Exendin-4 improves the oral glucose tolerance in diabetic rats: pancreas regeneration, better function of pancreatic islets, or impaired glucose uptake?

One major obstacle for successful clinical transplantation of isolated pancreatic islets (PI) is their limited survival in vivo. The aim of this study was to analyze the functional and morphological regeneration of PI in diabetic rats by Exendin-4 (Ex4) treatment in vivo. Male Wistar rats (n = 3/group) received 20 nmol/kg Ex4 i.p. for 20 days (day 0 to day +20). Diabetes was induced with 50 mg/kg streptozotocin i.v. on day -3 or on day +5. Diabetic and normal control rats received 0.9% NaCl i.p. instead. Body weight (BW), daily blood glucose (BG) and levels, oral glucose tolerance (OGT) were tested on day -5, day +10, day +20, and on day +22, ie, 48 hours after the last Ex4 injection. Histology of the pancreata ended the study on day +24. In vivo application of Ex4 could not prevent the development of diabetes. Injection of Ex4 led to a significant decrease in postprandial BG levels to 35% for 12 hours. Surprisingly, Ex4 increased postprandial BG levels up to 20% in normal rats. Ex4-treated rats showed better OGT than untreated controls. Interestingly, 48 hours after the last Ex4 injection on day +22 OGT was completely impaired. The Ex4-treated rats lost BW much faster than the untreated controls, and showed signs of gastroparesis at autopsy. Immunohistochemistry of the pancreata documented no signs of islet regeneration. Improvement of OGT in diabetic rats after Ex4 treatment may be explained by increased insulin release from the individual PI, which was confirmed by perifusion studies with isolated PI in vitro (data not shown). Yet, Ex4 may also exert an influence on the gastrointestinal tract as it delays the uptake of glucose during gastroparesis.