ABSTRACT. Background: Parenteral nutrition (PN) is known to induce villus atrophy, epithelial cell (EC) apoptosis, and increase mucosal permeability. The study hypothesized that increasing amounts of energy delivery to mice would result in the best outcome, with the least effects on the mucosa. Methods: Mice were randomized to enteral controls (saline infusion with ad libitum enteral food) or to 1 of 3 PN groups (with no enteral nutrition): full (100% of daily average energy intake for the mouse), reduced (75% of energy intake) or very low (50% of energy intake). Mice received PN for 7 days. Mucosal morphology, EC apoptosis, and bacterial translocation were assessed. Results: Villus height decreased significantly with decreasing levels of caloric intake and was significantly lower in all PN groups compared with controls. Body weight loss was significantly greater in PN groups vs controls and was greatest in mice with the lowest caloric delivery. A consistent trend toward a higher EC apoptotic index with decreasing caloric intake was observed, and apoptosis in all PN groups exceeded controls (2-fold). All PN groups demonstrated greater bacterial translocation than controls. Conclusions: PN induces intestinal EC apoptosis and villus and crypt atrophy, even at 100% of predicted energy needs, and such changes increased with greater reduction of energy intake. This study supports a concept that lack of enteral nutrition, rather than absolute caloric levels, is responsible for many of the adverse effects of PN. The study also allows the investigators to better optimize a mouse model of PN delivery. (Journal of Parenteral and Enteral Nutrition 30:474-479, 2006)

Patenterai nutrition (PN) is known to induce significant changes in mucosal structure and function. These changes include villus atrophy, epithelial cell (EC) apoptosis, and altered mucosal permeability. PN administration is associated not only with morphologic changes in the intestine but also with bacterial translocation and a loss of mucosal barrier function.1″3 The mechanisms of PN-associated loss of epithelial integrity and morphologic changes have been studied extensively using a mouse model.1″6 The use of a mouse model can offer great insights into these PN-associated effects on the gastrointestinal tract. Recent work by our group has helped to identify mechanisms that contribute to the occurrence of bacterial translocation and villus atrophy.1-3,6 Unlike other animal models, the use of mice is relatively inexpensive, allows for an extensive study of the alterations in their immune system, and can be managed in a small laboratory setting. Delivery of PN to mice, however, is a challenge and has been associated with a moderate mortality.7 To address this problem, this study was designed to determine the amount of energy delivery via PN using a mouse model, whereby the closest approximation to energy needs is met and with the fewest number of complications. We also sought to determine whether different levels of energy delivery, administered as PN, correlated with PN-associated intestinal changes. We further attempted to determine the most optimal amount of energy delivery while optimizing mouse survival and morbidity. We hypothesized that higher energy delivery via PN would result in fewer aberrations in intestinal morphology and improved survival.

MATERIALS AND METHODS

Animals

C57BL/6J male specific-pathogen-free mice (8 weeks old) were obtained from Jackson Laboratory (Bar Harbor, ME) and were maintained under temperature-, humidity-, and light-controlled conditions. Mice were initially fed ad libitum with standard mouse chow and water and allowed to acclimate. During the administration of IV solutions, mice were housed in metabolic cages to prevent coprophagia. The studies reported here conformed to the guidelines for the care and use of laboratory animals established by the University Committee on Use and Care of Animals at the University of Michigan, and protocols were approved by that committee (No. 7703).

Operative Procedures and Study Groups

Mice were anesthetized with sodium pentobarbital (50 mg/kg/body weight, intraperitoneal). All surgical procedures were performed under magnification in a sterile fashion. The left jugular vein was exposed and cannulated with a silicone rubber catheter (0.012-inch ID, 0.025-inch OD; Dow Corning, Midland, MI). The distal end of the catheter was tunneled subcutaneously and exited between the scapulae. The catheter was attached to a swivel spring, which allowed the mice freedom of movement in their individual cages (Metamount System, Instech Corp, Plymouth Meeting, MA). Catheterized mice were immediately connected to an infusion pump (AIM pain provider pump, generously donated by Abbott Laboratories, Abbott Park, IL) and saline (dextrose 5% in 0.45 NS with 20 mEq KCVL) was infused at an initial rate of 4.8 mL/d. After 24 hours, the animals were randomized to control or PN groups. Of note, the AIM pain provider pump is one of the most accurate available to date. Nevertheless, the delivery is cyclical and not continuous. Thus, as infusion rates increased, we found that some mice developed extravasation of IV PN. These mice were not included in the study results.