Reduced peripheral activity leading to hepato-preferential action of basal insulin peglispro compared with insulin glargine in patients with type 1 diabetes.

AIMS: Basal insulin peglispro (BIL), a novel PEGylated basal insulin with a large hydrodynamic size, has a delayed absorption and reduced clearance that prolongs the duration of action. The current study compared the effects of BIL and insulin glargine (GL) on endogenous glucose production (EGP), glucose disposal rate (GDR) and lipolysis in patients with type 1 diabetes. MATERIALS AND METHODS: This was a randomized, open-label, four-period, crossover study. Patients received intravenous infusions of BIL and GL, each at two dose levels selected for partial and maximal suppression of EGP, during an 8 to 10 h euglycemic clamp procedure with d-[3-3 H] glucose. RESULTS: Following correction for equivalent human insulin concentrations (EHIC), low-dose GL infusion resulted in similar EGP at the end of the clamp compared to low-dose BIL infusion (GL/BIL ratio of 1.03) but a higher GDR (GL/BIL ratio of 2.42), indicating similar hepatic activity but attenuated peripheral activity of BIL. Consistent with this, the EHIC-corrected GDR/EGP at the end of the clamp was 1.72-fold greater for GL than BIL following low-dose administration. At the lower dose of BIL and GL (concentrations in the therapeutic range), BIL produced less suppression of lipolysis compared with GL as indicated by free fatty acid and glycerol levels at the end of the clamp. CONCLUSIONS: Compared with GL, BIL restored the hepato-peripheral insulin action gradient seen in normal physiology via its peripherally restricted action on target tissues related to carbohydrate and lipid metabolism.

Repaglinide treatment amplifies first-phase insulin secretion and high-frequency pulsatile insulin release in Type 2 diabetes.

AIMS/HYPOTHESIS: First-phase insulin release and coordinated insulin pulsatility are disturbed in Type 2 diabetes. The present study was undertaken to explore a possible influence of the oral prandial glucose regulator, repaglinide, on first-phase insulin secretion and high-frequency insulin pulsatility in Type 2 diabetes. METHODS: We examined 10 patients with Type 2 diabetes in a double-blind placebo-controlled, cross-over design. The participants were treated for 6 weeks with either repaglinide [2-9 mg/day (average 5.9 mg)] or placebo in random order. At the end of each treatment period, first-phase insulin secretion was measured. Entrainment of insulin secretion was assessed utilizing 1-min glucose bolus exposure (6 mg/kg body weight every 10 min) for 60 min during (A) baseline conditions, i.e. 12 h after the last repaglinide/placebo administration, and (B) 30 min after an oral dose of 0.5 mg repaglinide/placebo with subsequent application of time-series analyses. RESULTS: Postprandial (2-h) blood glucose was significantly reduced by repaglinide after 5 weeks of treatment (P < 0.001). The fall in HbA(1c) did not reach statistical significance (P = 0.07). AUC(ins,0-12 min) during the first-phase insulin secretion test was enhanced (P < 0.05). In addition, glucose entrained insulin secretory burst mass and amplitude increased markedly (burst mass: repaglinide, 44.4 +/- 6.0 pmol/l/pulse vs. placebo, 31.4 +/- 3.3 pmol/l/pulse, P < 0.05; burst amplitude: repaglinide, 17.7 +/- 2.4 pmol/l/min vs. placebo, 12.6 +/- 1.3 pmol/l/min, P < 0.05) while basal insulin (non-pulsatile) secretion was unaltered. After acute repaglinide exposure (0.5 mg) basal insulin secretion increased significantly (P < 0.05). Neither acute nor chronic repaglinide administration influenced frequency or regularity of insulin pulses during entrainment. CONCLUSION/INTERPRETATION: Repaglinide augments first-phase insulin secretion as well as high-frequency insulin secretory burst mass and amplitude during glucose entrainment in patients with Type 2 diabetes, while regularity of the insulin release process was unaltered.

Beta-cell function and islet morphology in normal, obese, and obese beta-cell mass-reduced Göttingen minipigs.

Herein, we bridge beta-cell function and morphology in minipigs. We hypothesized that different aspects of beta-cell dysfunction are present in obesity and obesity with reduced beta-cell mass by using pulsatile insulin secretion as an early marker. Measures for beta-cell function (glucose and arginine stimulation plus baseline and glucose-entrained pulsatile insulin secretion) and islet morphology were studied in long-term (19-20 mo) obese (n = 5) and obese beta-cell-reduced [nicotinamide + streptozotocin (STZ), n = 5] minipigs and normal controls, representing different stages in the development toward type 2 diabetes. Acute insulin response (AIR) to glucose and arginine were, surprisingly, normal in obese (0.3 g/kg glucose: AIR = 246 +/- 119 vs. 255 +/- 61 pM in control; 67 mg/kg arginine: AIR = 230 +/- 124 vs. 214 +/- 85 pM in control) but reduced in obese-STZ animals (0.3 g/kg glucose: AIR = 22 +/- 36, P < 0.01; arginine: AIR = 87 +/- 92 pM, P < 0.05 vs. control). Baseline pulsatile insulin secretion was reduced in obese (59 +/- 16 vs. 76 +/- 16% in control, P < 0.05) and more so in obese-STZ animals (43 +/- 13%, P < 0.01), whereas regularity during entrainment was increased in obese animals (approximate entropy: 0.85 +/- 0.14 vs. 1.13 +/- 0.13 in control, P < 0.01). Beta-cell mass (mg/kg body wt) was normal in obese and reduced in obese-STZ animals, with pancreatic fat infiltration in both groups. In conclusion, obesity and insulin resistance are not linked with a general reduction of beta-cell function, but dynamics of insulin secretion are perturbed. The data suggest a sequence in the development of beta-cell dysfunction, with the three groups representing stages in the progression from normal physiology to diabetes, and assessment of pulsatility as the single most sensitive marker of beta-cell dysfunction.

Loss of beta-cell mass leads to a reduction of pulse mass with normal periodicity, regularity and entrainment of pulsatile insulin secretion in Göttingen minipigs.

AIMS/HYPOTHESIS: Type 2 diabetes is associated with impaired insulin action and secretion, including disturbed pulsatile release. Impaired pulsatility has been related to impaired insulin action, thus providing a possible link between release and action of insulin. Furthermore, progressive loss of beta-cell mass has been implicated in the pathogenesis of Type 2 diabetes. The aim of this study was to evaluate a possible link between loss of beta-cell mass and impaired pulsatile insulin secretion with special focus on glucose responsiveness of insulin secretion. METHODS: The kinetic and dynamic profiles of insulin in Göttingen minipigs are favourable for studies on pulsatility and a model of diabetes with reduced beta-cell mass has recently been established. Pigs were studied before (n=14) and after (n=10) reduction of beta-cell mass by nicotinamide (67 mg/kg) and streptozotocin (125 mg/kg) from 17.7+/-4.7 (normal animals, n=5) to 6.1+/-2.0 mg/kg. Pulsatile insulin secretion was examined during basal (n=8 normal, n=6 beta-cell reduced) and glucose entrained (n=6 normal, n=4 beta-cell reduced) conditions. Insulin concentration time series were analysed by autocorrelation and spectral analyses for periodicities and regularity, and by deconvolution for pulse frequency, mass and amplitude. RESULTS: Reduction of beta-cell mass and secondary hyperglycaemia resulted in correspondingly (r=0.7421, p=0.0275) reduced pulse mass (42% of normal during basal and 31% during entrained conditions) with normal periodicity (6.6+/-2.2 vs 5.8+/-2.4 min, p=0.50), regularity and entrainability of insulin secretion. CONCLUSION/INTERPRETATION: Neither beta-cell loss, nor 2 weeks of slight hyperglycaemia, as seen in the beta-cell-reduced minipig, probably accounts for the disturbed insulin pulsatility observed in human Type 2 diabetes.

The conscious Göttingen minipig as a model for studying rapid pulsatile insulin secretion in vivo.

AIMS/HYPOTHESIS: Pulsatile secretion is important for insulin action and suitable animal models are important tools for examining the role of impaired pulsatile insulin secretion as a possible link between beta-cell mass, function and morphology and insulin resistance. This study examines the vascular sampling site, insulin kinetics, pulsatility and the response to glucose pulse entrainment to evaluate the Göttingen minipig as a model for studying pulsatile insulin secretion. METHODS: Basal and glucose entrained insulin secretion was examined in normal minipigs and evaluated by autocorrelation, cross correlation and deconvolution. RESULTS: Cross correlation showed a relation between oscillations in insulin concentrations in the portal and jugular vein in anaesthetised animals ( p<0.001 in all animals), confirming the usefulness of jugular vein sampling for pulse detection. Jugular vein sampling in conscious animals showed obvious oscillations allowing estimates of burst shape and insulin kinetics. Glucose entrainment improved the pulsatile pattern (autocorrelation: 0.555+/-0.148 entrained vs 0.350+/-0.197 basal, p=0.054). Deconvolution analysis resolved almost all insulin release as secretory bursts (69+/-20 basal vs 99.5+/-1.2% entrained, p<0.01) with a pulse interval (min) of 6.6+/-2.2 (basal) and 9.4+/-1.5 (entrained) ( p<0.05) and a pulse mass (pmol/l per pulse) which was higher after entrainment (228+/-117 vs 41.2+/-18.6 basal, p<0.001). CONCLUSION/INTERPRETATION: The ability to fit kinetic parameters directly by deconvolution of peripheral endogenous insulin concentration time series in combination with the suitability of jugular vein sampling, rapid kinetics and entrainability makes the Göttingen minipig ideal for mechanistic studies of insulin pulsatility and its effects on insulin action.

The in vivo regulation of pulsatile insulin secretion.

The presence of oscillations in peripheral insulin concentrations has sparked a number of studies evaluating the impact of the insulin release pattern on the action of insulin on target organs. These have convincingly shown that equal amounts of insulin presented to target organs have improved action when delivered in a pulsatile manner. In addition, impaired (not absent) pulsatility of insulin secretion has been demonstrated in Type II (non-insulin-dependent) diabetes mellitus, suggesting a possible mechanism to explain impaired insulin action in Type II diabetes. Whereas the regulation of overall insulin secretion has been described in detail, the mechanisms by which this regulation affects the pulsatile insulin secretory pattern, and the relative and absolute contribution of changes in the characteristics of pulsatile insulin release have not been reviewed previously. This review will focus on the importance of the secretory bursts to overall insulin release, and on how insulin secretion is adjusted by changes in these secretory bursts. Detection and quantification of secretory bursts depend on methods, and the methodology involved in studies dealing with pulsatile insulin secretion is described. Finally, data suggest that impaired pulsatile insulin secretion is an early marker for beta-cell dysfunction in Type II diabetes, and the role of early detection of impaired pulsatility to predict diabetes or to examine mechanisms to cause beta-cell dysfunction is mentioned.

Glucocorticoid induced insulin resistance impairs basal but not glucose entrained high-frequency insulin pulsatility in humans.

AIMS/HYPOTHESIS: Type II (non-insulin-dependent) diabetes mellitus is characterized by abnormal insulin secretion, which involves a disrupted basal and glucose-entrained insulin pulsatility, and by insulin resistance. The aim of this study was to examine the influence of glucocorticoid-mediated insulin resistance on the regularity of high frequency insulin pulsatility. METHODS: Eight healthy men (means +/- SD; age 24.4 +/- 0.5 years, BMI 23.2 +/- 0.7 kg/m2) were examined after prednisolone treatment (30 mg/day) or placebo for 6 days in a double-blind, placebo controlled, cross-over study with a 6-week washout period. Blood was collected every minute for 60 min during baseline and glucose-entrainment. Time-series were assessed by spectral and autocorrelation analyses and a first-phase insulin secretion test was carried out. RESULTS: Prednisolone treatment led to insulin resistance as expected (HOMA-S; prednisolone vs placebo; 1.85 +/- 0.26 vs 1.02 +/- 0.10; p < 0.01) with exaggerated first-phase insulin secretion (3016 +/- 468 pmol/l vs 1688 +/- 207 pmol/l; p < 0.01), suggesting a stable disposition index. During baseline, normalized spectral power of serum insulin concentration time-series was reduced during prednisolone exposure compared with placebo (8.40 +/- 0.95 vs 11.79 +/- 1.66; p < 0.05) indicating a disturbed high-frequency oscillatory insulin release. A similar trend was observed using autocorrelation analysis (0.23 +/- 0.04 vs 0.32 +/- 0.07; p = 0.12). During glucose entrainment no difference in normalized spectral power or in the autocorrelation coefficient between prednisolone and placebo (p > 0.1) was observed. CONCLUSION/INTERPRETATION: Six days of prednisolone treatment resulted in a pertubed high-frequency insulin release in the fasting state whereas the ability of glucose to entrain insulin secretion was preserved. This indicates a mechanism of pertubed glucose-insulin feedback mechanism which causes irregular oscillatory insulin release.

Early changes in beta-cell function and insulin pulsatility as predictors for type 2 diabetes.

Peripheral insulin concentrations oscillate because of insulin secretory bursts every 6-10 min, which are the main source of overall insulin release. Regulation of insulin secretion occurs by modification of the mass and to some extent the frequency of the individual insulin secretory bursts. This oscillatory pattern, which is important for insulin action, is observed at the level of the individual beta-cells and in the islets, both in vitro (in the isolated perfused pancreas) and in vivo. It has therefore been suggested that beta-cells act as pacemakers and that co-ordination among islets is achieved by a neuronal network within the pancreas. Type 2 diabetes mellitus is characterised both by impaired release of insulin and by resistance to the action of insulin. Routine screening procedures for insulin secretion yield little predictive value for later development of diabetes but more sophisticated methods using time-series analysis of diurnal, ultradian and rapid oscillatory insulin secretion reveal the presence of profound defects in glucose-intolerant individuals. Furthermore, studies of rapid pulsatile insulin secretion have revealed defects in glucose-tolerant first-degree relatives of patients with Type 2 diabetes. Application of repeated minimal glucose infusions has further improved the discrimination between insulin release in health and diabetes, suggesting that this method may be suitable for smaller prospective studies of the development of diabetes. Repaglinide, a novel prandial glucose regulator, improves the insulin secretory burst mass in healthy people.

High-frequency insulin pulsatility and type 2 diabetes: from physiology and pathophysiology to clinical pharmacology.

Like many other hormones insulin is released in a pulsatile manner, which results in oscillatory concentrations in peripheral blood. The oscillatory pattern is believed to improve release control and to enhance the hormonal action. Insulin oscillates with a slow ultradian periodicity (approximately 140 min) and a high-frequency periodicity (approximately 6-10 min). Only the latter will be discussed in this review, which focusses almost exclusively on human data. Probably at least 75% of insulin secretion is released in a very regular pulsatile fashion in healthy people. In contrast, type 2 diabetic subjects exhibit in general irregular oscillations of basal plasma insulin. Furthermore, disturbed pulsatile insulin release is also a common feature in people prone to develop diabetes e.g. first-degree relatives of patients with type 2 diabetes. Tiny glucose oscillations (approximately 0.3mM) are capable of easily governing or entraining insulin oscillations in healthy people. This differs from type 2 diabetic individuals underlining a profound disruption of the beta-cells in type 2 diabetes to sense or respond to physiological glucose excursions. A pivotal question is how approximately 1,000,000 islets each containing from a few to several thousand beta-cells can be coordinated to secrete insulin in a pulsatile manner. Coordination is extremely complex involving the intrapancreatic neural network, the intraislet communication and metabolic oscillations in the individual beta-cell itself, but our understanding is still rather rudimentary. It is important to realize how antidiabetic treatment influences high-frequency insulin pulsatility. Only a few studies have been performed, but very recently it has been demonstrated that acute as well as long-term administration of the sulfonylurea compound gliclazide results in a substantial amplification (approximately 50%) of the pulsatile insulin secretion in type 2 diabetes. The fraction between pulsatile vs nonpulsatile insulin secretion is stable, which may be essential for controlling glucose and lipid homeostasis in type 2 diabetes. The influence of repaglinide, thiazolidinediones and a potential future antidiabetic compound (GLP-1) on pulsatile insulin secretion is also discussed briefly. Evaluation of high-frequency insulin pulsatility may be an important player in future tailoring of antidiabetic drugs and may turn out to be relevant as a predictor of type 2 diabetes in people at high risk for developing the disease.

Acute and short-term administration of a sulfonylurea (gliclazide) increases pulsatile insulin secretion in type 2 diabetes.

The high-frequency oscillatory pattern of insulin release is disturbed in type 2 diabetes. Although sulfonylurea drugs are widely used for the treatment of this disease, their effect on insulin release patterns is not well established. The aim of the present study was to assess the impact of acute treatment and 5 weeks of sulfonylurea (gliclazide) treatment on insulin secretory dynamics in type 2 diabetic patients. To this end, 10 patients with type 2 diabetes (age 53 +/- 2 years, BMI 27.5 +/- 1.1 kg/m(2), fasting plasma glucose 9.8 +/- 0.8 mmol/l, HbA(1c) 7.5 +/- 0.3%) were studied in a double-blind placebo-controlled prospective crossover design. Patients received 40-80 mg gliclazide/placebo twice daily for 5 weeks with a 6-week washout period intervening. Insulin pulsatility was assessed by 1-min interval blood sampling for 75 min 1) under baseline conditions (baseline), 2) 3 h after the first dose (80 mg) of gliclazide (acute) with the plasma glucose concentration clamped at the baseline value, 3) after 5 weeks of treatment (5 weeks), and 4) after 5 weeks of treatment with the plasma glucose concentration clamped during the sampling at the value of the baseline assessment (5 weeks-elevated). Serum insulin concentration time series were analyzed by deconvolution, approximate entropy (ApEn), and spectral and autocorrelation methods to quantitate pulsatility and regularity. The P values given are gliclazide versus placebo; results are means +/- SE. Fasting plasma glucose was reduced after gliclazide treatment (baseline vs. 5 weeks: gliclazide, 10.0 +/- 0.9 vs. 7.8 +/- 0.6 mmol/l; placebo, 10.0 +/- 0.8 vs. 11.0 +/- 0.9 mmol/l, P = 0.001). Insulin secretory burst mass was increased (baseline vs. acute: gliclazide, 43.0 +/- 12.0 vs. 61.0 +/- 17.0 pmol. l(-1). pulse(-1); placebo, 36.1 +/- 8.4 vs. 30.3 +/- 7.4 pmol. l(-1). pulse(-1), P = 0.047; 5 weeks-elevated: gliclazide vs. placebo, 49.7 +/- 13.3 vs. 37.1 +/- 9.5 pmol. l(-1). pulse(-1), P < 0.05) with a similar rise in burst amplitude. Basal (i.e., nonoscillatory) insulin secretion also increased (baseline vs. acute: gliclazide, 8.5 +/- 2.2 vs. 16.7 +/- 4.3 pmol. l(-1). pulse(-1); placebo, 5.9 +/- 0.9 vs. 7.2 +/- 0.9 pmol. l(-1). pulse(-1), P = 0.03; 5 weeks-elevated: gliclazide vs. placebo, 12.2 +/- 2.5 vs. 9.4 +/- 2.1 pmol. l(-1). pulse(-1), P = 0.016). The frequency and regularity of insulin pulses were not modified significantly by the antidiabetic therapy. There was, however, a correlation between individual values for the acute improvement of regularity, as measured by ApEn, and the decrease in fasting plasma glucose during short-term (5-week) gliclazide treatment (r = 0.74, P = 0.014, and r = 0.77, P = 0.009, for fine and coarse ApEn, respectively). In conclusion, the sulfonylurea agent gliclazide augments insulin secretion by concurrently increasing pulse mass and basal insulin secretion without changing secretory burst frequency or regularity. The data suggest a possible relationship between the improvement in short-term glycemic control and the acute improvement of regularity of the in vivo insulin release process.

Glucagon-like peptide 1 increases secretory burst mass of pulsatile insulin secretion in patients with type 2 diabetes and impaired glucose tolerance.

The insulinotropic gut hormone glucagon-like peptide (GLP)-1 increases secretory burst mass and the amplitude of pulsatile insulin secretion in healthy volunteers without affecting burst frequency. Effects of GLP-1 on secretory mechanisms in type 2 diabetic patients and subjects with impaired glucose tolerance (IGT) known to have impaired pulsatile release of insulin have not yet been studied. Eight type 2 diabetic patients (64+/-9 years, BMI 28.9+/-7.2 kg/m2, HbA1c 7.7+/-1.3%) and eight subjects with IGT (63+/-10 years, BMI 31.7+/-6.4 kg/m2, HbA1c 5.7+/-0.4) were studied on separate occasions in the fasting state during the continued administration of exogenous GLP-1 (1.2 pmol x kg(-1) x min(-1), started at 10:00 P.M. the evening before) or placebo. For comparison, eight healthy volunteers (62+/-7 years, BMI 27.7+/-4.8 kg/m2, HbA1c 5.4+/-0.5) were studied only with placebo. Blood was sampled continuously over 60 min (roller-pump) in 1-min fractions for the measurement of plasma glucose and insulin. Pulsatile insulin secretion was characterized by deconvolution, autocorrelation, and spectral analysis and by estimating the degree of randomness (approximate entropy). In type 2 diabetic patients, exogenous GLP-1 at approximately 90 pmol/l improved plasma glucose concentrations (6.4+/-2.1 mmol/l vs. placebo 9.8+/-4.1 mmol/l, P = 0.0005) and significantly increased mean insulin burst mass (by 68%, P = 0.007) and amplitude (by 59%, P = 0.006; deconvolution analysis). In IGT subjects, burst mass was increased by 45% (P = 0.019) and amplitude by 38% (P = 0.02). By deconvolution analysis, insulin secretory burst frequency was not affected by GLP-1 in either type 2 diabetic patients (P = 0.15) or IGT subjects (P = 0.76). However, by both autocorrelation and spectral analysis, GLP-1 prolonged the period (lag time) between subsequent maxima of insulin concentrations significantly from approximately 9 to approximately 13 min in both type 2 diabetic patients and IGT subjects. Under placebo conditions, parameters of pulsatile insulin secretion were similar in normal subjects, type 2 diabetic patients, and IGT subjects based on all methodological approaches (P > 0.05). In conclusion, intravenous GLP-1 reduces plasma glucose in type 2 diabetic patients and improves the oscillatory secretion pattern by amplifying insulin secretory burst mass, whereas the oscillatory period determined by autocorrelation and spectral analysis is significantly prolonged. This was not the case for the interpulse interval determined by deconvolution. Together, these results suggest a normalization of the pulsatile pattern of insulin secretion by GLP-1, which supports the future therapeutic use of GLP-1-derived agents.

Irregular circulating insulin concentrations in type 2 diabetes mellitus: an inverse relationship between circulating free fatty acid and the disorderliness of an insulin time series in diabetic and healthy individuals.

Insulin is released in a high-frequency pulsatile secretory pattern, which is reflected as quantifiable oscillations in peripheral circulating insulin concentrations. Type 2 diabetes mellitus is characterized by a broad spectrum of abnormalities in beta-cell function, including disturbed pulsatile insulin secretion as assessed by autocorrelation analysis. To achieve further insight into beta-cell pathophysiology in type 2 diabetes, we examined the orderliness of the baseline serum insulin time series (blood collection every minute for 75 minutes) in 16 type 2 diabetics (fasting plasma glucose, 170 +/- 10 mg/dL [mean +/- SE]; serum free fatty acid [FFA], 0.794 +/- 0.083 mmol/L; and known diabetes duration, 6 +/- 2 years) and 15 healthy controls (serum FFA, 0.523 +/- 0.055 mmol/L). We used approximate entropy (ApEn), a recently introduced scale- and model-independent measure of serial irregularity. ApEn was significantly increased in the type 2 diabetics compared with the controls (0.671 +/- 0.016 v 0.653 +/- 0.008, P = .04), indicating more irregular serum insulin time series in diabetics. Autocorrelation also discriminated between groups, although only when the data were pooled. Interestingly, an inverse relationship between ApEn and serum FFA was observed in the controls (r = -.63, P = .01) and diabetics (r = -.65, P < .01), whereas no relationships were found between ApEn and the age, body mass index (BMI), or plasma glucose. In conclusion, type 2 diabetes is characterized by an increased disorderliness of the fasting serum insulin time series, strongly suggesting perturbed rapid oscillatory insulin release. An inverse relationship between ApEn and fasting serum FFA among both groups might suggest a hitherto unknown stabilizing action of FFA on the high-frequency pulsatile insulin release process. This hypothesis needs to be tested in experimental designs that more specifically focus on this issue, eg, during changes in serum FFA.

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.

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.

Assessment of insulin secretion in relatives of patients with type 2 (non-insulin-dependent) diabetes mellitus: evidence of early beta-cell dysfunction.

To examine beta-cell function in glucose-tolerant offspring of type 2 diabetic families, 41 insulin-resistant (hyperinsulinemic-euglycemic clamp, P < .001) first-degree relatives and 32 controls underwent oral (OGTT) and intravenous (IVGTT) glucose tolerance tests and a constant intravenous glucose infusion (4.0 or 4.5 mg/kg/min) with blood sampling every minute for insulin determinations. Insulin concentration time-series were analyzed with complementary mathematical models (deconvolution and autocorrelation analysis, approximate entropy [ApEn], and coefficient of variation [CV] for a 6-point moving average, together with a combined index for regularity and stationarity [RaS] based on the last 2 measures). During the OGTT, the area under the curve (AUC) for plasma glucose was moderately (11%) but significantly (P < .01) elevated in the relatives despite a trend for increased serum insulin (AUC, P = .14). The acute-phase serum insulin response (IVGTT) did not differ between groups (2,055 +/- 330 v 1,766 +/- 229 pmol/L x 10 min, P = .84) but was inappropriately low (individually, P < .05 v control group) for the degree of insulin resistance in 16 relatives. Deconvolution analysis of the insulin time-series did not uncover differences in either the intersecretory pulse interval (5.8 +/- 0.2 v5.7 +/- 0.2 min/pulse) or the fractional secretory burst amplitude (133% +/- 10% v 116% +/- 7% over basal) between the 2 groups. Similarly, significant autocorrelation coefficients were observed in a comparable number of relatives and control subjects (P = .74). In contrast, the RaS index was significantly higher (ie, insulin time-series was more irregular and nonstationary) in the relatives (0.221 +/- 0.194) than in the controls (-0.318 +/- 0.176, P < .05), primarily attributed to the pattern of insulin secretion in relatives with a strong genetic burden. In conclusion, nonstationary and disorderly insulin secretion patterns during glucose stimulation and a low acute-phase serum insulin response associated with significant insulin resistance suggest early beta-cell regulatory dysfunction in individuals genetically predisposed to type 2 diabetes mellitus prior to any evident alterations in insulin secretory burst frequency or mass.

Repaglinide acutely amplifies pulsatile insulin secretion by augmentation of burst mass with no effect on burst frequency.

OBJECTIVE: Repaglinide is a new oral hypoglycemic agent that acts as a prandial glucose regulator proposed for the treatment of type 2 diabetes by stimulating insulin secretion. The aim of this study was to explore actions of repaglinide on the rapid pulsatile insulin release by high-frequency insulin sampling and analysis of insulin-concentration time series. RESEARCH DESIGN AND METHODS: We examined 8 healthy lean male subjects in a single-dose double-blind placebo-controlled crossover design. After the subjects underwent an overnight fast, blood sampling was initiated and continued every minute for 120 min. After 40 min, a single dose (0.5 mg) of repaglinide or placebo was given. Serum insulin-concentration time series were assessed by deconvolution analyses and the regularity statistic by approximate entropy (ApEn). RESULTS: Average insulin concentration was increased after repaglinide administration (basal vs. stimulated period, P values are placebo vs. repaglinide) (25.1 +/- 3.6 vs. 33.5 +/- 4.1 pmol/l, P < 0.001). Insulin secretory burst mass (15.8 +/- 2.2 vs. 19.6 +/- 2.8 pmol x l(-1) x pulse(-1), P = 0.02) and amplitude (6.1 +/- 0.9 vs. 7.7 +/- 1.2 pmol x l(-1) x min(-1), P = 0.008) were augmented after repaglinide administration. A concomitant trend toward an increase in basal insulin secretion was observed (2.5 +/- 0.3 vs. 3.2 +/- 0.4 pmol x l(-1) x min(-1), p = 0.06), while the interpulse interval was unaltered (6.8 +/- 1.0 vs. 5.4 +/- 0.4 min/pulse, P = 0.38). ApEn increased significantly after repaglinide administration (0.623 +/- 0.045 vs. 0.670 +/- 0.034, P = 0.04), suggesting less orderly oscillatory patterns of insulin release. CONCLUSIONS: In conclusion, a single dose of repaglinide amplifies insulin secretory burst mass (and basal secretion) with no change in burst frequency. The possible importance of these mechanisms in the treatment of type 2 diabetes characterized by disrupted pulsatile insulin secretion remains to be clarified.

Short-term treatment with GLP-1 increases pulsatile insulin secretion in Type II diabetes with no effect on orderliness.

AIMS/HYPOTHESIS: The enteric incretin hormone, glucagon-like peptide-1 (GLP-1), is a potent insulin secretagogue in healthy humans and patients with Type II (non-insulin-dependent) diabetes mellitus. In this study we assessed the impact of short-term GLP-1 infusion on pulsatile insulin secretion in Type II diabetic patients. METHODS: Type II diabetic patients (n = 8) were studied in a randomised cross-over design. Plasma insulin concentration time series were obtained during basal conditions and during infusion with saline or GLP-1 (1.2 pmol/l x kg(-1) x min(-1)) on 2 separate days. Plasma glucose was clamped at the initial concentration by a variable glucose infusion. Serum insulin concentration time series were evaluated by deconvolution analysis, autocorrelation analysis, spectral analysis and approximate entropy. RESULTS: Serum insulin concentrations increased by approximately 100% during GLP-1 infusion. Pulsatile insulin secretion was increased as measured by secretory burst mass (19.3 +/- 3.8 vs 53.0 +/- 10.7 pmol/l/ pulse, p = 0.02) and secretory burst amplitude (7.7 +/- 1.5 vs 21.1 +/- 4.3 pmol/l/min, p = 0.02). A similar increase in basal insulin secretion was observed (3.6 +/- 0.9 vs 10.2 +/- 2.2 pmol/l/min, p = 0.004) with no changes in the fraction of insulin delivered in pulses (0.50 +/- 0.06 vs 0.49 +/- 0.02, p = 0.84). Regularity of secretion was unchanged as measured by spectral analysis (normalised spectral power: 5.9 +/- 0.6 vs 6.3 +/- 0.8, p = 0.86), autocorrelation analysis (autocorrelation coefficient: 0.19 +/- 0.04 vs 0.18 +/- 0.05, p = 0.66) and the approximate entropy statistic (1.48 +/- 0.02 vs 1.51 +/- 0.02, p = 0.86). CONCLUSION/INTERPRETATION: Short-term stimulation with GLP-1 jointly increases pulsatile and basal insulin secretion, maintaining but not improving system regularity in Type II diabetic patients.

High-frequency oscillations in circulating amylin concentrations in healthy humans.

Amylin is stored in the pancreatic beta-cell granules and cosecreted with insulin in response to nutrient stimuli. To gain further insight into control of hormonal release in beta-cell physiology, we examined whether amylin, like insulin, circulates in a high-frequency oscillatory pattern, and if it does, to compare the secretory patterns of the two hormones. Eight overnight-fasted healthy individuals were studied during intravenous glucose infusion (2.0 mg. kg(-1). min(-1)). Blood was collected every minute for 90 min and analyzed in triplicate for amylin, total amylin immunoreactivity (TAI), and insulin. Mean plasma concentrations of amylin (nonglycosylated), TAI (nonglycosylated plus glycosylated), insulin, and glucose were 2.77 +/- 1.21 pmol/l, 7.60 +/- 1.73 pmol/l, 50.4 +/- 17.5 pmol/l, and 5.9 +/- 0.3 mmol/l, respectively. The 90-min time series of amylin, TAI, and insulin were analyzed for periodicity (by spectral analysis, autocorrelation analysis, and deconvolution analysis) and regularity [by approximate entropy (ApEn)]. Significant spectral density peaks were demonstrated by a random shuffling technique in 7 (out of 7), 8 (out of 8), and 8 (out of 8) time series, respectively, whereas autocorrelation analysis revealed significant pulsatility in 5 (out of 7), 7 (out of 8), and 5 (out of 8), respectively. The dominant periodicity of oscillations determined by spectral analysis was 4.6 +/- 0.3, 4.6 +/- 0.4, and 6. 5 +/- 1.1 min/pulse, respectively (amylin vs. insulin, P = 0.017, TAI vs. insulin, P = 0.018). By deconvolution analysis, amylin and insulin periodicities were, respectively, 6.3 +/- 1.0 and 5.5 +/- 0. 6 min. By application of the regularity statistic, ApEn, 6 (out of 7), 7 (out of 8), and 6 (out of 8), respectively, were found to be significantly different from random. In conclusion, like several other hormones, circulating amylin concentrations exhibit oscillations in the secretory patterns for nonglycosylated as well as glycosylated forms. Whether the high-frequency pulsatile release of amylin is disturbed in diabetes is not known.

Concordant induction of rapid in vivo pulsatile insulin secretion by recurrent punctuated glucose infusions.

Insulin is largely secreted as serial secretory bursts superimposed on basal release, insulin secretion is regulated through changes of pulse mass and frequency, and the insulin release pattern affects insulin action. Coordinate insulin release is preserved in the isolated perfused pancreas, suggesting intrapancreatic coordination. However, occurrence of glucose concentration oscillations may influence the process in vivo, as it does for ultradian oscillations. To determine if rapid pulsatile insulin release may be induced by minimal glucose infusions and to define the necessary glucose quantity, we studied six healthy individuals during brief repetitive glucose infusions of 6 and 2 mg x kg(-1) x min(-1) for 1 min every 10 min. The higher dose completely synchronized pulsatile insulin release at modest plasma glucose changes ( approximately 0.3 mM = approximately 5%), with large ( approximately 100%) amplitude insulin pulses at every single glucose induction (n = 54) at a lag time of 2 min (P < 0.05), compared with small (10%) and rare (n = 3) uninduced insulin excursions. The smaller glucose dose induced insulin pulses at lower significance levels and with considerable breakthrough insulin release. Periodicity shift from either 7- to 12-min or from 12- to 7-min intervals between consecutive glucose (6 mg x kg(-1) x min(-1)) infusions in six volunteers revealed rapid frequency changes. The orderliness of insulin release as estimated by approximate entropy (1.459 +/- 0.009 vs. 1.549 +/- 0.027, P = 0.016) was significantly improved by glucose pulse induction (n = 6; 6 mg x kg(-1) x min(-1)) compared with unstimulated insulin profiles (n = 7). We conclude that rapid in vivo oscillations in glucose may be an important regulator of pulsatile insulin secretion in humans and that the use of an intermittent pulsed glucose induction to evoke defined and recurrent insulin secretory signals may be a useful tool to unveil more subtle defects in beta-cell glucose sensitivity.