SECRETION OF GASTRIC-INHIBITORY POLYPEPTIDE (GIP) IN PATIENTS WITH INSULINOMA

The feedback regulation of gastric inhibitory polypeptide (GIP) releas e by endogenous insulin has been controversial. To examine the role of endogenous insulin in GIP release, GIP response to glucose ingestion was observed in five patients with benign insulinoma before and after removal of the tumor. Tumor was completely removed by the enucleation in four cases and by caudal pancreatectomy in one case. Seven healthy volunteers were served as controls. Plasma levels of GIP, Insulin (IRI ) and glucose were measured after glucose (50g) ingestion. Integrated incremental values of GIP and insulin were calculated. Basal plasma le vels of insulin before operation were significantly higher compared to those after operation and in normal controls. Both basal, plasma leve ls and integrated incremental values of GIP in insulinoma patients wer e not different from those in normal controls. These values of GIP did not alter significantly after removal of insulinoma, although basal p lasma levels of IRI were significantly decreased after operation. In c onclusion, endogenous hyperinsulinemia might have no effect on GIP rel ease after glucose ingestion. Secretion of GIP is not supposed to be r egulated by endogenous insulin.