No abstract

The structure of human preproglucagon, as deduced from nucleotide sequencing of the preproglucagon gene, contains two glucagon-like peptides (GLP-1 and GLP-2) in the portion C-terminal to glucagon. A rabbit antiserum was raised against synthetic GLP-1-(1-19) which had 20% cross-reactivity with synthetic GLP-1 and des-Gly37-GLP-1 amide, two possible forms of the GLP-1 whole molecule, but no significant cross-reactivity with glucagon or other pancreatic peptides. Immunocytochemistry revealed that the distribution of GLP-1-(1-19) immunoreactivity followed that of glucagon-like immunoreactivity in the normal human pancreas and in two human glucagon-secreting pancreatic tumors. Chromatography of human pancreas extracts on Sephadex G-50 gave peaks of cross-reactivity at Kav values of 0.06-0.16, 0.34-0.39, 0.54-0.58 (the elution position of synthetic GLP-1), and 0.64-0.70. The concentration of immunoreactivity in the Kav 0.54-0.58 peak measured by RIA using GLP-1 or des-Gly37-GLP-1 amide as standard was 94 +/- 7 pmol/g (mean +/- SEM), while the total pancreatic glucagon content was 4.8 +/- 0.8 nmol/g. One extract of a human glucagon-secreting pancreatic tumor contained a prominent peak of GLP-1-(1-19) peptide cross-reactivity with properties identical to those of GLP-1 or des-Gly37-GLP-1 amide on gel filtration and reverse phase high pressure liquid chromatography, but another tumor contained a preponderance of cross-reactive forms of greater molecular size. Pretreatment plasma from three patients with radiological and biochemical evidence of glucagon-secreting tumors contained a peak of cross-reactivity with the chromatographic properties of intact GLP-1. The low concentrations of intact GLP-1 in normal pancreas compared with pancreatic glucagon concentrations suggest that the majority of the proglucagon is cleaved in a manner that does not produce GLP-1, as defined by its delimiting pairs of basic amino acid residues.

Molecular forms of the glucagon‐like peptides (GLP) encoded by the human preproglucagon gene were analysed by chromatography combined with specific radioimmunoassays to the synthetic peptides. Whereas extracts of human pancreas and a glucagonoma contained a large proglucagon cleavage product possessing both GLP‐1 and GLP‐2 immunoreactivities, extracts of human intestine contained products corresponding to free GLP‐1 and a small amount ofchromatographically distinct GLP‐2 immunoreactivity. It is concluded that post‐translational processing of proglucagon differs in pancreas and intestine, so that the C‐terminal portion of the molecule is cleaved to liberate free GLP‐1 in the intestine. Further processing or degradation results in loss especially of GLP‐2 immunoreactivity.

The effects on pancreatic exocrine secretion of intravenous bolus injections of decapeptide of mammalian bombesin (also called neuromedin C) and neuromedin B, recently isolated mammalian bombesin-like peptides, have been studied and compared with those of amphibian bombesin in anaesthetized rats. Decapeptide of mammalian bombesin and neuromedin B stimulated the volume output from the pancreas with the same potency as that with which they stimulated protein output, as did amphibian bombesin. The maximal peak rates of volume and protein secretion observed in the 5- to 10-min period after the injection of 3 × 10^-10 mol/kg decapeptide of mammalian bombesin were 24.5 ± 1.2 μl/5 min and 8.5 ± 0.5 mg bovine serum albumin equivalents per 5 min (mean ± SEM, n = 5). These rates were equivalent to those produced by the same dose of amphibian bombesin, but the duration of responses to decapeptide of mammalian bombesin were shorter than those of equimolar doses of amphibian bombesin. The relative potencies of decapeptide of mammalian bombesin and neuromedin B, calculated from the doses producing 50% of maximum effect on total responses, were, respectively, 100 and 0.5% of that of amphibian bombesin. The results suggest that decapeptide of mammalian bombesin, and possibly neuromedin B, could play a regulatory role in the control of exocrine pancreatic secretion.

Glucagon-like peptide-1 does not have specific, high-affinity receptors on rat liver membranes, does not displace glucagon from glucagon receptors on these membranes and does not stimulate the production of cyclic AMP by isolated rat hepatocytes. In the presence of glucagon, high concentrations of glucagon-like peptide-1 do not significantly alter the production of cyclic AMP. Thus, glucagon-like peptide-1 appears unlikely to have a direct action on hepatic carbohydrate metabolism.