The pathophysiological role of incretin in diabetes mellitus has not been established. We therefore examined the effects of glucagonlike peptide l-(7–36)-amide (truncated GLP-I) and gastric inhibitory polypeptide (GIP) on insulin and glucagon release from isolated perfused pancreases of diabetic rats (12–14 wk of age, mean ± SE fasting plasma glucose 8.9 ± 0.6 mM, n = 25) after an injection of 90 mg/kg streptozocin on the 2nd day after birth and compared the results with those of nondiabetic control rats. In diabetic rats, the infusion of 1 nM GLP-I or GIP in perfusates with varying glucose concentrations (2.8, 5.6, 8.3,11.1, or 22.2 mM) caused a nearly equal degree of insulin stimulation from a similar basal insulin level. Meanwhile, basal and GLP-I- or GIPstimulated insulin release increased in correlation with the ambient glucose concentration in nondiabetic rats. The degree of stimulation of insulin release at glucose concentrations of 5.6 mM in diabetic rats was −33% that of nondiabetic rats. The stimulation potency was the same between GLP-I and GIP. The insulin treatment for diabetic rats (5 U/kg NPH insulin at 0900 and 2100 for 6 days) brought only a slight improvement in the glucose dependency of GLP-I-stimulated insulin release. The effects of GLP-I and GIP on glucagon release were completely opposite. GLP-I suppressed release; GIP stimulated it. In diabetic rats, the degree of suppression by GLP-I and stimulation by GIP were almost the same with similar basal glucagon levels in the perfusate with varying glucose concentrations. In nondiabetic rats, these parameters were inversely correlated with ambient glucose concentrations. Insulin treatment did not improve the glucose dependency of either the basal glucagon level or the degree of suppression by GLP-I. These data demonstrate that the incretin effect of GLP-I and GIP, glucose-dependent stimulation of insulin release, is reduced in neonatal streptozocin-induced diabetic rats and that conventional insulin treatment does not normalize this abnormality. Also, inhibition of glucagon release by GLP-I is glucose dependent in nondiabetic rats, and this glucose dependency is lost in diabetic rats.

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