Regulatory peptides (glucagon, somatostatin, substance P, and VIP) in the brain and gastrointestinal tract of Ambystoma mexicanum

Abstract

The concentrations of immunoreactive components of glucagon, somatostatin, substance P, and vasoactive intestinal polypeptide (VIP) in the brain, stomach, and gut of the neotenic Mexican axolotl ( Ambystoma mexicanum) were determined by radioimmunoassay using antibodies of defined regional specificity. The molecular forms of the immunoreactive components were analyzed by high-performance liquid chromatography (HPLC). The concentrations and molecular forms of somatostatin and VIP in axolotl brain were comparable to the concentrations in mammals but the substance P-like immunoreactivity was resolved by HPLC into components with the retention times of physalaemin and substance P together with their oxidized forms. No glucagon-like material was detected in the axolotl brain. The concentrations of substance P and VIP in the A. mexicanum digestive tract were appreciably lower than in the mammalian digestive tract and the VIP-like material did not coelute with porcine VIP. Somatostatin-14 represented the major molecular form in the axolotl stomach and gut. The distribution and molecular properties of the glucagon-like peptides in the axolotl digestive system were markedly different from these parameters in mammalian gut. Glucagon-like material is present only in low amounts in porcine and human stomach and, the concentration of enteroglucagon (N-GLI) in the gut is at least fiftyfold greater than pancreatic glucagon (C-GLI) concentrations. The axolotl stomach, in contrast, contains high levels of glucagon-like immunoreactive material and, in both stomach and gut, the levels of C-GLI and N-GLI were comparable. The glucagon-like material was heterogeneous on HPLC and was resolved into two major components but no component with the retention time of mammalian glucagon was present. The immunochemical properties of the axolotl glucagon-like peptides indicate that they possess strong homology with mammalian glucagon in the 10–18 and 25–29 regions of the molecule.