Prosomatostatin (pro-S) and its bioactive posttranslational products, somatostatin-14 (S-14), somatostatin-13 (S-13), and somatostatin-28 (S-28), were measured in human plasma by the use of immunoglobulins to the NH2-terminus of S-28 conjugated with agarose to separate them and, thereafter, by RIA with an antiserum recognizing the COOH-terminus of pro-S, and by specific RIA for the NH2-terminus of S-14 and pro-S. In healthy men, mean basal levels of pro-S were 4 pg equivalent S-14/ml; S-14/S-13 combined were 9 pg equivalent S-14/ ml; and S-28 levels were 16 pg/ml. After a 700-kcal meal, pro-S, S-14, and S-14/S-13 did not change, whereas S-28 levels doubled by 120 min and remained elevated for 240 min. To evaluate the origins of these peptides, their levels were compared in peripheral, portal, gastric, and mesenteric veins of anesthetized patients and in patients with total resection of stomach and pancreas before and after nutrient intake. The stomach and small intestine were sources of both peptides; however, most S-28 originated in the small intestine. These findings suggest that, in contrast to S-14, S-28 is a hormone and may modulate postprandial nutrient absorption and use.

Glucagon-like peptide 1 (GLP-1) is an insulinotropic hormone released after nutrient ingestion which is known to augment insulin secretion, inhibit glucagon release, and promote insulin-independent glucose disposition. To determine the overall effect of GLP-1 on glucose disposition after a meal we studied a group of healthy, conscious baboons before and after intragastric glucose administration during infusions of saline, and two treatments to eliminate the action of GLP-1: ( a ) exendin-

No abstract

Somatostatin-28 (S-28), secreted into the circulation from enterocytes after food, and S-14, released mainly from gastric and pancreatic D cells and enteric neurons, inhibit peripheral cellular functions. We hypothesized that S-28 is a humoral regulator of pancreatic B cell function during nutrient absorption. Consistent with this postulate, we observed in baboons a two to threefold increase in portal and peripheral levels of S-28 after meals, with minimal changes in S-14. We attempted to demonstrate a hormonal effect of these peptides by measuring their concentrations before and after infusing a somatostatin-specific monoclonal antibody (mAb) into baboons and comparing glucose, insulin, and glucagon-like peptide-1 levels before and for 4 h after intragastric nutrients during a control study and on 2 d after mAb administration (days 1 and 2). Basal growth hormone (GH) and glucagon levels and parameters of insulin and glucose kinetics were also measured. During immunoneutralization, we found that ( a ) postprandial insulin levels were elevated on days 1 and 2; ( b ) GH levels rose immediately and were sustained for 28 h, while glucagon fell; ( c ) basal insulin levels were unchanged on day 1 but were increased two to threefold on day 2, coincident with decreased insulin sensitivity; and ( d ) plasma glucose concentrations were similar to control values. We attribute the eventual rise in fasting levels of insulin to its enhanced secretion in compensation for the heightened insulin resistance from increased GH action. Based on the elevated postmeal insulin levels after mAb administration, we conclude that S-28 participates in the enteroinsular axis as a decretin to regulate postprandial insulin secretion.

Thrittene is a recently described peptide with a sequence homologous with somatostatin-28 ((1-13)) but is produced independent of the preprosomatostatin gene. It is localized in epithelial cells in stomach and gut mucosal crypts and in neuronal cell bodies in the myenteric plexus and enteric axons. It is also present in human plasma. The aim of this study was to determine whether the release of thrittene into the circulation was affected by the ingestion of nutrients and, if so, whether the pattern of release was distinct from the closely related peptide somatostatin-28 (S-28). Thrittene was indirectly measured in human plasma by an RIA using antiserum F4. F4 interacts with the Asn(5)-Pro(6) region shared by S-28 and thrittene. The contribution of S-28 to F4 immunoreactivity (F4-IR) was determined using a specific two-site assay, and this measure was subtracted from the total F4-IR to give an estimate of thrittene levels. Plasma for assay was taken from healthy men on 4 separate days before and after intake of: a mixed meal (715 kcal), and meals containing primarily fat (25 g; 225 kcal), carbohydrate (100 g; 454 kcal), and protein (22 g; 100 kcal). After the mixed meal, both S-28 and thrittene rose by 50-100% within 30 min and gradually declined by 4 h. These increments were mimicked after ingestion of fat. By contrast, thrittene levels increased after carbohydrate but not protein intake, whereas S-28 concentrations rose after protein but not carbohydrate ingestion. These findings indicate that thrittene is secreted into the mammalian circulation during food intake and raise the possibility that thrittene may play a role in nutrient disposition. The dichotomy in responses of thrittene and S-28 to ingestion of the major macronutrients suggests that they are secreted from different gastrointestinal cells.