Receptor occupancy and adenylate cyclase activation in rat liver and heart membranes by 10 glucagon analogs modified in position 2, 3, 4, 25, 27 and/or 29

Abstract

Rat liver and heart membranes were tested for adenylate cyclase activation by glucagon and 10 glucagon analogs mono- or polysubstituted in positions 2–4, 25, 27 and/or 29. The first membranes were, in addition, examined for the capacity of glucagon analogs to inhibit the binding of [ 125I]iodoglucagon. The monophasic slope of dose-effect curves suggested interaction with one class of glucagon receptors in both tissues, receptors in liver being more sensitive to the ligands and more efficiently coupled to adenylate cyclase than heart receptors. Structure-activity studies on liver membranes revealed that modifications of the β-turn potential in the 2–4 region by single residue substitutions could lead to partial agonists (with D-Gln 3 or Phe 4) or to a superagonist (with D-Phe 4). The importance of a proper α-helix conformation in the C-terminal part of glucagon for binding affinity was also obvious: replacing Trp 25, Met 27 and Thr 29 in combination by Phe 25, Leu 27 and Thr 29-NH 2 increased the affinity while single or combined substitutions with Gly 25 and/or Nle 27 sharply decreased the affinity. Similar trends were less evident but still obvious on heart membranes.