Interrelationships among insulin, glucagon, and gastric inhibitory polypeptide in insulinoma.

The interrelationships of serum insulin, glucagon, and gastric inhibitory polypeptide were examined in 13 patients with insulinoma during fed and fasted states. Compared with normal subjects, patients with insulinoma had significantly lower glucose and higher insulin levels during both the fed and the fasted states. Although glucagon concentration was higher at the completion of the fast in patients with insulinoma compared with normals, no significant differences were apparent during the fed state. No difference was noted in gastric inhibitory polypeptide either during the fed state or at the termination of the fast. Under the conditions of the study, no direct suppressive effect of insulin on glucagon or gastric inhibitory polypeptide secretion was apparent. In addition, as opposed to that in normals, the insulinotropic effect of glucagon did not appear to be blunted by hypoglycemia in most of the patients. After glucagon injection, all symptomatically hypoglycemic patients experienced an amelioration of symptoms and restoration of the plasma glucose into the normal range.