Advantageous metabolic effects of pulsatile insulin delivery in noninsulin-dependent diabetic patients

Similarities in Calcium Oscillations Between Neonatal Mouse Islets and Mature Islets Exposed to Chronic Hyperglycemia

Pulsatility is important to islet function. As islets mature into fully developed insulin-secreting micro-organs, their ability to produce oscillatory intracellular calcium ([Ca2+]i) patterns in response to glucose also matures. In this study, we measured [Ca2+]i using fluorescence imaging to characterize oscillations from neonatal mice on postnatal (PN) days 0, 4, and 12 in comparison to adult islets. Under substimulatory (3-mM) glucose levels, [Ca2+]i was low and quiescent for adult islets as expected, as well as for PN day 12 islets. In contrast, one-third of islets on PN day 0 and 4 displayed robust [Ca2+]i oscillations in low glucose. In stimulatory glucose (11 mM) conditions, oscillations were present on all neonatal days but differed from patterns in adults. By PN day 12, [Ca2+]i oscillations were approaching characteristics of fully developed islets. The immature response pattern of neonatal islets was due, at least in part, to differences in adenosine 5'-triphosphate (ATP)-sensitive K+-channel activity estimated by [Ca2+]i responses to KATP channel agents diazoxide and tolbutamide. Neonatal [Ca2+]i patterns were also strikingly similar to patterns observed in mature islets exposed to hyperglycemic conditions (20 mM glucose for 48 hours): elevated [Ca2+]i and oscillations in low glucose along with reduced pulse mass in high glucose. Since a hallmark of diabetic islets is dedifferentiation, we propose that diabetic islets display features of "reverse maturation," demonstrating similar [Ca2+]i dynamics as neonatal islets. Pulsatility is thus an important emergent feature of neonatal islets. Our findings may provide insight into reversing β-cell dedifferentiation and to producing better functioning β cells from pluripotent stem cells.

Efficacy Evaluation Study for Microburst Insulin Infusion: A Novel Model of Care

Objectives: This study aims to evaluate the impact of Microburst Insulin Infusion (MII) treatment on Type 1 and 2 diabetic patients' HbA1c, lipids, peripheral neuropathy, and patient-reported health status.

Methods: We reviewed clinical charts, including lab results, for more than 80 diabetic and pre-diabetic patients treated at one U.S. outpatient clinic in St. Louis, Missouri between February 2017 and December 2019. Data included patient demographics, treatment data, lab and neuropathy tests, and self-reported patient health status questions.

The explanatory variable was number of months of MII treatment. Treatments are 3–4 h in length, with two intensive infusions the first week and one treatment each week thereafter, usually for 12 weeks total. Lab tests were at 12-week intervals.

Generalized linear modeling and t-tests assessed the significance of differences between patients' baseline lab values, neuropathy measures, and health status before treatment vs. after final treatment.

Results: Number of MII treatments per patient ranged from 1 to 262, over 1–24 months. Time in MII treatment was significantly associated with reductions in HbA1c by nearly 0.04 points per month, and triglycerides declined 3 points per month. Neuropathy measures of large toe vibratory sensation (clanging tuning fork) improved significantly, as did patient-reported health and feelings of improvement since beginning treatment.

Discussion: The MII therapy appears to be efficacious in treating diabetic patients, particularly those with complications like neuropathy. Our findings affirmed several other studies. We uniquely incorporated patient health questionnaires, and empirically studied MII treatment efficacy for diabetes in a population large enough to permit statistically valid inferences. With multiple waves of data for over 80 patients, this is one of the most extensive quantitative studies of microburst insulin infusion therapy conducted to date, with protocols more uniformly implemented and survey instruments more consistently administered by the same clinical team. Given the advances in insulin infusion therapy brought by MII, and early indications of its efficacy, the time is right for more in-depth studies of the outcomes patients can achieve, the physiological mechanisms by which they occur, MII's comparative effectiveness vis-à-vis traditional treatments, and cost-effectiveness.

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.

Reducing Glucokinase Activity Restores Endogenous Pulsatility and Enhances Insulin Secretion in Islets From db/db Mice

An early sign of islet failure in type 2 diabetes (T2D) is the loss of normal patterns of pulsatile insulin release. Disruptions in pulsatility are associated with a left shift in glucose sensing that can cause excessive insulin release in low glucose (relative hyperinsulinemia, a hallmark of early T2D) and β-cell exhaustion, leading to inadequate insulin release during hyperglycemia. Our hypothesis was that reducing excessive glucokinase activity in diabetic islets would improve their function. Isolated mouse islets were exposed to glucose and varying concentrations of the glucokinase inhibitor d-mannoheptulose (MH) to examine changes in intracellular calcium ([Ca2+]i) and insulin secretion. Acutely exposing islets from control CD-1 mice to MH in high glucose (20 mM) dose dependently reduced the size of [Ca2+]i oscillations detected by fura-2 acetoxymethyl. Glucokinase activation in low glucose (3 mM) had the opposite effect. We then treated islets from male and female db/db mice (age, 4 to 8 weeks) and heterozygous controls overnight with 0 to 10 mM MH to determine that 1 mM MH produced optimal oscillations. We then used 1 mM MH overnight to measure [Ca2+]i and insulin simultaneously in db/db islets. MH restored oscillations and increased insulin secretion. Insulin secretion rates correlated with MH-induced increases in amplitude of [Ca2+]i oscillations (R2 = 0.57, P < 0.01, n = 10) but not with mean [Ca2+]i levels in islets (R2 = 0.05, not significant). Our findings show that correcting glucose sensing can restore proper pulsatility to diabetic islets and improved pulsatility correlates with enhanced insulin secretion.

In Vivo Decoding Mechanisms of the Temporal Patterns of Blood Insulin by the Insulin-AKT Pathway in the Liver

Cells respond to various extracellular stimuli through a limited number of signaling pathways. One strategy to process such stimuli is to code the information into the temporal patterns of molecules. Although we showed that insulin selectively regulated molecules depending on its temporal patterns using Fao cells, the in vivo mechanism remains unknown. Here, we show how the insulin-AKT pathway processes the information encoded into the temporal patterns of blood insulin. We performed hyperinsulinemic-euglycemic clamp experiments and found that, in the liver, all temporal patterns of insulin are encoded into the insulin receptor, and downstream molecules selectively decode them through AKT. S6K selectively decodes the additional secretion information. G6Pase interprets the basal secretion information through FoxO1, while GSK3β decodes all secretion pattern information. Mathematical modeling revealed the mechanism via differences in network structures and from sensitivity and time constants. Given that almost all hormones exhibit distinct temporal patterns, temporal coding may be a general principle of system homeostasis by hormones.

Episodic hormone secretion: a comparison of the basis of pulsatile secretion of insulin and GnRH

Rhythms govern many endocrine functions. Examples of such rhythmic systems include the insulin-secreting pancreatic beta-cell, which regulates blood glucose, and the gonadotropin-releasing hormone (GnRH) neuron, which governs reproductive function. Although serving very different functions within the body, these cell types share many important features. Both GnRH neurons and beta-cells, for instance, are hypothesized to generate at least two rhythms endogenously: (1) a burst firing electrical rhythm and (2) a slower rhythm involving metabolic or other intracellular processes. This review discusses the importance of hormone rhythms to both physiology and disease and compares and contrasts the rhythms generated by each system.

Essential elements of the native glucoregulatory system, which, if appreciated, may help improve the function of glucose controllers in the intensive care unit setting

In 2001, Van den Berghe and colleagues were able to show that tight glucose control decreases morbidity and mortality rates in the intensive care unit (ICU) setting. Several large, prospective, randomized controlled trials have failed to confirm these results. All of these studies attempted tight glucose control using expert-designed algorithms to adjust the rate of intravenous insulin. Unfortunately, these studies each had high rates of hypoglycemia, a high percentage of glucose values outside of the target range, and increased glucose variability. These three measurements have been shown to increase mortality rates in ICU patients. In order to achieve a high rate of success with regards to tight glucose control, a closed-loop system will need to be created. The two main elements of such a system are a continuous glucose sensor and a recursive glucose control algorithm. This review highlights the important elements of the native glucoregulatory system, which, if utilized, may help create a successful glucose control algorithm for a closed-loop system.

TRPM4 impacts on Ca2+ signals during agonist-induced insulin secretion in pancreatic beta-cells

TRPM4 is a Ca(2+)-activated non-selective cation (CAN) channel that functions in cell depolarization, which is important for Ca(2+) influx and insulin secretion in pancreatic beta-cells. We investigated TRPM4 expression and function in the beta-cell lines HIT-T15 (hamster), RINm5F (rat), beta-TC3 (mouse), MIN-6 (mouse) and the alpha-cell line INR1G9 (hamster). By RT-PCR, we identified TRPM4 transcripts in alpha- and beta-cells. Patch-clamp recordings with increasing Ca(2+) concentrations resulted in a dose-dependent activation of TRPM4 with the greatest depolarizing currents recorded from hamster-derived cells. Further, Ca(2+) imaging experiments revealed that inhibition of TRPM4 by a dominant-negative effect significantly decreased the magnitude of the Ca(2+) signals generated by agonist stimulation compared to control cells. The decrease in the [Ca(2+)](i) resulted in reduced insulin secretion. Our data suggest that depolarizing currents generated by TRPM4 are an important component in the control of intracellular Ca(2+) signals necessary for insulin secretion and perhaps glucagon from alpha-cells.

Oscillatory control of insulin secretion

Pancreatic beta-cells possess an inherent ability to generate oscillatory signals that trigger insulin release. Coordination of the secretory activity among beta-cells results in pulsatile insulin secretion from the pancreas, which is considered important for the action of the hormone in the target tissues. This review focuses on the mechanisms underlying oscillatory control of insulin secretion at the level of the individual beta-cell. Recent studies have demonstrated that oscillations of the cytoplasmic Ca(2+) concentration are synchronized with oscillations in beta-cell metabolism, intracellular cAMP concentration, phospholipase C activity and plasma membrane phosphoinositide lipid concentrations. There are complex interdependencies between the different messengers and signalling pathways that contribute to amplitude regulation and shaping of the insulin secretory response to nutrient stimuli and neurohormonal modulators. Several of these pathways may be important pharmacological targets for improving pulsatile insulin secretion in type 2 diabetes.

On high-frequency insulin oscillations

Insulin is released in a pulsatile manner, which results in oscillatory concentrations in blood. The oscillatory secretion improves release control and enhances the hormonal action. Insulin oscillates with a slow ultradian periodicity (approximately 140 min) and a high-frequency periodicity (approximately 6-10 min). Only the latter is reviewed in this article. At least 75% of the insulin secretion is released in a pulsatile manner. Individuals prone to developing diabetes or with overt type 2 diabetes are characterized by irregular oscillations of plasma insulin. Many factors have impact on insulin pulsatility such as age, insulin resistance and glycemic level. In addition, tiny glucose oscillations are capable of entraining insulin oscillations in healthy people in contrast to type 2 diabetic individuals emphasizing a profound disruption of the beta-cells in type 2 diabetes to sense or respond to physiological glucose excursions. A crucial question is how approximately 1,000,000 islets, each containing from a few to several thousand beta-cells, can be coordinated to secrete insulin in a pulsatile manner. This is blatantly a very complex operation to control involving an intra-pancreatic neural network, an intra-islet communication and metabolic oscillations in the beta-cell itself. Overnight beta-cell rest, e.g. during somatostatin administration, improves the disordered pulsatile insulin secretion in type 2 diabetes. Acute as well as long-term administration of sulphonylureas (SU) leads to substantial amplification (approximately 50%) of the pulsatile insulin secretion in type 2 diabetes. This is probably cardinal in terms of governing the hepatic glucose release in type 2 diabetes. Whether sulfonylureas also improve the ability of the beta-cells to sense glucose fluctuations remains to be explored. Thiazolidinediones reduce the pulsatile insulin secretion without affecting regularity, but appear to improve the ability of the beta-cell to be entrained by small glucose excursions. Finally, similar to SUs, the incretin hormone GLP-1 also results in an augmented pulsatile burst mass in both healthy and diabetic individuals, in the latter group, however, without influencing the disorderliness of pulses. This review will briefly describe the high-frequency insulin pulsatility during physiologic and pathophysiologic conditions as well as the influence of some hypoglycemic compounds on the insulin oscillations.

Relationship between beta-cell mass and diabetes onset

Regulation of blood glucose concentrations requires an adequate number of beta-cells that respond appropriately to blood glucose levels. beta-Cell mass cannot yet be measured in humans in vivo, necessitating autopsy studies, although both pre- and postmorbid changes may confound this approach. Autopsy studies report deficits in beta-cell mass ranging from 0 to 65% in type 2 diabetes (T2DM), and approximately 70-100% in type 1 diabetes (T1DM), and, when evaluated, increased beta-cell apoptosis in both T1DM and T2DM. A deficit of beta-cell mass of approximately 50% in animal studies leads to impaired insulin secretion (when evaluated directly in the portal vein) and induction of insulin resistance. We postulate three phases for diabetes progression. Phase 1: selective beta-cell cytotoxicity (autoimmune in T1DM, unknown in T2DM) leading to impaired beta-cell function and gradual loss of beta-cell mass through apoptosis. Phase 2: decompensation of glucose control when the pattern of portal vein insulin secretion is sufficiently impaired to cause hepatic insulin resistance. Phase 3: adverse consequences of glucose toxicity accelerate beta-cell dysfunction and insulin resistance. The relative contribution of beta-cell loss versus beta-cell dysfunction to diabetes onset remains an area of controversy. However, because cytotoxicity sufficient to induce beta-cell apoptosis predictably disturbs beta-cell function, it is naive to attempt to distinguish the relative contributions of these linked processes to diabetes onset.

Measurement of pulsatile insulin secretion in the rat: direct sampling from the hepatic portal vein

It has previously been shown that insulin is secreted in discrete secretory bursts by sampling directly from the portal vein in the dog and humans. Deficient pulsatile insulin secretion is the basis for impaired insulin secretion in type 2 diabetes. However, while novel genetically modified disease models of diabetes are being developed in rodents, no validated method for quantifying pulsatile insulin secretion has been established for rodents. To address this we 1) developed a novel rat model with chronically implanted portal vein catheters, 2) established the parameters to permit deconvolution of portal vein insulin concentrations profiles to measure insulin secretion and resolve its pulsatile components, and 3) measured total and pulsatile insulin secretion compared with that in the dog, the species in which this sampling and deconvolution approach was validated for quantifying pulsatile insulin secretion. In rats, portal vein catheter patency and function were maintained for periods up to 2-3 wk with no postoperative complications such as catheter tract infection. Rat portal vein insulin concentration profiles in the fasting state revealed distinct insulin oscillations with a periodicity of approximately 5 min and an amplitude of up to 600 pmol/l, which was remarkably similar to that in the dogs and in humans. Deconvolution analysis of portal vein insulin concentrations revealed that the majority of insulin ( approximately 70%) in the rat is secreted in distinct insulin pulses occurring at approximately 5-min intervals. This model therefore permits direct accurate measurements of pulsatile insulin secretion in a relatively inexpensive animal. With increased introduction of genetically modified rat models will be an important tool in elucidating the underlying mechanisms of impaired pulsatile insulin secretion in diabetes.

Mechanisms of impaired fasting glucose and glucose intolerance induced by an approximate 50% pancreatectomy

Impaired fasting glucose (IFG) and impaired glucose tolerance (IGT) often coexist and as such represent a potent risk factor for subsequent development of type 2 diabetes. beta-Cell mass is approximately 50% deficient in IFG and approximately 65% deficient in type 2 diabetes. To establish the effect of a approximately 50% deficit in beta-cell mass on carbohydrate metabolism, we performed a approximately 50% partial pancreatectomy versus sham surgery in 14 dogs. Insulin secretion was quantified from insulin concentrations measured in the portal vein at 1-min sampling intervals under basal conditions, after a 30-g oral glucose, and during a hyperglycemic clamp. Insulin sensitivity was measured by a hyperinsulinemic-euglycemic clamp combined with isotope dilution. Partial pancreatectomy resulted in IFG and IGT. After partial pancreatectomy both basal and glucose-stimulated insulin secretion were decreased through the mechanism of a selective approximately 50 and approximately 80% deficit in insulin pulse mass, respectively (P < 0.05). These defects in insulin secretion were partially offset by decreased hepatic insulin clearance (P < 0.05). Partial pancreatectomy also caused a approximately 40% decrease in insulin-stimulated glucose disposal (P < 0.05), insulin sensitivity after partial pancreatectomy being related to insulin pulse amplitude (r = 0.9, P < 0.01). We conclude that a approximately 50% deficit in beta-cell mass can recapitulate the alterations in glucose-mediated insulin secretion and insulin action in humans with IFG and IGT. These data support a mechanistic role of a deficit in beta-cell mass in the evolution of IFG/IGT and subsequently type 2 diabetes.

Pulsatile insulin secretion in elderly patients with diabetes

Insulin pulsation is impaired in type 2 diabetes. GLP-1 increases pulsatile insulin secretion in these patients. We conducted these studies with the hypothesis that GLP-1 would enhance pulsatile insulin secretion and alter glucose metabolism in elderly patients with type 2 diabetes. Experiments were conducted in nine patients (age: 72+/-5 years; BMI: 27+/-3kg/m(2); diabetes duration: 7+/-3 years; HbA(1c): 6.6+/-0.9%). Subjects underwent three glucose clamp studies. The first was a euglycemic clamp to determine individual insulin clearance. In the second, GLP-1 was infused from 0-240min (0.75pM/kg/min) and glucose was maintained at fasting levels. The third was similar except that octreotide (30ng/kg/min) was infused with GLP-1 to suppress pulsatile insulin. Insulin and glucose were given to match levels during the second study. 3-(3)H-glucose was infused to allow calculation of hepatic glucose production and glucose disposal rates. There was no significant difference in measurements of pulsatile insulin secretion or hepatic glucose production and glucose disposal rates between the studies. Because there was no difference in pulsatile insulin between experiments, we could not test the effect of pulsatile insulin on glucose metabolism. Further studies are required to determine the impact of insulin pulses on glucose metabolism.

The effect of manipulation of basal pulsatile insulin on insulin action in Type 2 diabetes

AIM

Disordered insulin pulsatility is associated with insulin resistant states including Type 2 diabetes. However, whether abnormal basal insulin pulses play a role in the pathogenesis of insulin resistance or are simply an associated feature remains undetermined. We investigated this relationship further by studying the effect of overnight (10 h) pulsatile insulin infusion on subsequent insulin sensitivity.

METHODS

We studied 17 Type 2 diabetic patients who underwent one of two protocols. In protocol A (10 patients) on two separate nights we infused insulin 0.1 mU/kg/min either in a constant infusion or in pulses every 13 min. Octreotide (0.43 microg/kg/h) was given to suppress endogenous insulin secretion and physiological replacement of glucagon (30 ng/kg/h) administered. Insulin sensitivity was measured using a hyperinsulinaemic euglycaemic clamp (2 mU/kg/min) next morning. In protocol B (seven patients), we employed the same experimental procedure but used a basal insulin infusion rate of 0.09 mU/kg/min in 7-min or 13-min pulses.

RESULTS

Appropriate pulse patterns were confirmed in each protocol. In protocol A, after overnight infusions, glucose infusion rates required to maintain euglycaemia at steady state hyperinsulinaemia were similar (33.9 +/- 5.2 vs. 31.2 +/- 4.1 micromol/kg/min; P = NS). In protocol B, after overnight infusions the glucose infusion rates required during hyperinsulinaemia were significantly lower during 7-min pulses (39.9 +/- 5.7 vs. 44.7 +/- 5.6 micromol/kg/min; P < 0.05).

CONCLUSION

There was no demonstrable priming effect derived from overnight pulsatile insulin compared with constant insulin infusion on subsequent insulin sensitivity in Type 2 diabetic subjects. The failure of 7-min pulses to exhibit an advantageous effect over 13-min pulses raises questions about the natural frequency of basal insulin pulses and their biological effect.

Mitochondrial metabolism reveals a functional architecture in intact islets of Langerhans from normal and diabetic Psammomys obesus

The cells within the intact islet of Langerhans function as a metabolic syncytium, secreting insulin in a coordinated and oscillatory manner in response to external fuel. With increased glucose, the oscillatory amplitude is enhanced, leading to the hypothesis that cells within the islet are secreting with greater synchronization. Consequently, non-insulin-dependent diabetes mellitus (NIDDM; type 2 diabetes)-induced irregularities in insulin secretion oscillations may be attributed to decreased intercellular coordination. The purpose of the present study was to determine whether the degree of metabolic coordination within the intact islet was enhanced by increased glucose and compromised by NIDDM. Experiments were performed with isolated islets from normal and diabetic Psammomys obesus. Using confocal microscopy and the mitochondrial potentiometric dye rhodamine 123, we measured mitochondrial membrane potential oscillations in individual cells within intact islets. When mitochondrial membrane potential was averaged from all the cells in a single islet, the resultant waveform demonstrated clear sinusoidal oscillations. Cells within islets were heterogeneous in terms of cellular synchronicity (similarity in phase and period), sinusoidal regularity, and frequency of oscillation. Cells within normal islets oscillated with greater synchronicity compared with cells within diabetic islets. The range of oscillatory frequencies was unchanged by glucose or diabetes. Cells within diabetic (but not normal) islets increased oscillatory regularity in response to glucose. These data support the hypothesis that glucose enhances metabolic coupling in normal islets and that the dampening of oscillatory insulin secretion in NIDDM may result from disrupted metabolic coupling.

Repaglinide is more efficient than glimepiride on insulin secretion and post-prandial glucose excursions in patients with type 2 diabetes. A short term study

OBJECTIVES

To compare the effect of Repaglinide vs Glimepiride on glucose- and meal-induced insulin secretion and on meal-test induced postprandial glucose excursions.

METHODS

After 2 weeks washout period, a 3-Month randomised, cross-over parallel group trial of R (1 mg x 2/die) vs G (2 mg/die) in 14 patients with type 2 diabetes "naive" in diet treatment was made.

RESULTS

Both R and G significantly but similarly lowered fasting glucose levels and improved fasting plasma insulin levels vs baseline. Hyperglycemic clamp showed that both 1st (129.15 +/- 23.6 vs 106.90 +/- 18.6 pmol/L; p=0.01) and 2nd phase (189.42 +/- 34.4 vs 144.21 +/- 37.3 pmol/L; p=0.003) B-cell response to glucose as well as area under the curve (52.07 +/- 10.86 vs 39.54 +/- 10.27 micromol/L x 120'; p=0.005) were greater in R than G groups. Insulin action (4.0 +/- 1.1 vs 3.2 +/- 0.9 mg x Kg x 60'/microU/mL; p=0.046) was also improved by R than G administration. In the meal test, R therapy produced a more rapId induction of insulin secretion during the first part. In fact, the mean rise in insulin secretion peaked at 45 min in R (p=0.001 vs G) and at 60 min in G (p=0.001 vs R). Consequently, glucose spike at 60 min was higher in G group compared to glucose spike at 45 min in R group (p=0.002).

CONCLUSIONS

Our study demonstrates that R is more efficient that G on improving glucose- and meal- induced insulin secretion as well as on controlling for postprandial glucose excursion.

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.

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.

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.

Augmented effect of short-term pulsatile versus continuous insulin delivery on lipid metabolism but similar effect on whole-body glucose metabolism in obese subjects

The present study was designed to examine the effect of pulsatile versus continuous insulin delivery on glucose and lipid metabolism in insulin-resistant subjects. Six obese women (body mass index, 40.0 +/- 2.8 kg/m2) underwent a euglycemic glucose clamp (plasma glucose, 90 mg/dL) twice. In random order, insulin was infused intravenously for 375 minutes either at a constant rate (0.4 mU/kg/min) or in a pulsatile manner (2.4 mU/kg/min for 2 minutes followed by an off interval of 10 minutes). Endogenous insulin release was suppressed by infusion of somatostatin (250 micrograms/h). Mean circulating insulin concentrations were similar during the two protocols (pulsatile v continuous infusion, 60 +/- 10 v 56 +/- 9 mU/L), but pulsatile infusion was accompanied by oscillations with an amplitude of 120 mU/L. After 6 hours of pulsatile versus continuous insulin, isotopically determined total glucose disposal (3-3H-glucose) and hepatic glucose production (HGP) were comparable (pulsatile v continuous, 2.80 +/- 0.56 v 2.82 +/- 0.51 and 0.37 +/- 0.14 v 0.32 +/- 0.17 mg/kg/min). However, the rate of glucose oxidation (indirect calorimetry) was augmented (P < .05), whereas lipid oxidation tended to be diminished (.10 > P > .05) following pulsatile infusion. In addition, blood glycerol was more suppressed with pulsatile (31 +/- 9 nmol/L) than with continuous infusion (36 +/- 10 nmol/L, P < .05), whereas blood lactate, alanine, and 3-hydroxybutyrate were similar in the two infusion protocols.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of physiological plasma insulin levels on glucose turnover parameters in familial hypercholesterolemia

Eight young, non-obese patients with primary familial hypercholesterolemia (FH) and 8 healthy subjects matched for age, body mass index, lean body mass, plasma triglyceride and HDL-levels and arterial blood pressure were selected from a lipid clinic. Patients with FH had higher plasma LDL-cholesterol (8.3 +/- 0.5 vs. 4.1 +/- 0.2 mmol/l, P < 0.001) than controls but similar plasma triglyceride (1.15 +/- 0.04 vs. 1.10 +/- 0.02 mmol/l P = NS) levels. Both study groups were submitted to a euglycaemic hyperinsulinemic glucose clamp combined with simultaneous infusion of [3H]glucose to measure insulin action on whole-body glucose uptake and on hepatic glucose production. Two insulin infusion rates (0.15 mU/kg per min from 0 to 120 min and 0.30 mU/kg per min from 121 to 240 min) were used resulting in similar plasma insulin levels in both groups studied. Our results demonstrate that both whole-body glucose uptake and hepatic glucose output are similar in the fasting state as well as during insulin administration in both groups of subjects. We conclude that, in the absence of other causes of insulin resistance, isolated hypercholesterolemia is associated with normal sensitivity to insulin in both liver and peripheral tissues.

Circadian rhythm and pulsatility of parathyroid hormone secretion in man

OBJECTIVE

We wished to investigate the circadian rhythm and pulsatility of parathyroid hormone (PTH) secretion in man, as conflicting results have been published.

DESIGN AND PATIENTS

To investigate the circadian rhythm during daytime, we sampled (a) peripheral blood at hourly intervals in 12 healthy young men from 0900 h until 1700 h. For observation of pulsatility, we sampled (b) peripheral blood at 1-minute intervals for 1 hour in three healthy men and three healthy women (mean 27.7 years, range 21-56 years) and (c) at 1-minute intervals for 30 minutes in 21 patients with surgically confirmed primary hyperparathyroidism (pHPT).

MEASUREMENTS

The serum levels of intact PTH were measured by two-site immunoradiometric assay and special care was taken to reduce intra-assay variability, especially at the normal PTH concentration. In series (a), ionized calcium, total calcium and phosphate were also determined.

RESULTS

A circadian rhythm during daytime was found for intact PTH in healthy men and women with a nadir at 0930 h and a peak in the afternoon. Ionized calcium and total calcium (protein-adjusted) decreased and phosphate increased in the afternoon. These changes were all statistically significant (P < 0.02). Pulsatility of PTH: Statistical cluster analysis of the data showed no pulsatility either in healthy persons or in patients with primary hyperparathyroidism. In two healthy women and one healthy man slight changes of longer duration were discovered, but no complete pulses. In five patients with primary hyperparathyroidism, larger differences between the highest and lowest concentrations of intact PTH were found, but no complete pulses.

CONCLUSIONS

Our data show a significant circadian rhythm during daytime of intact PTH and only minor changes from minute to minute. The alterations in PTH-levels occurred at longer time intervals in healthy persons. In some patients with primary hyperparathyroidism, decreases of PTH-levels were found. The circadian rhythm of PTH may be due to slight changes in calcium or phosphate concentration.

Metabolic and cardiovascular benefits deriving from beta-adrenergic blockade in chronic congestive heart failure

Ten patients with congestive heart failure were given metoprolol (50 mg/day) or placebo during a double-blind, crossover, randomized study. After a run-in period of 4 weeks, metoprolol and placebo were administered over a period of 3 months, which was separated by a washout period of 4 weeks. At the end of the run-in, metoprolol, and placebo periods, all patients underwent metabolic (oral glucose tolerance and hyperinsulinemic glucose clamp tests) and noninvasive cardiologic (New York Heart Association classification, bimodal echocardiographic left ventricular end-diastolic determination, maximal oxygen consumption, left ventricular radionuclide ejection fraction) tests. Our results show that beta-adrenergic blockade significantly enhances insulin-mediated suppression of hepatic glucose output (p less than 0.005) and increase in glucose uptake (p less than 0.01) with a concurrent improvement in New York Heart Association functional class (p less than 0.05) and the multistage exercise treadmill test score (p less than 0.05). After administration of metoprolol all changes in glucose turnover parameters were found to correlate with the decrease in basal plasma free fatty acid levels. In conclusion, our findings confirm the beneficial cardiologic effects of beta-adrenergic blockade in congestive heart failure and demonstrate that metoprolol is also useful for reversing the metabolic damage caused by exaggerated plasma norepinephrine levels.

Insulin resistance and hyperinsulinemia in patients with chronic congestive heart failure

Congestive heart failure is a condition associated with increased plasma norepinephrine levels. Moreover, norepinephrine has been recently demonstrated to affect glucose homeostasis by decreasing insulin sensitivity. In the present study, eight patients suffering from chronic congestive heart failure and 10 healthy age- and body mass index-matched subjected were submitted to both an oral glucose tolerance test (OGTT; 75 g) and a euglycemic hyperinsulinemic glucose clamp. During the 360 minutes of the glucose clamp, insulin was infused at three different rates (25, 50, and 100 mU/kg/h), while D-3H glucose infusion allowed determination of glucose turnover. In basal conditions, patients versus controls had similar plasma glucose (5.2 +/- 0.1 v 4.9 +/- 0.2 mmol/L,P = NS), but higher plasma insulin (125.7 +/- 9.2 v 35.7 +/- 3.3 pmol/L,P less than .01), norepinephrine (5.39 +/- 0.13 v 1.47 +/- 0.22 nmol/L,P less than .001), and free fatty acid (FFA) (927 +/- 79 v 792 +/- 88 mumol/L,P less than .05) levels. In patients, basal plasma norepinephrine correlated with FFA levels (r = .65, P less than .025). After loading glucose, plasma glucose and insulin levels were still significantly higher in patients than controls. Euglycemic hyperinsulinemic glucose clamp produced a lower insulin-mediated inhibition of endogenous (hepatic) glucose production (HGP) and a greater increase in both glucose disappearance rate (Rd) and glucose metabolic clearance rate (gMCR) in patients than in controls during the first two insulin infusion rates (25 and 50 mU/kg/h). By contrast, these differences disappeared during the highest insulin infusion rate (100 mU/kg/h). Insulin-mediated decrease in plasma FFA levels was also lower in patients than controls.(ABSTRACT TRUNCATED AT 250 WORDS)

Simvastatin reduces plasma lipid levels and improves insulin action in elderly, non-insulin dependent diabetics

Twelve elderly non-insulin dependent diabetic patients took part in a double-blind, cross-over, randomized study comparing simvastatin 30 mg/day and placebo. Each treatment period lasted 3 weeks and was separated by a 3 week wash-out period. At the end of each treatment period all subjects underwent in randomized order an oral glucose tolerance test (OGTT; 75 g) and an euglycaemic hyperinsulinaemic (50 mU/kg.h) glucose clamp. Simvastatin compared to placebo significantly reduced plasma total cholesterol (7.9 vs 5.3 mmol.l-1), LDL-cholesterol (7.2 vs 4.3 mmol.l-1), triglycerides (2.9 vs 2.1 mmol.l-1), free fatty acids (1106 vs 818 mmol-1) and glucose (7.4 vs 6.6 mmol.l-1) levels. After simvastatin, and in the last 60 min of the glucose clamp, there was an improvement in the action of insulin as demonstrated by stronger inhibition of hepatic glucose output (2.7 vs 5.2 mumol.kg-1.min-1) and stimulation both of the glucose disappearance rate (26.3 vs 19.5 mumol.kg-1.min-1) and glucose metabolic clearance rate (4.3 vs 3.6 ml.kg-1.min-1). The changes in glucose turnover parameters were significantly correlated with basal plasma free fatty acids and were independent of plasma glucoregulatory hormones. In conclusion, simvastatin seems to exert beneficial effects both on lipid and glucose metabolism.