Gastric inhibitory polypeptide responses and glucose turnover rates after natural meals in type II diabetic patients.

Gastric inhibitory polypeptide (GIP) has insulinotropic actions in the presence of hyperglycemia. However, its extrapancreatic effects on glucose homeostasis are controversial. We have studied the relationships between GIP and immunoreactive insulin (IRI) and glucose turnover rates (D3H-3 glucose technique) in five poorly controlled type II diabetic patients and five normal subjects before and after a breakfast containing 500 kcal including 42 g sucrose. Mean fasting serum glucose levels and glucose responses were significantly (P less than 0.001) higher in the diabetic patients than in normal subjects. Mean basal serum IRI levels were similar in both groups [12.8 +/- 2.9 (SEM) vs. 11.8 +/- 2 microU/ml, P = NS]. After meal ingestion, mean IRI levels rose significantly to a peak at 20 min in the normal subjects but the responses were blunted in the diabetic patients (74 +/- 10 vs. 24 +/- 6 microU/ml, P less than 0.001). At all other times studied (60-180 min), mean serum IRI levels were similar in the diabetic patients and the normal subjects except at 180 min. Mean basal serum GIP levels were similar in the diabetic patients and the normal subjects (538 +/- 100 vs. 400 +/- 50 pg/ml, P = NS). After meal ingestion, mean GIP levels rose between 0-60 min but were significantly higher in the diabetic patients only at 20 min (1200 +/- 190 vs. 566 +/- 76 pg/ml, P less than 0.01). Mean basal hepatic glucose output was higher (P less than 0.01) in the diabetic patients. However, the mean basal MCR values were similar. After meal ingestion, total splanchnic glucose output and rates of glucose utilization (RU) were significantly higher in the diabetic patients compared with the normal subjects (P less than 0.001, and P less than 0.001, respectively). Postmeal MCR values were not statistically different in both groups. There were significant positive correlations between postmeal splanchnic glucose output and both IRI (r = 0.805, P less than 0.005) and GIP (r = 0.749, P less than 0.02) in the diabetic patients but not in the normal subjects (r = 0.10, P = NS for both). Whereas no relationships existed between RU and IRI in either group, RU correlated strongly with GIP (r = 0.810, P less than 0.005) only in the diabetic patients. We hypothesize that GIP may play a compensatory role to improve both impaired beta-cell insulin release and peripheral glucose utilization which are the recognized pathogenetic mechanisms underlying type II diabetes mellitus.