Mechanisms of Glycemic Improvement Following Gastrointestinal Surgery

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Project Abstract Roux-en-Y gastric bypass surgery causes complete, durable remission of type 2 diabetes (T2DM) in 84% of cases, typically within a few days to weeks after surgery. Mounting evidence indicates that this dramatic phenomenon results from effects beyond those related to weight loss and reduced caloric intake alone. The mechanisms mediating the weight-independent anti-diabetes impact of RYGB are unknown, and elucidating them could lead to new diabetes medicines. The "lower intestinal hypothesis" postulates that RYGB improves T2DM by creating an intestinal shortcut to enhance nutrient delivery to the distal bowel, stimulating glucagon- like peptide-1. However, we and others have found that in rats, exclusion of a short segment of proximal small bowel (primarily the duodenum) from contact with ingested nutrients exerts direct anti-diabetic effects, independent of changes in food intake, body weight, or distal intestinal nutrient stimulation, leading to an alternate "upper intestinal hypothesis". Both hypotheses posit putative mechanisms that may involve the vagus nerve, the role of which in the effects of RYGB is unknown. We propose to determine whether the upper intestinal hypothesis is valid in humans and to clarify its mechanisms, as well as the role of the vagus in RYGB glycemic effects. Humans will undergo frequently sampled I.V. glucose tolerance tests (FS-IVGTT) and tracer- enhanced hyperinsulinemic/euglycemic clamps (to measure insulin secretion and sensitivity) before RYBG and 3 times in the first few weeks afterward, during which the proximal small bowel will either be excluded from nutrient contact or exposed to nutrients delivered through an indwelling gastric cannula. Related mechanistic studies will be performed in a novel long-term-survival RYGB model we have developed over the past 3 years in insulin-resistant pigs. Ossabaw pigs will undergo a gastrojejunostomy, which enhances nutrient delivery to the distal bowel in a manner similar to RYBG (but without affecting the stomach), performed either with or without duodenal exclusion from contact with ingested nutrients. Both operations increase distal bowel nutrient stimulation and neither causes weight loss; their only difference is the presence or absence of a modest proximal intestinal bypass. Effects of these procedures on glucose homeostasis will be quantified over time with FS-IVGTTs and minimal modeling. Long-term impacts on islets will be assessed with pre- and post- operative quantifications of ¿-cell proliferation, neogenesis, apoptosis, and mass. Beta-cell function and mechanisms of insulin secretion will be determined in vitro using perifused islets. Various GI tract segments will be examined to ascertain whether alterations in the development of enteroendocrine cells producing relevant gut peptides occur. To examine the role of the vagus nerve in the effects of RYGB, pigs will undergo this operation with or without a complete vagotomy, and all of the above pre- and post-mortem measurements will be made. Plasma levels of GLP-1, GIP. PYY, and ghrelin will be made during standardized meals throughout these experiments in both species, to clarify roles for these gut peptides in changes we observe.