Conflicting gastric inhibitory polypeptide data: possible causes

Postprandial glucose-dependent insulinotropic polypeptide and insulin responses in patients with chronic pancreatitis with and without secondary diabetes

OBJECTIVE

To study GIP and insulin release after a test meal in patients with chronic pancreatitis with and without secondary diabetes mellitus.

METHODS

28 patients with chronic pancreatitis were classified in groups I and II according to the presence or absence of secondary diabetes mellitus. Twelve healthy subjects were included as controls. After a test meal plasma GIP levels and serum insulin levels were determined at 0, 30, 60, 120 and 180 min.

RESULTS

A significant diminished GIP response was found in the groups of patients with respect to the control group. No association could be detected with severity of pancreatic insufficiency. Higher values of GIP were demonstrated at 60 and 120 min in patients without diabetes than in patients with it.

CONCLUSIONS

An abnormal GIP response is present in cases of chronic pancreatitis irrespective of the presence or severity of pancreatic insufficiency. This response is further affected if secondary diabetes mellitus is present.

The entero-insular axis in polycystic ovarian syndrome

We investigated the contributions made by the entero-insular axis, proinsulin and the fractional hepatic extraction of insulin to the hyperinsulinaemia characteristic of polycystic ovarian syndrome (PCOS). We measured plasma glucose, gastric inhibitory polypeptide (GIP), glucagon-like peptide-1 (7-36 amide) (GLP-1(7-36) amide), immunoreactive insulin (IRI), intact proinsulin (IPI), and C-peptide concentrations during a 75 g oral glucose tolerance test in seven normal weight women with PCOS and eight healthy women. Women with PCOS had higher fasting (P = 0.05) and integrated (P < 0.01) IRI concentrations than controls. Fasting C-peptide levels were similar in both groups but integrated C-peptide (P < 0.05) concentrations were greater in PCOS subjects than controls. Fasting and integrated concentrations of glucose, GIP and GLP-1(7-36) amide were similar in subjects with PCOS and controls. Although fasting IPI concentrations were similar in both groups, integrated IPI concentrations were higher (P = 0.05) in patients with PCOS. Women with PCOS had similar fasting but higher (P < 0.05) integrated IRI:C-peptide molar ratios than controls. Fasting and integrated IPI:IRI molar ratios were similar in both groups. These results confirm that lean women with PCOS have peripheral hyperinsulinaemia. The mild fasting hyperinsulinaemia is due to increased pancreatic secretion, whereas the stimulated hyperinsulinaemia is due to both pancreatic hypersecretion and reduced fractional hepatic extraction of insulin. Hyperproinsulinaemia is modest and appropriate in PCOS, GIP and GLP-1(7-36) amide do not contribute to the stimulated hyperinsulinaemia in PCOS.

Insulinotropic effects of cholecystokinin, gastric inhibitory polypeptide and glucagon-like peptide-1 during perifusion of short-term cultured canine isolated islets

Recent metabolic studies suggest that the incretin effect of gut hormones may account for most of the circulating insulin during mild postprandial hyperglycemia after transplantation of isolated islets. As yet, however, insulinotropic effects of pharmacological rather than physiological levels of the incretin candidates cholecystokinin-33 (CCK-33), gastric inhibitory polypeptide (GIP), and glucagon-like peptide-1 (GLP-1) on isolated perifused islets have been reported. We examined the insulinotropic effects of these peptides during perifusion of canine isolated islets. Exploration of beta-cell sensitivity in our model with graded 0.1-10 nM doses of CCK-33 and GIP at a 7.5 mM glucose level demonstrated insulinotropic effects from the lowest level. We, therefore, focused on the (near-) physiological effects of both CCK-33 (20 pM), GIP (500 pM), and GLP-1 (100 pM) during perifusion at a 2.5, 7.5, and 10 mM glucose level. No effects of CCK-33 were observed. GIP enhanced insulin release 1.1- and 1.2-fold, at the 7.5 and 10 mM glucose level, respectively. GLP-1 stimulated insulin output from the 2.5 mM glucose level; and a maximum, 2-fold, increase of insulin output was observed from the 7.5 mM glucose level. Thus, isolated perifused islets do respond to near-physiological beta-cell stimulation with gut hormones, and both GIP and GLP-1 may contribute to a hyperglycemia-enhanced activation of the enteroinsular axis after transplantation of isolated islets.