Direct effect of insulin on secretion of insulin, glucagon, gastric inhibitory polypeptide, and gastrin during maintenance of normoglycemia.

The direct effect of insulin on the secretion of insulin (as measured by C-peptide), glucagon, gastric inhibitory polypeptide, and gastrin was studied in normal subjects by infusing insulin while the plasma level of glucose was maintained in the normal fasting range (euglycemic clamp). Insulin-induced hypoglycemia resulted in increases in circulating glucagon and gastric inhibitory polypeptide, a decrease in C-peptide, and no change in gastrin levels. In contrast, during the euglycemic clamp, insulin was found to behave a direct suppressive effect on the secretion of glucagon, C-peptide, and gastrin, but no effect on levels of gastric inhibitory polypeptide.