Interactions between endogenous and exogenous insulin and human pancreatic polypeptide secretion.

The relationships between exogenous and endogenous insulin and plasma human pancreatic polypeptide (hPP) were investigated. Three tests were performed in 8 healthy subjects: 1 degree--1 g tolbutamide was injected i.v. in 1 min., 2 degrees--50 ml 50% glucose solution and 7 U of Insulin were contemporaneously infused at constant rate for 60 min., 3 degrees--50 ml saline solution were infused as control. After the tolbutamide injection plasma glucose levels were unchanged between 0-10 min. At 5, 10, 15 min. an increase in C-Peptide (CPR) value and a significant decrease of hPP were observed. At 15, 30 and 45 min. plasma glucose was significantly lower than baseline and hPP levels strongly rose until the end of the test. The glucose-insulin infusion testing resulted in a rise in plasma glucose and CPR levels at 15 min. and a fall in plasma hPP at 30 min. The present findings suggest that in the first part of tolbutamide test, the endogenous insulin secretion, valued as CPR, seems to inhibit hPP release probably through a paracrine pathway.