Intravenous glutamine or limited enteral feedings in piglets: amelioration of small intestinal disuse atrophy.

In hospitalized patients, a multitude of factors including loss of luminal trophic factors, altered splanchnic blood flow and nutritional deprivation lead to gastrointestinal mucosal atrophy. Clinical orders of nothing per os (NPO) or “bowel rest” cause intestinal atrophy, which leads to abnormal enteric flora with bacterial, endotoxin or cytokine translocation, altered host immune function and sepsis (Barber et al.1991, Zaloga 1994). The prevention or limitation of intestinal atrophy during small bowel disuse or “rest” would have clinical significance if fewer hospital days and lower infection rates resulted. Glutamine is the preferred respiratory fuel for enterocytes; administration of 2% glutamine solutions is known to be beneficial in the short term. However, glutamine is more like a conditionally essential amino acid, necessary only during recovery to stimulate DNA synthesis and increase mucosal mass (Lacey and Wilmore 1990). Glutamine administered intravenously (IV) long term to animals with intestinal atrophy has not been adequately investigated in clinically relevant species. Enteral feedings are also known to be beneficial in preventing intestinal disuse. Data in critically injured patients suggest that enteral nutrition had a positive effect on intestinal immune function and leads to decreased infectious morbidity and reduced hospital stay (Heyland et al.1993). Possible advantages of feeding small volumes enterally during prolonged periods of small bowel “rest” therapy have also not been investigated, but such feeding is quite possible in most hospitalized cases as a means of minimizing intestinal atrophy. The objective of this study was to compare the small intestinal response to intravenous glutamine vs. intermittent small volume enteral feedings, administered for .1 wk as a means of preventing intestinal atrophy. The piglet has become the preferred small intestinal research model because of the homology among the swine, human, canine, feline and equine small bowels. The small intestines of the dog, cat, pig and foal are very similar in morphology and physiology through the first year of life, and the atrophic response to NPO also appears to be similar. The information derived from a swine research model of small intestinal atrophy is, therefore, relevant to the small bowel changes after disuse in dogs, cats and foals. Materials and methods. Five litters of Hampshire × Duroc piglets were selected on the basis of body weight (BW) (1.8 –2.7 kg) at 4 d of age. Four litter mates were randomly assigned to one of the following four treatments: 1) 100% enterally fed sow’s milk replacer (SMR); 2) total parenteral nutrition (TPN); 3) TPN with 1.7% (wt/v) glutamine (GLN); or 4) 90% of total energy as TPN plus 10% enterally as SMR (PEN). The TPN treatment served as a negative control, whereas SMR served as a positive control. Because of logistical limitations, four litter mates (1 piglet/treatment) completed the 28-d feeding trial at a time. Piglets were maintained in accordance with the Animal Care Committee guidelines at the Johns Hopkins Medical Institutions and the Committee on Care and Use of Laboratory Animals from the Institute of Laboratory Animal Resources. Nutritional treatments. All piglets suckled sow’s colostrum/ milk until delivered to the laboratory at 4 d of age; they also received an iron dextran injection (100 mg intramuscularly) at 3 and 10 d of age. Catheters (1.0 mm i.d., 2.1 mm o.d., 91 cm) of medical grade silastic tubing (Dow Corning, Midland, MI) were surgically placed in the left and right external jugular veins of all piglets. The catheter tips were inserted into the cranial vena cava; the proximal end of the catheter was tunneled to the dorsum, passing cranially to the shoulder and exiting 5–10 cm posterior to the ear. One catheter was used for parenteral administration, and the second was used for blood sampling. Weekly blood samples were drawn for complete blood cell count with a differential and serum profile. All piglets were individually housed in stainless steel (60 × 60 × 60 cm3) cages with plastic mesh-covered screened floors, and maintained in nylon open mesh vests with an attached stainless steel tether line (Spalding Medical Research Products, Birmingham, AL) and swivel infusion system. All piglets were administered 630 kJ/(kg BW • d) of their respective nutritional treatments on the basis of daily BW and were maintained on these diets as the sole source of nutrition for 4 wk. TPN solution was formulated to have a nutrient profile recommended (NRC1988) for piglets <5 kg BW (Table 1). The amino acid component of the TPN solution was made by adding 56 g nonessential amino acids/L (Sigma Chemical, St. Louis, MO) to a commercial 11.4% amino acid product (Novamine, Clintec Nutrition, Deerfield, IL), whereas the GLN solution was formulated by adding 56 g glutamine/L. The 20% lipid solution (Intralipid, Kabivitrum, Alameda, CA) was infused by a separate pump and connection into the catheter line. The final parenteral solution administered to GLN piglets contained 17 g glutamine/L. The parenteral solutions were made once every 24 h and administered continuously for 28 d. The sow’s milk replacer (SMR) (Acidified Pig Milk Replacer, Milk Specialties, Dundee, IL) (Table 1) was a whey-based commercial product designed for young growing piglets. A 10% SMR solution was made daily and offered to the SMR piglets in amounts equal to the caloric intake of the TPN and GLN piglets. PEN piglets received 90% of their caloric intake similarly to the TPN piglets and the remaining 10% as SMR piglets. The daily allotted volume of SMR was offered at one time to the SMR and PEN piglets by using a plastic feeder attached to the cage and was consumed in a single-meal feeding pattern. Intestinal assays and data compilation. After 4 wk of feeding, piglets were anesthetized; a 20-cm jejunal segment was removed 30 cm distal to a suspensory ligament of the duodenum, and a 20-cm ileal segment was excised 10 cm proximal to the ileocecal sphincter. Segments were flushed with cold saline, weighed and the length was measured with a 15-g free-hanging weight. Pieces 2 cm in length from both segments were either fixed in 10% buffered formalin for histology or had mucosa scraped free, frozen in liquid nitrogen and stored at -70°C for mucosal disaccharidase assay. Piglets were killed by exsanguination while under general anesthesia as the visceral organs were removed and weighed. The remaining intestinal tract was flushed free of digesta with tap water and measured in sections as suspended with a 15-g free-hanging weight. The total small intestinal weight and length were calculated adding back all segments removed. Jejunal and ileal villous height and crypt depth were determined by a pathologist with the use of hematoxylin and eosin–stained tissues at an average of 12 vertically oriented sites. Sucrase, maltase and lactase brush border activities were measured in jejunal and ileal mucosal samples using a Tris/glucose oxidase reagent (Dahlqvist 1984). Mucosal protein was determined using a bovine plasma γ-globulin standard (Protein Assay Kit, Bio- Rad DC Laboratories, Richmond, CA). Data were analyzed statistically using a one-factor ANOVA, and results were reported as treatment means ± 1 SD. The mean separation Dunnett’s t test (2-tailed, a = 0.05) compared TPN, GLN and PEN treatments to the SMR control, and then GLN and PEN treatments to the TPN control with significance at P≤ 0.05. Results. The initial (2.37 ± 0.5 kg) and final (4.03 ± 1.4 kg) body weights of piglets were not different across assigned treatment groups; however, the daily intakes of energy, fat and amino acids tended to be lower in the TPN, GLN and PEN groups than in the SMR treatment group (Table 2). The nutrient intake differences between parenterally and enterally fed piglets can be explained by automatic parenteral pump shut-offs and occlusions that occurred over the 28-d study. Small bowel (SB) weights per unit of body weight (Table 2) were compared across treatments as a more conservative method by which to test relative gastrointestinal atrophy. SB weights of TPN-, GLN- and PEN-treated piglets were significantly less than that of SMR-treated piglets. There were no differences in other organ weights or intestinal lengths between SMR and parenterally fed (TPN, GLN and PEN) piglets, except for liver weights, which were significantly greater in parenterally fed piglets (>4% BW) compared with SMR fed piglets (2.5% BW). During wk 4, serum bilirubin and aspartate aminotransferase were not different between treatment groups; however, alanine aminotransferase and alkaline phosphatase in TPN and GLN serum were significantly greater than SMR serum values, whereas PEN serum values were intermediate. Small intestinal histology (Table 2) was affected by route of nutrition because parenteral administration decreased villous heights by 50% in the jejunum and by 25% in the ileum, and SMR piglets had jejunal and ileal villus:crypt ratios greater than the other three piglet groups. Within parenteral treatment groups, jejunal villous heights in the PEN group were greater than those in the TPN group; however, crypt depths were significantly greater in the PEN and GLN piglets than in the TPN group. Jejunal lactase and sucrase activities for TPN and GLN piglets were significantly less than those of SMR piglets, whereas activities of PEN-treated piglets were intermediate (Table 2). Ileal lactase and sucrase activities in SMR piglets were uniformly greater than those of TPN-, GLN- and PEN-treated piglets. Maltase activity in the jejunum and ileum, however, was not affected by the route of nutritional administration and remained uniform across treatments [150 ± 63/(min • g protein)]. Discussion. The 4-wk parenteral administration of nutrients to piglets did produce a small bowel (SB) disuse atrophy and hepatomegaly. Although there were no significant differences in final body weight or gain, TPN-fed piglets had significantly lower SB weights, shorter jejunal and ileal villous heights and lower SB enzyme activity than the positive control SMR-fed piglets. The negative control TPN treatment findings were consistent with previously reported results. In TPN-fed adult rats, glutamine has been shown to prevent decreased intestinal weight, DNA content, villous height, and sucrase and lactase activities when administered IV for 6–7 d. In this 4-wk swine study, SB weights and enzyme activities were not preserved with glutamine added to the TPN solution. Glutamine in the TPN solutions may have spared jejunal crypt cell atrophy; however, villous heights were not spared. The difference between the previous studies demonstrating some advantage in giving glutamine and this study demonstrating no clear advantage may be related to differences in the species (rats vs. pigs), age of the animal models (adult vs. young) but more likely, length of treatment time (1 vs. 4 wk). In dogs, there is an increased requirement for glutamine during the immediate postoperative phase (<7 d), but uptake returned to normal rates later during a recovery phase (>10 d post-surgery) (Souba et al. 1987 and 1990). The long-term use of IV glutamine was not effective in preventing NPO-induced disuse atrophy. Therefore, given the high cost of IV glutamine fluids and the apparent limited time of efficacy, the use of IV glutamine solutions for dogs, cats and foals recovering from small bowels diseases should be limited to early short-term use. Piglets fed small volume enteral feedings of milk (PEN) did have jejunal villous heights and crypt depths significantly greater than those in the TPN piglets, and jejunal sucrase and lactase activities in the PEN fed piglets were intermediate to those of the SMR and TPN piglets. Piglets are farrowed with negligible amounts of maltase and sucrase activities, which develop during the first few weeks of life; however, lactase activity is high at birth and decreases with age (Ulshen et al.1991). Lactase and sucrase are known to be diet-inducible enzymes, which decreased in TPN-fed piglets, whereas maltase is not a diet-inducible enzyme (Jackson and Grand 1991), and intestinal activity remained high in all piglets. The enteral feedings apparently had minimal effect on ileal parameters, which may indicate small volume feedings of a highly digestible product may not have reached the ileum as a result of proximal bowel digestion and absorption. Apparently, the use of a milk solution, providing only 10% of the total energy intake, resulted in intermediate jejunal lactase and sucrase activities with taller villi and deeper crypts; however, this solution had no effect on overall SB weight. On the basis of these results, we believe that limited enteral milk feeding during SB disuse, if possible, has important clinical ramifications and warrants further investigation. Feeding small volumes of a well-designed enteral product to dogs, cats and foals recovering from small intestinal diseases may be a beneficial alternative to NPO. [ABSTRACT FROM AUTHOR]