GLUCAGON AND GLUCAGON-LIKE PEPTIDE SIGNALING PATHWAYS IN THE LIVER OF2 FISH SPECIES, THE AMERICAN EEL AND THE BLACK BULLHEAD

Components of the signaling pathway for glucagon (GLU) and glucagon-li ke peptide-1 (GLP-1) were investigated in hepatocytes and membranes of two teleost fishes, the American eel (Anguilla rostrata) and the blac k bullhead (Ictalurus melas). Glucagon stimulated glucose release from isolated hepatocytes while increasing in a time-and dose-dependent fa shion cAMP and inositol 1,4,5-trisphosphate (IP3) concentrations. Phos pholipase C (PLC), but not adenylyl cyclase (ACase), activities were s tatistically increased in hepatic membranes. GLP-1 also stimulated glu cose release and a small but statistically significant increase in cAM P but not IP3 concentrations in hepatocytes. The glucagon-family pepti de also statistically stimulated PLC, but not ACase activities. Repons es to epinephrine (EPI) were generally similar with respect to glucose release and enzyme activation, but changes to cAMP were much greater than that of either GLU or GLP-1. These results support those in mamma lian hepatocytes, which suggest that; GLU may act through both the cAM P and the IP3 signaling pathways. However, they do not provide further insight into the mechanism by which GLP-1 may exert its metabolic eff ects on the fish liver, Although small changes in cAMP were noted, the possibility of an IP3 effect cannot be discounted. (C) 1997 Wiley-Lis s, Inc.