Differential regional metabolism of glucagon in anesthetized pigs

Glucagon metabolism under basal (endogenous) conditions and during intravenous glucagon infusion was studied in anesthetized pigs by use of midregion (M), COOH-terminal (C), and NH2-terminal (N)-RIAs. Arteriovenous concentration differences revealed a negative extraction of endogenous glucagon immunoreactivity across the portal bed (-35.4 ± 11.0, -40.3 ± 9.6, -35.6 ± 16.9%, M-, C-, N-RIA, respectively), reflecting net secretion of pancreatic glucagon and intestinal glicentin and oxyntomodulin, but under exogenous conditions, a net extraction occurred (11.6 ± 3.6 and 18.6 ± 5.7%, C- and N-RIA, respectively). Hindlimb extraction of endogenous (17.4 ± 3.7%, C-RIA) and exogenous (29.1 ± 4.8 and 19.8 ± 5.1%, C- and M-RIA) glucagon was detected, indicating M and C cleavage of the molecule. Renal extraction of glucagon was detected by all assays under endogenous (19.4 ± 6.7, 33.9 ± 7.1, 29.5 ± 6.7%, M-, C-, N-RIA) and exogenous conditions (46.9 ± 4.8, 46.4 ± 6.0, 47.0 ± 7.7%; M-, C-, N-RIA), indicating substantial elimination of the peptide. Hepatic glucagon extraction was undetectable under basal conditions and detected only by M-RIA (10.0 ± 3.8%) during glucagon infusion, indicating limited midregional cleavage of the molecule. The plasma half-life determined by C- and N-RIAs (2.7 ± 0.2 and 2.3 ± 0.2 min) were similar, but both were shorter than when determined by M-RIA (3.2 ± 0.2 min, P < 0.02). Metabolic clearance rates were similar regardless of assay (14.4 ± 1.1, 13.6 ± 1.7, 17.0 ± 1.7 ml·kg-1·min-1, M-, C-, N-RIA). Porcine plasma degraded glucagon, but this was not significantly affected by the dipeptidyl peptidase IV (DPP IV) inhibitor valine-pyrrolidide, and in anesthetized pigs, glucagon's metabolic stability was unchanged by DPP IV inhibition. We conclude that tissue-specific metabolism of glucagon occurs, with the kidney being the main site of removal and the liver playing little, if any, role. Furthermore, valine-pyrrolidide has no effect on glucagon stability, suggesting that DPP IV is unimportant in glucagon metabolism in vivo, in contrast to its significant role in the metabolism of the other proglucagon-derived peptides and glucose-dependent insulinotropic polypeptide.

Dynamics of pancreatic cell growth and differentiation during diabetes reversion in STZ-treated newborn rats

Cellular processes underlying ontogenesis and regression of streptozotocin (STZ)-induced diabetes in newborn rats were investigated at the most severe stage of diabetes at day 3 and after recovery of normoglycemia at day 8 by immunocytochemistry and quantitative analysis. A previously unknown endocrine cell type subpopulation (PEPS) was identified. It was characterized by granule polymorphism, coexpression of insulin and glucagon immunoreactivity, and a proliferative capacity transiently higher than in B cells. In STZ-treated rats at day 3, B cell mass decreased 14-fold, whereas PEPS cells were unaffected. The islet mass was restored to 55.7% by day 8, with a concomitant appearance of numerous small islets contiguous to small ducts. B cell mass increased by 6.9-fold compared with 1.8-fold in control rats, although proliferative capacities remained similar. Proliferation dropped considerably by day 8, preventing complete B cell mass recovery in STZ-treated rats. STZ-induced neonatal diabetes thus stimulates neogenesis of islets close to ducts and proliferation of PEPS cells. Those partially differentiated islet cells appear to be on the differentiation pathway of stem cells to fully differentiated B cells.

Release of glucagon-like peptide 1 immunoreactivity from the perfused rat pancreas

A reliable radioimmunoassay for glucagon-like peptide 1 (GLP-1) was developed with a detection limit of 12 pmol/l, which enabled us to detect the subtle change in concentrations of this peptide in the perfusate from the perfused rat pancreas. With this RIA, GLP-1 immunoreactivity (GLP-1 IR) was found to be secreted synchronously with glucagon immunoreactivity upon arginine stimulation from the perfused rat pancreas. Gel chromatographic analysis showed the presence of GLP-1 IR in the pancreatic extract which is eluted at the same position as synthetic GLP-1 (1-37). Moreover HPLC analysis confirmed the presence of GLP-1 IR, which eluted at exactly the same position as synthetic GLP-1, in the perfusate from pancreas perfusion upon stimulation of arginine. These results suggest that the pancreas stores and secretes GLP-1 IR concomitantly with glucagon as one of the cleavage products of their common precursor.