Isolation and characterization of exendin-4, an exendin-3 analogue, from Heloderma suspectum venom: Further evidence for an exendin receptor on dispersed acini from guinea pig pancreas

The recent identification in Heloderma horridum venom of exendin-3, a new member of the glucagon superfamily that acts as a pancreatic secretagogue, prompted a search for a similar peptide in Heloderma suepectum venom. An amino acid sequencing assay for peptides containing an amino-terminal histidine residue (His¹) was used to isolate a 39-amino acid peptide, exendin-4, from H. suspectum venom. Exendin-4 differs from exendin-3 by two amino acid substitutions, Gly²-Glu³ in place of Ser²-Asp³, but is otherwise identical. The structural differences make exendin-4 distinct from exendin-3 in its bioactivity. In dispersed acini from guinea pig pancreas, natural and synthetic exendin-4 stimulate a monophasic increase in cAMP beginning at 100 pM that plateaus at 10 nM. The exendin-4-induced increase in cAMP is inhibited progressively by increasing concentrations of the exendin receptor antagonist, exendin-(9-39) amide. Unlike exendin-3, exendin-4 does not stimulate a second rise in acinar cAMP at concentrations >100 nM, does not stimulate amylase release, and does not inhibit the binding of radiolabeled vasoactive intestinal peptide to acini. This indicates that in dispersed pancreatic acini, exendin-4 interacts only with the recently described exendin receptor.