Pediatric/congenital/developmentalTemporal Changes in the Intestinal Growth Promoting Effects of Glucagon-Like Peptide 2 Following Intestinal Resection
Introduction
The regulation of nutrient absorptive capacity is an important component of normal intestinal physiology. In the natural state, individuals are subjected to widely varying types and quantities of nutrients [1]. Following states of intestinal disease or inflammation, additional compensation is required, so that proximal intestinal dysfunction may be offset by increased uptake of nutrients in the distal intestine. An extreme variation of this is the response to the reduction of nutrient absorptive capacity associated with the loss of surface area following massive intestinal resection, known as short bowel syndrome (SBS) [2]. After resection, the residual intestine increases both intestinal surface area and absorptive enzymatic capacity, primarily by increasing the rate of crypt cell proliferation. In turn, this augments the nutrient absorptive capacity of the remnant intestine, a process known as “adaptation” [3, 4]. After a 90% resection of the proximal small intestine in the rat, the residual bowel up-regulates function so that after 7 d, nutrient absorption is restored to preresection efficiency [5]. The factors that control this adaptation are poorly understood, but they appear to be related to signals induced by enteral nutrients. Clinically, adaptation appears to only occur if luminal nutrition is administered [2, 3]. Previous studies from our group have shown that endogenous glucagon-like peptide 2 (GLP-2) is highly associated with the adaptive process, and exogenously administered GLP-2 can induce adaptation without the requirement for enteral nutrition [6, 7, 8, 9].
GLP-2 is a 33-amino acid member of the pituitary adenylate cyclase-activating glucagon superfamily [6, 9, 10]. GLP-2 is secreted from ileal and colonic enteroendocrine L cells in response to both proximal enteric neuronal signaling and the presence of luminal nutrients [6, 10, 11]. Previous studies have demonstrated that GLP-2 administration results in intestinal hypertrophy by increasing the crypt cell proliferation rate, which then results in increased villus height, crypt depth, and an overall increase in small intestinal length and weight [8, 12, 13]. This mimics the features of spontaneous adaptation, but occurred in animals maintained with entirely parenteral nutrition [7]. In addition, GLP-2 increased the expression of glucose transport protein (SGLT-1) much more than in enterally fed animals [7]. The mechanism(s) of action of GLP-2 in stimulating these effects is not clear; the receptor is confined to cells within the enteric neuronal system, entero-endocrine cells, and myofibroblasts in the submucosa [5, 9, 13]. In previous experimental studies, it has been shown that anti-inflammatory effects of GLP-2 are associated with activation of submucosal VIP expressing neurons of the enteric neuronal system [14]. However, in studies using knockout models, it has been shown that the trophic effects of GLP-2 on the intestinal mucosa appear to be dependent on local production of IGF-1, likely from myofibroblasts adjacent to the dividing crypt cells [15]. In clinical studies, GLP-2 has been shown to improve energy absorption in adult patients with SBS [16]. From this background, it is hypothesized that GLP-2 acts as a general regulator of nutrient absorptive capacity. It may become a useful therapy for human patients with impaired nutrient absorption [2, 6, 10]; however the optimal timing of administration postoperation is not known. The present studies examined (1) the hypothesis that the timing of GLP-2 administration would affect the trophic effects on the residual intestine, with early administration increasing the effects, and (2) the hypothesis that there would be an increase in the sensitivity of the residual intestine to exogenous GLP-2 administration, due to an increase in the expression of the GLP-2 receptor and an associated increase in the activation of the submucosal plexus of the enteric neuronal system. Herein, we show that early GLP-2 administration does have greater effects on intestinal growth following resection, and that GLP-2 receptor mRNA expression is up-regulated in the early postresection period, but that GLP-2 stimulation does not cause an increase in enteric neuronal activation within the submucosa in this same time period.
Section snippets
Animals
Male Sprague Dawley (SD) rats weighing 250 to 300 g were housed in metabolic caging with free access to food and water, and acclimatized to their environment for 5 d before experiments. Animals were maintained under standardized temperature, humidity, and 12 h light-dark cycles. Animal studies were conducted under the guidelines established by the Canadian Council of Animal Care following the approval of the Animal Care Committee at the University of Calgary.
Study Design
The effects of variations in timing
Results
In general, the animals did well. Two animals died from complications related to the intravenous catheter; these animals were replaced. Two further animals, one each from the SBR + TPN + GLP– 2 (first wk) and SBR + TPN + GLP-2 (second wk) groups had catheter occlusions; these catheters were replaced under a brief general anesthetic on d 3 and 4 of study, respectively, and their results used in all data fields.
Examining the effects of GLP-2 given immediately or delayed following resection, there
Discussion
In this study, we have demonstrated that following intestinal resection an early increase in the effect of exogenous GLP-2 on intestinal growth occurs, with persistent effects following withdrawal of the hormone. Similar, but less profound morphological effects, with increased mucosal proliferative and nutrient transporter effects were seen with delayed GLP-2 treatment. Both early and late GLP-2 administration increased weight accretion in TPN maintained animals. An increase in expression of
Acknowledgments
This study were supported by the Alberta Children's Hospital Research Foundation, and the Department of Surgery Research Fund, University of Calgary. Infrastructure support of the Sigalet Laboratory is provided by the Crohn's and Colitis Foundation of Canada. TK was supported by a Research Fellowship from the Department of Pediatric Surgery, Kagoshima University. HT was supported by a Research Scholarship from the Department of Pediatric Surgery, Kurume University School of Medicine. DLS has
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