Assessment of pulsatile insulin secretion derived from peripheral plasma C-peptide concentrations by nonparametric stochastic deconvolution.

The characteristics of pulsatile insulin secretion are important determinants of type 2 diabetes pathophysiology, but they are understudied due to the difficulties in measuring pulsatile insulin secretion noninvasively. Deconvolution of either peripheral C-peptide or insulin concentrations offers an appealing alternative to hepatic vein catheterization. However, to do so, there are a series of methodological challenges to overcome. C-peptide has a relatively long half-life and accumulates in the circulation. On the other hand, peripheral insulin concentrations reflect relatively fast clearance and hepatic extraction as it leaves the portal circulation to enter the systemic circulation. We propose a method based on nonparametric stochastic deconvolution of C-peptide concentrations, using individually determined C-peptide kinetics, to overcome these limitations. The use of C-peptide (instead of insulin) concentrations allows estimation of portal (and not post-hepatic) insulin pulses, whereas nonparametric stochastic deconvolution allows evaluation of pulsatile signals without any a priori assumptions of pulse shape and occurrence. The only assumption required is the degree of smoothness of the (unknown) secretion rate. We tested this method first on simulated data and then on 29 nondiabetic subjects studied during euglycemia and hyperglycemia and compared our estimates with the profiles obtained from hepatic vein insulin concentrations. This method produced satisfactory results both in the ability to fit the data and in providing reliable estimates of pulsatile secretion, in agreement with hepatic vein measurements. In conclusion, the proposed method enables reliable and noninvasive measurement of pulsatile insulin secretion. Future studies will be needed to validate this method in people with type 2 diabetes.

Performance of individually measured vs population-based C-peptide kinetics to assess ß-cell function in the presence and absence of acute insulin resistance.

AIMS: To compare the performance of population-based kinetics with that of directly measured C-peptide kinetics when used to calculate ß-cell responsivity indices, and to study people with and without acute insulin resistance to ensure that population-based kinetics apply to all conditions where ß-cell function is measured. METHODS: Somatostatin was used to inhibit endogenous insulin secretion in 56 people without diabetes. Subsequently, a C-peptide bolus was administered and the changing concentrations were used to calculate individual kinetic measures of C-peptide clearance. In addition, the participants were studied on 2 occasions in random order using an oral glucose tolerance test (OGTT). On one occasion, free fatty acid elevation, to cause insulin resistance, was achieved by infusion of Intralipid + heparin. The Disposition Index (DI) was then estimated by the oral minimal model using either population-based or individual C-peptide kinetics. RESULTS: There were marked differences in the exchange variables (k 12 and k 21 ) of the model describing C-peptide kinetics, but smaller differences in the fractional clearance; that is, the irreversible loss from the accessible compartment (k 01 ), obtained from population-based estimates compared with experimental measurement. Because it is predominantly influenced by k 01 , DI estimated using individual kinetics correlated well with DI estimated using population-based kinetics. CONCLUSIONS: These data support the use of population-based measures of C-peptide kinetics to estimate ß-cell function during an OGTT.

Glucose metabolism during rotational shift-work in healthcare workers.

AIMS/HYPOTHESIS: Shift-work is associated with circadian rhythm disruption and an increased risk of obesity and type 2 diabetes. We sought to determine the effect of rotational shift-work on glucose metabolism in humans. METHODS: We studied 12 otherwise healthy nurses performing rotational shift-work using a randomised crossover study design. On each occasion, participants underwent an isotope-labelled mixed meal test during a simulated day shift and a simulated night shift, enabling simultaneous measurement of glucose flux and beta cell function using the oral minimal model. We sought to determine differences in fasting and postprandial glucose metabolism during the day shift vs the night shift. RESULTS: Postprandial glycaemic excursion was higher during the night shift (381±33 vs 580±48 mmol/l per 5 h, p<0.01). The time to peak insulin and C-peptide and nadir glucagon suppression in response to meal ingestion was also delayed during the night shift. While insulin action did not differ between study days, the beta cell responsivity to glucose (59±5 vs 44±4 × 10-9 min-1; p<0.001) and disposition index were decreased during the night shift. CONCLUSIONS/INTERPRETATION: Impaired beta cell function during the night shift may result from normal circadian variation, the effect of rotational shift-work or a combination of both. As a consequence, higher postprandial glucose concentrations are observed during the night shift.

Hepatic insulin sensitivity in healthy and prediabetic subjects: from a dual- to a single-tracer oral minimal model.

Recently, a model was proposed to assess hepatic insulin sensitivity during a meal, i.e., the ability of insulin to suppress glucose production (EGP), SI (P). The model was developed on EGP data obtained from a triple-tracer meal and the tracer-to-tracee clamp technique and validated against the euglycemic hyperinsulinemic clamp. The aim of this study was to assess whether SI (P) can be obtained from plasma concentrations measured after a single-tracer meal by incorporating the above EGP model into the oral glucose minimal model by describing both glucose production and disposal (OMM(PD)). Triple-tracer meal data of two databases (20 healthy and 60 healthy and prediabetic subjects) were used. Virtually model-independent EGP estimates were calculated. OMM(PD) was identified on exogenous and endogenous glucose concentrations, providing indices of SI (P), disposal insulin sensitivity (SI (D)), and EGP. The model fitted the data well, and SI (P) and SI (D) were estimated with precision in both databases (SI (P) = 5.48 ± 0.54 10(-4) dl·kg(-1)·min(-1) per µU/ml and SI (D) = 9.93 ± 2.18 10(-4) dl·kg(-1)·min(-1) per µU/ml in healthy; SI (P) = 5.41 ± 3.55 10(-4) dl·kg(-1)·min(-1) per µU/ml and SI (D) = 5.34 ± 6.17 10(-4) dl·kg(-1)·min(-1) per µU/ml, in healthy and prediabetic subjects). Estimated SI (P) and that derived from the triple-tracer EGP model were very similar on average. Moreover, the time course of EGP normalized to basal EGP (EGPb), and EGP/EGPb agreed with the results obtained using the triple-tracer method. In this study, we have demonstrated that SI (P), SI (D), and EGP/EGPb time course can be estimated reliably from a single-tracer meal protocol in both healthy and prediabetic subjects.

Six and 12 Weeks of Caloric Restriction Increases ß Cell Function and Lowers Fasting and Postprandial Glucose Concentrations in People with Type 2 Diabetes.

BACKGROUND: Caloric restriction alone has been shown to improve insulin action and fasting glucose metabolism; however, the mechanism by which this occurs remains uncertain. OBJECTIVE: We sought to quantify the effect of caloric restriction on ß cell function and glucose metabolism in people with type 2 diabetes. METHODS: Nine subjects (2 men, 7 women) with type 2 diabetes [BMI (in kg/m(2)): 40.6 ± 1.4; age: 58 ± 3 y; glycated hemoglobin: 6.9% ± 0.2%] were studied using a triple-tracer mixed meal after withdrawal of oral diabetes therapy. The oral minimal model was used to measure ß cell function. Caloric restriction limited subjects to a pureed diet (<900 kcal/d) for the 12 wk of study. The studies were repeated after 6 and 12 wk of caloric restriction. RESULTS: Fasting glucose concentrations decreased significantly from baseline after 6 wk of caloric restriction with no further reduction after a further 6 wk of caloric restriction (9.8 ± 1.3, 5.9 ± 0.2, and 6.2 ± 0.3 mmol/L at baseline and after 6 and 12 wk of caloric restriction, respectively; P = 0.01) because of decreased fasting endogenous glucose production (EGP: 20.4 ± 1.1, 16.2 ± 0.8, and 17.4 ± 1.1 µmol · kg(-1) · min(-1) at baseline and after 6 and 12 wk of caloric restriction, respectively; P = 0.03). These changes were accompanied by an improvement in ß cell function measured by the disposition index (189 ± 51, 436 ± 68, and 449 ± 67 10(-14) dL · kg(-1) · min(-2) · pmol(-1) at baseline and after 6 and 12 wk of caloric restriction, respectively; P = 0.01). CONCLUSIONS: Six weeks of caloric restriction lowers fasting glucose and EGP with accompanying improvements in ß cell function in people with type 2 diabetes. An additional 6 wk of caloric restriction maintained the improvement in glucose metabolism. This trial was registered at clinicaltrials.gov as NCT01094054.

Effect of bilateral carotid body resection on the counterregulatory response to hypoglycaemia in humans.

NEW FINDINGS: What is the central question of this study? Hyperoxia blunts hypoglycaemia counterregulation in healthy adults. We hypothesized that this effect is mediated by the carotid bodies and that: (i) hyperoxia would have no effect on hypoglycaemia counterregulation in carotid body-resected patients; and (ii) carotid body-resected patients would exhibit an impaired counterregulatory response to hypoglycaemia. What is the main finding and its importance? Our data indicate that the effect of hyperoxia on hypoglycaemic counterregulation is mediated by the carotid bodies. However, a relatively normal counterregulatory response to hypoglycaemia in carotid body-resected patients highlights: (i) the potential for long-term adaptations after carotid body resection; and (ii) the importance of redundant mechanisms in mediating hypoglycaemia counterregulation. Hyperoxia reduces hypoglycaemia counterregulation in healthy adults. We hypothesized that this effect is mediated by the carotid bodies and that: (i) hyperoxia would have no effect on hypoglycaemia counterregulation in patients with bilateral carotid body resection; and (ii) carotid body-resected patients would exhibit an impaired counterregulatory response to hypoglycaemia. Five patients (three male and two female) with bilateral carotid body resection for glomus tumours underwent two 180 min hyperinsulinaemic, hypoglycaemic (∼ 3.3 mmol l(-1)) clamps separated by a minimum of 1 week and randomized to either normoxia (21% fractional inspired O2 ) or hyperoxia (100% fractional inspired O2). Ten healthy adults (seven male and three female) served as control subjects. Hypoglycaemia counterregulation in carotid body-resected patients was not significantly altered by hyperoxia (area under the curve expressed as a percentage of the normoxic response: glucose infusion rate, 111 ± 10%; cortisol, 94 ± 6%; glucagon, 107 ± 7%; growth hormone, 92 ± 10%; adrenaline, 89 ± 26%; noradrenaline, 79 ± 15%; main effect of condition, P > 0.05). This is in contrast to previously published results from healthy adults. However, the counterregulatory responses to hypoglycaemia during normoxia were not impaired in carotid body-resected patients when compared with control subjects (main effect of group, P > 0.05). Our data provide further corroborative evidence that the effect of hyperoxia on hypoglycaemic counterregulation is mediated by the carotid bodies. However, relatively normal counterregulatory responses to hypoglycaemia in carotid body-resected patients highlight the importance of redundant mechanisms in mediating hypoglycaemia counterregulation.

Effect of hypoxia on heart rate variability and baroreflex sensitivity during hypoglycemia in type 1 diabetes mellitus.

PURPOSE: Patients with type 1 diabetes mellitus exhibit impairments in autonomic and cardiovascular control which are worsened with acute hypoglycemia--thus increasing the risk of adverse cardiovascular events. Hypoxia, as seen with the common comorbidity of sleep apnea, may lead to further autonomic dysfunction and an increased risk of ventricular arrhythmias. Therefore, we hypothesized that heart rate variability (HRV) and baroreflex sensitivity (BRS) would be reduced during hypoglycemia in adults with type 1 diabetes, with a further decline when combined with hypoxia. METHODS: Subjects with type 1 diabetes (n = 13; HbA1c = 7.5 ± 0.3 %, duration of diabetes = 17 ± 5 yrs) completed two 180 min hyperinsulinemic (2 mU/kg TBW/min), hypoglycemic (~3.3 µmol/mL) clamps separated by a minimum of 1 week and randomized to normoxia (SpO2 ~98 %) or hypoxia (SpO2 ~85 %). Heart rate (electrocardiogram) and blood pressure (finger photoplethysmography) were analyzed at baseline and during the hypoglycemic clamp for measures of HRV and spontaneous cardiac BRS (sCBRS). RESULTS: Hypoglycemia resulted in significant reductions in HRV and sCBRS when compared with baseline levels (main effect of hypoglycemia: p < 0.05). HRV and sCBRS were further impaired during hypoxia (main effect of hypoxia: p < 0.05). CONCLUSIONS: Acute hypoxia worsens hypoglycemia-mediated impairments in autonomic and cardiovascular control in patients with type 1 diabetes and may increase the risk of cardiovascular mortality. These results highlight the potential cumulative dangers of hypoglycemia and hypoxia in this vulnerable population.

Modeling hepatic insulin sensitivity during a meal: validation against the euglycemic hyperinsulinemic clamp.

Recently, we proposed a model describing the suppression of endogenous glucose production (EGP) during a meal. It assumes that EGP suppression depends on glucose concentration and its rate of change and on delayed insulin action. Hepatic insulin sensitivity (S(I)(Lmeal)) can be derived from EGP model parameters. This model was shown to adequately describe EGP profiles measured with multiple tracer techniques; however, S(I)(Lmeal) has never been compared directly with its euglycemic hyperinsulinemic clamp counterpart (S(I)(Lclamp)). To do so, 62 subjects with different degrees of glucose tolerance underwent a triple-tracer mixed meal. Fifty-seven subjects also underwent a labeled ([3-(3)H]glucose) euglycemic hyperinsulinemic clamp. From the triple-tracer meal data, virtually model-independent estimates of EGP were obtained using the tracer-to-tracee clamp technique, and the EGP model was identified in each subject. Model fit was satisfactory, and S(I)(Lmeal) was estimated with good precision. Correlation between S(I)(Lclamp) and S(I)(Lmeal) was good (r = 0.72, P < 0.001); however, S(I)(Lmeal) was lower than S(I)(Lclamp) (4.60 ± 0.64 vs. 8.73 ± 1.07 10(-4) dl·kg(-1)·min(-1) per µU/ml, P < 0.01). This difference may be due to different ranges of insulin explored during the two tests (ΔI(clamp) = 15.60 ± 1.61 vs. ΔI(meal)= 83.37 ± 10.71 µU/ml) as well as steady- vs. non-steady-state glucose and insulin profiles. In conclusion, the new EGP model provides an estimate of hepatic insulin sensitivity during a meal that is in good agreement with that derived in the same individuals with a hyperinsulinemic clamp. When used in conjunction with the minimal model, the approach potentially enables estimation of hepatic insulin sensitivity from a single-tracer labeled meal or oral glucose tolerance test.

A concerted decline in insulin secretion and action occurs across the spectrum of fasting and postchallenge glucose concentrations.

AIMS/HYPOTHESIS: Individuals with impaired fasting glucose (IFG) are at increased risk of developing diabetes over the subsequent decade. However, there is uncertainty as to the mechanisms contributing to the development of diabetes. We sought to quantitate insulin secretion and action across the prediabetic range of fasting glucose. METHODS: We studied a cohort of 173 individuals with a fasting glucose concentration <7·0 mM after an overnight fast using a 75-g oral glucose tolerance test (OGTT). Insulin action (S(i)) was estimated using the oral glucose minimal model, and ß-cell responsivity indices (φ) were estimated using the oral C-peptide minimal model. The disposition index (DI) for each individual was calculated. The relationship of DI, φ and S(i) with fasting and postchallenge glucose, as well as other covariates, was explored using a generalized linear regression model. RESULTS: In this cross-sectional study, S(i) and DI were inversely related to fasting glucose concentrations. On the other hand, φ was unrelated to fasting glucose concentrations. S(i), φ and DI were all inversely related to area above basal glucose concentrations after glucose challenge. Multiple parameters including body composition and gender contributed to the variability of S(i) and DI at a given fasting or postchallenge glucose concentration. CONCLUSIONS/INTERPRETATION: Defects in insulin secretion and action interact with body composition and gender to influence postchallenge glucose concentrations. There is considerable heterogeneity of insulin secretion and action for a given fasting glucose likely because of patient subsets with isolated IFG and normal glucose tolerance.

Do the carotid bodies modulate hypoglycemic counterregulation and baroreflex control of blood pressure in humans?

Peripheral chemoreceptors in the carotid body regulate respiration, sympathetic outflow, and blood pressure in response to hypoxia. The carotid bodies play a role in the counterregulatory response to hypoglycemia in animal models and may interact with the arterial baroreflex. We hypothesized that desensitization of the carotid bodies by hyperoxia in humans would blunt hypoglycemic counterregulation and baroreflex control of blood pressure during hypoglycemia. Seven healthy adults (age 26.7 ± 0.39, BMI 25 ± 0.32, M/4, F/3) each underwent two 180 min hyperinsulinemic (2 mU/kg FFM/min), hypoglycemic (3.33 mmol/L) clamps 1 week apart, randomized to either normoxia (arterial P(O2) (P(a)O(2)) 111 ± 6.3 mmHg) or hyperoxia (P(a)O(2) 345 ± 80.6 mmHg) (p < 0.05). Plasma glucose concentrations were similar during normoxia and hyperoxia at baseline and during the clamp. The glucose infusion rate was 44.2 ± 3.5% higher (p < 0.01) during hyperoxia than normoxia during the clamp. Area under the curve values (expressed as % normoxia response) for counterregulatory hormones during hypoglycemia were significantly suppressed by hyperoxia. In addition, mean blood pressure during hypoglycemia was significantly lower with hyperoxia than with normoxia (delta reduction from baseline: -5.4 ± 3.4 mmHg normoxia vs. -13.8 ± 1.9 mmHg hyperoxia, p < 0.05). The typical baroreflex-mediated rise in heart rate and sympathetic activity with lower blood pressure did not occur when the CB were silenced. These data support the idea that the carotid bodies play a role in the counterregulatory response to hypoglycemia and in baroreflex control of blood pressure in humans.

The relationship between insulin and glucagon concentrations in non-diabetic humans.

Abnormal postprandial suppression of glucagon in Type 2 diabetes (T2DM) has been attributed to impaired insulin secretion. Prior work suggests that insulin and glucagon show an inverse coordinated relationship. However, dysregulation of α-cell function in prediabetes occurs early and independently of changes in ß-cells, which suggests insulin having a less significant role on glucagon control. We therefore, sought to examine whether hepatic vein hormone concentrations provide evidence to further support the modulation of glucagon secretion by insulin. As part of a series of experiments to measure the effect of diabetes-associated genetic variation in TCF7L2 on islet cell function, hepatic vein insulin and glucagon concentrations were measured at 2-minute intervals during fasting and a hyperglycemic clamp. The experiment was performed on 29 nondiabetic subjects (age = 46 ± 2 years, BMI 28 ± 1 Kg/m2 ) and enabled post-hoc analysis, using Cross-Correlation and Cross-Approximate Entropy (Cross-ApEn) to evaluate the interaction of insulin and glucose. Mean insulin concentrations rose from fasting (33 ± 4 vs. 146 ± 12 pmol/L, p < 0.01) while glucagon was suppressed (96 ± 8 vs. 62 ± 5 ng/L, p < 0.01) during the clamp. Cross-ApEn was used to measure pattern reproducibility in the two hormones using glucagon as control mechanism (0.78 ± 0.03 vs. 0.76 ± 0.03, fasting vs. hyperglycemia) and using insulin as a control mechanism (0.78 ± 0.02 vs. 0.76 ± 0.03, fasting vs. hyperglycemia). Values did not differ between the two scenarios. Cross-correlation analysis demonstrated a small in-phase coordination between insulin and glucagon concentrations during fasting, which inverted during hyperglycemia. This data suggests that the interaction between the two hormones is not driven by either. On a minute-to-minute basis, direct control and secretion of glucagon is not mediated (or restrained) by insulin.

Chromogranin A-positive hormone-negative endocrine cells in pancreas in human pregnancy.

Introduction: We sought to determine whether chromogranin A-positive hormone-negative (CPHN) endocrine cells are increased in the pancreas of pregnant women, offering potential evidence in support of neogenesis. Methods: Autopsy pancreata from pregnant women (n = 14) and age-matched non-pregnant control women (n = 9) were obtained. Staining of pancreatic sections for chromogranin A, insulin and a cocktail of glucagon, somatostatin, pancreatic polypeptide and ghrelin was undertaken, with subsequent evaluation for CPHN cell frequency. Results: The frequency of clustered ß-cells was increased in pregnant compared to non-pregnant subjects (46.6 ± 5.0 vs. 31.8 ± 5.0% clustered ß-cells of total clustered endocrine cells, pregnant vs. non-pregnant, p < .05). Frequency of endocrine cocktail cells was lower in pregnant women than non-pregnant women (36.2 ± 4.0 vs. 57.0 ± 6.8% clustered endocrine cocktail cells of total clustered endocrine cells, pregnant vs. non-pregnant, p < .01). No difference in frequency of CPHN cells was found in islets, nor in clustered or single cells scattered throughout the exocrine pancreas, between pregnant and non-pregnant women. The frequency of CPHN cells in pregnancy was independent of the number of pregnancies (gravidity). Conclusions: Our findings of no increase in CPHN cell frequency in pancreas of pregnant women suggest that this potential ß-cell regenerative mechanism is not that by which the increased ß-cell mass of pregnancy is achieved. However, an increase in the percentage of clustered ß-cells was found in pregnancy, with decreased frequency of other endocrine cells in clusters, suggesting a compensatory shift from other pancreatic endocrine cell types to ß-cells as a mechanism to meet the increased insulin demands of pregnancy.

Insulin Pulse Characteristics and Insulin Action in Non-diabetic Humans.

OBJECTIVE: Pulsatile insulin secretion is impaired in diseases such as type 2 diabetes that are characterized by insulin resistance. This has led to the suggestion that changes in insulin pulsatility directly impair insulin signaling. We sought to examine the effects of pulse characteristics on insulin action in humans, hypothesizing that a decrease in pulse amplitude or frequency is associated with impaired hepatic insulin action. METHODS: We studied 29 nondiabetic subjects on two occasions. On 1 occasion, hepatic and peripheral insulin action was measured using a euglycemic clamp. The deuterated water method was used to estimate the contribution of gluconeogenesis to endogenous glucose production. On a separate study day, we utilized nonparametric stochastic deconvolution of frequently sampled peripheral C-peptide concentrations during fasting to reconstruct portal insulin secretion. In addition to measuring basal and pulsatile insulin secretion, we used approximate entropy to measure orderliness and Fourier transform to measure the average, and the dispersion of, insulin pulse frequencies. RESULTS: In univariate analysis, basal insulin secretion (R2 = 0.16) and insulin pulse amplitude (R2 = 0.09) correlated weakly with insulin-induced suppression of gluconeogenesis. However, after adjustment for age, sex, and weight, these associations were no longer significant. The other pulse characteristics also did not correlate with the ability of insulin to suppress endogenous glucose production (and gluconeogenesis) or to stimulate glucose disappearance. CONCLUSIONS: Overall, our data demonstrate that insulin pulse characteristics, considered independently of other factors, do not correlate with measures of hepatic and peripheral insulin sensitivity in nondiabetic humans.

Hyperoxia blunts counterregulation during hypoglycaemia in humans: possible role for the carotid bodies?

Chemoreceptors in the carotid bodies sense arterial oxygen tension and regulate respiration. Isolated carotid body glomus cells also sense glucose, and animal studies have shown the carotid bodies play a role in the counterregulatory response to hypoglycaemia. Thus, we hypothesized that glucose infusion rate would be augmented and neuro-hormonal counterregulation blunted during hypoglycaemia when the carotid bodies were desensitized by hyperoxia. Seven healthy adults (four male, three female) underwent two 180 min hyperinsulinaemic (2 mU (kg fat-free mass (FFM))(-1) min(-1)), hypoglycaemic (3.33 mmol l(-1)) clamps 1 week apart, randomized to either normoxia (arterial P(O2) (P(aO2)) 111 ± 6.3 mmHg) or hyperoxia (P(aO2) 345 ± 80.6 mmHg) (P < 0.05). Plasma glucose concentrations were similar during normoxia and hyperoxia at baseline (5.52 ± 0.15 vs. 5.55 ± 0.13 µmol ml(-1)) and during the clamp (3.4 ± 0.05 vs. 3.3 ± 0.05 µmol ml(-1)). The glucose infusion rate was 44.2 ± 3.5% higher (P < 0.01) during hyperoxia than normoxia at steady state during the clamp (28.2 ± 0.15 vs. 42.7 ± 0.65 µmol (kg FFM)(-1) min(-1); P < 0.01). Area under the curve values (expressed as percentage normoxia response) for counterregulatory hormones during hypoglycaemia were significantly suppressed by hyperoxia (noradrenaline 50.7 ± 5.2%, adrenaline 62.6 ± 3.3%, cortisol 63.2 ± 2.1%, growth hormone 53.1 ± 2.7%, glucagon 48.6 ± 2.1%, all P < 0.05 vs. normoxia). These data support the idea that the carotid bodies respond to glucose and play a role in the counterregulatory response to hypoglycaemia in humans.

A model of GLP-1 action on insulin secretion in nondiabetic subjects.

Glucagon-like peptide-1 (GLP-1)-based therapies for diabetes have aroused interest because of their effects on insulin secretion and glycemic control. However, a mechanistic model enabling quantitation of pancreatic response to GLP-1 has never been developed. To develop such a model we studied 88 healthy individuals (age 26.3 +/- 0.6 yr, BMI 24.9 +/- 0.4 kg/m(2)) by use of a hyperglycemic clamp. A variable infusion maintained glucose concentrations at 150 mg/dl for 240 min. At 120 min, an intravenous infusion of GLP-1 was started (0.75 pmol kg(-1) min(-1) from 120-180 min, 1.5 pmol kg(-1) min(-1) from 181-240 min). Consequently, plasma C-peptide concentration rose from 1,852.0 +/- 62.8 pmol/l at 120 min to 4,272.2 +/- 176.4 pmol/l at 180 min and to 6,995.8 +/- 323.5 pmol/l at 240 min. Four models of GLP-1 action on insulin secretion were considered. All models share the common assumption that insulin secretion is made up of two components, one proportional to glucose rate of change through dynamic responsivity, Phi(d), and one proportional to glucose through static responsivity, Phi(s), but differing by modality of GLP-1 control. The model that best fit C-peptide data assumes that above-basal insulin secretion depends linearly on GLP-1 concentration and its rate of change. An index (Pi) measuring the percentage increase of secretion due to GLP-1 is derived. Before GLP-1 infusion, Phi(d) = 245.7 +/- 15.6 10(-9) and Phi(s) = 25.2 +/- 1.4 10(-9) min(-1). Under GLP-1 stimulus, Pi = 12.6 +/- 0.71% per pmol/l, meaning that an increase of 5 pmol/l in peripheral GLP-1 concentrations induces an approximately 60% increase in over-basal insulin secretion.

Effect of 11 beta-hydroxysteroid dehydrogenase-1 inhibition on hepatic glucose metabolism in the conscious dog.

Inactive cortisone is converted to active cortisol within the liver by 11 beta-hydroxysteroid dehydrogenase-1 (11 beta-HSD1), and impaired regulation of this process may be related to increased hepatic glucose production (HGP) in individuals with type 2 diabetes. The primary aim of this study was to investigate the effect of acute 11 beta-HSD1 inhibition on HGP and fat metabolism during insulin deficiency. Sixteen conscious, 42-h-fasted, lean, healthy dogs were studied. Somatostatin was infused to create insulin deficiency, and the animals were treated with a specific 11 beta-HSD1 inhibitor (compound 531) or placebo for 5 h. 11 beta-HSD1 inhibition completely suppressed hepatic cortisol production, and this attenuated the increase in HGP that occurred during insulin deficiency. PEPCK and glucose-6-phosphatase expression were decreased when 11 beta-HSD1 was inhibited, but gluconeogenic flux was unchanged, implying an effect on glycogenolysis. Since inhibition of hepatic cortisol production reduces HGP during insulin deficiency, 11 beta-HSD1 is a potential therapeutic target for the treatment of excess glucose production that occurs in diabetes.

The effect of DPP-4 inhibition with sitagliptin on incretin secretion and on fasting and postprandial glucose turnover in subjects with impaired fasting glucose.

OBJECTIVE: Low glucagon-like peptide-1 (GLP-1) concentrations have been observed in impaired fasting glucose (IFG). It is uncertain whether these abnormalities contribute directly to the pathogenesis of IFG and impaired glucose tolerance. Dipeptidyl peptidase-4 (DPP-4) inhibitors raise incretin hormone concentrations enabling an examination of their effects on glucose turnover in IFG. RESEARCH DESIGN AND METHODS: We studied 22 subjects with IFG using a double-blinded, placebo-controlled, parallel-group design. At the time of enrollment, subjects ate a standardized meal labelled with [1-(13)C]-glucose. Infused [6-(3)H] glucose enabled measurement of systemic meal appearance (MRa). Infused [6,6-(2)H(2)] glucose enabled measurement of endogenous glucose production (EGP) and glucose disappearance (Rd). Subsequently, subjects were randomized to 100 mg of sitagliptin daily or placebo. After an 8-week treatment period, the mixed meal was repeated. RESULTS: As expected, subjects with IFG who received placebo did not experience any change in glucose concentrations. Despite raising intact GLP-1 concentrations, treatment with sitagliptin did not alter either fasting or postprandial glucose, insulin or C-peptide concentrations. Postprandial EGP (18.1 +/- 0.7 vs 17.6 +/- 0.8 micromol/kg per min, P = 0.53), Rd (55.6 +/- 4.3 vs 58.9 +/- 3.3 micromol/kg per min, P = 0.47) and MRa (6639 +/- 377 vs 6581 +/- 316 micromol/kg per 6 h, P = 0.85) were unchanged. Sitagliptin was associated with decreased total GLP-1 implying decreased incretin secretion. CONCLUSIONS: DPP-4 inhibition did not alter fasting or postprandial glucose turnover in people with IFG. Low incretin concentrations are unlikely to be involved in the pathogenesis of IFG.

Higher muscle protein synthesis in women than men across the lifespan, and failure of androgen administration to amend age-related decrements.

We investigated age and sex effects and determined whether androgen replacement in elderly individuals (> or = 60 yr) could augment protein synthesis. Thirty young men and 32 young women (18-31 yr) were studied once, whereas 87 elderly men were studied before and after 1 yr of treatment with 5 mg/day testosterone (T), 75 mg/day dehydroepiandrosterone (DHEA), or placebo (P); and 57 elderly women were studied before and after 1 yr of treatment with 50 mg/day DHEA or P. [(15)N]Phenylalanine and [(2)H(4)]tyrosine tracers were infused, with measurements in plasma and vastus lateralis muscle. Whole-body protein synthesis per fat-free mass and muscle protein fractional synthesis rate (FSR) were lower in elderly than in young individuals (P<0.001), not significantly affected by hormone treatments, and higher in women than in men (P<0.0001), with no sex x age interaction. In regression analyses, peak O2 consumption (VO2peak), resting energy expenditure (REE), and sex were independently associated with muscle FSR, as were VO2peak, REE, and interactions of sex with insulin-like growth factor-II and insulin for whole-body protein synthesis. Women maintain higher protein synthesis than men across the lifespan as rates decline in both sexes, and neither full replacement of DHEA (in elderly men and women) nor partial replacement of bioavailable T (in elderly men) is able to amend the age-related declines.

Pancreatic alpha-cell mass across adult human lifespan.

AIM: To establish pancreatic alpha-cell mass in lean, non-diabetic humans over the adult lifespan, performed as a follow-up study to beta-cell mass across the adult human lifespan. METHODS: We examined human pancreatic autopsy tissue from 66 lean, non-diabetic individuals aged from 30 to 102 years, grouped into deciles: 3rd (30-39 years), 4th (40-49 years), 5th (50-59 years), 6th (60-69 years), 7th (70-79 years), 8th (80-89 years) and 9th deciles (90+ years). Sections of pancreas were immunostained for glucagon and analyzed for fractional alpha-cell area. Population-based pancreatic volume data were used to calculate alpha-cell mass. RESULTS: With advanced age, the exocrine pancreas undergoes atrophy demonstrated by increased fat area (as % exocrine area) (0.05 ± 0.01 vs 1.6 ± 0.7% fat area of total exocrine pancreas, 3rd vs 9th decile, P < 0.05). Consequently, islet density increases with age (2.7 ± 0.4 vs 10.5 ± 3.3 islets/mm2, 3rd vs 9th decile, P < 0.05). Alpha-cell fractional area increases with advanced age (0.34 ± 0.05% vs 0.73 ± 0.26%, 3rd vs 9th decile, P < 0.05). However, alpha-cell mass remains constant at ~190 mg throughout the adult lifespan in lean, non-diabetic humans. Within islets, alpha-cell distribution between mantle and core is unchanged across deciles (1862 ± 220 vs 1945 ± 200 vs 1948 ± 139 alpha cells in islet mantle/mm2, 3rd vs 6th vs 9th decile, P = 0.93 and 1912 ± 442 vs 1449 ± 123 vs 1514 ± 168 alpha cells in islet core/mm2, 3rd vs 6th vs 9th decile, P = 0.47), suggesting that human islets retain their structural organization in the setting of age-related exocrine atrophy. CONCLUSIONS: Consistent with our previous findings for beta-cell mass, alpha-cell mass remains constant in humans, even with advanced age. Pancreatic endocrine cells are much more robustly preserved than exocrine cells in aged humans, and islets maintain their structural integrity throughout life.