Niels Pørksen

In humans at least 75% of insulin secretion arises from punctuated insulin secretory bursts

Detection of insulin secretory bursts in peripheral blood is hampered by hepatic insulin extraction, dilution in the systemic insulin pool, and time-delayed damping of secretory burst amplitude. Previous studies in dogs in vivo and other experiments in vitro have shown that approximately 70% of all insulin is released within distinct insulin secretory bursts. To establish a method for detection and quantification of pulsatile insulin release in humans on the basis of peripheral insulin concentration measurements, we used a high-sensitivity, -specificity, and -precision insulin enzyme-linked immunosorbent assay (ELISA) and optimized an established deconvolution methodology to quantify the frequency, mass, and amplitude of insulin secretory bursts as well as to estimate the relative contribution of pulsatile insulin release to overall insulin secretion. By use of minutely sampled serum insulin concentrations measured by a highly sensitive insulin ELISA and insulin kinetics of 2.8 min (first half-life), 5.0 min (second half-life), and a fractional slow component of 0.28, the deconvolved insulin secretion rates in 20 healthy subjects during glucose infusion (4.5 mg.kg-1.min-1) could be resolved into a series (4.7 +/- 0.1 min/pulse) of approximately symmetric insulin secretory bursts with a mean mass of 87 +/- 12 pmol.l-1 pulse-1 and a mean amplitude (maximal release rate) of 35 +/- 4.7 pmol.l-1.min-1. The relative contribution of pulsatile to overall insulin secretion was 75 +/- 1.6% (range 59-85%). We conclude that in vivo insulin secretion in humans during nominal glucose stimulation consists of a series of punctuated insulin secretory bursts accounting for > or = 75% of total insulin secretion.Detection of insulin secretory bursts in peripheral blood is hampered by hepatic insulin extraction, dilution in the systemic insulin pool, and time-delayed damping of secretory burst amplitude. Previous studies in dogs in vivo and other experiments in vitro have shown that ∼70% of all insulin is released within distinct insulin secretory bursts. To establish a method for detection and quantification of pulsatile insulin release in humans on the basis of peripheral insulin concentration measurements, we used a high-sensitivity, -specificity, and -precision insulin enzyme-linked immunosorbent assay (ELISA) and optimized an established deconvolution methodology to quantify the frequency, mass, and amplitude of insulin secretory bursts as well as to estimate the relative contribution of pulsatile insulin release to overall insulin secretion. By use of minutely sampled serum insulin concentrations measured by a highly sensitive insulin ELISA, and insulin kinetics of 2.8 min (first half-life), 5.0 min (second half-life), and a fractional slow component of 0.28, the deconvolved insulin secretion rates in 20 healthy subjects during glucose infusion (4.5 mg ⋅ kg-1 ⋅ min-1) could be resolved into a series (4.7 ± 0.1 min/pulse) of approximately symmetric insulin secretory bursts with a mean mass of 87 ± 12 pmol ⋅ l-1 ⋅ pulse-1and a mean amplitude (maximal release rate) of 35 ± 4.7 pmol ⋅ l-1 ⋅ min-1. The relative contribution of pulsatile to overall insulin secretion was 75 ± 1.6% (range 59-85%). We conclude that in vivo insulin secretion in humans during nominal glucose stimulation consists of a series of punctuated insulin secretory bursts accounting for ≥75% of total insulin secretion.

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

Abstract.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. [Diabetologia (2002) 45: 49–55]

Glucagon-Like Peptide 1 Increases Mass But Not Frequency or Orderliness of Pulsatile Insulin Secretion

Glucagon-like peptide 1 (GLP-1) is a peptide hormone that is released from the gut after luminal stimulation. The hormone is a potent insulin secretagogue and is a potential novel pharmaceutical adjuvant in the treatment of NIDDM. Insulin is secreted as a series of punctuated secretory bursts superimposed on a basal insulin release. Recently, the contribution of these secretory bursts to overall insulin secretion has been evaluated, and studies using catheterization across the pancreas in a canine model and studies using deconvolution in humans have revealed that the majority of insulin is released during these secretory bursts. Moreover, the main regulation of insulin secretion is through perturbation of mass and frequency of these secretory bursts. The mode of delivery of insulin into the circulation seems important for insulin action, and it is therefore important to know the impact of a potential therapeutic insulin secretagogue on the mode of insulin secretion. To assess the effects of GLP-1 on the mass, frequency, amplitude, and overall contribution of pulsatile insulin secretion, we used a recently validated deconvolution model to examine these variables before and during infusion of GLP-1 in eight healthy men (age 28 ± 2 years; BMI 24 ± 2 kg/m2). At a constant glucose infusion (2.5 mg · kg−1 · min−1), near-steady state was reached at 75 min, and sampling was performed every minute at t = 75–115 and 145–185 min. At t = 115 min, an infusion of saline or GLP-1 (50 pmol · kg−1 · min−1) was given. The regularity of insulin secretion was measured by approximate entropy, a recently developed mathematical statistic, applied herein to assess the regularity in a hormone concentration time series. After GLP-1 infusion, there was an abrupt increase in the peripheral concentrations of serum Cpeptide (696 ± 65 vs. 1,538 ± 165 pmol/1) and insulin (49 ± 8 vs. 138 ±21 pmolA) concentrations. This increase was mainly due to an increase in the pulsatile component of insulin secretion that was achieved by a fourfold increase in secretory burst mass (28.2 ± 4.4 vs. 100.1 ± 15.8 pmol · .l−1· pulse−1; P < 0.001), and amplitude (12.7 ± 2.2 vs. 4.3 ± 7.7 pmol · l−1 · min−1; P < 0.002), whereas the secretory burst frequency was not affected by GLP-1 (11.5 ± 0.7 vs. 12.6 ± 0.6 pulses/h; P = 0.4). As a consequence, the detected contribution of pulsatile to overall insulin secretion was increased from 56 ± 4 to 77 ± 4% (P < 0.005). The orderliness of the insulin release process was not deteriorated by short-term GLP-1 infusion as assessed by approximate entropy (1.19 ± 0.04 vs. 1.18 ± 0.04; P = 0.7).

Glucose turnover, fuel oxidation and forearm substrate exchange in patients with thyrotoxicosis before and after medical treatment.

OBJECTIVEu2003Accelerated metabolism is a hallmark of thyrotoxicosis, but the underlying bioche mical mechanisms are incompletely understood and the majority of studies have investigated normal subjects rendered only modestly hyperthyroid for a brief period of time. We have therefore studied a group of thyrotoxic patients using several different techniques.

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/hypothesisType 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.MethodsThe 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 (67xa0mg/kg) and streptozotocin (125xa0mg/kg) from 17.7±4.7 (normal animals,n=5) to 6.1±2.0xa0mg/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.ResultsReduction 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±2.4xa0min, p=0.50), regularity and entrainability of insulin secretion.Conclusion/interpretationNeither 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.

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 · kg–1· 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. [Diabetologia (2000) 43: 583–588]

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

Abstractn 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.n Methods. Basal and glucose entrained insulin secretion was examined in normal minipigs and evaluated by autocorrelation, cross correlation and deconvolution.n 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).n 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.

Evaluation of a mathematical model of diabetes progression against observations in the Diabetes Prevention Program.

The seminal publication of the Diabetes Prevention Program (DPP) results in 2002 has provided insight into the impact of major therapies on the development of diabetes over a time span of a few years. In the present work, the publicly available DPP data set is used to calibrate and evaluate a recently developed mechanistic mathematical model for the long-term development of diabetes to assess the models ability to predict the natural history of disease progression and the effectiveness of preventive interventions. A general population is generated from which virtual subject samples corresponding to the DPP enrollment criteria are selected. The model is able to reproduce with good fidelity the observed time courses of both diabetes incidence and average glycemia, under realistic hypotheses on evolution of disease and efficacy of the studied therapies, for all treatment arms. Model-based simulations of the long-term evolution of the disease are consistent with the transient benefits observed with conventional therapies and with promising effects of radical improvement of insulin sensitivity (as by metabolic surgery) or of β-cell protection. The mechanistic diabetes progression model provides a credible tool by which long-term implications of antidiabetic interventions can be evaluated.