Gastric Inhibitory Polypeptide, Insulin, and Glucose Changes Produced by Growth Hormone, Prednisolone, Glucagon, Insulin, Fasting, or Diazoxide

Because of the strong augmen-tary action of gastric inhibitory polypeptide (GIP) on insulin secretion, we studied in foxhounds the effect on GIP secretion of several factors known to affect insulin secretion and plasma glucose. Growth hormone infusion decreased plasma GIP, yet increased insulin; no change in plasma glucose was found. Growth hormone inhibited oral glucose-induced increments in plasma GIP and glucose. Prednisolone did not affect levels of GIP or insulin, whether given alone or preceding oral glucose. Diazoxide decreased glucose-induced GIP and insulin secretion, but increased plasma glucose. Fasting for 112 hr decreased GIP but did not decrease the glucose-induced GIP rise. Infusion of glucagon or insulin in amounts sufficient to produce high physiologic or pharmacologic plasma levels did not change GIP concentration, nor did they alter GIP rises with glucose. In conclusion, GIP was not inhibited by insulin or glucagon but was decreased by growth hormone, fasting, or diazoxide. Whereas fasting increased glucose-stimulated insulin secretion, diazoxide decreased it. The insulin-releasing activity of a number of factors does not correlate with increases in plasma GIP, and increases in plasma insulin or glucagon may not suppress increases in plasma GIP. There was not a good correlation of the plasma levels of GIP, insulin, glucose, and glucagon.