Insulinoma-bearing Rats Research Articles

Insulin modulates glucose-dependent insulinotropic polypeptide (GIP) secretion from enteroendocrine K cells in rats

Effects of insulin excess and deficiency on glucose-dependent insulinotropic polypeptide (GIP) was examined in rats following insulinoma transplantation or streptozotocin (STZ) administration. Over 14 days, food intake was increased (p < 0.001) in both groups of rats, with decreased body weight (p < 0.01) in STZ rats. Non-fasting plasma glucose levels were decreased (p < 0.01) and plasma insulin levels increased (p < 0.001) in insulinoma-bearing rats, whereas STZ treatment elevated glucose (p < 0.001) and decreased insulin (p < 0.01). Circulating GIP concentrations were elevated (p < 0.01) in both animal models. At 14 days, oral glucose resulted in a decreased glycaemic excursion (p < 0.05) with concomitant elevations in insulin release (p < 0.001) in insulinoma-bearing rats, whereas STZ-treated rats displayed similar glucose-lowering effects but reduced insulin levels (p < 0.01). GIP concentrations were augmented in STZ rats (p < 0.05) following oral glucose. Plasma glucose and insulin concentrations were not affected by oral fat, but fat-induced GIP secretion was particularly (p < 0.05) increased in insulinoma-bearing rats. Exogenous GIP enhanced (p < 0.05) glucose-lowering in all groups of rats accompanied by insulin releasing (p < 0.001) effects in insulinoma-bearing and control rats. Both rat models exhibited increased (p < 0.001) intestinal weight but decreased intestinal GIP concentrations. These data suggest that circulating insulin has direct and indirect effects on the synthesis and secretion of GIP.

Studies on the Chemical and Immunological Destruction of Insulin-Secreting Islet Cell Tumours

Chemical and immunological destruction of insulin-secreting islet cell tumours were evaluated in vivo and in vitro using the transplantable radiation-induced NEDH rat insulinoma and the derived clonal RINm5F cell line. Administration of a large amount of polyclonal insulin antibody did not affect the development of hypoglycaemia, tumour weights or the survival of insulinoma-bearing rats. Streptozotocin (100 mg/kg body weight) evoked a rapid and sustained decrease of insulin concentrations, accompanied by tumour regression and elevation of plasma glucose. Administration of alloxan (200 mg/kg) was without effect. In vitro, streptozotocin and alloxan exerted approximately equipotent time-dependent and concentration-dependent cytotoxic effects on cultured insulinoma cells as established by cell staining with trypan blue. The cytotoxic actions of both drugs were decreased by agents believed to scavenge free radicals or to act as inhibitors of poly(ADP-ribose)synthetase. Exposure of clonal RINm5F cells to the nitrosocompounds, N-methyl-N'-nitro-N-nitrosoguanidine and N-nitroso-N-methylurea, resulted in a particularly marked reduction in cell viability compared with streptozotocin.

Peripheral neuropathy and microangiopathy in rats with insulinoma: association with chronic hyperinsulinemia.

Hypoglycemia can precipitate or worsen peripheral neuropathy in patients with insulinoma or in diabetic patients on an intensive insulin regimen. It still remains unclear as to whether hyperinsulinemia itself is involved in neuropathic changes in these patients. We, therefore, explored the possible isolated effects of chronic hyperinsulinemia on neuropathic changes in insulinoma-bearing rats (I-rats). I-rats were generated by a combined treatment with nicotinamide and streptozotocin. At 15 months after the treatment, they showed a wide range of the plasma insulin (PI) level with or without a decrease in the blood glucose (BG) level and were divided into three groups on the basis of the presence of hypoglycemia (BG < 2.5 mmol/L) or hyperinsulinemia (PI > 100 mU/L): the first exhibited only hypoglycemia, the second exhibited only hyperinsulinemia, and the third exhibited neither. Peripheral nerve function and structure as well as microvasculature were evaluated among these groups in addition to age-matched untreated control rats (C-rats). The first group of hypoglycemic I-rats showed a decrease (p < 0.05) in the axon/myelin ratio and an increase (p < 0.0001) in fibers undergoing axonal degeneration compared to C-rats, while the other two groups did not. On the other hand, the second group of hyperinsulinemic I-rats showed a decrease (p < 0.05) in the myelinated axonal size and an increase (all p < 0.05) in the F-wave latency and the densities of myelinated fibers and endoneurial microvessels exhibiting endothelial hyperplasia, vascular wall thickening, or pericytes debris compared to the third group of isoglycemic I-rats without hyperinsulinemia. These results suggest that hypoglycemia is associated with increased myelinated axonal damage, while hyperinsulinemia is associated with increased densities of small myelinated axons and endoneurial microvessels with microangiopathic changes in I-rats. We, therefore, propose that the observed findings may be relevant to the complicated features of neuropathy in diabetic patients with chronic hyperinsulinemia.

The hypothalamic satiety peptide CART is expressed in anorectic and non-anorectic pancreatic islet tumors and in the normal islet of Langerhans

The hypothalamic satiety peptide CART (cocaine and amphetamine regulated transcript) is expressed at high levels in anorectic rat glucagonomas but not in hypoglycemic insulinomas. However, a non-anorectic metastasis derived from the glucagonoma retained high CART expression levels and produced circulating CART levels comparable to that of the anorectic tumors. Moreover, distinct glucagonoma lines derived by stable HES-1 transfection of the insulinoma caused severe anorexia but retained low circulating levels of CART comparable to that of insulinoma bearing or control rats. Islet tumor associated anorexia and circulating CART levels are thus not correlated, and in line with this peripheral administration of CART (5–50 mg/kg) produced no effect on feeding behavior. In the rat two alternatively spliced forms of CART mRNA exist and quantitative PCR revealed expression of both forms in the hypothalamus, in the different islet tumors, and in the islets of Langerhans. Immunocytochemistry as well as in situ hybridization localized CART expression to the somatostatin producing islet D cell. A potential endocrine/paracrine role of islet CART remains to be clarified.

Peripheral nerve pathology in rats with streptozotocin-induced insulinoma

Peripheral nerve structure was systematically examined in rats with insulinoma induced by streptozotocin (STZ). Normal Wistar rats, aged 3 months (n = 10), were treated with intravenous injections of STZ (20 mg/kg) and housed in plastic cages with free access to water and chow until 24 months of age. Three rats with insulinoma survived and were examined pathologically. Age-matched normal Wistar rats (n = 6) were used for comparison. The insulinoma-bearing rats showed a marked increase in body weight and decrease in blood glucose. In a teased nerve fiber study of the sciatic nerve, the percentage of abnormal fibers undergoing axonal degeneration and de- and remyelination in age-matched normal control rats was 3.9 +/- 2.5% (means +/- SD), whereas in the three insulinoma-bearing rats 49%, 50%, and 24%, respectively, of the fibers showed such changes. Regenerating fibers were also numerous in each insulinoma-bearing rat (36%, 42% and 27%, respectively). Morphometric analysis revealed smaller mean myelinated fiber and axonal areas in all the nerves examined (sciatic, tibial and sural) in insulinoma-bearing rats as compared to those in age-matched normal rats. Fiber area frequency histograms showed a decrease in large myelinated fibers and an increase in small regenerated fibers in insulinoma-bearing rats. Ultrastructurally, endoneurial microvessels exhibited a narrowed vascular lumen with swollen endothelial cells and vacuolar degeneration of pericytes, suggesting an involvement of vascular changes in the neuropathic development. The present study demonstrated marked structural changes in both motor and sensory peripheral nerves of rats bearing experimentally induced insulinoma. We consider that axonal degeneration, regeneration and demyelination constitute the main pathology in the peripheral nerves of insulinoma-bearing rats, although no particular difference in severity of the lesions between sensory and motor and between proximal and distal nerves was apparent.

Hyperinsulinaemia causes a preferential increase in hepatic p4501A2 activity

Male NEDH (New England Deaconess Hospital) rats were transplanted with a radiationinduced tumour from a donor male rat and were killed 18 days following transplantation. At the time of killing the insulinoma-bearing animals were severely hypoglycaemic but plasma ketone levels were normal. Insulinoma-bearing animals exhibited higher hepatic O-deethylation of ethoxyresorufin and N-demethylation of ethylmorphine activities when compared to control animals. Similarly, hepatic microsomal preparations from insulinoma-bearing rats were more efficient than control animals in converting the promutagen 2-amino-6-methyldipyrido[1,2- a:3′,2′']imidazole (Glu-P-1) to mutagenic intermediates in the Ames test. Immunoblot analysis employing polyclonal antibodies against the P4501A and P453A families revealed that insulinoma-bearing rats had higher hepatic P4501A2 apoprotein levels. No major differences in P4503A1 apoprotein levels between insulinoma-bearing and control rats were noted. Subcutaneous administration of insulin to male Wistar rats gave rise to a modest increase in ethoxyresorufin O-deethylase activity and in the ability to activate Glu-P-1 to mutagens in the Ames test. Immunoblot analysis revealed an increase in hepatic P4501A2 apoprotein levels following the treatment with insulin. It is concluded that insulinoma-bearing rats display high P4501A2 activity and the hyperinsulinaemia that characterize this condition is responsible for the effect. Moreover, administration of insulin to other strains of rat, such as Wistar, also enhances P4501A2 activity, presumably as a result of hyperinsulinaemia.

Somatostatin, Gastrin-Releasing Peptide and Gastrin in the Stomach of Rats with Streptozotocin-Induced Diabetes and Insulinoma

Somatostatin, gastrin-releasing peptide (GRP) and gastrin were measured in the stomach of rats with streptozotocin-induced diabetes, insulinoma-bearing rats and their respective controls. Rats injected with streptozotocin exhibited hyperphagia, insulinopenia and severe hyperglycemia. Stomach weights, and the concentrations and total amounts of GRP and gastrin in the stomach, were similar to nondiabetic control rats. The concentration of somatostatin in the stomach of diabetic rats was 25% greater, but the total stomach content of somatostatin was similar to that of control rats. Insulinoma-bearing rats exhibited hyperphagia, hyperinsulinemia and hypoglycemia. Concentrations of GRP and gastrin in the stomach were 72% and 19% lower, respectively, than in control rats. Despite 45% greater stomach weight, the total stomach content of GRP was 61% lower. Stomach concentrations of somatostatin, and total stomach contents of somatostatin and gastrin, were similar in insulinoma-bearing and control rats. The results demonstrate abnormalities in the stomach concentrations of regulatory peptides in rats with diabetes and insulinoma. These abnormalities are not attributable to changes in food intake alone, suggesting specific effects of these metabolic diseases on gastric regulatory peptides and gastric function.

Effects of hypoglycemia and prolonged fasting on insulin and glucagon gene expression. Studies with in situ hybridization.

In situ hybridization of proinsulin and proglucagon mRNA was performed in rat pancreas to assess prohormone gene expression during various glucopenic conditions. During a 4-d fast mean blood glucose declined by 48 mg/dl; proinsulin mRNA signal density remained normal while proglucagon mRNA signal density more than doubled. At the end of a continuous 12-d insulin infusion blood glucose averaged 53 +/- 12 mg/dl; proinsulin mRNA signal density declined to 30% of controls while proglucagon mRNA signal density more than doubled. In insulinoma-bearing NEDH rats blood glucose averaged 34 +/- 3.5 mg/dl; the proinsulin mRNA signal was virtually undetectable and proglucagon mRNA signal density was more than twice the controls. There was no detectable change in either beta-cell area or islet number in rats subjected to fasting or insulin infusion, but in insulinoma-bearing rats beta cell area was markedly reduced. Thus compensation during 4 d of starvation involves an increase in glucagon gene expression without change in insulin gene expression or beta cell mass. In moderate insulin-induced hypoglycemia glucagon gene expression is increased and insulin gene expression decreased. In more profound insulinoma-induced hypoglycemia, in addition to the foregoing changes in hormone gene expression, there is a profound reduction in the number of insulin-expressing cells.

Effects of transplantation and resection of a radiation-induced rat insulinoma on glucose homeostasis and the endocrine pancreas.

Twenty-one days after s.c. subscapular transplantation of a radiation-induced insulinoma, male NEDH rats exhibited hyperinsulinaemia and hypoglycaemia. These features were associated with islet atrophy, degenerative changes in pancreatic A and B cells, and decreases in the pancreatic contents of insulin, glucagon and somatostatin. The immunoreactive glucagon and somatostatin contents of extrapancreatic tissues of insulinoma-bearing rats were unchanged. Surgical resection of the tumour resulted in an immediate fall of plasma insulin, attaining concentrations similar to those of anaesthetised control rats by 10 min. The estimated half-life of insulin was 3.5 min. Hypoglycaemia persisted until 60 min after resection, followed by hyperglycaemia of 1-2 days duration. Glucose tolerance was impaired 1 day after tumour resection despite the coexistence of raised insulin concentrations. Evidence for abnormal pancreatic B cell function was gained by injection of arginine which failed to evoke a plasma insulin response in the resected rats. Two days after resection, plasma glucose and insulin concentrations were similar to those of control rats. Plasma glucose and insulin responses to glucose and arginine were suggestive of tumour recurrence by 12 days. A single large encapsulated tumour was eventually observed in each rat, with resection giving a 17-56 day prolongation of life.

Reversal of diabetes by syngeneic transplantation of a radiation-induced rat insulinoma

The growth and metabolic effects of a radiation-induced rat insulinoma were examined after subcutaneous subscapular transplantation into normal and streptozotocin diabetic NEDH rats. Streptozotocin diabetic rats exhibited hyperglycaemia, hypoinsulinaemia, impaired glucose tolerance without an insulin response, polyuria, polydipsia, hyperphagia and weight loss. Transplantation of tumour fragments gradually improved the physical and metabolic state over the following 3 weeks. Coincident with a progressive rise in plasma insulin between 10 and 17 days, the diabetic rats gained weight and reduced their food intake. The rats remained hyperglycaemic during this time, but developed hypoglycaemia with marked hyperinsulinaemia by 24 days. Furthermore, plasma glucose and insulin concentrations were not increased by an intraperitoneal glucose challenge, indicating greatly accelerated glucose clearance. Both the streptozotocin-treated and normal insulinoma-bearing rats incurred a fatal hypoglycaemic coma by 28-33 days after transplantation. Final body weights, tumour weights and concentrations of glucose and insulin were similar in the two groups. This study demonstrates reversal of streptozotocin diabetes by insulinoma transplantation. The hyperglycaemia and the accompanying diabetic environment did not modify tumour growth and development.

Effects of cytotoxic drugs and inhibitors of insulin secretion on a serially transplantable rat insulinoma and cultured rat insulinoma cells

1. 1. The effects of cytotoxic drugs and inhibitors of insulin secretion were examined in vivo in rats with a radiation-induced transplantable insulinoma, and in vitro using cultured rat insulinoma cells and the derived RINm5F insulin-secreting cell line. 2. 2. Administration of diazoxide to insulinoma-bearing rats resulted in a transient decrease of plasma insulin with a temporary rise of glucose concentrations. Mannoheptulose and somatostatin failed to affect the marked hyperinsulinaemia and hypoglycaemia. 3. 3. Streptozotocin produced a rapid and sustained decrease of insulin concentrations in insulinoma-bearing rats, accompanied by a progressive elevation of plasma glucose. Administration of alloxan failed to affect circulating insulin or glucose concentrations. 4. 4. In vitro, streptozotocin and alloxan exerted approximately equipotent time-dependent and concentration-dependent cytotoxic effects on insulinoma cells and RINm5F cells as established by cell staining with trypan blue. The cytotoxic actions of both drugs were decreased by agents believed to scavenge free radicals or to act as inhibitors of poly(ADP-ribose) synthetase. 5. 5. The results suggest that the cytotoxic actions of streptozotocin and alloxan on rat insulinoma cells and RINm5F cells are mediated by the generation of hydroxyl free radicals and DNA strand breaks. The ineffectiveness of alloxan in insulinoma-bearing rats probably reflects the high rate of decomposition of the drug in vivo.

Defective diurnal changes of food intake, plasma glucose and insulin in rats with a transplantable islet cell tumour.

Subcutaneous implantation of small fragments of a radiation-induced transplantable rat insulinoma into the subscapular region of 16- to 17-week-old male NEDH rats resulted, over a 22-day period, in the progressive development of marked hyperinsulinaemia and severe hypoglycaemia, despite a compensatory increase in food intake. Diurnal changes were examined at 3-hourly intervals for 24 h in control rats and tumour-bearing rats at 20-21 days after transplantation. The control animals exhibited distinct diurnal changes of food intake, glucose and insulin concentrations. Food intake was greatest between 17.00 and 23.00 h; plasma insulin was greatest between 20.00 and 23.00 h, and plasma glucose was raised at 20.00, 02.00 and 05.00 h, compared with the other times. In contrast, insulinoma-bearing rats displayed no diurnal changes other than a small decrease in food intake between 05.00 and 11.00 h. Plasma glucose and insulin concentrations were significantly different from control rats at all times, and food intake was significantly increased between 23.00 and 17.00 h. These observations demonstrate that the transplantable insulinoma not only causes hyperinsulinaemia and hypoglycaemia but results in hyperphagia and defective diurnal changes of food intake, plasma glucose and insulin concentrations. Interruption of nutrient intake by withdrawal of food for 6 h exacerbated the hypoglycaemia of insulinoma-bearing rats leading to coma.

Metabolic effects and secretory properties of a radiation-induced transplantable rat insulinoma

1. 1. The metabolic effects and secretory properties of a radiation-induced transplantable insulinoma were examined in 16–17 week old NEDH rats. 2. 2. Subcutaneous subscapular implantation of tumour fragments resulted in hyperphagia, increased body weight gain, marked hyperinsulinaemia and severe hypoglycaemia, with the resulting death of the recipient by 27 days. Ultimate tumour size was 2.1 ± 0.4 g (mean ± SEM). 3. 3. At 3 days after transplantation, plasma glucose and insulin responses to intraperitoneal glucose, insulin, arginine and adrenaline were similar to control rats. 4. 4. At 20 days, plasma glucose concentrations of insulinoma-bearing rats remained low throughout glucose tolerance tests, and insulin responsiveness to glucose stimulation was absent. 2-Deoxy- d-glucose produced only a small rise of glucose concentrations in tumour-bearing rats. 5. 5. Insulin sensitivity was not appreciably impaired at 20 days despite severe hyperinsulinaemia and hypoglycaemia. The ability of adrenaline and propranolol to suppress plasma insulin and raise plasma glucose concentrations was also retained. 6. 6. At 20 days, glucagon evoked a marked plasma insulin response with no change in plasma glucose concentrations. In contrast, arginine and glibenclamide failed to stimulate insulin above high basal concentrations.

Measurement of Glycosylated Haemoglobins and Glycosylated Plasma Proteins in Animal Models with Diabetes or Inappropriate Hypoglycaemia

Stable glycosylated haemoglobins and glycosylated plasma proteins were determined by affinity chromatography using Glycogel B in animal models with diabetes or inappropriate hypoglycaemia. Adult Aston ob/ob mice and C57BL/KsJ db/db mice exhibited 1.5-1.9 fold increases of body weight, 2.5-3.4 fold elevations of plasma glucose and 20.9-29.3 fold elevations of plasma insulin. Glycosylated haemoglobins and glycosylated plasma proteins were raised 7.2-8.2 fold and 6.6-6.7 fold respectively. In adult NEDH rats, administration of streptozotocin or implantation of transplantable insulinoma fragments produced reciprocal changes in insulin and glucose concentrations either resulting in the onset of insulin deficiency (5.9 fold decrease) and hyperglycaemia (3.2 fold increase) by 2 days, or hyperinsulinaemia (2.1 fold increase) and hypoglycaemia (1.4 fold decrease) by 6 and 8 days, respectively. Glycosylated plasma proteins were increased (1.2 fold) rapidly after streptozotocin treatment followed by glycosylated haemoglobins (1.6 fold increase) after 8 days. In contrast, the decreases in glycosylated plasma proteins and glycosylated haemoglobins (4.4 fold and 1.4 fold, respectively) in insulinoma-bearing rats preceded the demonstration of hypoglycaemia by 5 and 2 days, respectively. Glycosylated plasma proteins in insulinoma-bearing rats returned to pretransplantation values at 10-16 days. Good correlations were observed in mice and rats between glucose and both glycosylated haemoglobins (r = 0.92 and r = 0.90, respectively) and glycosylated plasma proteins (r = 0.85 and r = 0.93), and between the glycosylated blood proteins themselves (r = 0.95 and r = 0.91). The results show that the measurement of glycosylated blood proteins by affinity chromatography using Glycogel B provides a sensitive and reliable indicator of the recent glycaemic environment.

Differential regulation of glucokinase activity in pancreatic islets and liver of the rat.

The differential tissue-specific regulation of glucokinase activity in liver and pancreatic islet cells was investigated in the insulinoma-bearing rat. A transplantable insulinoma caused hyperinsulinemia and hypoglycemia in the host by 2-3 months after implantation. Suppression of the pancreatic B-cells by the high insulin and/or low glucose manifested itself by a decrease of insulin in islet tissue. Removal of the tumor initiated transient insulin deficiency and hyperglycemia with extremes of these changes at 24 h after tumor resection. These conditions markedly affected glucose phosphorylation in the islet cells: glucokinase activity was reduced 71% in islet samples from insulinoma-bearing rats, and the enzyme fully recovered within 24 h after tumor resection. Hexokinase activity, by contrast, was not affected by these manipulations. To evaluate the relative contributions of hypoglycemia and hyperinsulinemia in islet glucokinase adaptation, glucose was intravenously infused to insulinoma-bearing rats; glycemia in excess of 150 mg/100 ml combined with excessive hyperinsulinemia resulted in a partial recovery of islet glucokinase activity, first apparent after 9 h of glucose infusion and with doubling of the activity after 24 h after glucose loading. In contrast, liver glucokinase was increased nearly 4-fold at the time of extreme hypoglycemia and hyperinsulinemia and rapidly fell to control rates following tumor removal. Intravenous infusion of glucose for 24 h into the tumor-bearing rat (i.e. hyperglycemia combined with excessive plasma insulin) had no influence on liver glucokinase activity. Liver hexokinase was not influenced by any of these experimental manipulations. The data indicate that the activities of pancreatic islet and liver glucokinase are regulated in a differential manner. Insulin is apparently the primary determinant of liver glucokinase and glucose seems to control islet glucokinase. Biochemical mechanisms for differential organ-specific regulation of glucokinase activity seem to have evolved such that this enzyme may play a dual role in glucose homeostasis, namely to serve as insulin-dependent glucose sensor in the B-cells and as insulin-sensitive determinant of hepatic glucose use.

Effects of a transplantable insulinoma upon regulatory peptide concentrations in the gastrointestinal tract of the rat.

The rapid growth (0.8 +/- 0.3 g/day) of a transplantable insulinoma, which also contained substance P (2.9 +/- 2.3 pmol/g) and gastrin-releasing peptide (3.2 +/- 2.1 pmol/g), resulted in the development of hyperphagia, hyperinsulinaemia and hypoglycaemia in rats (n = 8). After a 14-day growth period, the insulinoma-bearing rats showed an increase (49%; p less than 0.01) in the weight of the small intestine but no significant change in stomach weight compared with control animals. The content (pmol/organ) of somatostatin, substance P, neurokinin A and vasoactive intestinal peptide in the stomachs of the tumour rats was unchanged. A depletion in the content (53% p less than 0.01) and concentration (57%; p less than 0.01) of gastrin-releasing peptide, however, suggested either hypersecretion, possibly mediated through hypoglycaemia-induced vagal stimulation, or inhibition of synthesis. The concentration and content of glucagon-like immunoreactivity (enteroglucagon) in the small intestine of the insulinoma rats increased markedly (47%; p less than 0.01 and 120%; p less than 0.01). This increase is consistent with a proposed role of this peptide as a factor trophic to the intestinal mucosa. No significant changes in the concentrations of somatostatin, substance P, neurokinin A, vasoactive intestinal peptide and gastrin-releasing peptide in the small intestine were observed. However, the increase in gut weight resulted in a greater content of vasoactive intestinal peptide (40%; p less than 0.01) and substance P (37%; p less than 0.05) in the insulinoma rats.

Induction of the glucokinase-glucose sensor in pancreatic islets of insulinoma-bearing rats following tumor removal.

Blood glucose concentrations in mammals are maintained within a narrow range because the glucose homeostat in the organism controls glucose storage and usage by a variety of cells.

Differential insulin responses to oral and intravenous glucose in the transplantable rat insulinoma — a role for GIP

We have previously demonstrated an impaired insulin response to intraperitoneal glucose and arginine by the transplantable NEDH rat insulinoma. The nature of this tumour B-cell defect has been further studied by investigating the response of insulinoma-bearing rats to intravenous and intragastric glucose. Intravenous glucose failed to stimulate plasma immunoreactive insulin (IRI) above high basal levels (14.5 ± 1.1 μg/L). However, significant elevation of the plasma IRI concentration was observed following an intragastric glucose load (17.1 ± 1.5 μg/L; P < 0.02). In view of the different effects of oral and intravenous glucose on insulin secretion in the RIN, implicating an involvement of incretin factors from the gut, the response of the tumour to GIP was investigated. Plasma IRI concentrations rose significantly in these animals (20.6 ± 2.5 μg/L at 5 min, P < 0.02). We conclude that (a) the transplantable rat insulinoma is responsive to GIP, and (b) that whilst the tumour B-cell has lost its insulin responsiveness to hyperglycaemia produced by intraperitoneal or intravenous glucose, it retains its ability to respond to intragastric glucose. This could be due to incretin factors from the gut of which GIP is currently the strongest candidate.