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

A mechanism for slow rhythms in coordinated pancreatic islet activity

Insulin levels in the blood oscillate with a variety of periods, including rapid (5-10 min), ultradian (50-120 min), and circadian (24 h). Oscillations of insulin are beneficial for lowering blood glucose and disrupted rhythms are found in people with type 2 diabetes and their close relatives. These in vivo secretion dynamics imply that the oscillatory activity of individual islets of Langerhans are synchronized, although the mechanism for this is not known. One mechanism by which islets may synchronize is negative feedback of insulin on whole-body glucose levels. In previous work, we demonstrated that a negative feedback loop with a small time delay, to account for the time required for islets to be exposed to a new glucose concentration in vivo, results in small 3-6 islet populations synchronizing to produce fast closed-loop oscillations. However, these same islet populations could also produce slow closed-loop oscillations with periods longer than the natural islet oscillation periods. Here, we investigate the origin of the slow oscillations and the bistability with the fast oscillations using larger islet populations (20-50 islets). In contrast to what was observed earlier, larger islet populations mainly synchronize to longer-period oscillations that are approximately twice the delay time used in the feedback loop. A mean-field model was also used as a proxy for a large islet population to uncover the underlying mechanism for the slow rhythm. The heterogeneous intrinsic oscillation periods of the islets interferes with this rhythm mechanism when islet populations are small, and is similar to adding noise to the mean-field model. Thus, the effect of a time delay in the glucose feedback mechanism is similar to other examples of time-delayed systems in biology and may be a viable mechanism for ultradian oscillations.

Perspectives on Prediabetes and Aging

Diabetes prevention programs (DPPs) have been shown to effectively delay, and sometimes prevent, the progression from prediabetes to diabetes; however, labeling someone with prediabetes comes with potential negative psychological, financial, and self-perception consequences. Many older adults with prediabetes nowadays have a relatively "low-risk" form of prediabetes that rarely progresses to diabetes and may regress to normoglycemia. In this article, we review the impact of aging on glucose metabolism and provide a holistic approach to cases of prediabetes in older adults that maximizes the benefit-risk balance of interventions aimed at addressing prediabetes.

Measurement of Pulsatile Insulin Secretion: Rationale and Methodology

Pancreatic β-cells are responsible for the synthesis and exocytosis of insulin in response to an increase in circulating glucose. Insulin secretion occurs in a pulsatile manner, with oscillatory pulses superimposed on a basal secretion rate. Insulin pulses are a marker of β-cell health, and secretory parameters, such as pulse amplitude, time interval and frequency distribution, are impaired in obesity, aging and type 2 diabetes. In this review, we detail the mechanisms of insulin production and β-cell synchronization that regulate pulsatile insulin secretion, and we discuss the challenges to consider when measuring fast oscillatory secretion in vivo. These include the anatomical difficulties of measuring portal vein insulin noninvasively in humans before the hormone is extracted by the liver and quickly removed from the circulation. Peripheral concentrations of insulin or C-peptide, a peptide cosecreted with insulin, can be used to estimate their secretion profile, but mathematical deconvolution is required. Parametric and nonparametric approaches to the deconvolution problem are evaluated, alongside the assumptions and trade-offs required for their application in the quantification of unknown insulin secretory rates from known peripheral concentrations. Finally, we discuss the therapeutical implication of targeting impaired pulsatile secretion and its diagnostic value as an early indicator of β-cell stress.

Oscillations of C-peptide in the euglycemic clamp and their effect on the pharmacodynamic assessment of insulin preparations

C-peptide should be continuously suppressed. However, increased postdosing C-peptide is not an uncommon phenomenon in euglycemic clamp studies involving healthy participants. This study aimed to determine the extent to which the postdosing C-peptide increases from the baseline that could affect the accuracy of glucodynamics in euglycemic clamp studies involving healthy subjects. First, 10 healthy males underwent a 10-h euglycemic clamp without exogenous insulin administration to obtain a reference interval (RI) for the ratio of C-peptide after 0 min (CP_t ) to baseline C-peptide (CP₀ ). Then, the data from a pharmacokinetic and pharmacodynamic study of insulin aspart (IAsp) were analyzed, and 70 eligible clamps were grouped by CP_t /CP₀ : group A ([CP_t /CP₀ ]_max   > upper limit of RI), group B (1<[CP_t /CP₀ ]_max  ≤ upper limit of RI), and group C ([CP_t /CP₀ ]_max  ≤ 1). The differences in basal and clamped blood glucose, CP_t /CP₀ , and the pharmacokinetics and pharmacodynamics of IAsp were compared, and the relationship between elevated CP_t and the accuracy of pharmacodynamics was analyzed. The RI of CP_t /CP₀ was 22.7%-152.1%; 1.5 × baseline might be a ceiling for the increase in CP_t under stable conditions. The maximum glucose infusion rate (GIR) in group A tended to be higher than that in group B or C (P_{for trend}  = 0.033). The AUC_GIR,0-10h in groups A, B, and C was 1983 ± 650,1682 ± 454, and 1479 ± 440 mg/kg (P = 0.047), respectively, under comparable IAsp exposure. No intergroup difference was detected in clamped glucose, IAsp dose, or body mass index. In conclusion, postdosing C-peptide over 1.5× baseline indicates insufficient inhibition of endogenous insulin secretion, which could compromise the pharmacodynamics of insulin preparations.

Time-Restricted Eating to Prevent and Manage Chronic Metabolic Diseases

Molecular clocks are present in almost every cell to anticipate daily recurring and predictable changes, such as rhythmic nutrient availability, and to adapt cellular functions accordingly. At the same time, nutrient-sensing pathways can respond to acute nutrient imbalance and modulate and orient metabolism so cells can adapt optimally to a declining or increasing availability of nutrients. Organismal circadian rhythms are coordinated by behavioral rhythms such as activity-rest and feeding-fasting cycles to temporally orchestrate a sequence of physiological processes to optimize metabolism. Basic research in circadian rhythms has largely focused on the functioning of the self-sustaining molecular circadian oscillator, while research in nutrition science has yielded insights into physiological responses to caloric deprivation or to specific macronutrients. Integration of these two fields into actionable new concepts in the timing of food intake has led to the emerging practice of time-restricted eating. In this paradigm, daily caloric intake is restricted to a consistent window of 8-12 h. This paradigm has pervasive benefits on multiple organ systems.

Age-Related Changes in Glucose Metabolism, Hyperglycemia, and Cardiovascular Risk

Aging and diabetes mellitus are 2 well-known risk factors for cardiovascular disease (CVD). During the past 50 years, there has been an dramatic increase in life expectancy with a simultaneous increase in the prevalence of diabetes mellitus in the older population. This large number of older individuals with diabetes mellitus is problematic given that CVD risk associated with aging and diabetes mellitus. In this review, we summarize epidemiological data relating to diabetes mellitus and CVD, with an emphasis on the aging population. We then present data on hyperglycemia as a risk factor for CVD and review the current knowledge of age-related changes in glucose metabolism. Next, we review the role of obesity in the pathogenesis of age-related glucose dysregulation, followed by a summary of the results from major randomized controlled trials that focus on cardiovascular risk reduction through glycemic control, with a special emphasis on older adults. We then conclude with our proposed model of aging that body composition changes and insulin resistance link possible dysregulation of physiological pathways leading to obesity and diabetes mellitus-both forms of accelerated aging-and risks for CVD.

Circadian Transcription from Beta Cell Function to Diabetes Pathophysiology

The mammalian circadian clock plays a central role in the temporal coordination of physiology across the 24-h light-dark cycle. A major function of the clock is to maintain energy constancy in anticipation of alternating periods of fasting and feeding that correspond with sleep and wakefulness. While it has long been recognized that humans exhibit robust variation in glucose tolerance and insulin sensitivity across the sleep-wake cycle, experimental genetic analysis has now revealed that the clock transcription cycle plays an essential role in insulin secretion and metabolic function within pancreatic beta cells. This review addresses how studies of the beta cell clock may elucidate the etiology of subtypes of diabetes associated with circadian and sleep cycle disruption, in addition to more general forms of the disease.

Mathematical investigation of diabetically impaired ultradian oscillations in the glucose-insulin regulation

We study the effect of diabetic deficiencies on the production of an oscillatory ultradian regime using a deterministic nonlinear model which incorporates two physiological delays. It is shown that insulin resistance impairs the production of oscillations by dampening the ultradian cycles. Four strategies for restoring healthy regulation are explored. Through the introduction of an instantaneous glucose-dependent insulin response, explicit conditions for the existence of periodic solutions in the linearised model are formulated, significantly reducing the complexity of identifying an oscillatory regime. The model is thus shown to be suitable for representing the effect of diabetes on the oscillatory regulation and for investigating pathways to reinstating a physiological healthy regime.

Intranasal insulin decreases circulating cortisol concentrations during early sleep in elderly humans

Aging is associated with increases in hypothalamic-pituitary-adrenal (HPA) axis activity that can predispose to metabolic and cognitive impairments. We investigated in elderly and young subjects whether intranasal insulin administration to the human brain reduces early-sleep nadir concentrations of adrenocorticotropin and cortisol, that is, indicators of baseline HPA axis activity. In within-subject comparisons, intranasal insulin (160 IU) or placebo was administered to 14 elderly (mean age 70.0 years) and 30 young (23.6 years) healthy subjects before bedtime. Sleep was polysomnographically assessed and blood samples were repeatedly collected. Elderly compared with young participants displayed increased early-sleep cortisol concentrations (p < 0.04) and reductions in slow wave and REM sleep (p < 0.001). Insulin administration reduced cortisol levels between 2300 hours and 0020 hours in the elderly (p = 0.03) but not young participants (p = 0.56; p = 0.003 for interaction). Findings indicate that central nervous insulin acts as an inhibitory signal in basal HPA axis activity regulation and suggest that intranasal insulin may normalize sleep-associated stress axis activity in older age.

Morning Circadian Misalignment during Short Sleep Duration Impacts Insulin Sensitivity

Short sleep duration and circadian misalignment are hypothesized to causally contribute to health problems including obesity, diabetes, metabolic syndrome, heart disease, mood disorders, cognitive impairment, and accidents. Here, we investigated the influence of morning circadian misalignment induced by an imposed short nighttime sleep schedule on impaired insulin sensitivity, a precursor to diabetes. Imposed short sleep duration resulted in morning wakefulness occurring during the biological night (i.e., circadian misalignment)-a time when endogenous melatonin levels were still high indicating the internal circadian clock was still promoting sleep and related functions. We show the longer melatonin levels remained high after wake time, insulin sensitivity worsened. Overall, we find a simulated 5-day work week of 5-hr-per-night sleep opportunities and ad libitum food intake resulted in ∼20% reduced oral and intravenous insulin sensitivity in otherwise healthy men and women. Reduced insulin sensitivity was compensated by an increased insulin response to glucose, which may reflect an initial physiological adaptation to maintain normal blood sugar levels during sleep loss. Furthermore, we find that transitioning from the imposed short sleep schedule to 9-hr sleep opportunities for 3 days restored oral insulin sensitivity to baseline, but 5 days with 9-hr sleep opportunities was insufficient to restore intravenous insulin sensitivity to baseline. These findings indicate morning wakefulness and eating during the biological night is a novel mechanism by which short sleep duration contributes to metabolic dysregulation and suggests food intake during the biological night may contribute to other health problems associated with short sleep duration.

Comparison of the pharmacokinetics of three concentrations of insulin aspart during continuous subcutaneous insulin infusion (CSII) in a pig model

OBJECTIVES

The aim of the study was to investigate the pharmacokinetic properties of insulin aspart (IAsp) in three different concentrations given as a continuous subcutaneous insulin infusion (CSII).

METHODS

A randomized cross-over study was performed in pigs, where IAsp U200, U100 or U20 was given for 8 h with the same total dose. Six pigs were included and blood was sampled during the CSII and 3 h after.

KEY FINDINGS

The half-life (t(1/2) ) was 24.3 (range 17.3-41.3), 28.8 (range 19.6-54.3) and 23.6 (range 17.4-36.8) min for U200, U100 and U20, respectively. The area under the curve per dose (AUC/D) was determined to be 51.2 ± 19.5, 52.3 ± 12.5 and 51.6 ± 6.7 pm × min/kg for U200, U100 and U20, respectively. The steady state plasma concentration (C(ss) ) was 57.5 ± 27.1, 54.3 ± 10.3 and 55.1 ± 8.0 pm (mean ± SD) for U200, U100 and U20, respectively. Time to steady state (T(ss) ) was 110 ± 36, 98 ± 48 and 90 ± 27 min for U200, U100 and U20, respectively.

CONCLUSIONS

In conclusion, no significant difference was found in t(1/2) , AUC/D, C(ss) or T(ss) between the three IAsp concentrations when given at a basal rate in CSII.

Continuous operation of microfabricated electrophoresis devices for 24 hours and application to chemical monitoring of living cells

Microchip electrophoresis is an emerging analytical technology with several useful attributes including rapid separation time, small sample requirements, and automation. In numerous potential applications, such as chemical monitoring or high-throughput screening, it may be desirable to use a system for many analyses without operator intervention; however, long-term operation of microchip electrophoresis systems has received little attention. We have developed a microchip electrophoresis system that can automatically inject samples at 6 s intervals for 24 h resulting in collection of 14,400 assays in one session. Continuous operation time of a prototype of the device was limited to 2 h due to degradation of reagents and electrophoresis buffers on the chip; however, modification so that all reagents were continuously perfused into reservoirs on the device ensured fresh reagents were always used for analysis and enabled extended operating sessions. The electrophoresis chip incorporated a cell perfusion chamber and reagent addition channels to allow chemical monitoring of fluid around cells cultured on the chip by serial electrophoretic immunoassays. The immunoassay had detection limits of 0.4 nM for insulin and generated approximately 4% relative standard deviation over an entire 24 h period with no evidence of signal drift. The combined system was used to monitor insulin secretion from single islets of Langerhans for 6-39 h. The monitoring experiments revealed that islets have secretion dynamics that include spontaneous oscillations after extended nonoscillating periods and possible ultradian rhythms.

Pharmacokinetics following continuous subcutaneous insulin infusion of insulin aspart with or without initial subcutaneous bolus

AIM

To evaluate time to steady state insulin concentration (C(ss)) following continuous subcutaneous insulin infusion (CSII) of insulin aspart (IAsp) with or without an initial s.c. bolus.

METHODS

In random order 10 healthy volunteers were given a basal insulin infusion rate (0.5 U/h) for 8 h with or without an initial s.c. bolus (1.4 U). Serum IAsp was measured until 3 h after infusion was stopped.

RESULTS

An overshoot of IAsp was seen before C(ss) was achieved following an initial bolus of insulin as compared to no bolus. The apparent half-life (t((1/2))) with or without bolus did not differ (p = 0.15). Time to steady state (T(ss)) was evaluated in two ways: (1) T(ss) defined as the first point within an interval of C(ss)+/- 2 x CV was 233 vs. 166 min with and without a bolus respectively (p = 0.068). (2) A t-test was performed for each concentration-time point vs. mean C(ss), and the first point with no significance was defined, T(ss). This gave 208 (p = 0.09) and 178 min (p = 0.24) with and without bolus respectively. Mathematical modelling suggests that an ideal mean bolus should be 0.89 U, and that this bolus dose may result in a shorter T(ss).

CONCLUSION

A bolus of 1.4 U resulted in an overshoot of serum IAsp before C(ss) and a longer period before C(ss) is achieved. Mathematical modelling suggests that a mean bolus of 0.89 U would result in a faster achievement of C(ss) compared to no bolus.

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.

Enhanced modelling of the glucose–insulin system and its applications in insulin therapies

It is well known that Michaelis–Menten kinetics is suitable for the response function in chemical reaction, when the reaction rate does not increase indefinitely when an excess of resource is available. However, the existing models for insulin therapies assume that the response function of insulin clearance is proportional to the insulin concentration. In this paper, we propose a new model for insulin therapy for both type 1 and type 2 diabetes mellitus, in which the insulin degradation rate assumes Michaelis–Menten kinetics. Our analysis shows that it is possible to mimic pancreatic insulin secretion by exogenous insulin infusions, and our numerical simulations provide clinical strategies for insulin–administration practices.

Chronobiology in the endocrine system

Biological signaling occurs in a complex web with participation and interaction of the central nervous system, the autonomous nervous system, the endocrine glands, peripheral endocrine tissues including the intestinal tract and adipose tissue, and the immune system. All of these show an intricate time structure with rhythms and pulsatile variations in multiple frequencies. Circadian (about 24-hour) and circannual (about 1-year) rhythms are kept in step with the cyclic environmental surrounding by the timing and length of the daily light span. Rhythmicity of many endocrine variables is essential for their efficacy and, even in some instances, for the qualitative nature of their effects. Indeed, the continuous administration of certain hormones and their synthetic analogues may show substantially different effects than expected. In the design of drug-delivery systems and treatment schedules involving directly or indirectly the endocrine system, consideration of the human time organization is essential. A large amount of information on the endocrine time structure has accumulated, some of which is discussed in this review.

Awakening and counterregulatory response to hypoglycemia during early and late sleep

OBJECTIVE

Nocturnal hypoglycemia represents an important problem for diabetic patients, which has been primarily attributed to an attenuated hormonal counterregulation during sleep. So far, hypoglycemia counterregulation has been exclusively examined during early nocturnal sleep, although early sleep differs markedly in sleep stage architecture from late sleep. Here, we investigated whether awakening and counterregulatory responses differ between early and late sleep.

RESEARCH DESIGN AND METHODS

Sixteen healthy subjects were tested on three occasions. On two nights, a linear fall in plasma glucose to a nadir of 2.2 mmol/l within 60 min was induced by insulin infusion. On one night, this was done immediately after sleep onset and on the other night after approximately 3.5 h of sleep. In a further control night, no hypoglycemia was induced.

RESULTS

During early sleep, 10 subjects awoke in response to hypoglycemia, whereas no subject awoke during the corresponding interval of the control night (P < 0.004). During late sleep, all subjects awoke upon hypoglycemia, and four subjects awoke spontaneously during the corresponding control interval (P < 0.001). The pattern indicates that the frequency of awakenings caused by hypoglycemia is similar for early and late sleep. Increases in epinephrine, norepinephrine, ACTH, cortisol, and growth hormone were distinctly weaker during late than early hypoglycemia (all P < 0.05).

CONCLUSIONS

Diminished hormonal counterregulation during late sleep could be one factor contributing to the clinically observed accumulation of hypoglycemic episodes in the later part of the night in patients with diabetes.

Mathematical modeling and qualitative analysis of insulin therapies

Several insulin therapies are widely in clinical use with the basic strategy that mimics insulin secretion in a normal glucose-insulin endocrine metabolic regulatory system. In this paper, we model the insulin therapies using a delay differential equation model. We study the dynamics of the model both qualitatively and quantitatively. The analytical results show the existence and uniqueness of a stable periodic solution that corresponds to ultradian insulin secretion oscillations. Numerically we simulate the insulin administration based on our model. The numerical simulation results are in agreement with findings of clinical studies.

Defective awakening response to nocturnal hypoglycemia in patients with type 1 diabetes mellitus

BACKGROUND

Nocturnal hypoglycemia frequently occurs in patients with type 1 diabetes mellitus (T1DM). It can be fatal and is believed to promote the development of the hypoglycemia-unawareness syndrome. Whether hypoglycemia normally provokes awakening from sleep in individuals who do not have diabetes, and whether this awakening response is impaired in T1DM patients, is unknown.

METHODS AND FINDINGS

We tested two groups of 16 T1DM patients and 16 healthy control participants, respectively, with comparable distributions of gender, age, and body mass index. In one night, a linear fall in plasma glucose to nadir levels of 2.2 mmol/l was induced by infusing insulin over a 1-h period starting as soon as polysomnographic recordings indicated that stage 2 sleep had been reached. In another night (control), euglycemia was maintained. Only one of the 16 T1DM patients, as compared to ten healthy control participants, awakened upon hypoglycemia (p = 0.001). In the control nights, none of the study participants in either of the two groups awakened during the corresponding time. Awakening during hypoglycemia was associated with increased hormonal counterregulation. In all the study participants (from both groups) who woke up, and in five of the study participants who did not awaken (three T1DM patients and two healthy control participants), plasma epinephrine concentration increased with hypoglycemia by at least 100% (p < 0.001). A temporal pattern was revealed such that increases in epinephrine in all participants who awakened started always before polysomnographic signs of wakefulness (mean +/- standard error of the mean: 7.5 +/- 1.6 min).

CONCLUSIONS

A fall in plasma glucose to 2.2 mmol/l provokes an awakening response in most healthy control participants, but this response is impaired in T1DM patients. The counterregulatory increase in plasma epinephrine that we observed to precede awakening suggests that awakening forms part of a central nervous system response launched in parallel with hormonal counterregulation. Failure to awaken increases the risk for T1DM patients to suffer prolonged and potentially fatal hypoglycemia.

On-line adaptive algorithm with glucose prediction capacity for subcutaneous closed loop control of glucose: evaluation under fasting conditions in patients with Type 1 diabetes

AIMS

To evaluate an algorithm with glucose prediction capacity and continuous adaptation of patient parameters-a model predictive control (MPC) algorithm-to control blood glucose concentration during fasting conditions in patients with Type 1 diabetes. In the subcutaneous (sc) route within a closed loop system.

METHODS

Paired experiments were performed in six patients. Over 8 h the MPC algorithm was used to control glucose with s.c. insulin administration and two different glucose monitoring protocols: first, the algorithm was provided with intravenous (i.v.) glucose values for insulin dosage calculation directly (i.v.-s.c. route). Then, in the second experiment, i.v. glucose values were fed to the MPC with a delay of 30 min to simulate s.c. glucose measurements ('s.c.'-s.c. route). In both experiments plasma glucose, insulin dosage, and serum insulin levels were analysed.

RESULTS

Glucose concentration was brought from hyper- to normoglycaemia and kept in the physiological range (6-7 mmol/l) with both routes in all subjects. Mean glucose concentration reached the threshold of 7 mmol/l approximately 2 (i.v.-s.c. route) and 3 ('s.c.'-s.c. route) hours after the start of glucose control with the MPC. During the last 2 h of automated glucose control, mean glucose concentration was 6.3 +/- 0.2 mmol/l and 6.6 +/- 0.3 mmol/l for i.v.-s.c. and 's.c.'-s.c. route, respectively. Glucose concentration, insulin doses, and serum insulin levels did not differ significantly between routes (P > 0.05).

CONCLUSIONS

The MPC algorithm is suitable for glucose control during fasting within an extracorporeal artificial beta-cell in the subcutaneous route Type 1 diabetic patients.

Effects of dietary polyunsaturated n-3 fatty acids on dyslipidemia and insulin resistance in rodents and humans. A review

For many years, clinical and animal studies on polyunsaturated n-3 fatty acids (PUFAs), especially those from marine oil, eicosapentaenoic acid (20:5,n-3) and docosahexaenoic acid (22:6,n-3), have reported the impact of their beneficial effects on both health and diseases. Among other things, they regulate lipid levels, cardiovascular and immune functions as well as insulin action. Polyunsaturated fatty acids are vital components of the phospholipids of membrane cells and serve as important mediators of the nuclear events governing the specific gene expression involved in lipid and glucose metabolism and adipogenesis. Besides, dietary n-3 PUFAs seem to play an important protecting role against the adverse symptoms of the Plurimetabolic syndrome. This review highlights some recent advances in the understanding of metabolic and molecular mechanisms concerning the effect of dietary PUFAs (fish oil) and focuses on the prevention and/or improvement of dyslipidemia, insulin resistance, impaired glucose homeostasis, diabetes and obesity in experimental animal models, with some extension to humans.

Emergence of complex dynamics in a simple model of signaling networks

Various physical, social, and biological systems generate complex fluctuations with correlations across multiple time scales. In physiologic systems, these long-range correlations are altered with disease and aging. Such correlated fluctuations in living systems have been attributed to the interaction of multiple control systems; however, the mechanisms underlying this behavior remain unknown. Here, we show that a number of distinct classes of dynamical behaviors, including correlated fluctuations characterized by 1/f scaling of their power spectra, can emerge in networks of simple signaling units. We found that, under general conditions, complex dynamics can be generated by systems fulfilling the following two requirements, (i) a "small-world" topology and (ii) the presence of noise. Our findings support two notable conclusions. First, complex physiologic-like signals can be modeled with a minimal set of components; and second, systems fulfilling conditions i and ii are robust to some degree of degradation (i.e., they will still be able to generate 1/f dynamics).

Physiological complexity, aging, and the path to frailty

Healthy physiological processes require the complex interaction of multiple control systems operating over multiple time scales. The output of these processes (for example, heart rate, blood pressure, hormonal rhythms, or postural sway) demonstrates complex variability that can be quantified using the concept of fractals, derived from the field of nonlinear dynamics. Complex physiological dynamics enable an organism to rapidly respond to the internal and external perturbations of everyday life. Aging and disease are associated with a loss of complexity in the dynamics of many physiological systems. This loss of complexity may reduce the ability to adapt to stress and lead to the syndrome of frailty.

Fish Oil Affects Pancreatic Fat Storage, Pyruvate Dehydrogenase Complex Activity and Insulin Secretion in Rats Fed a Sucrose-Rich Diet

Rats fed a sucrose-rich diet (SRD) develop hypertriglyceridemia and a marked decline in beta cell function. The purpose of this study was to determine whether changes in triglyceride concentration and/or altered pyruvate dehydrogenase complex (PDHc) activity contribute to the beta cell dysfunction, and to analyze the effect of dietary fish oil on the altered patterns of insulin secretion and peripheral insulin resistance. Rats were fed an SRD for 210 d. One-half of the rats continued consuming the SRD until d 270. The other half received an SRD in which fish oil (FO) was partially substituted for corn oil until d 270. A group of rats was fed a control diet (CD) throughout the experiment. The islets of rats fed the SRD had a greater triglyceride concentration and lower PDHc activity than those fed the CD. Insulin secretion patterns under the stimulus of glucose, palmitate or L-arginine were impaired in SRD-fed compared with CD-fed rats. This was accompanied by peripheral insulin resistance, mild hyperglycemia, a sharp increase of plasma triglyceride and free fatty acid levels and greater epididymal and retroperitoneal fat weights. FO normalized and/or improved these variables. Our results indicate that the increased fat storage and decreased PDHc activity in the beta cells play a key role in the abnormal insulin secretion of rats chronically fed an SRD. This is consistent with the reversion of these alterations by dietary FO.

Acute hyperglycemia alters the ability of the normal beta-cell to sense and respond to glucose

Impaired glucose tolerance (IGT) and non-insulin-dependent diabetes mellitus (NIDDM) are associated with an impaired ability of the beta-cell to sense and respond to small changes in plasma glucose. The aim of this study was to establish whether acute hyperglycemia per se plays a role in inducing this defect in beta-cell response. Seven healthy volunteers with no family history of NIDDM were studied on two occasions during a 12-h oscillatory glucose infusion with a periodicity of 144 min. Once, low-dose glucose was infused at a mean rate of 6 mg x kg(-1) x min(-1) and amplitude 33% above and below the mean rate, and, once, high-dose glucose was infused at 12 mg x kg(-1) x min(-1) and amplitude 16% above and below the mean rate. Mean glucose levels were significantly higher during the high-dose compared with the low-dose glucose infusion [9.5 +/- 0.8 vs. 6.8 +/- 0.2 mM (P < 0.01)], resulting in increased mean insulin secretion rates [ISRs; 469.1 +/- 43.8 vs. 268.4 +/- 29 pmol/min (P < 0.001)] and mean insulin levels [213.6 +/- 46 vs. 67.9 +/- 10.9 pmol/l (P < 0.008)]. Spectral analysis evaluates the regularity of oscillations in glucose, insulin secretion, and insulin at a predetermined frequency. Spectral power for glucose, ISR, and insulin was reduced during the high-dose glucose infusion [11.8 +/- 1.4 to 7.0 +/- 1.6 (P < 0.02), 7.6 +/- 1.5 to 3.2 +/- 0.5 (P < 0.04), and 10.5 +/- 1.6 to 4.6 +/- 0.7 (P < 0.01), respectively]. In conclusion, short-term infusion of high-dose glucose to obtain glucose levels similar to those previously seen in IGT subjects results in reduced spectral power for glucose, ISR, and insulin. The reduction in spectral power previously observed for ISR in IGT or NIDDM subjects may be due partly to hyperglycemia.

Dynamics of stability: the physiologic basis of functional health and frailty

Under basal resting conditions most healthy physiologic systems demonstrate highly irregular, complex dynamics that represent interacting regulatory processes operating over multiple time scales. These processes prime the organism for an adaptive response, making it ready and able to react to sudden physiologic stresses. When the organism is perturbed or deviates from a given set of boundary conditions, most physiologic systems evoke closed-loop responses that operate over relatively short periods of time to restore the organism to equilibrium. This transiently alters the dynamics to a less complex, dominant response mode, which is denoted "reactive tuning." Aging and disease are associated with a loss of complexity in resting dynamics and maladaptive responses to perturbations. These alterations in the dynamics of physiologic systems lead to functional decline and frailty. Nonlinear mathematical techniques that quantify physiologic dynamics may predict the onset of frailty, and interventions aimed toward restoring healthy dynamics may prevent functional decline.

Ultradian oscillations of insulin secretion in humans

Ultradian rhythmicity appears to be characteristic of several endocrine systems. As described for other hormones, insulin release is a multioscillatory process with rapid pulses of about 10 min and slower ultradian oscillations (50--120 min). The mechanisms underlying the ultradian circhoral oscillations of insulin secretion rate (ISR), which arise in part from a rhythmic amplification of the rapid pulses, are not fully understood. In humans, included in the same period range is the alternation of rapid eye movement (REM) and non-REM (NREM) sleep cycles and the associated opposite oscillations in sympathovagal balance. During sleep, the glucose and ISR oscillations were amplified by about 150%, but the REM-NREM sleep cycles did not entrain the glucose and ISR ultradian oscillations. Also, the latter were not related to either the ultradian oscillations in sympathoagal balance, as inferred from spectral analysis of cardiac R-R intervals, or the plasma fluctuations of glucagon-like peptide-1 (GLP-1), an incretin hormone known to potentiate glucose-stimulated insulin. Other rhythmic physiological processes are currently being examined in relation to ultradian insulin release.

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.

Non-genomic effects of glucocorticoids in the neural system. Evidence, mechanisms and implications

Complementing the classical concept of genomic steroid actions, here we (i) review evidence showing that important neural effects of glucocorticoids are exerted by non-genomic mechanisms; (ii) describe known mechanisms that may underlie such effects; (iii) summarize the functions and implications of non-genomic mechanisms and (iv) outline future directions of research. The role of non-genomic mechanisms is to shape the response of the organism to challenges that require a substantial reorganization of neural and somatic functions and involve massive behavioral shifts. Non-genomic effects may (i) prepare the cell for subsequent glucocorticoid-induced genomic changes, (ii) bridge the gap between the early need of change and the delay in the expression of genomic effects and (iii) may induce specific changes that in some instances are opposite to those induced by genomic mechanisms. The latter can be explained by the fact that challenging situations require different responses in early (acute) and later (chronic) phases. Data show that non-genomic mechanisms of glucocorticoid action play a role in both pathological phenomena and the expression of ameliorative pharmacological effects. Non-genomic mechanisms that underlie many glucocorticoid-induced neural changes constitute a for long overlooked controlling factor. Despite the multitude and the variety of accumulated data, important questions remain to be answered.

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.

Modeling the insulin-glucose feedback system: the significance of pulsatile insulin secretion

A mathematical model of the insulin-glucose feedback regulation in man is used to examine the effects of an oscillatory supply of insulin compared to a constant supply at the same average rate. We show that interactions between the oscillatory insulin supply and the receptor dynamics can be of minute significance only. It is possible, however, to interpret seemingly conflicting results of clinical studies in terms of their different experimental conditions with respect to the hepatic glucose release. If this release is operating near an upper limit, an oscillatory insulin supply will be more efficient in lowering the blood glucose level than a constant supply. If the insulin level is high enough for the hepatic release of glucose to nearly vanish, the opposite effect is observed. For insulin concentrations close to the point of inflection of the insulin-glucose dose-response curve an oscillatory and a constant insulin infusion produce similar effects.

Ultradian corticosterone rhythm and the propensity to behave aggressively in male rats

Ultradian fluctuations in plasma glucocorticoids have been demonstrated in a variety of species including humans. The significance of such rhythms is poorly known, although disorganized ultradian glucocorticoid rhythms have been associated with behavioural disorders. Here we report that ultradian glucocorticoid rhythms may establish the propensity to behave aggressively in male rats. Male rats were significantly more aggressive in the increasing phase of their corticosterone fluctuation when confronting a male intruder than counterparts in the decreasing phase of their corticosterone fluctuations facing such opponents. Corticosterone fluctuations were mimicked by a combination of treatments with the corticosterone synthesis inhibitor metyrapone and corticosterone. Again, males with increased plasma corticosterone levels were more aggressive than counterparts with a decreased plasma corticosterone concentration. These data suggest that the behavioural response to an aggressive challenge may vary in the same animal across the day due to the pulsating nature of corticosterone secretion. Aggressive behaviour is also episodic in humans; moreover, intermittent explosive behaviour is recognized as a psychological disorder. It can be hypothesized that a temporal coincidence between the occurrence of a challenge and a surge in plasma corticosterone concentration may be one of the factors that promote episodic aggressive outbursts.

Rapid oscillations in omental lipolysis are independent of changing insulin levels in vivo

Abnormal fat metabolism plays an important role in the pathogenesis of obesity-related type 2 diabetes mellitus. This study examined whether free fatty acid levels (FFAs), like insulin levels, oscillate rapidly in plasma. Peripheral and portal blood samples from dogs were assayed for FFA, glycerol, glucose, and insulin. FFA and glycerol showed correlated oscillatory profiles, with about 8 pulses/hour. Omental lipolysis was also pulsatile, with about 10 pulses/hour, and insulin levels oscillated rapidly in plasma with about 7 pulses/hour. We applied an insulin clamp, beta-adrenergic blockade, or both together, to determine the driving force behind the FFA oscillation, and we analyzed our findings by approximate entropy (ApEn) for which lower values suggest regular pulses and higher values suggest disorder. Under basal conditions, ApEn was 0.3 +/- 0.2. With insulin not oscillating, FFA still cycled at about 9 pulses/hour and the ApEn was 0.2 +/- 0.1. In contrast, beta-blockade, either in the presence or absence of an insulin clamp, removed the FFA oscillation in three of nine dogs. In the other six dogs, the oscillatory profile was unchanged, but ApEn was significantly higher than basal values, suggesting that the regularity of the profile was disrupted. These results suggest that the FFA oscillation is driven by the central nervous system, not by insulin.

Circadian control of insulin secretion is independent of the temporal distribution of feeding

To investigate whether there is a circadian regulation of insulin secretion, rats were adapted to a feeding regimen of six meals equally distributed over 24 h. Under these conditions basal glucose and insulin levels increased during the light phase and decreased during the dark phase. Maximal blood glucose responses were fairly similar during the six different meals, but glucose increments were clearly delayed during the last two meals consumed during the light period. Insulin increments were highest during the dark phase and clearly diminished during the second half of the light phase. This situation was reversed when the scheduled meals were replaced by i.v. glucose infusions, i.e., no significant differences were detected between insulin responses, whereas glucose increments were reduced during the dark period. These results show that there is a circadian regulation of basal blood glucose and feeding-induced insulin responses, which is independent of the temporal distribution of feeding activity.

Increased disorderliness of basal insulin release, attenuated insulin secretory burst mass, and reduced ultradian rhythmicity of insulin secretion in older individuals

Insulin is secreted in a pulsatile fashion. Rapid pulses are considered to be important for inhibiting hepatic glucose output, and ultradian pulses for stimulating peripheral glucose disposal. Aging is characterized by a progressive impairment in carbohydrate tolerance. We undertook the current studies to determine whether alterations in pulsatile insulin release accompany the age-related changes in carbohydrate metabolism. Healthy young (n = 8; body mass index, 21 +/- 1 kg/m2; age, 24 +/- 1 yr) and old (n = 9; body mass index, 24 +/- 1 kg/m2; age, 77 +/- 2 yr) volunteers underwent two studies. In the first study, insulin was sampled every 1 min for 150 min, and pulse analysis was conducted using a recently validated multiparameter deconvolution technique. In the second study, insulin was sampled every 10 min for 600 min, and insulin release was evaluated by Cluster analysis. In the 150-min studies, insulin secretory burst mass (P < 0.05) and amplitude (P < 0.01) were reduced in the elderly. In addition, approximate entropy, a measure of irregularity or disorderliness of insulin release, was increased in the aged (P < 0.01). In the 600-min studies, interpulse interval was greater in the aged (P < 0.05), and burst number was less (P < 0.05). We conclude that normal aging is characterized by more disorderly insulin release, a reduction in the amplitude and mass of rapid insulin pulses, and a decreased frequency of ultradian pulses. Whether these alterations in insulin pulsatility contribute directly to the age-related changes in carbohydrate metabolism will require further study.

Alterations in the ultradian oscillations of insulin secretion and plasma glucose in aging

Normal insulin secretion includes oscillations with a period length of 80-150 min which are tightly coupled to glucose oscillations of similar period. To determine whether normal aging is associated with alterations in these ultradian oscillations, eight, modestly overweight, older men (65 +/- 5 years) and eight weight-matched young control subjects (25 +/- 4 years) were studied during 53 h of constant glucose infusion. Blood samples were collected every 20 min and insulin secretion rates were calculated by deconvolution. Ultradian oscillations of glucose and insulin secretion were evident in both groups. Pulse frequency was similar for glucose and insulin secretion, and was not affected by age. The absolute amplitude of the glucose oscillations was similar in both groups but their relative amplitude was slightly dampened in the older adults. Both the absolute and the relative amplitudes of insulin secretory oscillations were markedly reduced in the older subjects. The normal linear increase in the amplitude of insulin oscillations occurring with increasing amplitudes of glucose oscillations was still present in the older adults but analysis of covariance indicated that the slope was significantly lower than in the young control subjects (p < 0.0005), reflecting a decreased responsiveness of the beta cell to glucose changes. The temporal concordance between insulin and glucose oscillations, as estimated by pulse concomitancy and cross-correlation, was also lower in older subjects. The similarities between the alterations in the ultradian oscillations of insulin secretion and glucose in older healthy adults and those occurring in diabetic patients suggest that an impairment of beta-cell function may play a primary role in the deterioration of glucose tolerance in aging.

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.

An integrated view of beta-cell dysfunction in type-II diabetes

Type-II (non-insulin-dependent) diabetes mellitus (NIDDM) is a heterogeneous disease resulting from insulin resistance and beta-cell dysfunction. beta-Cell dysfunction in Type-II diabetes is characterized by a specific lack of first-phase glucose-induced insulin secretion. This defect is readily reversible upon normalization of blood glucose levels. Chronic hyperglycemia itself is harmful to the beta-cell and affects both insulin biosynthesis and exocytosis. No unique intracellular defect has been demonstrated to be responsible for all common forms of the disease. However, mutations of the glucokinase gene have been identified in maturity onset diabetes in the young, a particular form of NIDDM.