Evidence for early impairment of glucagon-like peptide 1-induced insulin secretion in human type 2 (non insulin-dependent) diabetes.

To investigate a possible role of an enteroinsular axis involvement in the pathogenesis of type 2 diabetes, plasma glucagon-like peptide 1 (GLP-1) 7-36 amide response to nutrient ingestion was evaluated in type 2 diabetics affected by different degrees of beta-cell dysfunction. METHODS: 14 patients on oral hypoglycaemic treatment (group A: HbA1C = 8.1 +/- 1.8 %) and 11 age-matched diabetic patients on diet only (group B: HbA1C = 6.4 +/- 0.9) participated in the study. 10 healthy volunteers were studied as controls. In the postabsorptive state, a mixed meal (700 kCal) was administered to all subjects, and blood samples were regularly collected up to 180' for plasma glucose, insulin, glucagon, and GLP-1 determination. RESULTS: In the control group, the test meal induced a significant increase in plasma GLP-1 at 30' and 60' (p < 0.01); the peptide concentrations then returning toward basal levels. beta-cell function estimation by HOMA score confirmed a more advanced involvement in group A than in group B (p < 0.01). In contrast, the insulin resistance degree showed a similar result in the two groups (HOMA-R). In group A, first-phase postprandial insulin secretion (0 - 60') resulted, as expected, in being significantly reduced compared to healthy subjects (p < 0.001). In the same patients the mean fasting GLP-1 value was similar to controls, but the meal failed to increase plasma peptide levels, which even tended to decrease during the test (p < 0.01). In group B, food-mediated early insulin secretion was higher than in group A (p < 0.001), although significantly reduced when compared to controls (p < 0.01). Like group A, no GLP-1 response to food ingestion occurred in group B patients in spite of maintained basal peptide secretion. Whereas the test-meal did not significantly modify plasma glucagon levels in the control group, glucagon concentrations increased at 30' and 60' in both diabetic groups (p < 0.01). CONCLUSIONS: 1) The functional integrity of GLP-1 cells results as being seriously impaired even in the condition of mild diabetes; 2) the early peptide failure could contribute to the development of beta-cell deterioration which characterizes overt type 2 diabetes.     

Diminished glucagon suppression after β-cell reduction is due to impaired α-cell function rather than an expansion of α-cell mass

Impaired suppression of glucagon levels after oral glucose or meal ingestion is a hallmark of type 2 diabetes. Whether hyperglucagonemia after a β-cell loss results from a functional upregulation of glucagon secretion or an increase in α-cell mass is yet unclear. CD-1 mice were treated with streptozotocin (STZ) or saline. Pancreatic tissue was collected after 14, 21, and 28 days and examined for α- and β-cell mass and turnover. Intraperitoneal (ip) glucose tolerance tests were performed at day 28 as well as after 12 days of subcutaneous insulin treatment, and glucose, insulin, and glucagon levels were determined. STZ treatment led to fasting and post-challenge hyperglycemia (P < 0.001 vs. controls). Insulin levels increased after glucose injection in controls (P < 0.001) but were unchanged in STZ mice (P = 0.36). Intraperitoneal glucose elicited a 63.1 ± 4.1% glucagon suppression in control mice (P < 0.001), whereas the glucagon suppression was absent in STZ mice (P = 0.47). Insulin treatment failed to normalize glucagon levels. There was a significant inverse association between insulin and glucagon levels after ip glucose ingestion (r(2) = 0.99). β-Cell mass was reduced by ～75% in STZ mice compared with controls (P < 0.001), whereas α-cell mass remained unchanged (P > 0.05). α-Cell apoptosis (TUNEL) and replication (Ki67) were rather infrequently noticed, with no significant differences between the groups. These studies underline the importance of endogenous insulin for the glucose-induced suppression of glucagon secretion and suggest that the insufficient decline in glucagon levels after glucose administration in diabetes is primarily due to a functional loss of intraislet inhibition of α-cell function rather than an expansion of α-cell mass.     

The human amylin analog, pramlintide, reduces postprandial hyperglucagonemia in patients with type 2 diabetes mellitus.

AIMS: Amylin is a second beta-cell hormone that is normally co-secreted with insulin in response to meals; it complements the effects of insulin in postprandial glucose control, in part by suppressing glucagon secretion. In patients with type 2 diabetes, mealtime administration of the human amylin analog pramlintide markedly improves postprandial glucose excursions. The aim of this study was to examine whether pramlintide reduces the postprandial hyperglucagonemia that is often seen in this patient population. METHODS: Utilizing a single-blind, placebo-controlled crossover design, 24 patients with type 2 diabetes, 12 insulin-treated and 12 non-insulin-treated, underwent a standardized mixed meal test on 2 occasions during which they received, in randomized order, a five-hour intravenous infusion of placebo or pramlintide (100 microg/h). RESULTS: During the placebo infusion, plasma glucose and plasma glucagon concentrations increased substantially after the meal. During the pramlintide infusion, postprandial plasma glucose and plasma glucagon responses were significantly (p < 0.05, all) reduced following ingestion of the same meal, both in the insulin-treated and non-insulin-treated subgroups. CONCLUSION: Supplementation of mealtime amylin with pramlintide reduces postprandial hyperglucagonemia in patients with type 2 diabetes, a mechanism that likely contributes to pramlintide's postprandial glucose-lowering effect.     

Zinc Supplementation Does Not Affect Glucagon Response to Intravenous Glucose and Insulin Infusion in Patients with Well-Controlled Type 2 Diabetes

Glucagon dysregulation is an essential component in the pathophysiology of type 2 diabetes. Studies in vitro and in animal models have shown that zinc co-secreted with insulin suppresses glucagon secretion. Zinc supplementation improves blood glucose control in patients with type 2 diabetes, although there is little information about how zinc supplementation may affect glucagon secretion. The objective of this study was to evaluate the effect of 1-year zinc supplementation on fasting plasma glucagon concentration and in response to intravenous glucose and insulin infusion in patients with type 2 diabetes. A cross-sectional study was performed after 1-year of intervention with 30 mg/day zinc supplementation or a placebo on 28 patients with type 2 diabetes. Demographic, anthropometric, and biochemical parameters were determined. Fasting plasma glucagon and in response to intravenous glucose and insulin infusion were evaluated. Patients of both placebo and supplemented groups presented a well control of diabetes, with mean values of fasting blood glucose and glycated hemoglobin within the therapeutic goals established by ADA. No significant differences were observed in plasma glucagon concentration, glucagon/glucose ratio or glucagon/insulin ratio fasting, after glucose or after insulin infusions between placebo and supplemented groups. No significant effects of glucose or insulin infusions were observed on plasma glucagon concentration. One-year zinc supplementation did not affect fasting plasma glucagon nor response to intravenous glucose or insulin infusion in well-controlled type 2 diabetes patients with an adequate zinc status.     

GIP and GLP-1 as incretin hormones: lessons from single and double incretin receptor knockout mice

Glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1) are gut-derived incretins secreted in response to nutrient ingestion. Both incretins potentiate glucose-dependent insulin secretion and enhance beta-cell mass through regulation of beta-cell proliferation, neogenesis and apoptosis. In contrast, GLP-1, but not GIP, inhibits gastric emptying, glucagon secretion, and food intake. Furthermore, human subjects with Type 2 diabetes exhibit relative resistance to the actions of GIP, but not GLP-1R agonists. The physiological importance of both incretins has been investigated through generation and analysis of incretin receptor knockout mice. Elimination of incretin receptor action in GIPR-/- or GLP-1R-/- mice produces only modest impairment in glucose homeostasis. Similarly, double incretin receptor knockout (DIRKO) mice exhibit normal body weight and normal levels of plasma glucagon and hypoglycemic responses to exogenous insulin. However, glucose-stimulated insulin secretion is significantly decreased following oral but not intraperitoneal glucose challenge in DIRKO mice and the glucose lowering actions of dipeptidyl peptidase-IV (DPP-IV) inhibitors are extinguished in DIRKO mice. Hence, incretin receptor signaling exerts physiologically relevant actions critical for glucose homeostasis, and represents a pharmacologically attractive target for development of agents for the treatment of Type 2 diabetes.     

Study of insulin response to oral glucose load after acute and chronic glycemic control in type 2 diabetic subjects

To investigate whether correction of fasting hyperglycemia per se improves the insulin secretion in type 2 diabetic subjects, plasma insulin response to 75 g oral glucose load has been studied after acute and chronic normalization of fasting plasma glucose levels in 7 overt type 2 diabetic subjects. For the acute normalization of elevated fasting plasma glucose levels, an artificial endocrine pancreas was employed. Although fasting plasma glucose concentrations were normalized before the oral glucose challenge, insulin response to oral glucose was not improved compared to those without normalization of fasting plasma glucose levels. After 1-3 month control of hyperglycemia, the insulin response to glucose in the subjects was significantly improved compared to those without treatments. Results indicate that chronic metabolic control is essential for the improvement of insulin response to glucose in type 2 diabetic subjects, and also suggest that the impaired insulin secretion in type 2 diabetes is not due to hyperglycemia per se, but due to the metabolic derangements which lead to chronic hyperglycemia.     

Modulation of A and B cell functions by tolbutamide and arginine in the pancreas of thiamine-deficient rats

The effects of administration of glucose orally and tolbutamide or arginine intravenously on insulin and glucagon secretion and blood glucose level were studied in normal and thiamine-deficient rats. In thiamine deficiency, insulin secretion and glucose tolerance were impaired during glucose ingestion. Tolbutamide decreased the blood glucose level in both control and thiamine-deficient rats but its stimulatory effect on insulin secretion was minimal in thiamine-deficient rats unlike the control animals. Arginine did not alter substantially the blood glucose or insulin in thiamine-deficient rats, whereas it increased the insulin level in control rats. The fasting plasma glucagon level was high in thiamine deficiency. Tolbutamide increased the plasma glucagon in control rats, but did so only marginally in thiamine-deficient rats. Arginine also increased the glucagon secretion throughout the period of study in control rats. In thiamine-deficient rats the glucagon secretion was pronounced only after 20 min of arginine administration. These results suggest that an unimpaired glucose metabolism is a prerequisite to induce proper insulin secretion. Only proper insulin secretion can check the glucagon secretion rather than the increased glucose level. Hypoglycemia can induce glucagon secretion independent of the insulin level.     

Effect of lowering postprandial hyperglycemia on insulin secretion in older people with impaired glucose tolerance

Glucose tolerance declines with age, resulting in a high prevalence of diabetes and impaired glucose tolerance (IGT) in the older population. Hyperglycemia per se can lead to impaired beta-cell function (glucose toxicity). We tested the role of glucose toxicity in age-related beta-cell dysfunction in older people (65 +/- 8 yr) with IGT treated with the alpha-glucosidase inhibitor acarbose (n = 14) or placebo (n = 13) for 6 wk in a randomized, double-blind study. Baseline and posttreatment studies included 1) an oral glucose tolerance test (OGTT), 2) 1-h postprandial glucose monitoring, 3) a frequently sampled intravenous glucose tolerance test (insulin sensitivity, or S(I)), and 4) glucose ramp clamp (insulin secretion rates, or ISR), in which a variable glucose infusion increases plasma glucose from 5 to 10 mM. The treatment groups had similar baseline body mass index; fasting, 2-h OGTT, and 1-h postprandial glucose levels; and S(I). In these carefully matched older people with IGT, both fasting (5.7 +/- 0.2 vs. 6.3 +/- 0.2 mM, P = 0.002) and 1-h postprandial glucose levels (6.9 +/- 0.3 vs. 8.2 +/- 0.4 mM, P = 0.02) were significantly lower in the acarbose than in the placebo group. Despite this reduction of chronic hyperglycemia in the acarbose vs. placebo group, measures of insulin secretion (ISR area under the curve: 728 +/- 55 vs. 835 +/- 81 pmol/kg, P = 0.9) and acute insulin response to intravenous glucose (329 +/- 67 vs. 301 +/- 54 pM, P = 0.4) remained unchanged and impaired. Thus short-term improvement of chronic hyperglycemia does not reverse beta-cell dysfunction in older people with IGT.     

Overnight inhibition of insulin secretion restores pulsatility and proinsulin/insulin ratio in type 2 diabetes

Impaired insulin secretion in type 2 diabetes is characterized by decreased first-phase insulin secretion, an increased proinsulin-to-insulin molar ratio in plasma, abnormal pulsatile insulin release, and heightened disorderliness of insulin concentration profiles. In the present study, we tested the hypothesis that these abnormalities are at least partly reversed by a period of overnight suspension of beta-cell secretory activity achieved by somatostatin infusion. Eleven patients with type 2 diabetes were studied twice after a randomly ordered overnight infusion of either somatostatin or saline with the plasma glucose concentration clamped at approximately 8 mmol/l. Controls were studied twice after overnight saline infusions and then at a plasma glucose concentration of either 4 or 8 mmol/l. We report that in patients with type 2 diabetes, 1) as in nondiabetic humans, insulin is secreted in discrete insulin secretory bursts; 2) the frequency of pulsatile insulin secretion is normal; 3) the insulin pulse mass is diminished, leading to decreased insulin secretion, but this defect can be overcome acutely by beta-cell rest with somatostatin; 4) the reported loss of orderliness of insulin secretion, attenuated first-phase insulin secretion, and elevated proinsulin-to-insulin molar ratio also respond favorably to overnight inhibition by somatostatin. The results of these clinical experiments suggest the conclusion that multiple parameters of abnormal insulin secretion in patients with type 2 diabetes mechanistically reflect cellular depletion of immediately secretable insulin that can be overcome by beta-cell rest.     

Glycaemic effects of incretins in Type 1 diabetes mellitus: a concise review, with emphasis on studies in humans

The remission phase of Type 1 diabetes mellitus is associated with substantial recovery of beta-cell function and with marked improvement of endogenous insulin responses to meals in the early months after diagnosis, accompanied by little or no improvement in the insulin response to parenteral glucose, suggesting that the incretin function may be important in glycaemic regulation in this phase of diabetes. Preservation of the insulin response to parenteral glucagon-like peptide-1 (GLP-1), contrasting with lack of stimulation of insulin secretion by the other known incretin gastric inhibitory polypeptide (GIP), prompted studies with exogenous GLP-1 in recent-onset Type 1 diabetes. These studies showed substantial reduction of glycaemic excursions after ingestion of mixed nutrients during intravenous infusion of GLP-1 without administration of insulin, in subjects with a range of endogenous secretion of insulin in response to meals as demonstrated by blood levels of the insulin-connecting peptide (CP). These effects were independent of stimulation of blood levels of CP and were reproduced in volunteers with no endogenous release of CP in response to meals. The glycaemic effects were associated with inhibition of abnormal rises of blood levels of glucagon, and with suppression of endogenous release of human pancreatic polypeptide (HPP), by GLP-1. It was hypothesized that a major component of the glycaemic effect is attributable to the known action of GLP-1 to inhibit gastric emptying and to inhibit glucagon secretion. Studies of the effects of GLP-1 agonists (GLP-1 and exendin-4) given together with established insulin doses before a meal supported the hypothesis. The more prolonged actions of exendin-4 were accompanied by greater and more prolonged reduction of glycaemic effects of ingestion of meals in volunteers with CP-negative Type 1 diabetes mellitus, during intensive insulin therapy, in whom delay of gastric emptying was confirmed by studies of blood levels of acetaminophen ingested with the meals. Side effect-free doses of exendin-4 given together with insulin in volunteers with CP-negative Type 1 diabetes receiving continuing intensive insulin therapy demonstrated the capacity of this combination therapy to normalize blood glucose levels after ingestion of meals that were consistent with the dietary program of the volunteers, without apparent increased risk of hypoglycaemia within a normal between-meals interval. It is suggested that further and more prolonged studies of the use of long-acting GLP-1 agonists as congeners with insulin in Type 1 diabetes mellitus are indicated.     

Long-term high-fat feeding leads to severe insulin resistance but not diabetes in Wistar rats

Although lipid excess can impair beta-cell function in vitro, short-term high-fat feeding in normal rats produces insulin resistance but not hyperglycemia. This study examines the effect of long-term (10-mo) high polyunsaturated fat feeding on glucose tolerance in Wistar rats. The high fat-fed compared with the chow-fed group was 30% heavier and 60% fatter, with approximately doubled fasting hyperinsulinemia (P < 0.001) but only marginal fasting hyperglycemia (7.5 +/- 0.1 vs. 7.2 +/- 0.1 mmol/l, P < 0.01). Insulin sensitivity was approximately 67% lower in the high-fat group (P < 0.01). The acute insulin response to intravenous arginine was approximately double in the insulin-resistant high-fat group (P < 0.001), but that to intravenous glucose was similar in the two groups. After the intravenous glucose bolus, plasma glucose decline was slower in the high fat-fed group, confirming mild glucose intolerance. Therefore, despite severe insulin resistance, there was only a mildly elevated fasting glucose level and a relative deficiency in glucose-stimulated insulin secretion; this suggests that a genetic or congenital susceptibility to beta-cell impairment is required for overt hyperglycemia to develop in the presence of severe insulin resistance.     

A coordinated control strategy for insulin and glucagon delivery in type 1 diabetes

Type 1 diabetes is an autoimmune condition characterised by a pancreatic insulin secretion deficit, resulting in high blood glucose concentrations, which can lead to micro- and macrovascular complications. Type 1 diabetes also leads to impaired glucagon production by the pancreatic α-cells, which acts as a counter-regulatory hormone to insulin. A closed-loop system for automatic insulin and glucagon delivery, also referred to as an artificial pancreas, has the potential to reduce the self-management burden of type 1 diabetes and reduce the risk of hypo- and hyperglycemia. To date, bihormonal closed-loop systems for glucagon and insulin delivery have been based on two independent controllers. However, in physiology, the secretion of insulin and glucagon in the body is closely interconnected by paracrine and endocrine associations. In this work, we present a novel biologically-inspired glucose control strategy that accounts for such coordination. An in silico study using an FDA-accepted type 1 simulator was performed to evaluate the proposed coordinated control strategy compared to its non-coordinated counterpart, as well as an insulin-only version of the controller. The proposed coordinated strategy achieves a reduction of hyperglycemia without increasing hypoglycemia, when compared to its non-coordinated counterpart.     

Insulin secretion and islet endocrine cell content at onset and during the early stages of diabetes in the BB rat: effect of the level of glycemic control.

Although it is agreed that autoimmune destruction of pancreatic islets in diabetic BB rats is rapid, reports of endocrine cell content of islets from BB diabetic rats at the time of onset of diabetes vary considerably. Because of the rapid onset of the disease (hours) and the attendant changes in islet morphology and insulin secretion, it was the aim of this study to compare islet beta-cell numbers to other islet endocrine cells as close to the time of onset of hyperglycemia as possible (within 12 h). As it has been reported that hyperglycemia renders the beta cell insensitive to glucose, the early effects of different levels of insulin therapy (well-controlled vs. poorly controlled glycemia) on islet morphology and insulin secretion were examined. When measured within 12 h of onset, insulin content of BB diabetic islets, measured by morphometric analysis or pancreatic extraction, was 60% of insulin content of control islets. Despite significant amounts of insulin remaining in the pancreas, 1-day diabetic rats exhibited fasting hyperglycemia and were glucose intolerant. The insulin response from the isolated perfused pancreas to glucose and the glucose-dependent insulinotropic hormone, gastric inhibitory polypeptide (GIP), was reduced by 95%. Islet content of other endocrine peptides, glucagon, somatostatin, and pancreatic polypeptide, was normal at onset and at 2 weeks post onset. A group of diabetic animals, maintained in a hyperglycemic state for 7 days with low doses of insulin, were compared with a group kept normoglycemic by appropriate insulin therapy. No insulin could be detected in islets of poorly controlled diabetics, while well-controlled animals had 30% of the normal islet insulin content.(ABSTRACT TRUNCATED AT 250 WORDS)     

Reference intervals for glucose, beta-cell polypeptides, and counterregulatory factors during prolonged fasting

To establish reference intervals for the pancreatic beta-cell response and the counterregulatory hormone response to prolonged fasting, we studied 33 healthy subjects (16 males, 17 females) during a 72-h fast. Glucose, insulin, C-peptide, and proinsulin levels decreased (P < 0.001), and the levels of counterregulatory factors increased during the fast [P < 0.05; glucagon and free fatty acids (FFA) with a linear increase and epinephrine, norepinephrine, and cortisol with a clear underlying circadian rhythm]. Growth hormone secretion increased from the first to third day of fasting (P < 0.05) but actually decreased from the second to third day of fasting (P = 0.03). Males had higher glucose and glucagon levels and lower FFA levels during the fast (P < 0.05), whereas no effect of gender on beta-cell polypeptides was observed. A high body mass index resulted in higher insulin and C-peptide levels during the fast (P < 0.05). In conclusion, we have provided reference intervals for glucoregulatory factors during a 72-h fast. We observed a diminished beta-cell response concomitant with an increased secretion of counterregulatory hormones. These results should be of clinical and scientific value in the investigation of hypoglycemic disorders.     

Reduction of hepatic insulin clearance after oral glucose ingestion is not mediated by glucagon-like peptide 1 or gastric inhibitory polypeptide in humans

Changes in hepatic insulin clearance can occur after oral glucose or meal ingestion. This has been attributed to the secretion and action of gastric inhibitory polypeptide (GIP) and glucagon-like peptide (GLP)-1. Given the recent availability of drugs based on incretin hormones, such clearance effects may be important for the future treatment of type 2 diabetes. Therefore, we determined insulin clearance in response to endogenously secreted and exogenously administered GIP and GLP-1. Insulin clearance was estimated from the molar C-peptide-to-insulin ratio calculated at basal conditions and from the respective areas under the curve after glucose, GIP, or GLP-1 administration. Oral glucose administration led to an approximately 60% reduction in the C-peptide-to-insulin ratio (P < 0.0001), whereas intravenous glucose administration had no effect (P = 0.09). The endogenous secretion of GIP or GLP-1 was unrelated to the changes in insulin clearance. The C-peptide-to-insulin ratio was unchanged after the intravenous administration of GIP or GLP-1 in the fasting state (P = 0.27 and P = 0.35, respectively). Likewise, infusing GLP-1 during a meal course did not alter insulin clearance (P = 0.87). An inverse nonlinear relationship was found between the C-peptide-to-insulin ratio and the integrated insulin levels after oral and during intravenous glucose administration. Insulin clearance is reduced by oral but not by intravenous glucose administration. Neither GIP nor GLP-1 has significant effects on insulin extraction. An inverse relationship between insulin concentrations and insulin clearance suggests that the secretion of insulin itself determines the rate of hepatic insulin clearance.     

Is the insulin resistance of patients with noninsulin-dependent diabetes mellitus secondary to insulin deficiency?

Defects in both insulin secretion and action have been documented in patients with noninsulin-dependent diabetes mellitus (NIDDM), leading to the suggestion that both fasting hyperglycemia and insulin resistance in NIDDM are secondary to insulin deficiency. In order to test this hypothesis, insulin secretion (plasma insulin response to oral glucose) and insulin action (insulin clamp) were determined in 25 patients with NIDDM. The results documented relationships between incremental plasma insulin response to glucose and degree of fasting hyperglycemia (r = -.045, P less than 0.05) and insulin-stimulated glucose utilization (r = 0.25, P = NS). These data indicate that differences in insulin secretory response accounted for only approximately 20% of the variance in fasting plasma glucose level and 6% of the variance in insulin resistance in NIDDM. Thus, differences in insulin-secretory response contribute modestly to magnitude of glycemia, and not at all to variations in insulin resistance in NIDDM, permitting rejection of the hypothesis that insulin resistance is secondary to insulin deficiency.     

Qualitative abnormality of insulin secretion in a case with insulinoma.

We have presented here a case of atypical insulinoma. Despite the recurrent episodes of hypoglycemic symptoms, the plasma level of insulin has never been excessive at fasting or by regular provocative tests. Detailed examination had demonstrated qualitative abnormality of insulin secretion. Hyposuppressibility of insulin secretion by hypoglycemia, borderline diabetic curve of glucose tolerance test, blunted response ot insulin to glucagon and leucine were the principle characteristics of these abnormalities. After removal of adenoma, insulin response to glucose, glucagon and leucine was improved. Only secretion provoked a high level of insulin and this abnormal elevation was no longer seen after the removal of adenoma. A removed elevation was no longer seen after the removal of adenoma. A removed insulinoma contained 25 U of immunoreactive insulin per gram tissue, but was negative for aldehyde-fuchsin staining. On electromicroscopy only atypical beta-cell granules were seen.     

Reduced beta-cell mass and altered glucose sensing impair insulin-secretory function in betaIRKO mice

Pancreatic beta-cell-restricted knockout of the insulin receptor results in hyperglycemia due to impaired insulin secretion, suggesting that this cell is an important target of insulin action. The present studies were undertaken in beta-cell insulin receptor knockout (betaIRKO) mice to define the mechanisms underlying the defect in insulin secretion. On the basis of responses to intraperitoneal glucose, approximately 7-mo-old betaIRKO mice were either diabetic (25%) or normally glucose tolerant (75%). Total insulin content was profoundly reduced in pancreata of mutant mice compared with controls. Both groups also exhibited reduced beta-cell mass and islet number. However, insulin mRNA and protein were similar in islets of diabetic and normoglycemic betaIRKO mice compared with controls. Insulin secretion in response to insulin secretagogues from the isolated perfused pancreas was markedly reduced in the diabetic betaIRKOs and to a lesser degree in the nondiabetic betaIRKO group. Pancreatic islets of nondiabetic betaIRKO animals also exhibited defects in glyceraldehyde- and KCl-stimulated insulin release that were milder than in the diabetic animals. Gene expression analysis of islets revealed a modest reduction of GLUT2 and glucokinase gene expression in both the nondiabetic and diabetic mutants. Taken together, these data indicate that loss of functional receptors for insulin in beta-cells leads primarily to profound defects in postnatal beta-cell growth. In addition, altered glucose sensing may also contribute to defective insulin secretion in mutant animals that develop diabetes.