Amphibian glucagon family peptides: potent metabolic regulators in fish hepatocytes

Peptides analogous to glucagon-like peptide-1 (GLP-1) have been isolated fr om amphibian pancreas and intestine, and their amino acid sequences and cDN A structures elucidated. Just like their mammalian counterpart, these pepti des are potent insulinotropins in mammalian pancreatic cells. We show here that these peptides also exert strong glycogenolytic actions when applied t o dispersed fish hepatocytes. We compared the potencies of three synthetic GLP-1s from Xenopus laevis and two native GLP-1s from Bufo marinus in the a ctivation of glycogenolysis in the hepatocytes of a marine rockfish (Sebast es caurinus) and two freshwater catfish (Ameiurus nebulosus and A. melas), and demonstrated their effectiveness in increasing the degree of phosphoryl ation of glycogen phosphorylase. We also compared the glycogenolytic potenc y of the peptides with those of human GLP-1 and glucagons from human and B. marinus. Sensitivity to these peptides is species-specific, with the rockf ish responding at lower concentrations to GLP-1s and the two catfish reacti ng better to glucagons. However, the relative potency of the amphibian GLP- 1s and glucagons is similar in the three species. Xenopus GLP-1C (chi GLP-1 C) is consistently more potent than chi GLP-1B, while chi GLP-1A displays t he smallest activation of glycogenolysis. Similarly, Bufo GLP-1(32)-the pep tide with the highest amino acid sequence identity to chi GLP-1C-always sho ws a higher potency than Bufo GLP-1(37), which is closely related to chi GL P-1B. The relative hierarchy of these glycogenolytic GLP-1s differs from th eir ranking as insulinotropins in mammalian beta -cells. In the rockfish system, Bufo glucagon-36, a C-terminally extended glucagon, is more potent than the shorter bovine glucagon and Bufo glucagon-29 in th e activation of glycogenolysis; when tested in A. nebulosus hepatocytes, bo vine and amphibian glucagons are equipotent. Amphibian GLP-1s and glucagons activate glycogenolysis in fish hepatocytes through increased phosphorylat ion of glycogen phosphorylase, implying involvement of the adenylyl cyclase /protein kinase A system in signal transduction. We conclude that the broad physiological effectiveness of GLP-1 has been retained throughout vertebra te evolution, and that both insulinotropic activity and glycogenolytic acti ons belong to the repertoire of GLP-1. (C) 2001 Elsevier Science B.V. All r ights reserved.