9.9+/-4.9; IGF-1, 6.0+/-9.6; and GLP-2, 0.8+/-2.7 v. -6.2+/-4.7 in untreated resected animals [weight as percentile initial weight]). This was associated with a significant alteration in intestinal morphology in the IGF-1-treated animals, and an increase in glucose transport rates in all hormonally treated animals when compared with untreated control resected animals.

CONCLUSIONS

These results show that IGF-1, GH, and GLP-2 all improve short-term weight gain after massive bowel resection in a rat model. The effects seen on weight gain may be caused by improved dietary nutrient absorption from an increase in the intestinal surface area or increase in transporting activity or alterations in the metabolic efficiency of the animal. These findings suggest further studies of these therapies as treatment for short-bowel syndrome are indicated.

Impact of FK506 and steroids on adaptation after intestinal resection or segmental transplantation

Segmental small intestinal transplantation (SIT) using living related donors (LRD) is being evaluated as a therapy, clinically. Advantages of this technique include an increase in the donor pool, optimization of the timing of transplants, and potential immunologic benefits. However, the ability of a short segment of intestine to function after transplantation has not been investigated in large animal models. This study evaluates the impact of immunosuppression on the adaptive process and the ability of a transplanted segment of intestine to adapt. A pig model of segmental SIT was used. Animals were resected, leaving 150 cm of distal ileum (n = 5), resected and treated with FK506 (n = 4), or steroids (n = 4), or with FK506 + steroids (n = 7), or transplanted using a similar segment of ileum and treated with FK506 + steroid immunosuppression (n = 9). Animals undergoing resection, or resection plus steroid treatment, did well, gaining weight post-operatively (37% and 15% of preoperative weight, respectively). However, animals undergoing resection and treated with FK506 or FK506 + steroids did poorly, losing weight (-14% and -22% of preoperative weight, respectively) and showing significant impairment of intestinal adaptation, morphologically and functionally. Furthermore, FK506-treated animals developed inflammatory changes in the intestinal mucosa, mimicking rejection. Segmental SIT animals had a high rate of rejection (66%) and showed a similar impairment in adaptation. Hence, segmental SIT is a stringent physiological test of intestinal adaptation. FK506 appears to impair gut function after resection, either directly, or by interfering with the adaptive process. In this model of segmental SIT, FK506 and steroids at the doses tested did not provide adequate immunosuppression to prevent rejection and the graft did not function adequately to allow growth. Further studies are required to evaluate the mechanisms underlying these findings, and to determine if similar effects occur in humans.

Intestinal transplantation

Intestinal transplantation might become the alternative to definitive parenteral nutrition in patients with permanent intestinal failure. Indeed, recent advances in immunosuppressive treatment and better monitoring and control of acute rejection have brought intestinal transplantation into the realms of standard treatment of intestinal failure. This procedure may be performed in adult or paediatric patients under certain conditions. This short review focuses on the current clinical results and indications for intestinal transplantation and discusses the strategy regarding this challenging procedure.

Fat absorption after small intestinal transplantation in the rat

BACKGROUND

Intestinal transplantation is now used for patients with severe malabsorption, however, little data exists quantifying the ability of the graft to absorb fat. This study tested the hypothesis that intestinal transplantation would not affect the lymphatic or venous uptake of fatty acids.

METHODS

A syngeneic rat model of intestinal transplantation (SIT) with caval drainage of the graft was used. Control animals underwent intestinal division and reanastomosis (n=15 in each group). The animals were followed for 6 weeks, and fat absorption in vivo was quantified. The animals were anesthetized, sampling catheters were placed in the jugular and superior mesenteric veins and in the mesenteric lymphatic duct, and a feeding tube was passed into the duodenum. Animals were allowed to recover, and a steady-state duodenal infusion of lauric (C12:0) and palmitic (C16:0) fatty acid emulsion was begun. A radiolabeled pulse of lauric (C12:0) and palmitic (C16:0) fatty acid was then given, and the subsequent appearance in the lymphatic and venous systems was quantified.

RESULTS

In vivo absorption of dietary fat was preserved, but after transplantation the mesenteric lymphatic flow and cumulative lymphatic appearance of both labels was significantly reduced (flow reduced from 4.8+/-1.1 in controls to 1.0+/-0.29 ml/hr in transplant animals, whereas lauric acid absorption was 33+/-11.4% in controls vs. 7.5+/-2.5% in transplant animals). There was a modest increase in the jugular venous appearance of the fatty acids (2.0+/-1.1% in transplant animals vs. 0.75+/-0.55% in controls for lauric acid; P<0.05 for all comparisons). Absorption of lauric and palmitic acids was very similar, and there was no preferential absorption detected in the portal venous system. Dye studies demonstrated lymphatic recannulization around the vascular anastomosis, into the retroperitoneum.

CONCLUSIONS

These results suggest that in this model of SIT, fat absorption via the mesenteric duct is reduced, but that compensatory collaterals form into the retroperitoneal lymphatics. There was no evidence of any significant increase in portal venous uptake of fatty acids after SIT, nor of preferential absorption of medium-chain fatty acids. These results may have implications for patients after SIT.

Nutritional and intestinal effects of the novel immunosuppressive agents: deoxyspergualin, rapamycin, and mycophenolate mofetil

OBJECTIVES

Transplantation of the small intestine would be an attractive therapeutic option for treatment of short bowel syndrome if effective, nontoxic immunosuppressive agents could be developed. This study examines the effect of three newly developed immuno-suppressive agents: rapamycin, deoxyspergualin, and mycophenolate mofetil, on the nutritional status and intestinal function of normal juvenile rats.

DESIGN & METHODS

Rapamycin (2 mg/kg every second day), deoxyspergualin (2 mg/kg every second day) and mycophenolate mofetil (MM) (25 mg/kg every second day) were injected subcutaneously for six weeks.

RESULTS

Rapamycin and deoxyspergualin caused significant reductions in weight gain without impairing feed intake. Both drugs caused small decreases in fat absorption; treatment with DSG induced an increase in permeability to 99Tc-DTPA. However, the permeability to other markers, such as mannitol and lactulose, was decreased in the rapamycin and mycophenolate mofetil-treated animals. Intestinal function in vitro was quantified using glucose flux (absorption). In the rapamycin group, there was a significant decrease in ileal uptake of glucose, with the net flux (absorption) being zero; there was an associated loss of villous size histologically. In the deoxyspergualin-treated groups, there was a decrease in the jejunal glucose flux. In the mycophenolate mofetil-treated animals, there was a decrease in jejunal with a compensatory increase in ileal glucose absorption. There were minor variations in intestinal morphology, but these were not consistent.

CONCLUSIONS

Rapamycin and deoxyspergualin in these doses cause a significant reduction in weight gain in healthy juvenile animals, and all the drugs caused changes in the active transport characteristics of the intestine. Accordingly, the use of these drugs for intestinal transplantation should be evaluated carefully for their nutritional impact.