The Effect of Calcium on the Release of Gastric Inhibitory Polypeptide (GIP)

Plasma gastric inhibitory polypeptide (GIP), insulin, glucagon concentrations and blood glucose levels in response to the ingestion of 100 g glucose were measured in 5 patients with hyperparathyroidism in order to elucidate the effect of hypercalcemia on the release of these hormones. In addition, the effect of acute hypercalcemia on the release of these hormones in response to glucose ingestion was investigated in normal subjects. Fasting plasma GIP concentration in patients with hyperparathyroidism was significantly greater than the value in seventeen normal subjects. Significantly higher responses of plasma GIP and insulin were observed after the glucose ingestion in the patients with hyperparathyroidism as compared with the values in the normal subjects, and integrated GIP and insulin responses to the glucose ingestion for 120 min in the patients with hyperparathyroidism were significantly greater than the values in the normal subjects. On the other hand, plasma glucagon concentration after the glucose ingestion in the patients with hyperparathyroidism remained unchanged, although plasma glucagon concentrations after the glucose ingestion decreased significantly from the basal value in the normal subjects. Blood glucose levels after the glucose ingestion in two groups increased significantly from the basal value in the same manner. In nine normal subjects calcium infusion (4 mg/kg bolus injection followed by continuous infusion of 4 mg/kg/hr for 3 hr) caused a significantly high concentration of plasma calcium (11.5 approximately 13.0 mg/dl) from the basal value. Significantly higher responses of plasma GIP and insulin to the glucose ingestion were observed during calcium infusion as compared with the values during saline infusion. On the other hand, plasma glucagon concentration after the glucose ingestion was not significantly changed during calcium infusion in contrast with a significant decrease of plasma glucagon after the glucose ingestion during saline infusion. Consequently, calcium was considered to play a major part in the release of GIP and insulin. The characteristic response of plasma glucagon during calcium infusion was considered, at least in part, to protect the hypoglycemia caused by hyperinsulinemia.