Impaired beta-cell function, incretin effect, and glucagon suppression in patients with type 1 diabetes who have normal fasting glucose

We have recently described a novel phenotype in a group of subjects with type 1 diabetes that is manifested by glucose >11.1 mmol/l 120 min after an oral glucose load, but with normal fasting glucose levels. We now describe the metabolic characteristics of these subjects by comparing parameters of islet hormone secretion and glucose disposal in these subjects to age-matched nondiabetic control subjects. The patients with type 1 diabetes had fasting glucose, insulin, and glucagon values similar to those of control subjects. Additionally, the insulin secretory response to intravenous arginine at euglycemia was similar in the control and diabetic groups (264 +/- 33.5 and 193 +/- 61.3 pmol/l; P = 0.3). However, marked differences in beta-cell function were found in response to hyperglycemia. Specifically, the first-phase insulin response was lower in diabetic subjects (329.1 +/- 39.6 vs. 91.3 +/- 34.1 pmol/l; P < 0.001), as was the slope of glucose potentiation of the insulin response to arginine (102 +/- 18.7 vs. 30.2 +/- 6.1 pmol/l per mmol/l; P = 0.005) and the maximum insulin response to arginine (2,524 +/- 413 vs. 629 +/- 159 pmol/l; P = 0.001). Although plasma levels of glucagon-like peptide (GLP)-1 and gastric inhibitory peptide (GIP) did not differ between control and diabetic subjects, the incretin effect was lower in the diabetic patients (70.3 +/- 5.4 vs. 52.1 +/- 5.9%; P = 0.03). Finally, there was a lack of suppression of glucagon in the patients after both oral and intravenous glucose administration, which may have contributed to their postprandial hyperglycemia. Glucose effectiveness did not differ between patients and control subjects, nor did insulin sensitivity, although there was a tendency for the patients to be insulin resistant (9.18 +/- 1.59 vs. 5.22 +/- 1.17 pmol.(-1).min(-1); P = 0.08). These data characterize a novel group of subjects with type 1 diabetes manifested solely by hyperglycemia following an oral glucose load in whom islet function is normal at euglycemia, but who have marked defects in both alpha- and beta-cell secretion at hyperglycemia. This pattern of abnormalities may be characteristic of islet dysfunction early in the development of type 1 diabetes.     

beta-cell function in morbidly obese subjects during free living: long-term effects of weight loss

Insulin hypersecretion and insulin resistance are physiologically linked features of obesity. We tested whether extreme hypersecretion impairs beta-cell function under free-living conditions and whether major weight loss modifies insulin hypersecretion, insulin sensitivity, and beta-cell function. Plasma glucose, C-peptide, and free fatty acid concentrations were measured at hourly intervals during 24 h of normal life (including calorie-standardized meals) in 20 morbidly obese nondiabetic patients (BMI 48.4 +/- 1.7 kg/m2) and 7 nonobese age- and sex-matched control subjects; 8 of the obese patients were restudied 6 months and 2 years following biliopancreatic diversion. Insulin secretion was reconstructed from C-peptide levels by deconvolution and related to concurrent glucose levels through a mathematical model incorporating key features of beta-cell function: rate sensitivity, beta-cell glucose sensitivity, and potentiation. Insulin sensitivity (by the euglycemic insulin clamp technique) was reduced by 50% in obese subjects (23.1 +/- 2.5 of obese subjects vs. 52.9 +/- 4.9 micromol.min(-1) . kg(FFM)(-1) of control subjects, means +/- SE, P = 0.0004) as was mean 24-h insulin clearance (median 809 [interquartile range 451] vs. 1,553 [520] ml.min(-1) . m(-2), P < 0.001) due to a 50% reduction in hepatic insulin extraction (P < 0.01). Over 24 h, insulin secretion was doubled in obese subjects (468 nmol [202] in obese subjects vs. 235 [85] of control subjects, P=0.0002). Despite the hypersecretion, beta-cell glucose sensitivity, rate sensitivity, and potentiation were similar in obese and control subjects. Six months postoperatively (weight loss = 33 +/- 3 kg), both insulin hypersecretion (282 nmol [213]) and insulin sensitivity (51.6 +/- 3.7 micromol.min(-1).kg(FFM)(-1)) were normalized. At 2 years (weight loss = 50 +/- 8 kg), insulin sensitivity was supernormal (68.7 +/- 3.3 micromol.min(-1).kg(FFM)(-1)) and insulin secretion was lower than normal (167 nmol [37]) (both P < 0.05 vs. control subjects). In conclusion, severe uncomplicated obesity is characterized by gross insulin hypersecretion and insulin resistance, but the dynamic aspects of beta-cell function are intact. Malabsorptive bariatric surgery corrects both the insulin hypersecretion and the insulin resistance at a time when BMI is still high. With continued weight loss over a 2-year period, moderately obese subjects become supersensitive to insulin and, correspondingly, insulin hyposecretors.     

The GLP-1 derivative NN2211 restores beta-cell sensitivity to glucose in type 2 diabetic patients after a single dose

Glucagon-like peptide 1 (GLP-1) stimulates insulin secretion in a glucose-dependent manner, but its short half-life limits its therapeutic potential. We tested NN2211, a long-acting GLP-1 derivative, in 10 subjects with type 2 diabetes (means +/- SD: age 63 +/- 8 years, BMI 30.1 +/- 4.2 kg/m(2), HbA(1c) 6.5 +/- 0.8%) in a randomized, double-blind, placebo-controlled, crossover study. A single injection (7.5 micro g/kg) of NN2211 or placebo was administered 9 h before the study. beta-cell sensitivity was assessed by a graded glucose infusion protocol, with glucose levels matched over the 5-12 mmol/l range. Insulin secretion rates (ISRs) were estimated by deconvolution of C-peptide levels. Findings were compared with those in 10 nondiabetic volunteers during the same glucose infusion protocol. In type 2 diabetic subjects, NN2211, in comparison with placebo, increased insulin and C-peptide levels, the ISR area under the curve (AUC) (1,130 +/- 150 vs. 668 +/- 106 pmol/kg; P < 0.001), and the slope of ISR versus plasma glucose (1.26 +/- 0.36 vs. 0.54 +/- 0.18 pmol x l[min(-1) x mmol(-1) x kg(-1)]; P < 0.014), with values similar to those of nondiabetic control subjects (ISR AUC 1,206 +/- 99; slope of ISR versus plasma glucose, 1.44 +/- 0.18). The long-acting GLP-1 derivative, NN2211, restored beta-cell responsiveness to physiological hyperglycemia in type 2 diabetic subjects.     

Effect of acute hyperglycemia on insulin secretion in humans

First-phase insulin response to intravenous glucose is impaired both in type 2 diabetic patients and in subjects at risk for the disease. Hyperglycemia can modify beta-cell response by either inhibiting or potentiating both first- and second-phase insulin release. In normal subjects, the effect of acute hyperglycemia on insulin secretion is controversial. We measured (in 13 healthy volunteers) insulin secretion (by deconvolution of plasma C-peptide concentrations) during three consecutive 30-min hyperglycemic steps (2.8, 2.8, and 5.6 mmol/l), followed by an intravenous arginine bolus. First-phase insulin secretion in response to the first hyperglycemic step (456 +/- 83 pmol.min(-1).m(-2)) was significantly larger than that in response to the second step (311 +/- 37 pmol.min(-1).m(-2), P < 0.01); the subsequent increase in glycemia failed to stimulate first-phase secretion any further (377 +/- 60 pmol.min(-1).m(-2), NS vs. the previous value). This inhibition was also evident when insulin release rates were corrected for the respective increments (absolute or percentage) in plasma glucose levels and was not due to beta-cell exhaustion because the arginine bolus still elicited a large peak of insulin secretion (4,790 +/- 2,330 pmol.min(-1).m(-2)). In contrast, second-phase insulin secretion was related to the prevailing glucose levels across the three hyperglycemic steps in a direct quasilinear manner. We conclude that first-phase insulin secretion is inhibited by short-term modest hyperglycemia, whereas the second-phase insulin secretion increases linearly with hyperglycemia.     

The Gly972Arg polymorphism in the insulin receptor substrate-1 gene contributes to the variation in insulin secretion in normal glucose-tolerant humans

The Gly972Arg polymorphism in the insulin receptor substrate (IRS)-1 was found in some studies to have a higher prevalence in type 2 diabetic subjects than in control subjects. Previously, transfection of IRS-1 with this polymorphism into insulin-secreting cells resulted in a marked reduction of glucose-stimulated insulin secretion compared with the wild-type transfected cells. In the present study, we compared insulin secretion in well-matched normal glucose-tolerant subjects with and without this polymorphism. Several validated indexes of beta-cell function from the oral glucose tolerance test were significantly lower in X/Arg (n = 31) compared with Gly/Gly (n = 181) (P between 0.002 and 0.05), whereas insulin sensitivity (measured with a euglycemic clamp) was not different. During a modified hyperglycemic clamp, insulin secretion rates were significantly lower in Gly/Arg (n = 8) compared with Gly/Gly (n = 36) during the first phase (1,711+/-142 vs. 3,014+/-328 pmol/min, P = 0.05) and after maximal stimulation with arginine (5,340+/-639 vs. 9,075+/-722 pmol/min, P = 0.03). In summary, our results suggest that the Gly972Arg polymorphism in IRS-1 is associated with decreased insulin secretion in response to glucose but not with insulin sensitivity. It is possible that this polymorphism causes insulin resistance at the level of the beta-cell and contributes to the polygenic etiology of type 2 diabetes.     

Beta-cell function is a major contributor to oral glucose tolerance in high-risk relatives of four ethnic groups in the U.S

First-degree relatives of individuals with type 2 diabetes are at increased risk of developing hyperglycemia. To examine the prevalence and pathogenesis of abnormal glucose homeostasis in these subjects, 531 first-degree relatives with no known history of diabetes (aged 44.1 +/- 0.7 years; BMI 29.0 +/- 0.3 kg/m(2)) underwent an oral glucose tolerance test (OGTT). Newly identified diabetes was found in 19% (n = 100), and impaired fasting glucose (IFG) and/or impaired glucose tolerance (IGT) was found in 36% (n = 191). Thus, only 45% (n = 240) had normal glucose tolerance (NGT). The homeostasis model assessment of insulin resistance (HOMA-IR) was used to estimate insulin sensitivity; beta-cell function was quantified as the ratio of the incremental insulin to glucose responses over the first 30 min during the OGTT (DeltaI(30)/DeltaG(30)). This latter measure was also adjusted for insulin sensitivity as it modulates beta-cell function ([DeltaI(30)/DeltaG(30)]/HOMA-IR). Decreasing glucose tolerance was associated with increasing insulin resistance (HOMA: NGT 12.01 +/- 0.54 pmol/mmol; IFG/IGT 16.14 +/- 0.84; diabetes 26.99 +/- 2.62; P < 0.001) and decreasing beta-cell function (DeltaI(30)/DeltaG(30): NGT 157.7 +/- 9.7 pmol/mmol; IFG/IGT 100.4 +/- 5.4; diabetes 57.5 +/- 7.3; P < 0.001). Decreasing beta-cell function was also identified when adjusting this measure for insulin sensitivity ([DeltaI(30)/DeltaG(30)]/HOMA-IR). In all four ethnic groups (African-American, n = 55; Asian-American, n = 66; Caucasian, n = 217; Hispanic-American, n = 193), IFG/IGT and diabetic subjects exhibited progressively increasing insulin resistance and decreasing beta-cell function. The relationships of insulin sensitivity and beta-cell function to glucose disposal, as measured by the incremental glucose area under the curve (AUCg), were examined in the whole cohort. Insulin sensitivity and AUCg were linearly related so that insulin resistance was associated with poorer glucose disposal (r(2) = 0.084, P < 0.001). In contrast, there was a strong inverse curvilinear relationship between beta-cell function and AUCg such that poorer insulin release was associated with poorer glucose disposal (log[DeltaI(30)/DeltaG(30)]: r(2) = 0.29, P < 0.001; log[(DeltaI(30)/DeltaG(30))/HOMA-IR]: r(2) = 0.45, P < 0.001). Thus, abnormal glucose metabolism is common in first-degree relatives of subjects with type 2 diabetes. Both insulin resistance and impaired beta-cell function are associated with impaired glucose metabolism in all ethnic groups, with beta-cell function seeming to be more important in determining glucose disposal.     

Disturbances in beta-cell function in impaired fasting glycemia

In a cross-sectional study, we assessed beta-cell function and insulin sensitivity index (ISI) with hyperglycemic clamps (10 mmol/l) in 24 subjects with impaired fasting glycemia (IFG, fasting plasma glucose [FPG] between 6.1 and 7.0 mmol/l), 15 type 2 diabetic subjects (FPG >7.0 mmol/l), and 280 subjects with normal fasting glycemia (NFG, FPG <6.1 mmol/l). First-phase insulin release (0-10 min) was lower in IFG (geometric mean 541 pmol/l.10 min; 95% confidence interval [CI] 416-702 pmol/l.10 min) and in type 2 diabetes (geometric mean 376 pmol/l.10 min; 95% CI 247-572 pmol/l.10 min) than NFG (geometric mean 814 pmol/l.10 min; 95% CI 759-873 pmol/l.10 min) (P < 0.001). Second-phase insulin secretion (140-180 min) was also lower in IFG (geometric mean 251 pmol/l; 95% CI 198-318 pmol/l; P = 0.026) and type 2 diabetes (geometric mean 157 pmol/l; 95% CI 105-235 pmol/l; P < 0.001) than NFG (geometric mean 295 pmol/l; 95% CI 276-315 pmol/l). IFG and type 2 diabetic subjects had a lower ISI (0.15 plus minus 0.02 and 0.16 plus minus 0.02 micromol/kg fat-free mass [FFM]/min/pmol/l, respectively) than NFG (0.24 plus minus 0.01 micromol/kg FFM/min/pmol/l, P < 0.05). We found a stepwise decline in first-phase (and second-phase) secretion in NFG subjects with progressive decline in oral glucose tolerance (P < 0.05). IFG subjects with impaired glucose tolerance (IGT) had lower first-phase secretion than NFG subjects with IGT (P < 0.02), with comparable second-phase secretion and ISI. NFG and IFG subjects with a diabetic glucose tolerance (2-h glucose >11.1 mmol/l) had a lower ISI than their respective IGT counterparts (P < 0.05). We conclude that the early stages of glucose intolerance are associated with disturbances in beta-cell function, while insulin resistance is seen more markedly in later stages.     

Failure of physiological plasma glucose excursions to entrain high-frequency pulsatile insulin secretion in type 2 diabetes

Insulin is released in high-frequency pulsatile bursts at intervals of 6-13 min. Intrapancreatic mechanisms are assumed to coordinate pulsatile insulin release, but small oscillations in plasma glucose concentrations may contribute further. To gain additional insight into beta-cell (patho)physiology, we explored the ability of repetitive small glucose infusions (6 mg/kg over 1 min every 10 min) to modify rapid pulsatile insulin secretion in 10 type 2 diabetic individuals (plasma glucose 9.3 +/- 1.0 mmol/l, HbA1c 7.9 +/- 0.5%, mean +/- SE) and 10 healthy subjects. All subjects were investigated twice in randomly assigned order: during saline and during glucose exposure. Blood was collected every minute for 90 min to create a plasma insulin concentration time-series for analysis using 3 complementary algorithms: namely, spectral analysis, autocorrelation analysis, and approximate entropy (ApEn). During saline infusion, none of the algorithms were able to discriminate between diabetic and control subjects (P > 0.20). During glucose entrainment, spectral density peaks (SP) and autocorrelation coefficients (AC) increased significantly (P < 0.001), and ApEn decreased (P < 0.01), indicating more regular insulin time-series in the healthy volunteers. However, no differences were observed in the diabetic individuals between the glucose and saline conditions. Furthermore, in spite of identical absolute glucose excursions (approximately 0.3 mmol/l) glucose pulse entrainment led to a complete (SP: 4.76 +/- 0.62 [range 2.08-7.60] vs. 17.24 +/- 0.93 [11.70-20.58], P < 0.001; AC: 0.01 +/- 0.05 [0.33-0.24] vs. 0.64 +/- 0.05 [0.35-0.83], P < 0.001) or almost complete (ApEn: 1.59 +/- 0.02 [1.48-1.67] vs. 1.42 +/- 0.05 [1.26-1.74], P < 0.005) separation of the insulin time-series in diabetic and control subjects. Even elevating the glucose infusion rate in the diabetic subjects to achieve comparable relative (and hence higher absolute) glucose excursions (approximately 4.9%) failed to entrain pulsatile insulin secretion in this group. In conclusion, the present study demonstrates that failure to respond adequately with regular oscillatory insulin secretion to recurrent high-frequency and (near)-physiological glucose excursion is a manifest feature of beta-cell malfunction in type 2 diabetes. Whether the model will be useful in unmasking subtle (possible prediabetic) defects in beta-cell sensitivity to glucose drive remains to be determined.     

The influence of GLP-1 on glucose-stimulated insulin secretion: effects on beta-cell sensitivity in type 2 and nondiabetic subjects

The intestinally derived hormone glucagon-like peptide 1 (GLP-1) (7-36 amide) has potent effects on glucose-mediated insulin secretion, insulin gene expression, and beta-cell growth and differentiation. It is, therefore, considered a potential therapeutic agent for the treatment of type 2 diabetes. However, the dose-response relationship between GLP-1 and basal and glucose-stimulated prehepatic insulin secretion rate (ISR) is currently not known. Seven patients with type 2 diabetes and seven matched nondiabetic control subjects were studied. ISR was determined during a graded glucose infusion of 2, 4, 6, 8, and 12 mg x kg(-1) x min(-1) over 150 min on four occasions with infusion of saline or GLP-1 at 0.5, 1.0, and 2.0 pmol x kg(-1) x min(-1). GLP-1 enhanced ISR in a dose-dependent manner during the graded glucose infusion from 332 +/- 51 to 975 +/- 198 pmol/kg in the patients with type 2 diabetes and from 711 +/- 123 to 2,415 +/- 243 pmol/kg in the control subjects. The beta-cell responsiveness to glucose, expressed as the slope of the linear relation between ISR and the glucose concentration, increased in proportion to the GLP-1 dose to 6 times relative to saline at the highest GLP-1 dose in the patients and 11 times in the control subjects, but it was 3 to 5 times lower in the patients with type 2 diabetes compared with healthy subjects at the same GLP-1 dose. During infusion of GLP-1 at 0.5 pmol x kg(-1) x min(-1) in the patients, the slope of ISR versus glucose became indistinguishable from that of the control subjects without GLP-1. Our results show that GLP-1 increases insulin secretion in patients with type 2 diabetes and control subjects in a dose-dependent manner and that the beta-cell responsiveness to glucose may be increased to normal levels with a low dose of GLP-1 infusion. Nevertheless, the results also indicate that the dose-response relation between beta-cell responsiveness to glucose and GLP-1 is severely impaired in patients with type 2 diabetes.     

Mechanisms of glucose hypersensitivity in beta-cells from normoglycemic, partially pancreatectomized mice.

Increased beta-cell sensitivity to glucose precedes the loss of glucose-induced insulin secretion in diabetic animals. Changes at the level of beta-cell glucose sensor have been described in these situations, but it is not clear whether they fully account for the increased insulin secretion. Using a euglycemic-normolipidemic 60% pancreatectomized (60%-Px) mouse model, we have studied the ionic mechanisms responsible for increased beta-cell glucose sensitivity. Two weeks after Px (Px14 group), Px mice maintained normoglycemia with a reduced beta-cell mass (0.88 +/- 0.18 mg) compared with control mice (1.41 +/- 0.21 mg). At this stage, the dose-response curve for glucose-induced insulin release showed a significant displacement to the left (P < 0.001). Islets from the Px14 group showed oscillatory electrical activity and cytosolic Ca2+ ([Ca2+]i) oscillations in response to glucose concentrations of 5.6 mmol/l compared with islets from the control group at 11.1 mmol/l. All the above changes were fully reversible both in vitro (after 48-h culture of islets from the Px14 group) and in vivo (after regeneration of beta-cell mass in islets studied 60 days after Px). No significant differences in the input resistance and ATP inhibition of ATP-sensitive K+ (K(ATP)) channels were found between beta-cells from the Px14 and control groups. The dose-response curve for glucose-induced MTT (C,N-diphenyl-N'-4,5-dimethyl thiazol 2 yl tetrazolium bromide) reduction showed a significant displacement to the left in islets from the Px14 group (P < 0.001). These results indicate that increased glucose sensitivity in terms of insulin secretion and Ca2+ signaling was not due to intrinsic modifications of K(ATP) channel properties, and suggest that the changes are most likely to be found in the glucose metabolism.     

Insulin secretion and insulin sensitivity in relation to glucose tolerance: lessons from the Botnia Study

Recently, a new stage in glucose tolerance, impaired fasting glucose (IFG) (fasting plasma glucose level of 6.1-6.9 mmol/l), was introduced in addition to impaired glucose tolerance (IGT) (2-h glucose level of 7.8-11.0 mmol/l). It is not clear whether IFG and IGT differ with respect to insulin secretion or sensitivity. To address this question, we estimated insulin secretion (by measuring both insulin levels and the ratio of insulin-to-glucose levels in 30-min intervals) and insulin sensitivity (by using the homeostasis model assessment [HOMA] index) from an oral glucose tolerance test (OGTT) in 5,396 individuals from the Botnia Study who had varying degrees of glucose tolerance. There was poor concordance between IFG and IGT: only 36% (303 of 840) of the subjects with IFG had IGT, whereas 62% (493 of 796) of the subjects with IGT did not have IFG. Compared with subjects with normal glucose tolerance (NGT), subjects with IFG were more insulin resistant (HOMA-insulin resistance [IR] values 2.64 +/- 0.08 vs. 1.73 +/- 0.03, P < 0.0005), had greater insulin responses during an OGTT (P = 0.0001), had higher waist-to-hip ratios (P < 0.005), had higher triglyceride and total cholesterol concentrations (P < 0.0005), and had lower HDL cholesterol concentrations (P = 0.0001). Compared with subjects with IFG, subjects with IGT had a lower incremental 30-min insulin-to-glucose area during an OGTT (13.8 +/- 1.7 vs. 21.7 +/- 1.7, P = 0.0008). Compared with subjects with IGT, subjects with mild diabetes (fasting plasma glucose levels <7.8 mmol/l) showed markedly impaired insulin secretion that could no longer compensate for IR and elevated glucose levels. A progressive decline in insulin sensitivity was observed when moving from NGT to IGT and to subjects with diabetes (P < 0.05 for trend), whereas insulin secretion followed an inverted U-shaped form. We conclude that IFG is characterized by basal IR and other features of the metabolic syndrome, whereas subjects with IGT have impaired insulin secretion in relation to glucose concentrations. An absolute decompensation of beta-cell function characterizes the transition from IGT to mild diabetes.     

Decrease in beta-cell mass leads to impaired pulsatile insulin secretion, reduced postprandial hepatic insulin clearance, and relative hyperglucagonemia in the minipig

Most insulin is secreted in discrete pulses at an interval of approximately 6 min. Increased insulin secretion after meal ingestion is achieved through the mechanism of amplification of the burst mass. Conversely, in type 2 diabetes, insulin secretion is impaired as a consequence of decreased insulin pulse mass. beta-cell mass is reported to be deficient in type 2 diabetes. We tested the hypothesis that decreased beta-cell mass leads to decreased insulin pulse mass. Insulin secretion was examined before and after an approximately 60% decrease in beta-cell mass achieved by a single injection of alloxan in a porcine model. Alloxan injection resulted in stable diabetes (fasting plasma glucose 7.4 +/- 1.1 vs. 4.4 +/- 0.1 mmol/l; P < 0.01) with impaired insulin secretion in the fasting and fed states and during a hyperglycemic clamp (decreased by 54, 80, and 90%, respectively). Deconvolution analysis revealed a selective decrease in insulin pulse mass (by 54, 60, and 90%) with no change in pulse frequency. Rhythm analysis revealed no change in the periodicity of regular oscillations after alloxan administration in the fasting state but was unable to detect stable rhythms reliably after enteric or intravenous glucose stimulation. After alloxan administration, insulin secretion and insulin pulse mass (but not insulin pulse interval) decreased in relation to beta-cell mass. However, the decreased pulse mass (and pulse amplitude delivered to the liver) was associated with a decrease in hepatic insulin clearance, which partially offset the decreased insulin secretion. Despite hyperglycemia, postprandial glucagon concentrations were increased after alloxan administration (103.4 +/- 6.3 vs. 92.2 +/- 2.5 pg/ml; P < 0.01). We conclude that an alloxan-induced selective decrease in beta-cell mass leads to deficient insulin secretion by attenuating insulin pulse mass, and that the latter is associated with decreased hepatic insulin clearance and relative hyperglucagonemia, thereby emulating the pattern of islet dysfunction observed in type 2 diabetes.     

Functional significance of the UCSNP-43 polymorphism in the CAPN10 gene for proinsulin processing and insulin secretion in nondiabetic Germans

Recently, an association of the G allele in UCSNP-43 of calpain 10 with type 2 diabetes and decreased glucose disposal was reported. Calpain 10 is also expressed in pancreatic islets. It is not known, however, whether and how this polymorphism contributes to the biological variation of beta-cell function. We studied 73 nondiabetic subjects from the southwest region of Germany (G/G, n = 41; G/A, n = 29; and A/A, n = 3) using a modified hyperglycemic clamp (10 mmol/l glucose, added glucagon-like peptide 1, final arginine bolus). The genotype distribution was not different between subjects with normal glucose tolerance (n = 56) and those with impaired glucose tolerance (n = 17; P = 0.74, chi2 test). First-phase insulin secretion (adjusted for sex and insulin sensitivity from hyperglycemic clamp) was greater in G/G (2,747 +/- 297 pmol/min) than in G/A + A/A (1,612 +/- 156 pmol/min, P = 0.003). Insulin secretion in response to arginine (adjusted for insulin sensitivity) was also greater in G/G (9,648 +/- 1,186 pmol/min) than in G/A + A/A (5,686 +/- 720 pmol/min, P = 0.04). The acute poststimulus proinsulin-to-insulin ratio was lower in G/G (1.6 +/- 0.4% first phase; 1.6 +/- 0.2% arginine) than in G/A + A/A (4.0 +/- 0.5% first phase, P < 0.001; 2.5 +/- 0.4% arginine, P = 0.03). In conclusion, it appears unlikely that any association of the UCSNP-43 polymorphism alone with type 2 diabetes involves impairment of insulin secretion in our population of German Caucasians. This may be entirely different with specific haplotype combinations.     

Evaluation of the effect of gain on the meal response of an automated closed-loop insulin delivery system

A continuous closed-loop insulin delivery system using subcutaneous insulin delivery was evaluated in eight diabetic canines. Continuous glucose profiles were obtained by extrapolation of blood glucose measurements. Insulin delivery rate was calculated, using a model of beta-cell insulin secretion, and delivered with a Medtronic MiniMed subcutaneous infusion pump. The model acts like a classic proportional-integral-derivative controller, delivering insulin in proportion to glucose above target, history of past glucose values, and glucose rate of change. For each dog, a proportional gain was set relative to the open-loop total daily dose (TDD) of insulin. Additional gains based on 0.5 x TDD and 1.5 x TDD were also evaluated (gain dose response). Control was initiated 4 h before the meal with a target of 6.7 mmol/l. At the time of the meal, glucose was similar for all three gains (6.0 +/- 0.3, 5.2 +/- 0.3, and 4.9 +/- 0.5 mmol/l for 0.5 x TDD, TDD, and 1.5 x TDD, respectively; P > 0.05) with near-target values restored at the end of experiments (8.2 +/- 0.9, 6.0 +/- 0.6, and 6.0 +/- 0.5, respectively). The peak postprandial glucose level decreased significantly with increasing gain (12.1 +/- 0.6, 9.6 +/- 1.0, and 8.5 +/- 0.6 mmol/l, respectively; P < 0.05). The data demonstrate that closed-loop insulin delivery using the subcutaneous site can provide stable glycemic control within a range of gain.     

Effects of aging on insulin secretion

Aging is associated with hyperinsulinemia, but reports vary on the contributions of altered insulin clearance versus insulin secretion to this phenomenon. To elucidate the role of insulin secretion in the hyperinsulinemia of aging, 10 elderly (age 66 +/- 4 yr, body mass index 25 +/- kg/m2) and 8 young (age 30 +/- 5 yr, body mass index 24 +/- 3 kg/m2) subjects were studied to determine rates of insulin secretion in response to fasting, mixed meals, and intravenous glucose administration. Insulin secretion was determined with a two-compartment model based on individual C-peptide kinetic parameters derived after bolus injection of biosynthetic human C-peptide. Basal insulin secretion rates were increased in elderly subjects (82.5 +/- 9.0 vs. 62.8 +/- 6.1 pmol.min-1.m-2; P less than .05). This was reflected in elevated serum insulin levels in elderly subjects (62.8 +/- 10.1 vs. 41.1 +/- 5.0 pM, P less than .05). During a 24-h mixed-meal profile, elderly subjects had an increase in their glucose response (P less than .01 by analysis of variance [ANOVA]) and total insulin secretion (261 +/- 28 vs. 195 +/- 22 nmol.24 h-1.m-2; P less than .05) compared with young subjects. However, the relative total increases in both glycemia and insulin secretion, calculated as a function of basal levels, were similar between the groups (both NS). To experimentally control for differences in glycemia, both groups underwent a 16.8-mM hyperglycemic clamp and a stepped intravenous glucose infusion to match glycemia. Under these steady-state and dynamic conditions, insulin secretion profiles were nearly identical (NS by ANOVA).(ABSTRACT TRUNCATED AT 250 WORDS)     

Evaluation of insulin sensitivity and beta-cell function indexes obtained from minimal model analysis of a meal tolerance test

Modeling analysis of glucose, insulin, and C-peptide following a meal has been proposed as a means to estimate insulin sensitivity (S(i)) and beta-cell function from a single test. We compared the model-derived meal indexes with analogous indexes obtained from an intravenous glucose tolerance test (IVGTT) and hyperglycemic clamp (HGC) in 17 nondiabetic subjects (14 men, 3 women, aged 50 +/- 2 years [mean +/- SE], BMI 25.0 +/- 0.7 kg/m(2)). S(i) estimated from the meal was correlated with S(i) estimated from the IVGTT and the HGC (r = 0.59 and 0.76, respectively; P < 0.01 for both) but was approximately 2.3 and 1.4 times higher (P < 0.05 for both). The meal-derived estimate of the beta-cell's response to a steady-state change in glucose (static secretion index) was correlated with the HGC second-phase insulin response (r = 0.69; P = 0.002), but the estimated rate-of-change component (dynamic secretion index) was not correlated with first-phase insulin release from either the HGC or IVGTT. Indexes of beta-cell function obtained from the meal were significantly higher than those obtained from the HGC. In conclusion, insulin sensitivity and beta-cell indexes derived from a meal are not analogous to those from the clamp or IVGTT. Further work is needed before these indexes can be routinely used in clinical and epidemiological studies.     

Markedly reduced beta-cell function does not result in insulin resistance in islet autografted dogs.

Autotransplantation of islets of Langerhans has resulted in long-term normoglycemia in pancreatectomized dogs. This canine model is useful in evaluating both the progress of islet transplantation and the effect of a reduced islet mass upon the determinants of glucose tolerance: i.e., insulin secretion, insulin sensitivity, and glucose effectiveness. To determine the effect of a reduced islet mass on these factors, we measured the acute insulin response to arginine (AIRa) and glucose (AIRg), the slope of glycemic potentiation of AIRa (SP), insulin sensitivity (Sl), and glucose effectiveness (SG) in control (CN), diabetic (DM), and pancreatectomized dogs rendered normoglycemic with transplanted autografts of islets of Langerhans (TX). Normal fasting plasma glucose (FPG) (TX 4.7 +/- 0.2 mM; CN 4.9 +/- 0.1 mM; P greater than 0.05) was maintained despite a markedly reduced insulin secretion in TX (AIRa 24%, AIRg 15%, and SP 11% of CN). All measures of insulin secretion were significantly correlated (SP vs. AIRg, r = 0.80, P less than 0.0001; AIRa vs. AIRg, r = 0.92, P less than 0.0001) across all animals, but none of the measures of secretion were significantly correlated with either the number of islets transplanted or time posttransplant (P greater than 0.10). Insulin sensitivity was normal in islet autografted dogs (TX: 136 +/- 12 min-1/(nmol/ml); CN: 101 +/- 11 min-1/(nmol/ml), P greater than 0.05) but SG was reduced (TX: 1.93 +/- 0.28 x 100 min-1; CN: 3.53 +/- 0.35 x 100 min-1, P less than 0.05), as determined by the minimal-model method. In diabetic animals (FPG = 16.1 +/- 1.3 mM), insulin secretion was negligible by all measures (P greater than 0.05), and was associated with insulin resistance (Sl = 28 +/- 8 min-1/(nmol/ml)) and reduced SG (1.72 +/- 0.11 x 100 min-1). These studies indicate that across a range of insulin secretion in dogs, the secretagogues arginine and glucose provide similar estimates of beta-cell function. This markedly reduced beta-cell function does not result in insulin resistance when fasting normoglycemia is maintained, but is associated with a decrease in glucose action at basal insulin.     

Impact of intra-abdominal fat and age on insulin sensitivity and beta-cell function

The prevalence of glucose intolerance and type 2 diabetes increases with age. To determine whether the hyperbolic relationship between insulin sensitivity and the insulin response is affected by age and whether the decline in beta-cell function with age is related to increases in intra-abdominal fat or age per se, we studied 220 healthy subjects with fasting glucose <6.1 mmol/l (89 men and 131 women, aged 26-75 years, BMI 18.7-40.4 kg/m(2)). The insulin sensitivity index (S(i)) and the acute insulin response to glucose (AIRg) were determined, and from these beta-cell function was estimated as the disposition index (S(i) x AIRg). Intra-abdominal fat and subcutaneous fat areas were quantified by computed tomography. S(i) (5.40 +/- 0.5 vs. 7.86 +/- 0.7 x10(-5) min(-1)/[pmol/l]), P < 0.01) was decreased and intra-abdominal fat (117 +/- 10 vs. 81 +/- 9 cm(2), P < 0.05) was increased in the oldest (age 60-75 years) versus the youngest (age 26-44 years) quartile. The hyperbolic relationship between S(i) and AIRg was present independent of age; thus, beta-cell function measured as the disposition index (1,412 +/- 120 vs. 2,125 +/- 150 x10(-5) min(-1), P < 0.01) was lower in the oldest versus the youngest quartile. In multiple regression, intra-abdominal fat (r = -0.470, P < 0.001) but not age was associated with S(i), but both intra-abdominal fat (r = -0.198, P = 0.003) and age (r = -0.131, P = 0.05) were correlated with the disposition index. These data suggest that although intra-abdominal fat is a strong determinant of insulin sensitivity and beta-cell function, age has an independent effect on beta-cell function that may contribute to the increased prevalence of type 2 diabetes in older populations.