Control of pulsatile insulin secretion in man

Cx36-mediated coupling reduces beta-cell heterogeneity, confines the stimulating glucose concentration range, and affects insulin release kinetics

We studied the effect of gap junctional coupling on the excitability of beta-cells in slices of pancreas, which provide a normal environment for islet cells. The electrophysiological properties of beta-cells from mice (C57Bl/6 background) lacking the gap junction protein connexin36 (Cx36(-/-)) were compared with heterozygous (Cx36(+/-)) and wild-type littermates (Cx36(+/+)) and with frequently used wild-type NMRI mice. Most electrophysiological characteristics of beta-cells were found to be unchanged after the knockout of Cx36, except the density of Ca(2+) channels, which was increased in uncoupled cells. With closed ATP-sensitive K(+) (K(ATP)) channels, the electrically coupled beta-cells of Cx36(+/+) and Cx36(+/-) mice were hyperpolarized by the membrane potential of adjacent, inactive cells. Additionally, the hyperpolarization of one beta-cell could attenuate or even stop the electrical activity of nearby coupled cells. In contrast, beta-cells of Cx36(-/-) littermates with blocked K(ATP) channels rapidly depolarized and exhibited a continuous electrical activity. Absence of electrical coupling modified the electrophysiological properties of beta-cells consistent with the reported increase in basal insulin release and altered the switch on/off response of beta-cells during an acute drop of the glucose concentration. Our data indicate an important role for Cx36-gap junctions in modulating stimulation threshold and kinetics of insulin release.

Relationship between Gestational Diabetes Mellitus and Type 2 Diabetes: Evidence of Mitochondrial Dysfunction

Background: We examined the pathogenesis of gestational diabetes mellitus (GDM) in a large Dutch multiethnic cohort.

Methods: We used a 2-step testing procedure to stratify 2031 consecutive pregnant women into 4 groups according to American Diabetes Association criteria: (a) normal glucose tolerance (NGT), (b) mild gestational hyperglycemia (MGH), (c) GDM without early postpartum diabetes within 6 months of delivery (GDM1), and (d) GDM with early postpartum diabetes (GDM2). Antepartum and postpartum clinical characteristics and measures of glucose tolerance were documented.

Results: Overall, 1627 women had NGT, 237 had MGH, 156 had GDM1, and 11 had GDM2. Prepregnancy body mass index values progressively increased from NGT to MGH to GDM1. The fasting plasma glucose concentration, the 100-g oral glucose tolerance test (OGTT) area under the curve, and the mean glucose concentration during the OGTT all increased progressively among the 4 groups. The fasting C-peptide concentration displayed an inverted-U pattern, with a maximum at a mean plasma glucose concentration during the OGTT of 9.6 mmol/L in the transition from GDM1 to GDM2. The fasting C-peptide/glucose concentration ratio decreased by 42% in GDM patients compared with NGT patients, whereas the ratios in MGH and NGT women were similar.

Conclusions: Progressive metabolic derangement of glucose tolerance 1st detected during pregnancy mimics the pathogenesis of type 2 diabetes. In addition, our results imply an impaired basal glucose effectiveness in the early prediabetic state. To explain the parallel in both metabolic derangements, we postulate that GDM, like type 2 diabetes, is attributable to the same inherited mitochondrial dysfunction.

Interaction of glycolysis and mitochondrial respiration in metabolic oscillations of pancreatic islets

Insulin secretion from pancreatic beta-cells is oscillatory, with a typical period of 2-7 min, reflecting oscillations in membrane potential and the cytosolic Ca(2+) concentration. Our central hypothesis is that the slow 2-7 min oscillations are due to glycolytic oscillations, whereas faster oscillations that are superimposed are due to Ca(2+) feedback onto metabolism or ion channels. We extend a previous mathematical model based on this hypothesis to include a more detailed description of mitochondrial metabolism. We demonstrate that this model can account for typical oscillatory patterns of membrane potential and Ca(2+) concentration in islets. It also accounts for temporal data on oxygen consumption in islets. A recent challenge to the notion that glycolytic oscillations drive slow Ca(2+) oscillations in islets are data showing that oscillations in Ca(2+), mitochondrial oxygen consumption, and NAD(P)H levels are all terminated by membrane hyperpolarization. We demonstrate that these data are in fact compatible with a model in which glycolytic oscillations are the key player in rhythmic islet activity. Finally, we use the model to address the recent finding that the activity of islets from some mice is uniformly fast, whereas that from islets of other mice is slow. We propose a mechanism for this dichotomy.

Familial factors in diabetic nephropathy: an offspring study

AIMS

Familial clustering of diabetic nephropathy in patients with Type 2 diabetes suggests that inherited factors predispose to diabetic nephropathy, but the nature of these factors is uncertain. The aim of the study was to compare the prevalence of known risk factors for nephropathy in non-diabetic offspring of Type 2 diabetic patients with and without nephropathy and in control subjects.

METHODS

Three groups of patients were recruited with 40 or 41 subjects in each group. These were subjects having one Type 2 diabetic parent with nephropathy (DN); subjects having one parent with Type 2 diabetes without nephropathy (DnoN), and non-diabetic unrelated control subjects with no personal or parental history of diabetes (Control subjects).

RESULTS

The median (interquartile range) albumin/creatinine ratio (ACR) was 1.40 (0.96-2.90) mg/mmol in DN; 0.94 (0.50-1.46) mg/mmol in DnoN and 1.22 (0.66-1.83) mg/mmol in Controls (ANOVA: P = 0.03). ACR was higher in group DN than in DnoN (P < 0.006) and in Control subjects (P < 0.03), but there was no difference between DnoN and Control subjects. Twenty-four-hour ambulatory blood pressure monitoring showed mean daytime systolic blood pressure to be significantly higher in group DN than in DnoN (P < 0.02) or Control subjects (P < 0.01) (ANOVA: P = 0.004). Fasting insulin, HOMA-IR, interleukin-6 (IL-6) and C-reactive protein (CRP) were similar in the three groups.

CONCLUSION

Our data provide further evidence that genetic factors are important in determining urinary albumin excretion and renal disease associated with Type 2 diabetes and suggest that genes that affect systemic arterial blood pressure but not those relating to insulin resistance or inflammation are likely to be implicated.

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.

Loss of connexin36 channels alters beta-cell coupling, islet synchronization of glucose-induced Ca2+ and insulin oscillations, and basal insulin release

Normal insulin secretion requires the coordinated functioning of beta-cells within pancreatic islets. This coordination depends on a communications network that involves the interaction of beta-cells with extracellular signals and neighboring cells. In particular, adjacent beta-cells are coupled via channels made of connexin36 (Cx36). To assess the function of this protein, we investigated islets of transgenic mice in which the Cx36 gene was disrupted by homologous recombination. We observed that compared with wild-type and heterozygous littermates that expressed Cx36 and behaved as nontransgenic controls, mice homozygous for the Cx36 deletion (Cx36(-/-)) featured beta-cells devoid of gap junctions and failing to exchange microinjected Lucifer yellow. During glucose stimulation, islets of Cx36(-/-) mice did not display the regular oscillations of intracellular calcium concentrations ([Ca(2+)](i)) seen in controls due to the loss of cell-to-cell synchronization of [Ca(2+)](i) changes. The same islets did not release insulin in a pulsatile fashion, even though the overall output of the hormone in response to glucose stimulation was normal. However, under nonstimulatory conditions, islets lacking Cx36 showed increased basal release of insulin. These data show that Cx36-dependent signaling is essential for the proper functioning of beta-cells, particularly for the pulsatility of [Ca(2+)](i) and insulin secretion during glucose stimulation.

Two SUR1-specific Histidine Residues Mandatory for Zinc-induced Activation of the Rat KATP Channel

Zinc at micromolar concentrations hyperpolarizes rat pancreatic beta-cells and brain nerve terminals by activating ATP-sensitive potassium channels (KATP). The molecular determinants of this effect were analyzed using insulinoma cell lines and cells transfected with either wild type or mutated KATP subunits. Zinc activated KATP in cells co-expressing rat Kir6.2 and SUR1 subunits, as in insulinoma cell lines. In contrast, zinc exerted an inhibitory action on SUR2A-containing cells. Therefore, SUR1 expression is required for the activating action of zinc, which also depended on extracellular pH and was blocked by diethyl pyrocarbonate, suggesting histidine involvement. The five SUR1-specific extracellular histidine residues were submitted to site-directed mutagenesis. Of them, two histidines (His-326 and His-332) were found to be critical for the activation of KATP by zinc, as confirmed by the double mutation H326A/H332A. In conclusion, zinc activates KATP by binding itself to extracellular His-326 and His-332 of the SUR1 subunit. Thereby zinc could exert a negative control on cell excitability and secretion process of pancreatic beta-and alpha-cells. In fact, we have recently shown that such a mechanism occurs in hippocampal mossy fibers, a brain region characterized, like the pancreas, by an important accumulation of zinc and a high density of SUR1-containing KATP.

Prolonged cardiac repolarisation during spontaneous nocturnal hypoglycaemia in children and adolescents with type 1 diabetes

AIMS/HYPOTHESIS

It has been postulated that hypoglycaemia-related cardiac dysrhythmia and, in particular, prolonged cardiac repolarisation, may contribute to increased mortality rates in children and adolescents with type 1 diabetes.

METHODS

We examined the prevalence of prolonged QT interval on ECG during spontaneous hypoglycaemia in 44 type 1 diabetic subjects (aged 7-18 years), and explored the relationships between serial overnight measurements of QT interval corrected for heart rate (QTc) and serum glucose, potassium and epinephrine levels. Each subject underwent two overnight profiles; blood was sampled every 15 min for glucose measurements and hourly for potassium and epinephrine. Serial ECGs recorded half-hourly between 23.00 and 07.00 hours were available on 74 nights: 29 with spontaneous hypoglycaemia (defined as blood glucose <3.5 mmol/l) and 45 without hypoglycaemia.

RESULTS

Mean overnight QTc was longer in females than in males (412 vs 400 ms, p=0.02), but was not related to age, diabetes duration or HbA(1)c. Prolonged QTc (>440 ms) occurred on 20 out of 74 (27%) nights, with no significant differences between male and female subjects, and was more prevalent on nights with hypoglycaemia (13/29, 44%) than on nights without (7/45, 15%, p=0.0008). Potassium levels were lower on nights when hypoglycaemia occurred (minimum potassium 3.4 vs 3.7 mmol/l, p=0.0003) and were inversely correlated with maximum QTc (r=-0.40, p=0.03). In contrast, epinephrine levels were not higher on nights with hypoglycaemia and were not related to QTc.

CONCLUSIONS/INTERPRETATION

In young type 1 diabetic subjects, prolonged QTc occurred frequently with spontaneous overnight hypoglycaemia and may be related to insulin-induced hypokalaemia.

Calcium and glycolysis mediate multiple bursting modes in pancreatic islets

Pancreatic islets of Langerhans produce bursts of electrical activity when exposed to stimulatory glucose levels. These bursts often have a regular repeating pattern, with a period of 10-60 s. In some cases, however, the bursts are episodic, clustered into bursts of bursts, which we call compound bursting. Consistent with this are recordings of free Ca2+ concentration, oxygen consumption, mitochondrial membrane potential, and intraislet glucose levels that exhibit very slow oscillations, with faster oscillations superimposed. We describe a new mathematical model of the pancreatic beta-cell that can account for these multimodal patterns. The model includes the feedback of cytosolic Ca2+ onto ion channels that can account for bursting, and a metabolic subsystem that is capable of producing slow oscillations driven by oscillations in glycolysis. This slow rhythm is responsible for the slow mode of compound bursting in the model. We also show that it is possible for glycolytic oscillations alone to drive a very slow form of bursting, which we call "glycolytic bursting." Finally, the model predicts that there is bistability between stationary and oscillatory glycolysis for a range of parameter values. We provide experimental support for this model prediction. Overall, the model can account for a diversity of islet behaviors described in the literature over the past 20 years.

Loss of entrainment of high-frequency plasma insulin oscillations in type 2 diabetes is likely a glucose-specific beta-cell defect

Spontaneous high-frequency insulin oscillations are easily entrainable to exogenous glucose in vitro and in vivo, but this property is lost in type 2 diabetes (2-DM). We hypothesized that this lack of entrainment in 2-DM would be specific to glucose. This was tested in nine control and ten 2-DM subjects. Serial blood sampling at 1-min intervals was carried out for 60 min in the basal state and for 120 min while small (1-60 mg/kg) boluses of arginine were injected intravenously at exactly 29-min intervals. Samples were analyzed for insulin concentrations, and time series analysis was carried out using spectral analysis. In control subjects, the mean period of basal plasma insulin oscillations was 10.3 +/- 1.3 min and was entrained by arginine to a mean period of 14.9 +/- 0.6 min (P < 0.00001 vs. basal). Similarly, in 2-DM subjects, spontaneous insulin oscillations were entrained by arginine; mean basal insulin period was 10.0 +/- 1.0 min and 14.5 +/- 1.8 min with arginine boluses (P < 0.00001). All of the primary peaks observed in spectral analysis were statistically significant (P < 0.05). Percent total power of primary peaks ranged from 17 to 68%. Thus arginine boluses entrain spontaneous high-frequency insulin oscillations in 2-DM subjects. This represents a distinct and striking difference from the resistance of the beta-cell to glucose entrainment in 2-DM. We conclude that loss of entrainment of spontaneous high-frequency insulin oscillations in 2-DM is likely a glucose-specific manifestation of beta-cell secretory dysfunction.

Multi-pulse drug delivery from a resorbable polymeric microchip device

Controlled-release drug delivery systems have many applications, including treatments for hormone deficiencies and chronic pain. A biodegradable device that could provide multi-dose drug delivery would be advantageous for long-term treatment of conditions requiring pulsatile drug release. In this work, biodegradable polymeric microchips were fabricated that released four pulses of radiolabelled dextran, human growth hormone or heparin in vitro. Heparin that was released over 142 days retained on average 96 +/- 12% of its bioactivity. The microchips were 1.2 cm in diameter, 480-560 microm thick and had 36 reservoirs that could each be filled with a different chemical. The devices were fabricated from poly(L-lactic acid) and had poly(D,L-lactic-co-glycolic acid) membranes of different molecular masses covering the reservoirs. A drug delivery system can be designed with the potential to release pulses of different drugs at intervals after implantation in a patient by using different molecular masses or materials for the membrane.

Dihydroxyacetone-induced oscillations in cytoplasmic free Ca2+ and the ATP/ADP ratio in pancreatic beta-cells at substimulatory glucose

Glucose stimulation of pancreatic beta-cells causes oscillatory influx of Ca2+, leading to pulsatile insulin secretion. We have proposed that this is due to oscillations of glycolysis and the ATP/ADP ratio, which modulate the activity of ATP-sensitive K+ channels. We show here that dihydroxyacetone, a secretagogue that feeds into glycolysis below the putative oscillator phosphofructokinase, could cause a single initial peak in cytoplasmic free Ca2+ ([Ca2+]i) but did not by itself cause repeated oscillations in [Ca2+]i in mouse pancreatic beta-cells. However, in the presence of a substimulatory concentration of glucose (4 mm), dihydroxyacetone induced [Ca2+]i oscillations. Furthermore, these oscillations correlated with oscillations in the ATP/ADP ratio, as seen previously with glucose stimulation. Insulin secretion in response to dihydroxyacetone was transient in the absence of glucose but was considerably enhanced and somewhat prolonged in the presence of a substimulatory concentration of glucose, in accordance with the enhanced [Ca2+]i response. These results are consistent with the hypothesized role of phosphofructokinase as the generator of the oscillations. Dihydroxyacetone may affect phosphofructokinase by raising the free concentration of fructose 1,6-bisphosphate to a critical level at which it activates the enzyme autocatalytically, thereby inducing the pulses of phosphofructokinase activity that cause the metabolic oscillations.

Determination of the glycaemic index of foods: interlaboratory study

OBJECTIVE

Practical use of the glycaemic index (GI), as recommended by the FAO/WHO, requires an evaluation of the recommended method. Our purpose was to determine the magnitude and sources of variation of the GI values obtained by experienced investigators in different international centres.

DESIGN

GI values of four centrally provided foods (instant potato, rice, spaghetti and barley) and locally obtained white bread were determined in 8-12 subjects in each of seven centres using the method recommended by FAO/WHO. Data analysis was performed centrally.

SETTING

University departments of nutrition.

SUBJECTS

Healthy subjects (28 male, 40 female) were studied.

RESULTS

The GI values of the five foods did not vary significantly in different centres nor was there a significant centrexfood interaction. Within-subject variation from two centres using venous blood was twice that from five centres using capillary blood. The s.d. of centre mean GI values was reduced from 10.6 (range 6.8-12.8) to 9.0 (range 4.8-12.6) by excluding venous blood data. GI values were not significantly related to differences in method of glucose measurement or subject characteristics (age, sex, BMI, ethnicity or absolute glycaemic response). GI values for locally obtained bread were no more variable than those for centrally provided foods.

CONCLUSIONS

The GI values of foods are more precisely determined using capillary than venous blood sampling, with mean between-laboratory s.d. of approximately 9.0. Finding ways to reduce within-subject variation of glycaemic responses may be the most effective strategy to improve the precision of measurement of GI values.

Glucagon-like peptide 1 and fatty acids amplify pulsatile insulin secretion from perifused rat islets

Glucose-induced insulin secretion from isolated, perifused rat islets is pulsatile with a period of about 5-10 min, similar to the insulin oscillations that are seen in healthy humans but which are impaired in Type II diabetes. We evaluated the pattern of enhancement by the potent incretin, glucagon-like peptide 1 (GLP-1). GLP-1 increased the amplitude of pulses and the magnitude of insulin secretion from the perifused islets, without affecting the average time interval between pulses. Forskolin and the phosphodiesterase inhibitor isobutylmethylxanthine had the same effect, suggesting that the effect was due to elevated cAMP levels. The possibility that cAMP might enhance the amplitude of pulses by reducing phosphofructo-2-kinase (PFK-2) activity was eliminated when the liver isoform of PFK-2 was shown to be absent from beta-cells. The possibility that cAMP enhanced pulsatile secretion, at least in part, by stimulating lipolysis was supported by the observations that added oleate had a similar effect on secretion, and that the incretin effect of GLP-1 was inhibited by the lipase inhibitor orlistat. These data show that the physiological incretin GLP-1 preserves and enhances normal pulsatile insulin secretion, which may be essential in proposed therapeutic uses of GLP-1 or its analogues.

Pancreatic beta-cell loss and preservation in type 2 diabetes

BACKGROUND

In most individuals, the need to respond to progressive states of insulin resistance is met by increasing insulin production. For insulin-resistant patients, however, the balance between insulin supply and demand may fail from the progressive loss of pancreatic beta-cell function, eventually leading to type 2 diabetes mellitus.

OBJECTIVE

The aim of this review was to discuss the current concepts underlying potential pancreatic beta-cell failure in the progression toward type 2 diabetes and therapies that may alter the process.

METHODS

Data included in this review were identified through a MEDLINE search for articles published from 1966 to April 2003. Search terms used were beta cell, diabetes, insulin resistance, obesity, cardiovascular disease, thiazolidinediones, and metformin.

RESULTS

Evidence of the progressive loss of beta-cell function may include altered conversion of proinsulin to insulin, changes in pulsed and oscillatory insulin secretion, and quantitative reductions in insulin release. Potential underlying mechanisms are glucose toxicity, lipotoxicity, poor tolerance of increased secretory demand, and a reduction in beta-cell mass.

CONCLUSION

Current clinical management of type 2 diabetes is focused on treatment of the signs and symptoms of late-stage disease rather than addressing potential underlying causes, which may be amenable to currently available therapies, based on a broad understanding of existing data, practice experience, and rational speculation.

Primary in vivo oscillations of metabolism in the pancreas

The role of metabolism in the generation of plasma insulin oscillations was investigated by simultaneous in vivo recordings of oxygen tension (pO(2)) in the endocrine and exocrine pancreas and portal blood insulin concentrations in the anesthetized rat. At the start of the experiment, the blood glucose concentration of seven rats was 6.2 +/- 0.1 mmol/l and the arterial blood pressure was 116 +/- 5 mmHg. These values did not differ from those obtained at the end of the experiment. Islet pO(2) was measured by impaling superficially located islets with a miniaturized Clark electrode. The pO(2) measurements revealed slow (0.21 +/- 0.03 min(-1)) with superimposed rapid (3.1 +/- 0.3 min(-1)) oscillations. The average pO(2) was 39 +/- 5 mmHg. Simultaneous recordings of pO(2) in the exocrine pancreas were significantly lower (16 +/- 6 mmHg), but showed a slow and a rapid oscillatory activity with similar frequencies as seen in the endocrine pancreas. Corresponding measurements of portal insulin concentrations revealed insulin oscillations at a frequency of 0.22 +/- 0.02 min(-1). The results are the first in vivo recordings of an oscillatory islet parameter with a frequency corresponding to that of plasma insulin oscillations; they support a primary role of metabolic oscillations in the induction of plasma insulin oscillations.

Pulsatile insulin secretion: detection, regulation, and role in diabetes

Insulin concentrations oscillate at a periodicity of 5-15 min per oscillation. These oscillations are due to coordinate insulin secretory bursts, from millions of islets. The generation of common secretory bursts requires strong within-islet and within-pancreas coordination to synchronize the secretory activity from the beta-cell population. The overall contribution of this pulsatile mechanism dominates and accounts for the majority of insulin release. This review discusses the methods involved in the detection and quantification of periodicities and individual secretory bursts. The mechanism by which overall insulin secretion is regulated through changes in the pulsatile component is discussed for nerves, metabolites, hormones, and drugs. The impaired pulsatile secretion of insulin in type 2 diabetes has resulted in much focus on the impact of the insulin delivery pattern on insulin action, and improved action from oscillatory insulin exposure is demonstrated on liver, muscle, and adipose tissues. Therefore, not only is the dominant regulation of insulin through changes in secretory burst mass and amplitude, but the changes may affect insulin action. Finally, the role of impaired pulsatile release in early type 2 diabetes suggests a predictive value of studies on insulin pulsatility in the development of this disease.

Control mechanisms of the oscillations of insulin secretion in vitro and in vivo

The mechanisms driving the pulsatility of insulin secretion in vivo and in vitro are still unclear. Because glucose metabolism and changes in cytosolic free Ca(2+) ([Ca(2+)](c)) in beta-cells play a key role in the control of insulin secretion, and because oscillations of these two factors have been observed in single isolated islets and beta-cells, pulsatile insulin secretion could theoretically result from [Ca(2+)](c) or metabolism oscillations. We could not detect metabolic oscillations independent from [Ca(2+)](c) changes in beta-cells, and imposed metabolic oscillations were poorly effective in inducing oscillations of secretion when [Ca(2+)](c) was kept stable, which suggests that metabolic oscillations are not the direct regulator of the oscillations of secretion. By contrast, tight temporal and quantitative correlations between the changes in [Ca(2+)](c) and insulin release strongly suggest that [Ca(2+)](c) oscillations are the direct drivers of insulin secretion oscillations. Metabolism may play a dual role, inducing [Ca(2+)](c) oscillations (via changes in ATP-sensitive K(+) channel activity and membrane potential) and amplifying the secretory response by increasing the efficiency of Ca(2+) on exocytosis. The mechanisms underlying the oscillations of insulin secretion by the isolated pancreas and those observed in vivo remain elusive. It is not known how the functioning of distinct islets is synchronized, and the possible role of intrapancreatic ganglia in this synchronization requires confirmation. That pulsatile insulin secretion is beneficial in vivo, by preventing insulin resistance, is suggested by the greater hypoglycemic effect of exogenous insulin when it is infused in a pulsatile rather than continuous manner. The observation that type 2 diabetic patients have impaired pulsatile insulin secretion has prompted the suggestion that such dysregulation contributes to the disease and justifies the efforts toward understanding of the mechanism underlying the pulsatility of insulin secretion both in vitro and in vivo.

Role of oscillations in membrane potential, cytoplasmic Ca2+, and metabolism for plasma insulin oscillations

A model for the relationship between ionic and metabolic oscillations and plasma insulin oscillations is presented. It is argued that the pancreatic beta-cell in vivo displays two intrinsic frequencies that are important for the regulation of plasma insulin oscillations. The rapid oscillatory activity (2--7 oscillations [osc] per minute), which is evident in both ionic and metabolic events, causes the required elevation in cytoplasmic Ca(2+) concentration ([Ca(2+)](i)) for the exocytosis of insulin granules. This activity is important for regulation of the amplitude of plasma insulin oscillations. The frequency of the rapid oscillatory ionic activities is regulated by glucose and allows the beta-cell to respond in an analogous way, with gradual changes in [Ca(2+)](i) and insulin release in response to the alterations in glucose concentration. The slower oscillatory activity (0.2--0.4 osc/min), which is evident in the metabolism of the beta-cell, has a frequency corresponding to the frequency observed in plasma insulin oscillations. The frequency is not affected by changes in the glucose concentration. This activity is suggested to generate energy in a pulsatile fashion, which sets the frequency of the plasma insulin oscillations. It is proposed that the slow oscillations in [Ca(2+)](i) observed in vitro are a manifestation of the metabolic oscillations and do not represent an in vivo phenomenon.

Repeatability characteristics of simple indices of insulin resistance: implications for research applications

The objectives of this study were to evaluate test characteristics, such as normality of distribution, variation, and repeatability, of simple fasting measures of insulin sensitivity and to use the results to choose among these measures. Duplicate fasting samples of insulin and glucose were collected before 4 h of euglycemic hyperinsulinemic clamping using insulin infusion rates ranging from 40-600 mU/m2 x min. Currently recommended estimates of insulin sensitivity, including the fasting insulin, 40/insulin, the homeostasis model assessment, the logarithmic transformation of the homeostasis model assessment, and the Quantitative Insulin Sensitivity Check Index, were evaluated. The normality of distribution and the variability of the tests (coefficient of variation and discriminant ratio) were compared between the measures and against the "gold standard" hyperinsulinemic clamp. Data from 253 clamp studies in 152 subjects were examined, including 79 repeated studies for repeatability analysis. In subjects ranging from lean to diabetic, the log transformed fasting measures combining insulin and glucose had normal distributions and test characteristics superior to the other simple indices (logarithmic transformation of the homeostasis model assessment coefficient of variation, 0.55; discriminant ratio, 13; Quantitative Insulin Sensitivity Check Index coefficient of variation, 0.05; discriminant ratio, 10) and statistically comparable to euglycemic hyperinsulinemic clamps (coefficient of variation, 0.10; discriminant ratio, 6.4). These favorable characteristics helped explain the superior correlations of these measures with the hyperinsulinemic clamps among insulin-resistant subjects. Furthermore, therapeutic changes in insulin sensitivity were as readily demonstrated with these simple measures as with the hyperinsulinemic clamp. The test characteristics of the logarithmic transformation of the homeostasis model assessment and the Quantitative Insulin Sensitivity Check Index are superior to other simple indices of insulin sensitivity. This helps explain their excellent correlations with formal measures both at baseline and with changes in insulin sensitivity and supports their broader application in clinical research.

The role of IGF-binding proteins in mediating the effects of recombinant human IGF-I on insulin requirements in type 1 diabetes mellitus

To determine the role of IGF-binding proteins in mediating the direct effects of recombinant human IGF-I on insulin requirements in type 1(insulin-dependent) diabetes mellitus, overnight changes in IGF-I, IGF-II, and IGF-binding protein-1, -2, and -3, collected under euglycemic conditions, were compared in nine subjects after double blind, randomized, sc administration of recombinant human IGF-I (40 microg/kg) or placebo at 1800 h. On both nights a somatostatin analog infusion (300 ng/kg x h) suppressed endogenous GH production, and three timed discrete GH pulses (total, 0.029 IU/kg x night) ensured identical GH levels. After recombinant human IGF-I administration, IGF-I levels and the IGF-I/IGF-binding protein-3 ratio increased [mean +/- SEM:IGF-I, 401 +/- 22 ng/ml; placebo, 256 +/- 20 ng/ml (P = 0.0002); IGF-I, 0.108 +/- 0.006; placebo, 0.074 +/- 0.004 (P = 0.0003), respectively], and insulin requirements decreased (IGF-I, 0.12 +/- 0.03; placebo, 0.23 +/- 0.03 U/kg x min; P = 0.008). The normal within-individual inverse relationships between insulin and IGF-binding protein-1 levels were observed (lag time 2 h: r = -0.34; P < 0.01). Yet despite reduced free insulin levels (8.5 +/- 1.5; placebo, 12.2 +/- 1.2 mU/liter; P = 0.03), IGF-binding protein-1 levels were reduced after recombinant human IGF-I administration (53.7 +/- 6.8; placebo, 82.2 +/- 11.8 ng/ml; P = 0.008). The largest reductions in free insulin levels after recombinant human IGF-I and thus putative improvement in insulin sensitivity occurred in subjects with the smallest increase in the plasma IGF-I/IGF-binding protein-3 ratio (r = 0.7; P = 0.03). Taken together, these data are consistent with the hypothesis that transcapillary movement of IGF-I (perhaps mediated by IGF-binding protein-1), out of the circulation facilitates altered insulin sensitivity. These data have important implications for risk-benefit assessment of recombinant human IGF-I therapy in type 1 diabetes mellitus.

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.

Insulin signalling through ultradian oscillations

Periodic oscillations appear to be a characteristic of insulin secretion at various different levels. Very rapid pulsations are seen in the isolated beta-cell and islet, while rapid (10- to 15-min) pulsations are seen both in the intact organism and in the isolated pancreas. Ultradian oscillations, particularly evident in situations of sustained exogenous glucose loading, appear to be a characteristic of intact organisms and have been hypothesized to be intrinsic to the normal glucose-insulin feedback system. Many of the features seen in experimental situations and in abnormalities of the system can be predicted by computer modelling of this system, supporting this hypothesis. A further theoretical feature of this hypothesis, borne out by experiment, is the ability to entrain insulin pulsatility by oscillations in an exogenous glucose infusion. Identification of defective ultradian oscillations and entrainment can identify subtle abnormalities of insulin sensitivity and pancreatic function, and restoration of normal function can be demonstrated after pharmaceutical intervention.

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.

Amplitude modulation of pulsatile insulin secretion by intrapancreatic ganglion neurons

Neuron activity and insulin release were measured simultaneously from 33 preparations of intrapancreatic canine ganglia and pancreatic parenchyma adjacent to the ganglia. The electrical activity of single neurons of the ganglia was recorded with intracellular microelectrodes, and insulin release from the attached islets was determined with an enzyme-linked immunosorbent assay. Insulin release was 62 +/- 18 fmol preparation/min in the presence of 10 mmol/l glucose and pulsatile (3.7 +/- 0.4 min/pulse). Corresponding measurements of neuronal electrical activity showed a stable membrane potential of -53.5 +/- 0.6 mV. Short, high-frequency (20 Hz) preganglionic nerve stimulation evoked action potentials and, in 46% of the preparations, a threefold rise in the insulin secretory rate associated with increased amplitude of the insulin pulses. The effects were blocked by 10 micromol/l tetrodotoxin (TTX). In other preparations, continuous low-frequency (0.05-0.5 Hz) preganglionic nerve stimulation evoked action potentials and, in 50% of the preparations, a gradual increase of insulin release associated with augmentation of insulin pulse amplitude without alteration of the duration. The effects were blocked by 50 micromol/l hexamethonium (HEX). In the remaining preparations, no change in insulin release was observed during nerve stimulation. In the absence of stimulation, neither TTX nor HEX affected the membrane potential or insulin secretion. These first simultaneous measurements of intrapancreatic ganglion activity and insulin secretion are consistent with amplitude modulation of pulsatile insulin secretion induced by changes in electrical activity in a population of intrapancreatic ganglion neurons.

Pathophysiology of impaired pulsatile insulin release

Plasma insulin displays 5-10 min oscillations. In Type 2 diabetes the regularity of the oscillations disappears, which may lead to insulin receptor down-regulation and glucose intolerance and explain why pulsatile delivery of the hormone has a greater hypoglycemic effect than continuous delivery. The rhythm is intrinsic to the islet. Variations in metabolism, cytoplasmic Ca(2+) concentration ([Ca(2+)](i)), other hormones, neuronal signaling and possibly beta-cell insulin receptor expression have been implicated in the regulation of plasma insulin oscillations. Most of these factors are important for amplitude-regulation of the insulin pulses. Although evidence exists supporting a role of both metabolism and [Ca(2+)](i) as pacemakers of the pulses, metabolic oscillations probably have a primary role and [Ca(2+)](i) oscillations a permissive role. Results from islets from animal models of diabetes suggest that altered plasma insulin pattern could be due to lowering of pulse amplitude of insulin oscillations rather than alterations in their frequency. Supporting a role of metabolism, altered plasma insulin oscillations were found in MODY2, MIDD and glycogenosis Type VII, which are linked to alterations in glucokinase, mitochondrial tRNALeu(UUR) and phosphofructokinase. Plasma insulin oscillations require coordination of islet secretory activities in the pancreas. The intrapancreatic ganglia have been suggested as coordinators. The diabetes-associated neuropathy may contribute to the deranged pattern as indicated by glucose intolerance in chagasic patients. Continued investigation of the role and regulation of pulsatile insulin release will lead to better understanding of the pathophysiology of impaired pulsatile insulin release, which could lead to new approaches to restore normal plasma insulin oscillations in diabetes and related diseases.

Oxygen microsensor and its application to single cells and mouse pancreatic islets

An oxygen microsensor with a < 3-micron tip diameter was developed for monitoring oxygen levels at single cells and mouse pancreatic islets. The sensor was fabricated by electrochemically recessing an etched Pt wire inside a pulled glass micropipet and then coating with cellulose acetate. This fabrication process was found to be simpler than previous oxygen electrode designs of comparable size. The microsensors had a average sensitivity of 0.59 +/- 0.29 pA/mmHg (mean +/- SD, n = 42), signals that were minimally perturbed by convection, and response times of < 1 s. The electrode was used to measure the oxygen gradient around and inside single mouse islets. The measurements demonstrate that oxygen levels within even the largest islets at maximal glucose stimulation are 67 +/- 1.6 mmHg (mean +/- SD, n = 5), indicating that islets have adequate oxygen supplies by diffusion under tissue culture conditions to support insulin secretion. The electrode was also used to record the dynamics of oxygen level at single islets as a function of glucose concentration. As glucose level was changed from 3 to 10 mM, oxygen level decreased by 15.8 +/- 2.3 mmHg (mean +/- SEM, n = 6) and oscillations with a period of 3.3 +/- 0.6 min (mean +/- SEM, n = 6) appeared in the oxygen level. In islets bathed in quiescent solutions containing 10 mM glucose, similar oscillations could be observed. In addition, in the quiet solutions it was possible to detect faster oscillations with a period of 12.1 +/- 1.7 s (mean +/- SEM, n = 6) superimposed on the slower oscillations. Oxygen consumption could also be observed at single insulinoma cells using the electrode. Individual cells also showed oscillations in oxygen consumption with a period of a few seconds. The results demonstrate that the electrode can be used for dynamic oxygen level recordings in biological microenvironments.

Pharmacodynamic aspects of sustained release preparations

The sustained release (SR) mode of drug administration has certain features that have an important impact on the magnitude of the pharmacologic response: (a) it minimizes fluctuation in blood drug concentrations (i.e. between peak and trough). However, due to the pronounced non-linear relationship between drug concentration and pharmacologic effect (i.e. pharmacodynamics) the impact of this property differs considerably as a function of the shape of the pharmacodynamic profile and the position of the specific range of concentrations on the curve of this profile; (b) it produces a slow input rate which tends to minimize the body's counteraction to the drug's intervening effect on regulated physiological processes; and (c) it provides a continuous mode of drug administration. This important pharmacodynamic characteristic may produce, in certain cases, an opposite clinical effect than that attained by an intermittent (pulsatile) mode of administration of the same drug. For many drugs with non-concentration-dependent pharmacodynamics, the exposure time, rather than the AUC, is the relevant parameter and it can therefore be optimized by SR preparations. The slow input function may minimize hysteresis in cases where the site of action is not in a rapid equilibrium with the blood circulation. The pharmacodynamics of the desired effect(s) and/or adverse effect(s) may also be influenced by the site of administration, especially in cases where the drug is delivered directly to its site of action. These factors demonstrate the important influence of the mode of administration on the pharmacological and clinical outcomes. In addition, they highlight the need to include these pharmacodynamic considerations in all stages from drug development to the optimization of their clinical use.

Basal insulin-level oscillations in normotensive individuals with genetic predisposition to essential hypertension exhibit an irregular pattern

BACKGROUND

Insulin is secreted in regular pulses at intervals of 12-14 min in normal fasting subjects. An abnormal pattern has been found in subjects with non-insulin-dependent diabetes mellitus (NIDDM) and in young individuals predisposed to NIDDM. It has been suggested that there might be a causal relationship between insulin-secretion abnormalities and insulin resistance.

OBJECTIVE

To examine whether insulin-secretion abnormalities are also present in offspring of patients with essential hypertension.

METHODS

Eleven young (aged 18-35 years) normotensive individuals each of whom had two parents with essential hypertension were compared with 10 age- and sex-matched controls each of whom had two normotensive parents. We verified that diabetes and morbid obesity were absent among the subjects and their parents. We studied basal insulin-secretion patterns during a 60 min period, glucose tolerance by administering an oral glucose-tolerance test, insulin resistance by using an isoglycaemic hyperinsulinaemic clamp and basal plasma catecholamine levels.

RESULTS

Autocorrelation analysis of insulin concentrations showed that the hypertension-prone subjects had a significantly reduced or irregular oscillatory pattern compared with the regular insulin-level oscillations with a period of 12-14 min in control subjects. The hypertension-prone subjects had significantly higher systolic blood pressures and tended to be insulin-resistant.

CONCLUSION

This is the first evidence of early insulin-secretion abnormalities in young normotensive individuals with a genetic predisposition to essential hypertension, but with a normal glucose tolerance and without a genetic predisposition to NIDDM. Early insulin-secretion abnormalities may be the very first step towards the development of insulin resistance and an important factor initiating the hypertension in hypertension-prone individuals.

Relation of functional ovarian hyperandrogenism to non-insulin dependent diabetes mellitus

Up to 40% of women with polycystic ovary syndrome (PCOS) demonstrate some degree of glucose intolerance, either impaired glucose tolerance (IGT) or non-insulin dependent diabetes mellitus (NIDDM). Defects in insulin action have long-been recognized as characteristic in these women. Recently, evidence has been obtained which documents that insulin secretory dysfunction also contributes significantly to the observed glucose intolerance. This chapter will focus on the recent evidence supporting the specific roles of disordered insulin secretion and action, in the development of glucose intolerance in PCOS. In addition, the use of pharmacological agents that modify insulin action as therapeutic options for women with PCOS, will be discussed.

Influence of glucose concentration on the inhibitory effect of amylin on insulin secretion. Study in the perfused rat pancreas

The effect of amylin on insulin secretion is a matter of controversy. Short-term experiments have shown that amylin, at 75 pmol/l, inhibits the insulin release elicited by a modest increase in the perfusate glucose concentration (from 5.5 mmol/l to 9 mmol/l). The present work was undertaken to further investigate the effect of amylin on glucose-induced insulin release at different glucose concentrations. The study was performed in the isolated perfused rat pancreas. Amylin, at 75 pmol/l, markedly blocked the insulin response when the perfusate glucose concentration was increased from 3.2 mmol/l to 7 mmol/l (by 90%; P < 0.01) or from 5.5 mmol/l to 9 mmol/l (by 80%; P < 0.01). At the same amylin concentration, no significant inhibition of insulin output was observed when the perfusate glucose level was augmented from 5.5 mmol/l to 16.6 mmol/l, from 7 mmol/l to 11 mmol/l or from 9 mmol/l to 13 mmol/l. At a higher concentration (750 pmol/l), amylin also failed to inhibit the insulin response induced by increasing glucose levels from 5.5 mmol/l to 16.6 mmol/l or from 9 to 13 mmol/l. These findings indicate that, in the rat pancreas, amylin only inhibits insulin release when evoked by elevations of glucose levels comparable to those occurring in normal subjects under physiological conditions.

Delayed endocrine pancreas graft function after simultaneous pancreas-kidney transplantation. Incidence, risk factors, and impact on long-term outcome

BACKGROUND

The incidence of delayed endocrine pancreas graft function and its impact on long-term outcome after simultaneous pancreas-kidney transplantation are unknown.

METHODS

We studied 54 technically successful adult type I insulin-dependent diabetic recipients of cadaver, whole organ, bladder-drained simultaneous pancreas-kidney transplants (mean age, 37.6 years; 65% male, 35% female; 9% pancreas retransplants; 63% on chronic pretransplant dialysis; mean duration of diabetes, 25.1 years). Insulin was administered during the first 2 weeks after transplantation, as needed, to keep blood glucose < 150 mg/dl. Delayed endocrine pancreas graft function was defined as total, cumulative insulin requirement of > 30 U between day 5 and day 10, and/or > 15 U between day 11 and 15. Quadruple immunosuppression was used for all recipients.

RESULTS

The incidence of delayed endocrine pancreas graft function was 69%. By univariate analysis, delayed endocrine graft function was associated with pretransplant recipient weight > 80 kg (P = 0.04), donor age > 45 years (P = 0.02), and cardiocerebrovascular (P = 0.06) and nontraumatic causes of donor death (P = 0.02). The incidence of acute pancreas rejection episodes was similar for recipients without and with delayed endocrine pancreas graft function. Pancreas graft survival at 1 and 3 years was 94% and 82% without versus 76% and 59% with delayed endocrine graft function (P = 0.03).

CONCLUSIONS

Increased pancreas graft failure after delayed endocrine function was a consequence of insufficient functional reserve (e.g., older donors) rather than increased immunogenicity. Pretransplant reduction of recipient weight and careful donor selection are therefore crucial in order to decrease the incidence of delayed endocrine pancreas graft function and its negative impact on long-term outcome.

Pulsatile intravenous insulin replacement in streptozotocin diabetic rats is more efficient than continuous delivery: effects on glycaemic control, insulin-mediated glucose metabolism and lipolysis

Short-term exposure of tissues to pulses of insulin generally leads to an enhancement of insulin action. We have investigated the possible beneficial effects of long-term near-physiological continuous vs pulsatile intravenous insulin treatment of insulin-deficient streptozotocin (70 mg/kg) diabetic rats on blood glucose control, in vivo insulin action and in vitro insulin action in isolated adipocytes. First, we determined the 24-h peripheral plasma insulin profiles in normal rats under precisely controlled mealfeeding conditions. Basal plasma insulin levels (40 +/- 9 microU/ml) oscillate with a periodicity of 11.9 +/- 0.9 min (p < 0.05), and an amplitude of 60 +/- 10%. Subsequently, the 24-h insulin profile was mimicked in diabetic (D) rats by a continuous (c) or pulsatile (p) (6-min double, 6-min off) insulin infusion rate for 2 weeks, using a programmable pumpswivel unit. Control (C) rats received vehicle treatment. In Cc, Dc, Cp and Dp daily urinary glucose loss and average plasma glucose levels were 0 +/- 0, 7.5 +/- 4.4, 0 +/- 0, 0.8 +/- 0.4 mmol and 6.7 +/- 0.2, 11.5 +/- 2.7, 6.6 +/- 0.1, 5.9 +/- 1.4 mmol/l, respectively. Hypoglycaemia (< 3 mmol/l) was observed in 10 and 20% of the blood samples collected from Dc and Dp rats, respectively. After 2 weeks of treatment, in vivo peripheral and hepatic insulin action was measured by the hyperinsulinaemic euglycaemic (6 mmol/l) clamp with [3-3H]-glucose infusion. Pre-clamp counter-regulatory hormone levels were similar among rats. Compared to Cc and Cp, Dc showed a reduction in insulin sensitivity and responsiveness for peripheral glucose uptake whereas Dp only showed a reduction in insulin sensitivity. Suppression of hepatic glucose production by insulin was similar among rats. After 2.5 weeks of treatment, epididymal adipocytes were isolated. Specific [125I]-insulin binding, basal and insulin-stimulated [U-14C]-glucose uptake and isoproterenol-stimulated glycerol output were comparable among rat adipocytes. The inhibition of glycerol output by insulin was identical in Cp and Dp (V(max) = 48.6 +/- 6.1 and 42.3 +/- 4.6%) but blunted in Dc vs Cc (V(max) = 8.2 +/- 4.6 vs 44.0 +/- 7.2%, p < 0.01) adipocytes, suggesting a post-binding defect in the antilipolytic action of insulin in Dc rats. In conclusion, long-term near-physiological pulsatile intravenous insulin replacement in insulin-deficient diabetic rats is more efficient than continuous delivery in reducing blood glucose, lowering glucosuria, increasing insulin sensitivity and inhibiting lipolysis.

Temporal patterns of changes in ATP/ADP ratio, glucose 6-phosphate and cytoplasmic free Ca2+ in glucose-stimulated pancreatic beta-cells

Closure of ATP-sensitive K+ (K(ATP)) channels is part of the stimulus-secretion coupling mechanism in the pancreatic beta-cell, leading to membrane depolarization and influx of Ca2+ through voltage-sensitive L-type Ca2+ channels. The elevated ATP/ADP ratio seen in the presence of high levels of glucose has been postulated to mediate the glucose-induced closure of the K(ATP) channels and rise in cytoplasmic free Ca2+ concentration ([Ca2+]i), or alternatively to be a consequence of activation of mitochondrial dehydrogenases by the increase in [Ca2+]i. To distinguish between these two possibilities, the time course of the change in the ATP/ADP ratio was determined in comparison with that of [Ca2+]i. We here show that a severalfold rise in the ATP/ADP ratio occurs rapidly on stimulation of suspensions of mouse pancreatic beta-cells with glucose. The change in the ATP/ADP ratio is an early event that begins within 20-40 s and precedes the rise in [Ca2+]i. The temporal relationship indicates that the adenine nucleotide changes cannot be a consequence of the [Ca2+]i changes and may indeed be the connecting link between glucose metabolism and [Ca2+]i changes. When the cells were sequentially treated with high glucose concentration, clonidine and finally high extracellular Ca2+ concentration to induce synchronized oscillations in [Ca2+]i in the cell suspension, corresponding oscillations in the ATP/ADP ratio were observed. Glucose 6-phosphate levels oscillated out of phase with the ATP/ADP ratio. These results support the hypothesis that the Ca2+ oscillations previously observed in glucose-stimulated single islets or beta-cells may reflect oscillations in the ATP/ADP ratio that accompany oscillatory glycolysis.

The effect of high-molecular-weight guar gum on net apparent glucose absorption and net apparent insulin and gastric inhibitory polypeptide production in the growing pig: relationship to rheological changes in jejunal digesta

The present study was designed to determine the quantitative effects of starchy meals containing guar gum on rates of net apparent glucose absorption and net apparent insulin and gastric inhibitory polypeptide (GIP) production in growing pigs. The effects of these meals on the viscosity of jejunal digesta were also examined and correlated to changes in glucose absorption. Four growing pigs were each given either a low-fat semi-purified diet (control) or the same diet supplemented with a high-molecular-weight guar gum at concentrations in the diet of 20 or 40 g/kg. Blood samples were removed simultaneously via indwelling catheters from the mesenteric artery and the hepatic portal vein. Samples of jejunal digesta were removed via a T-piece cannula and used immediately for viscosity measurements at 39 degrees. The 'zero-shear' viscosity of each sample was then calculated. Blood-flow measurements were made using an ultrasonic flow probe fitted to the hepatic portal vein. All measurements were made at intervals of 10 or 30 min during a 4 h postprandial period. Meals containing guar gum significantly increased (P < 0.05) the viscosity of jejunal digesta, an effect that was strongly dependent on the concentration of guar gum in the original diet. No significant differences in blood-flow rates were found between the control and guar-containing diets. Both concentrations of guar gum significantly reduced (P < 0.05) glucose absorption and insulin and GIP secretion rates over the 4 h postprandial period. An inverse relationship between the rate of glucose absorption and the 'zero-shear' viscosity of jejunal digesta was found. This study also provides direct evidence for the important role played by the enteroinsular axis in modifying the glycaemic response to a meal containing guar gum.

24-hour glucose profiles during continuous or oscillatory insulin infusion. Demonstration of the functional significance of ultradian insulin oscillations

Under basal and stimulated conditions, normal insulin secretion oscillates with periods in the ultradian 100-150-min range. To test the hypothesis that oscillatory insulin delivery is more efficient in reducing blood glucose levels than continuous administration, nine normal young men were each studied on two occasions during a 28-h period including a period of polygraphically recorded sleep. Endogenous insulin secretion was suppressed by somatostatin, a constant intravenous glucose infusion was administered, and exogenous insulin was infused either at a constant rate or in a sinusoidal pattern with a period of 120 min. The mean glucose level over the 28-h period was 0.72 +/- 0.31 mmol/liter lower when insulin was infused in an oscillatory pattern than when the rate of infusion was constant (P < 0.05). The greater hypoglycemic effect of oscillatory versus constant infusion was particularly marked during the daytime, with the difference averaging 1.04 +/- 0.38 mmol/liter (P < 0.03). Serum insulin levels tended to be lower during oscillatory than constant infusion, although the same amount of exogenous insulin was administered under both conditions. Ultradian insulin oscillations appear to promote more efficient glucose utilization.

Valid parameters for investigation of the pupillary light reflex in normal and diabetic subjects shown by factor analysis and partial correlation

Pupillary test data of 103 normal and 119 diabetic subjects (47 IDDM, 72 NIDDM) were evaluated by factor analysis. From a total of nine pupillary parameters three factors were extracted in the analysis. Factor 1 represents maximal pupillary area, contraction velocity at 1 s, dilation velocity at 6 s and minimal pupillary area--static and simple dynamic parameters; factor 2 amplitude of pupillary unrest, area under the detrended curve of pupillary unrest and period of pupillary unrest--parameters of pupillary unrest; factor 3 fusion frequency of pupillary response following flicker stimuli and latency time of pupillary light reflex--second order dynamic parameters. Factor analysis was then applied to investigate diabetic patients with a high percentage of autonomic neuropathic participants (about 39% had pupillary and about 35% had cardiorespiratory function disorders), which revealed the same three factors as those identified in normal subjects. Furthermore, an age-related database of parameters of pupillary unrest is given. It demonstrates that normal subjects and diabetic patients did not differ in the period of pupillary unrest (normal vs diabetic (mean +/- SEM): 1550 +/- 29 vs 1536 +/- 27 ms; 2p > 0.5). The difference in amplitude (47.8 +/- 2.8 vs 41.0 +/- 2.6% percentile; 2p = 0.071) and area under the detrended curve of pupillary unrest (47.9 +/- 2.8 vs 40.8 +/- 2.6% percentile, 2p = 0.062) seems to show a trend but was not significant. In conclusion, factor analysis revealed three different pupillary test factors.(ABSTRACT TRUNCATED AT 250 WORDS)

Lack of control by glucose of ultradian insulin secretory oscillations in impaired glucose tolerance and in non-insulin-dependent diabetes mellitus

Normal subjects demonstrate the presence of ultradian oscillations (period 80-150 min) in insulin secretion rate (ISR) tightly coupled to glucose oscillations of similar period. These oscillations appear to be a function of the feedback loop linking glucose and insulin. The present study was undertaken to determine whether the control by glucose of the ultradian oscillations in insulin secretion is altered in impaired glucose tolerance IGT and in non-insulin-dependent diabetes mellitus (NIDDM). Patients with NIDDM (n = 7), IGT (n = 4), and matched nondiabetic controls (n = 5) were studied under three separate protocols that involved administration of glucose at either a constant rate of 6 mg/kg per min for 28 h or in one of two oscillatory patterns at the same overall mean rate. The amplitude of the oscillations was 33% above and below the mean infusion rate, and their respective periods were 144 min (slow oscillatory infusion) or 96 min (rapid oscillatory infusion). Insulin, C-peptide, and glucose were sampled at 10-min intervals during the last 24 h of each study. ISRs were calculated by deconvolution of C-peptide levels. Analysis of the data showed that (a) the tight temporal coupling between glucose and ISR in the nondiabetic controls was impaired in the IGT and NIDDM groups as demonstrated by pulse analysis, cross-correlation analysis, and spectral analysis; (b) the absolute amplitude of the ISR pulses progressively declined with the transition from obesity to IGT to NIDDM; and (c) the absolute amplitude of the ISR oscillations failed to increase appropriately with increasing absolute amplitude of glucose oscillations in the IGT and NIDDM subjects compared with the control group. In conclusion, the present study demonstrates that important dynamic properties of the feedback loop linking insulin secretion and glucose are disrupted not only in established NIDDM but also in conditions where glucose tolerance is only minimally impaired. Further studies are needed to determine how early in the course of beta-cell dysfunction this lack of control by glucose of the ultradian oscillations in insulin secretion occurs and to define more precisely if this phenomenon plays a pathogenetic role in the onset of hyperglycemia in genetically susceptible individuals.

Periodicity of insulin secretion comprises multiple cycles with different duration in perfused rat islets

Insulin secretion from pancreatic islets has been found to be periodic by in vivo and in vitro experiments. The pacemaker which regulates the periodicity may be localized in the central nervous system or in the pancreas, though the precise location and the mechanisms of generating pacing have not been determined. In order to solve these problems, we examined the period of secretory cycles of insulin in isolated islets using a prolonged perfusion system, and investigated the effects of glucose and other agents on these periods. Isolated islets from male Wistar rats were enclosed in a millipore holder and were perfused with MEM containing 1 mg/ml glucose at a flow rate of 0.3 ml/min for 240 min. The effluent was collected at 1-min intervals to measure insulin secretion. The results were analyzed by the maximum entropy method to demonstrate the periodicity of insulin secretion. When islets were perfused with 1 mg/ml glucose, the periodicity comprised five cycles with different duration: 71.5 +/- 14.6 min, 29.8 +/- 3.4 min, 19.2 +/- 1.5 min, 11.6 +/- 2.1 min and 4.3 +/- 0.4 min. This indicates the presence of a pacemaker within the islets, although, in vivo, participation of a higher center to control periodicity has to be taken into account. Further, the presence of a long cycle (71.5 +/- 14.6 min) of insulin secretion which previously has only been observed in vivo was first demonstrated in this in vitro study. The cycles were consistent even in islets which were desensitized to glucose by cultivating in a high glucose medium for 5 days before perfusion.(ABSTRACT TRUNCATED AT 250 WORDS)

Loss of regular oscillatory insulin secretion in islet cell antibody positive non-diabetic subjects

Basal insulin secretion was compared in nine islet-cell antibody positive, non-diabetic first-degree relatives of children with Type 1 (insulin-dependent) diabetes mellitus and nine normal control subjects matched for age, sex and weight. Acute insulin responses to a 25 g intravenous glucose tolerance test were similar in the two groups (243 (198-229) vs 329 (285-380) mU.l-1 x 10 min-1, mean (+/- SE), p = 0.25). Fasting plasma insulin was assayed in venous samples taken at one min intervals for 2 h. Time series analysis was used to demonstrate oscillatory patterns in plasma insulin. Autocorrelation showed that regular oscillatory activity was generally absent in the islet-cell antibody-positive group, whereas a regular 13 min cycle was shown in control subjects (p less than 0.0001). Fourier transformation did, however, show a 13 min spectral peak in the islet-cell antibody positive group, consistent with intermittent pulsatility. We conclude that overall oscillatory patterns of basal insulin secretion are altered in islet-cell antibody positive subjects even when the acute insulin response is within the normal range.

Pattern of secretion of bioactive and immunoreactive gonadotrophins in normal pubertal children

OBJECTIVE

The aim was to investigate the relationship between the nocturnal pulsatile secretory patterns of immunoreactive and bioactive luteinizing hormone in normal children at various stages of puberty.

DESIGN

Blood samples were taken at 15-minute intervals from 2000 hours to 0800 hours. Pubertal stage was assessed by the method of Tanner (1962).

PATIENTS

Thirty-four healthy siblings (17 males, 17 females) of diabetic children were recruited (median age 13.1, range 9.1-20.9 years). They were of normal height, non-obese, and covered the range of puberty.

MEASUREMENTS

Follicle stimulating and luteinizing hormone levels were measured by radioimmunoassay in all 34 subjects; bioactive LH (B-LH) was assayed in a subgroup of 13 subjects selected to encompass the range of normal puberty. Oestradiol (girls) and testosterone (boys) were also measured at hourly intervals.

RESULTS

Immunoreactive luteinizing and follicle stimulating hormone concentrations showed a progressive rise during puberty in both sexes. FSH concentrations were significantly higher in females than in males at all stages of puberty. Overnight mean bioactive luteinizing hormone concentrations were higher than immunoreactive luteinizing hormone levels in all the girls studied (n = 7). Although the number of bioactive luteinizing hormone pulses (31) was greater than immunoreactive pulses (27), the profiles were generally very similar. In the early pubertal girls an increase in the bioactive: immunoreactive ratio was observed during the middle of the night with the onset of pulsatility. Oestrogen was detected in the girls in breast stage 4-5 but not in two of the early pubertal girls, despite pulses of immunoreactive and bioactive luteinizing hormone. The boys had higher mean bioactive than immunoreactive luteinizing hormone levels and overall bioactive and immunoreactive luteinizing hormone and testosterone concentrations increased with puberty stage. Concordance between bioactive and immunoreactive hormone pulses was good although more immunoreactive pulses (16) were seen than bioactive pulses (14). As in the girls, an increase in the bioactive: immunoreactive ratio was observed in the middle of the night with the onset of pulsatility at genital stage 2 but, in contrast to the oestrogen data in the girls, testosterone secretion always followed luteinizing hormone pulsatility overnight.

CONCLUSION

We conclude that mean overnight immunoreactive luteinizing and follicle stimulating hormone concentrations increase during puberty in both sexes. Bioactive luteinizing hormone levels are two to three times higher than immunoreactive luteinizing hormone in both sexes, but there is very little discordance between immunoreactive and bioactive luteinizing hormone pulsatility. The bioactive: immunoreactive ratio increases with the occurrence of pulsatility overnight in early pubertal children. The relationship between these changes in bioactive and immunoreactive luteinizing hormone and sex steroids is clearest in boys where the nocturnal testosterone rise always follows pulsatile LH secretion.

Entrainment of pulsatile insulin secretion by oscillatory glucose infusion

Ultradian "oscillations" or "pulses" of insulin secretion with periods around 120 min occur in man. It is not known whether glucose plays an active role in generating these oscillations, or if an intrapancreatic pacemaker generates oscillations in insulin secretion that entrain glucose passively. To determine if the frequency of pulses of insulin secretion could be modified by oscillatory glucose infusion, seven normal men were studied on three separate occasions. The first study involved a constant glucose infusion administered at a rate of 6 mg/kg per min for 28 h. During the two subsequent studies, the subjects received an oscillatory glucose infusion for 28 h with the same mean rate, an amplitude of 33% above and below the mean infusion rate, a sinusoidal waveshape and a period either 20% longer ("slow oscillatory infusion") or 20% shorter ("rapid oscillatory infusion") than the periodicity observed during constant glucose infusion. Samples for insulin, C-peptide, and glucose were drawn at 10-min intervals during the last 24 h of each study. Insulin secretion rates were calculated by deconvolution of C-peptide levels. During constant glucose infusion, the respective periods of oscillation of glucose and insulin secretion averaged 126 +/- 5 min and 118 +/- 3 min (mean +/- SEM). During the slow oscillatory infusion, the period of infusion was 155 +/- 7 min and the periods of insulin secretion and glucose were, respectively, 155 +/- 7 min and 150 +/- 5 min. During rapid oscillatory infusion, the period of infusion was 103 +/- 5 min and the period of both insulin secretion and glucose was 105 +/- 5 min. Thus the periodicity of both insulin secretion and plasma glucose changed in parallel with the exogenous periodicity, indicating complete entrainment of the secretory oscillations. These results suggest that the ultradian oscillations of insulin secretion are caused by the feedback loop linking glucose and insulin.