Synthesis of Two Glucagon Antagonists: Receptor Binding, Adenylate Cyclase, and Effects on Blood Plasma Glucose Levels

In diabetes mellitus, hyperglycemia is often associated with elevated levels of glucagon in the blood. This suggests that glucagon (1) is a contributing factor in the metabolic abnormalities of diabetes mellitus. A glucagon-receptor antagonist would provide direct evidence for glucagon's role in diabetes mellitus. On the basis of careful consideration of conformational, arnphiphilic and structural factors, we have synthesized two new glucagon analogues with antagonist biological activities by using solid-phase methodology. These two new analogues, [Asp³,D-Phe⁴,Ser⁵,Lys^17>18,Glu²¹]glucagon (2) and [D-Phe⁴,Tyr⁵,3,5-I₂-Tyr¹⁰,Arg¹²,Lys¹⁷'^I8,Glu²¹]glucagon (3), had IC₅₀values 5.4% and 50% those of glucagon, respectively, and showed no measurable adenylate cyclase activity. When tested in normal rats, 2 lowered plasma glucose levels and suppressed glucagon-mediated hyperglycemia 105 ±8%, back to basal levels. Analogue 3, which lowered the basal adenylate cyclase activity in rat liver plasma membranes, increased plasma glucose levels at very high concentration in vivo and inhibited glucagon-mediated hyperglycemia in normal rats by 50%. However, neither of the new glucagon antagonists lowered the plasma glucose levels of diabetic animals. The data would suggest these new glucagon-receptor antagonists may have two actions: (a) in normal rats they can act as standard glucagon-receptor inhibitors of glucagon-mediated glycogenolysis; (b) in diabetic rats, however, because of the low levels of glycogen in the liver, the antagonists apparently have little or no antagonist effect or enhancement on glucagon-mediated glucose production.