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

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.

Spectro-spatial features in distributed human intracranial activity proactively encode peripheral metabolic activity

Mounting evidence demonstrates that the central nervous system (CNS) orchestrates glucose homeostasis by sensing glucose and modulating peripheral metabolism. Glucose responsive neuronal populations have been identified in the hypothalamus and several corticolimbic regions. However, how these CNS gluco-regulatory regions modulate peripheral glucose levels is not well understood. To better understand this process, we simultaneously measured interstitial glucose concentrations and local field potentials in 3 human subjects from cortical and subcortical regions, including the hypothalamus in one subject. Correlations between high frequency activity (HFA, 70-170 Hz) and peripheral glucose levels are found across multiple brain regions, notably in the hypothalamus, with correlation magnitude modulated by sleep-wake cycles, circadian coupling, and hypothalamic connectivity. Correlations are further present between non-circadian (ultradian) HFA and glucose levels which are higher during awake periods. Spectro-spatial features of neural activity enable decoding of peripheral glucose levels both in the present and up to hours in the future. Our findings demonstrate proactive encoding of homeostatic glucose dynamics by the CNS.

Effectiveness and safety of pulsatile intravenous insulin therapy for the improvement of respiratory quotient in Chinese patients with diabetes mellitus

Pulsatile intravenous insulin therapy (PIVIT) is a means of imitating naturally occurring insulin pulses artificially. It is thought to improve carbohydrate metabolism, which can be assessed using the respiratory quotient (RQ). The aim of this present study was to assess the efficacy and safety of PIVIT for the improvement of RQ in Chinese patients with diabetes mellitus (DM). This 12-week, multi-center, prospective, randomized, open-label, parallel-group study involved 110 DM patients (both type 1 and type 2) whose RQ was <0.8. Of these, 53 patients formed the control group, in which standard anti-diabetic therapy was maintained, and 54 patients formed the treatment group, which underwent weekly PIVIT in addition to the administration of standard anti-diabetic therapy. RQ was evaluated monthly in control subjects, and before and after every PIVIT treatment in the treatment group. After weekly PIVIT for 12 weeks, the mean RQ increased from 0.70 to 0.90 in the treatment group, but did not change in the control group. The percentage of subjects reporting adverse events (AEs) was 31.5% (17/54) in the treatment group and 9.43% (5/53) in the control group (P=0.0053). The most frequently reported AE (by 12 subjects) was a gastroenteric reaction when these individuals were receiving 50% glucose during the PIVIT treatment. The majority of AEs were mild and did not interfere with the ongoing treatment. Thus, PIVIT can be viewed as tolerated and effective for the improvement of RQ in Chinese DM patients. This study was retrospectively registered with the Chinese Clinical Trial Registry (http://www.chictr.org.cn) on November 13th 2019 (registration no. ChiCTR1900027510).

Free-Living Humans Cross Cardiovascular Disease Risk Categories Due to Daily Rhythms in Cholesterol and Triglycerides

Cardiovascular disease risk assessment relies on single time-point measurement of risk factors. Although significant daily rhythmicity of some risk factors (e.g., blood pressure and blood glucose) suggests that carefully timed samples or biomarker timeseries could improve risk assessment, such rhythmicity in lipid risk factors is not well understood in free-living humans. As recent advances in at-home blood testing permit lipid data to be frequently and reliably self-collected during daily life, we hypothesized that total cholesterol, HDL-cholesterol or triglycerides would show significant time-of-day variability under everyday conditions. To address this hypothesis, we worked with data collected by 20 self-trackers during personal projects. The dataset consisted of 1,319 samples of total cholesterol, HDL-cholesterol and triglycerides, and comprised timeseries illustrating intra and inter-day variability. All individuals crossed at least one risk category in at least one output within a single day. 90% of fasted individuals (n = 12) crossed at least one risk category in one output during the morning hours alone (06:00-08:00) across days. Both individuals and the aggregated group show significant, rhythmic change by time of day in total cholesterol and triglycerides, but not HDL-cholesterol. Two individuals collected additional data sufficient to illustrate ultradian (hourly) fluctuation in triglycerides, and total cholesterol fluctuation across the menstrual cycle. Short-term variability of sufficient amplitude to affect diagnosis appears common. We conclude that cardiovascular risk assessment may be augmented via further research into the temporal dynamics of lipids. Some variability can be accounted for by a daily rhythm, but ultradian and menstrual rhythms likely contribute additional variance.

Evidence for a Coupled Oscillator Model of Endocrine Ultradian Rhythms

Whereas long-period temporal structures in endocrine dynamics have been well studied, endocrine rhythms on the scale of hours are relatively unexplored. The study of these ultradian rhythms (URs) has remained nascent, in part, because a theoretical framework unifying ultradian patterns across systems has not been established. The present overview proposes a conceptual coupled oscillator network model of URs in which oscillating hormonal outputs, or nodes, are connected by edges representing the strength of node-node coupling. We propose that variable-strength coupling exists both within and across classic hormonal axes. Because coupled oscillators synchronize, such a model implies that changes across hormonal systems could be inferred by surveying accessible nodes in the network. This implication would at once simplify the study of URs and open new avenues of exploration into conditions affecting coupling. In support of this proposed framework, we review mammalian evidence for (1) URs of the gut-brain axis and the hypothalamo-pituitary-thyroid, -adrenal, and -gonadal axes, (2) UR coupling within and across these axes; and (3) the relation of these URs to body temperature. URs across these systems exhibit behavior broadly consistent with a coupled oscillator network, maintaining both consistent URs and coupling within and across axes. This model may aid the exploration of mammalian physiology at high temporal resolution and improve the understanding of endocrine system dynamics within individuals.

Investigation of glucose fluctuations by approaches of multi-scale analysis

Glucose variability provides detailed information on glucose control and fluctuation. The aim of this study is to investigate the glucose variability by multi-scale analysis approach on 72-h glucose series captured by continuous glucose monitoring system (CGMS), gaining insights into the variability and complexity of the glucose time series data. Ninety-eight type 2 DM patients participated in this study, and 72-h glucose series from each subject were recorded by CGMS. Subjects were divided into two subgroups according to the mean amplitude of glycemic excursions (MAGE) value threshold at 3.9 based on Chinese standard. In this study, we applied two types of multiple scales analysis methods on glucose time series: ensemble empirical mode decomposition (EEMD) and refined composite multi-scale entropy (RCMSE). With EEMD, glucose series was decomposed into several intrinsic mode function (IMF), and glucose variability was examined on multiple time scales with periods ranging from 0.5 to 12 h. With RCMSE, complexity of the structure of glucose series was quantified at each time scale ranging from 5 to 30 min. Subgroup with higher MAGE value (>3.9) presented higher glycemic baseline and variability. There were significant differences in glycemic variability on IMFs3-5 between subgroups with MAGE>3.9 and MAGE < = 3.9 (p<0.001), but no significant differences in variability on IMFs1-2. The complexity of glucose series quantified by RCMSE showed statistically difference on each time scale from 5 to 30 min between subgroups (p<0.05). Glucose series from subjects with higher MAGE value represented higher variability but lower complexity on multiple time scales. Compared with traditional matrices measuring the glucose variability, approaches of EEMD and RCMSE can quantify the dynamic glycemic fluctuation in multiple time scales and provide us more detailed information on glycemic variability and complexity.

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.

Long-term activation of PKA in β-cells provides sustained improvement to glucose control, insulin sensitivity and body weight

Type 2 diabetes is associated with obesity, insulin resistance and β-cell failure. Therapeutic aims are to reduce adiposity, improve insulin sensitivity and enhance β-cell function. However, it has been proposed that chronically increasing insulin release leads to β-cell exhaustion and failure. We previously developed mice to have increased activity of the cAMP-dependent protein kinase (PKA), specifically in β-cells (β-caPKA mice). β-caPKA mice have enhanced acute phase insulin release, which is the primary determinant of the efficacy of glucose clearance. Here these mice were used to determine the sustainability of enhanced insulin secretion, and to characterize peripheral effects of enhanced β-cell function. Increased PKA activity was induced by tamoxifen administration at 10 weeks of age. Male mice were aged to 12 months of age and female mice to 16 months. Glucose control in both male and female β-caPKA mice was significantly improved relative to littermate controls with ad libitum feeding, upon refeeding after fasting, and in glucose tolerance tests. In female mice insulin release was both greater and more rapid than in controls. Female mice were more insulin sensitive than controls. Male and female β-caPKA mice had lower body weights than controls. DEXA analysis of male mice revealed that this was due to reduced adiposity and not due to changes in lean body mass. This study indicates that targeting β-cells to enhance insulin release is sustainable, maintains insulin sensitivity and reduces body weight. These data identify β-cell PKA activity as a novel target for obesity therapies.

Multi-scale glycemic variability: a link to gray matter atrophy and cognitive decline in type 2 diabetes

OBJECTIVE

Type 2 diabetes mellitus (DM) accelerates brain aging and cognitive decline. Complex interactions between hyperglycemia, glycemic variability and brain aging remain unresolved. This study investigated the relationship between glycemic variability at multiple time scales, brain volumes and cognition in type 2 DM.

RESEARCH DESIGN AND METHODS

Forty-three older adults with and 26 without type 2 DM completed 72-hour continuous glucose monitoring, cognitive tests and anatomical MRI. We described a new analysis of continuous glucose monitoring, termed Multi-Scale glycemic variability (Multi-Scale GV), to examine glycemic variability at multiple time scales. Specifically, Ensemble Empirical Mode Decomposition was used to identify five unique ultradian glycemic variability cycles (GVC1-5) that modulate serum glucose with periods ranging from 0.5-12 hrs.

RESULTS

Type 2 DM subjects demonstrated greater variability in GVC3-5 (period 2.0-12 hrs) than controls (P<0.0001), during the day as well as during the night. Multi-Scale GV was related to conventional markers of glycemic variability (e.g. standard deviation and mean glycemic excursions), but demonstrated greater sensitivity and specificity to conventional markers, and was associated with worse long-term glycemic control (e.g. fasting glucose and HbA1c). Across all subjects, those with greater glycemic variability within higher frequency cycles (GVC1-3; 0.5-2.0 hrs) had less gray matter within the limbic system and temporo-parietal lobes (e.g. cingulum, insular, hippocampus), and exhibited worse cognitive performance. Specifically within those with type 2 DM, greater glycemic variability in GVC2-3 was associated with worse learning and memory scores. Greater variability in GVC5 was associated with longer DM duration and more depression. These relationships were independent of HbA1c and hypoglycemic episodes.

CONCLUSIONS

Type 2 DM is associated with dysregulation of glycemic variability over multiple scales of time. These time-scale-dependent glycemic fluctuations might contribute to brain atrophy and cognitive outcomes within this vulnerable population.

Diabetes and altered glucose metabolism with aging

Diabetes and impaired glucose tolerance affect a substantial proportion of older adults. Abnormal glucose metabolism is not a necessary component of aging. Older adults with diabetes and altered glucose status likely represent a subset of the population at high risk for complications and adverse geriatric syndromes. Goals for treatment of diabetes in the elderly include control of hyperglycemia, prevention and treatment of diabetic complications, avoidance of hypoglycemia, and preservation of quality of life. Research exploring associations of dysglycemia and insulin resistance with the development of adverse outcomes in the elderly may ultimately inform use of future glucose-lowering therapies in this population.

Age-associated changes in pancreatic exocrine secretion of the isolated perfused rat pancreas

Gut functions, such as gastrointestinal motility, gastric secretion and pancreatic secretion, were reduced with age. Glucose tolerance is impaired, and the release of insulin and β-cell's sensitivity on glucose are reduced with age. However, a lot of controversial data have been reported as insulin concentrations after glucose ingestion are either higher or no different in elderly and young subjects. Thus, this study was aimed to investigate whether aging could affect pancreatic exocrine secretion and its action mechanisms. An isolated perfused rat pancreatic model was used to exclude the effects of external nerves or hormones. Pancreatic secretion was increased by CCK under 5.6 mM glucose background in the isolated perfused pancreas of young (3 months), 12 months and 18 months aged rats. There was no significant difference between young and aged rats. In 3 months old rats, CCK-stimulated pancreatic secretion was potentiated under 18 mM glucose background. However, the potentiation effects of endogenous insulin and CCK were not observed in 12 and 18 months old rats. Exogenous insulin also potentiated CCK-stimulated pancreatic secretion in 3 months old rats. Similarly, exogenous insulin failed to potentiate CCK-stimulated pancreatic secretion as that of 3 months old rats. Wet weight of pancreas and amylase content in pancreatic tissue were not changed with age. These results indicate that pancreatic exocrine secretion is reduced with age and endogenous insulin secretion and/or action is involved in this phenomenon.

Shape of glucose, insulin, C-peptide curves during a 3-h oral glucose tolerance test: any relationship with the degree of glucose tolerance?

We aimed to analyze the shape of the glucose, insulin, and C-peptide curves during a 3-h oral glucose tolerance test (OGTT). Another aim was defining an index of shape taking into account the whole OGTT pattern. Five-hundred ninety-two OGTT curves were analyzed, mainly from women with former gestational diabetes, with glycemic concentrations characterized by normal glucose tolerance (n = 411), impaired glucose metabolism (n = 134), and Type 2 diabetes (n = 47). Glucose curves were classified according to their shape (monophasic, biphasic, triphasic, and 4/5-phases), and the metabolic condition of the subjects, divided according to the glucose shape stratification, was analyzed. Indices of shape based on the discrete second-order derivative of the curve patterns were also defined. We found that the majority of the glucose curves were monophasic (n = 262). Complex shapes were less frequent but not rare (n = 37 for the 4/5-phases shape, i.e., three peaks). There was a tendency toward the amelioration of the metabolic condition for increasing complexity of the shape, as indicated by lower glucose concentrations, improved insulin sensitivity and β-cell function. The shape index computed on C-peptide, WHOSH(CP) (WHole-Ogtt-SHape-index-C-peptide), showed a progressive increase [monophasic: 0.93 ± 0.04 (dimensionless); 4/5-phases: 1.35 ± 0.14], and it showed properties typical of β-cell function indices. We also found that the type of glucose shape is often associated to similar insulin and C-peptide shape. In conclusion, OGTT curves can be characterized by high variability, and complex OGTT shape is associated with better glucose tolerance. WHOSH(CP) (WHole-Ogtt-SHape-index) may be a powerful index of β-cell function much simpler than model-based indices.

Blood glucose dynamics

Measurement of blood glucose concentration is central to the diagnosis and treatment of diabetes. Although there are large numbers of historic glucose measurements in individuals with diabetes, until recently there have been very few data sets that were recorded continuously or sampled frequently enough to reveal intrinsic blood glucose dynamics, or the change in blood glucose with time. There have even fewer such recordings from individuals not having diabetes to serve as a therapeutic target. As a result, blood glucose dynamics have generally not been used in the diagnosis or treatment of the disease. Although present blood glucose monitoring is based largely on discrete measurements, future monitoring will likely focus on analysis of blood glucose excursions. New measurements are now being obtained, and there is a need for new methods of analysis to extract the maximal information from the data. Several approaches are demonstrated here for characterization of blood glucose dynamics, and a patient profiling system is proposed. An example of new insights is the observation that there are four time scales of blood glucose variations in individuals without diabetes, and these time scales are modified or lost in diabetes.

Gastric emptying, diabetes, and aging

Gastric emptying is mildly slowed in healthy aging, although generally remains within the normal range for young people. The significance of this is unclear, but may potentially influence the absorption of certain drugs, especially when a rapid effect is desired. Type 2 diabetes is common in the elderly, but there is little data regarding its natural history, prognosis, and management. This article focuses on the interactions between gastric emptying and diabetes, how each is influenced by the process of aging, and the implications for patient management.

Evaluation of neuroendocrine status in longevity

It is well known that physiological changes in the neuroendocrine system may be related to the process of aging. To assess neuroendocrine status in aging humans we studied a group of 155 women including 78 extremely old women (centenarians) aged 100-115 years, 21 early elderly women aged 64-67 years, 21 postmenopausal women aged 50-60 years and 35 younger women aged 20-50 years. Plasma NPY, leptin, glucose, insulin and lipid profiles were evaluated, and serum concentrations of pituitary, adrenal and thyroid hormones were measured. Our data revealed several differences in the neuroendocrine and metabolic status of centenarians, compared with other age groups, including the lowest serum concentrations of leptin, insulin and T3, and the highest values for prolactin. We failed to find any significant differences in TSH and cortisol levels. On the other hand, LH and FSH levels were comparable with those in the elderly and postmenopausal groups, but they were significantly higher than in younger subjects. GH concentrations in centenarians were lower than in younger women. NPY values were highest in the elderly group and lowest in young subjects. We conclude that the neuroendocrine status in centenarians is markedly different from that found in early elderly or young women.

Diabetes mellitus in the elderly: insulin resistance and/or impaired insulin secretion?

Elderly people are more glucose intolerant and insulin resistant than young individuals, and many of them will develop type 2 diabetes. It remains, however, controversial whether this decrease in function is due to an inevitable consequence of "biological aging" or due to environmental or lifestyle variables. Indeed, increased adiposity/altered fat distribution, decreased fat free mass/abnormal muscle composition, poor dietary habits and physical inactivity all contribute to reduce insulin sensitivity. Insulin resistance in elderly people appears to predominate in skeletal muscle, whereas hepatic glucose output seems to be almost unaffected. Several abnormalities in islet beta-cell and insulin secretion were also pointed out in elderly people such as increased amyloid deposition and decreased amylin secretion, impaired insulin secretion pulsatility, decreased insulin sensitivity of pancreatic beta-cells to insulinotropic gut hormones and diminished insulin response to non-glucose stimuli such as arginine. Controversial results were reported concerning the effects of aging on absolute insulin secretion in response to oral or intravenous glucose. However, insulin secretion appears to decrease with age, with significantly diminished beta-cell sensitivity and acute insulin response to glucose, provided it is analyzed relative to concomitant decreased insulin sensitivity. Thus, there is an interplay between decreased insulin secretion and increased insulin resistance that largely explains the abnormal glucose metabolism seen in elderly. Weight loss, especially reduction of abdominal adiposity, and increased physical activity may contribute to improve insulin sensitivity and glucose tolerance, and prevent the development of type 2 diabetes in elderly people.

Effect of donor age on function of isolated human islets

This study intended to evaluate the impact of donor age on the function of isolated islets. Analysis of human islets from cadaveric donors (age 16-70 years) was performed using glucose-stimulated insulin release (GSIR) (n = 93), islet ATP content (n = 27), diabetic nude mouse bioassay (n = 72), and the insulin secretory function after single-donor clinical islet allotransplantation (n = 7). The GSIR index was significantly higher in younger donors (age < or =40 years) than in older donors and negatively correlated with the donor age (r = -0.535). Islet ATP was higher in younger donors (115.7 +/- 17.7 vs. 75.7 +/- 6.6 pmol/microg DNA). The diabetes reversal rate of mice with 2,000 IE was significantly higher in younger donors (96 vs. 68%). C-peptide increment to glucose during intravenous glucose tolerance test at days 90-120 after clinical transplantation showed negative correlation with donor age (r = -0.872) and positive correlation with the islet mass (r = 0.832). On the other hand, acute insulin response to arginine only showed correlation with the islet mass and not with donor age. These results show that insulin secretory response to glucose deteriorates with increasing age and that it may be related to changes in ATP generation in beta-cells.

Prevalence and Determinants of Impaired Glucose Metabolism in Frail Elderly Patients: The Belgian Elderly Diabetes Survey (BEDS)

BACKGROUND

Although diabetes in elderly persons is generally type 2, the metabolic abnormalities associated with aging suggest that elderly persons may differ from younger persons with type 2 diabetes. In addition, nonobese elderly persons with type 2 diabetes show a marked impairment in insulin release accompanied by mild insulin resistance, whereas obese elderly persons have marked insulin resistance in the presence of "adequate" levels of insulin. Other factors that could adversely affect glucose tolerance in aging include drug use, associated disease, and other stressful conditions commonly encountered in geriatric inpatients units. The authors' objectives in this study were 1) to prospectively assess the prevalence of glucose homeostasis abnormalities among elderly hospitalized patients and the degree to which it reflects abnormalities in insulin secretion or insulin sensitivity using homeostasis model assessment of fasting glucose, insulin, and C-peptide; and 2) to define the social, functional, pathologic, and nutritional characteristics of persons with impaired glucose tolerance or diabetes.

METHODS

Ninety-eight patients underwent a comprehensive geriatric assessment. Determinants of glucose homeostasis were assessed using the homeostasis model assessment, which provides estimates of beta-cell function (%B) and insulin sensitivity (%S).

RESULTS

Twelve patients (12%) had fasting glucose concentrations greater than 110 mg/dl. Four patients had impaired fasting glucose levels greater than 110 mg/dl but less than 126 mg/dl (IFG group), and 8 patients had levels greater than 126 mg/dl (type 2 diabetes group). Except for a higher proportion of women in the IFG-diabetes group, the latter did not exhibit significant differences in functional, morbidity, or nutritional characteristics compared with the normal glucose tolerance group. The entire cohort (n=98) presented with a mean (+/-SD) %B of 71%+/-47% and a mean %S of 208%+/-198%. Compared with the normal glucose tolerance group, the IFG-diabetes group had a fasting glycemia level of 142+/-24 mg/dl (vs 92+/-9 mg/dl), a %B of 43%+/-21% (vs 74%+/-45%), and a mean %S of 126%+/-113% (vs 219%+/-205%).

CONCLUSIONS

These data confirm the high prevalence of impaired glucose metabolism among elderly people, although the usual risk factors were not significantly increased. Marked beta secretory defects seem to be the rule, whereas a significant degree of insulin resistance is unusual.

Unique characteristics of the geriatric diabetic population and the role for therapeutic strategies that enhance glucagon-like peptide-1 activity

PURPOSE OF REVIEW

Care for elderly diabetic patients poses a unique clinical challenge. This review highlights distinct aspects of the pathophysiology and the risks for secondary complications in the geriatric diabetic population. Based on these considerations, we discuss emerging therapeutic options based on the actions of the incretin hormone glucagon-like peptide (GLP)-1, which may be ideal for achieving glycemic control in the elderly diabetic patient.

RECENT FINDINGS

Aging is associated with diminished capacity of pancreatic beta-cells to respond to glucose. This functional decline in beta-cell insulin secretion is a major contributor to the development of diabetes in the older patient. In addition, elderly diabetics suffer from a broader range of diabetic complications than do younger diabetics, warranting aggressive glycemic control. GLP-1 is known to improve beta-cell insulin secretion, increase beta-cell mass, and suppress glucagon secretion. Recent studies investigating improved GLP-1 activity have yielded promising results, with improved glycemic control in elderly patients with type 2 diabetes and without significant risk for hypoglycemia.

SUMMARY

Elderly diabetics are a growing subset of the type 2 diabetic population with unique pathophysiologic characteristics and diabetic risk profiles. Therapeutic strategies that incorporate enhancement of GLP-1 action on beta-cells to improve beta-cell insulin secretion and glycemic control may be ideal for this distinct population and should be validated with further long-term clinical studies.

Endogenous glucose production, insulin sensitivity, and insulin secretion in normal glucose-tolerant Pima Indians with low birth weight

Individuals with low birth weight (LBW) are at increased risk of developing type 2 diabetes in later life. Whether impairments in endogenous glucose production (EGP), insulin action, insulin secretion, or a combination thereof account for this association is unclear. We, therefore, examined these parameters in Pima Indians with normal glucose tolerance. Body composition, glucose and insulin responses during a 75-g oral glucose tolerance test (OGTT), EGP, insulin-stimulated glucose disposal during low- and high-dose insulin infusion (M-low and M-high, hyperinsulinemic glucose clamp), and acute insulin response (AIR) to a 25-g intravenous glucose challenge were measured in 230 Pima Indians (147 men and 83 women, aged 25 +/- 0.4 years [mean +/- SE; range, 18 to 44]) with normal glucose tolerance. A subgroup of 63 subjects additionally underwent biopsies of subcutaneous adipose tissue for determination of adipocyte cell size and lipolysis. Subjects in the lowest quartile of birth weight (birth weight: 2,891 +/- 33 g, LBW, n = 58) were compared to those whose birth weight was in the upper 3 quartiles (birth weight: 3,657 +/- 28 g, NBW, n = 172). Age- and sex-adjusted body mass index (BMI), percent body fat, and waist-to-thigh ratio (WTR) were similar in LBW and NBW subjects. Suppression of EGP during the clamp was less in LBW than in NBW subjects before (P = .002) and after adjustment for age, sex, percent body fat, and M-low (P = .02). M-low and M-high were less in LBW than in NBW subjects before (P = .05 and P = .01) and after adjustment for age, sex, percent body fat, and WTR (P = .04 and P = .05). AIR was not different in LBW compared to NBW subjects before adjustments (P = .06), but it was lower in LBW than in NBW subjects after adjustment for age, sex, percent body fat, and M-low (P = .02), suggesting that AIR did not increase appropriately for the decrease in insulin-stimulated glucose disposal (M). In addition, average adipocyte cell size (P = .08) and basal lipolysis (P = .02) were higher in the LBW than in the NBW group. These results show that Pima Indians with LBW manifest a variety of impairments in metabolism in adulthood. Among these, a lesser insulin-stimulated suppression of EGP and a lesser insulin secretory capacity are the predominant ones. We conclude that interaction of multiple defects may contribute to increased susceptibility to type 2 diabetes among individuals with LBW.

Aging and insulin secretion

Glucose tolerance progressively declines with age, and there is a high prevalence of type 2 diabetes and postchallenge hyperglycemia in the older population. Age-related glucose intolerance in humans is often accompanied by insulin resistance, but circulating insulin levels are similar to those of younger people. Under some conditions of hyperglycemic challenge, insulin levels are lower in older people, suggesting beta-cell dysfunction. When insulin sensitivity is controlled for, insulin secretory defects have been consistently demonstrated in aging humans. In addition, beta-cell sensitivity to incretin hormones may be decreased with advancing age. Impaired beta-cell compensation to age-related insulin resistance may predispose older people to develop postchallenge hyperglycemia and type 2 diabetes. An improved understanding of the metabolic alterations associated with aging is essential for the development of preventive and therapeutic interventions in this population at high risk for glucose intolerance.

L-arginine: an ultradian-regulated substrate coupled with insulin oscillations in healthy volunteers

OBJECTIVE

Coupled oscillations of 50-110 min in insulin and glucose have been found previously in healthy men under continuous enteral nutrition. Because L-arginine induces insulin release as glucose does, we tested the hypothesis that L-arginine can also display such an ultradian rhythm.

RESEARCH DESIGN AND METHODS

Seven healthy male subjects participated in one experimental night during which blood was sampled every 10 min from 2300 to 0700. Plasma glucose, C-peptide, and L-arginine levels were measured simultaneously. The insulin secretion rate (ISR) was calculated from plasma C-peptide levels by a deconvolution procedure.

RESULTS

Plasma glucose followed the recognizable profiles, with oscillations closely linked to similar changes in the ISR. Pulse analysis of L-arginine profiles revealed significant oscillations linked to glucose and ISR oscillations, with the highest cross-correlation coefficients at time lag 0 ranging from 0.380 to 0.680 for glucose and L-arginine and from 0.444 to 0.726 for ISR and L-arginine (P < 0.01). The mean period of L-arginine oscillations was 77.2 +/- 6.2 min, and their mean amplitude was 19.9 +/- 1.7%, similar to that of glucose (17.0 +/- 1.9%), when expressed as the percentage of mean overnight levels.

CONCLUSIONS

This newly discovered ultradian rhythm of L-arginine and its coupling with glucose and ISR oscillations sheds new light on the regulation of L-arginine, the substrate of numerous metabolic pathways, including nitric oxide synthesis. These oscillations may be of significance in conditions of hyperinsulinemia or abnormal glucose tolerance.

Pulsatile insulin release from islets isolated from three subjects with type 2 diabetes

Plasma insulin in healthy subjects shows regular oscillations, which are important for the hypoglycemic action of the hormone. In individuals with type 2 diabetes, these regular variations are altered, which has been implicated in the development of insulin resistance and hyperglycemia. The origin of the change is unknown, but derangement of the islet secretory pattern has been suggested as a contributing cause. In the present study, we show the dynamics of insulin release from individually perifused islets isolated from three subjects with type 2 diabetes. Insulin release at 3 mmol/l glucose was 10.5 +/- 4.5 pmol.g(-1).s(-1) and pulsatile (0.26 +/- 0.05 min(-1)). In islets from one subject, 11 mmol/l glucose transiently increased insulin release by augmentation of the insulin pulses without affecting the frequency. Addition of 1 mmol/l tolbutamide did not increase insulin release. In islets from the remaining subjects, insulin release was not affected by 11 mmol/l glucose. Tolbutamide transiently increased insulin release in islets from one subject. Insulin release from four normal subjects at 3 mmol/l glucose was 4.3 +/- 0.8 pmol.g(-1).s(-1) and pulsatile (0.23 +/- 0.03 min(-1)). At 11 mmol/l glucose, insulin release increased in islets from all subjects. Tolbutamide further increased insulin release in islets from two subjects. It is concluded that islets from the three individuals with type 2 diabetes release insulin in pulses. The impaired secretory response to glucose may be related to impaired metabolism before mitochondrial degradation of the sugar.

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.

Relationships among age, proinsulin conversion, and beta-cell function in nondiabetic humans

The aim of the present study was to examine the relationships among beta-cell function, proinsulin conversion to insulin, and age. We studied insulin and proinsulin secretion in nondiabetic subjects during an oral glucose tolerance test (OGTT) using published indexes of beta-cell function (n = 379, age 16--68 years) and a modified hyperglycemic clamp (10 mmol/l, additional glucagon-like peptide [GLP-1] infusion, final arginine bolus; n = 50, age 19--68 years). Proinsulin conversion to insulin was assessed using proinsulin/insulin (PI/I) ratios immediately after an acute stimulus (OGTT, 30 min; hyperglycemic clamp, 2.5-5.0 min after glucose and arginine). There was a negative correlation between age and beta-cell function (adjusted for insulin sensitivity, BMI, and fasting glucose) in the OGTT (r = -0.21, P < 0.001) and first phase of the hyperglycemic clamp (r = -0.30, P = 0.03), but not second phase (r = -0.08, P = 0.6) or arginine-induced insulin secretion (r = 0.06, P = 0.7). There was a positive correlation between age and the PI/I ratio in the OGTT (r = 0.24, P < 0.001). Analogously, there was also a positive correlation between age and the PI/I ratio during first phase (r = 0.37, P = 0.009) and arginine stimulation (r = 0.33, P = 0.01) of the hyperglycemic clamp. First-phase insulin secretion of the hyperglycemic clamp was inversely correlated with the PI/I ratio (r = -0.60, P < 0.001). Interestingly, adjusting first-phase secretion rate for the PI/I ratio abolished the linear relationship with age (r = -0.06, P = 0.7). In conclusion, aging is associated with deteriorating beta-cell function and deteriorating proinsulin conversion to insulin. The age effect on insulin secretion appears to be attributable at least in part to an impairment of proinsulin conversion to insulin.

Do oscillations of insulin secretion occur in the absence of cytoplasmic Ca2+ oscillations in beta-cells?

That oscillations of the cytoplasmic free Ca(2+) concentration ([Ca(2+)](i)) in beta-cells induce oscillations of insulin secretion is not disputed, but whether metabolism-driven oscillations of secretion can occur in the absence of [Ca(2+)](i) oscillations is still debated. Because this possibility is based partly on the results of experiments using islets from aged, hyperglycemic, hyperinsulinemic ob/ob mice, we compared [Ca(2+)](i) and insulin secretion patterns of single islets from 4- and 10-month-old, normal NMRI mice to those of islets from 7- and 10-month-old ob/ob mice (Swedish colony) and their lean littermates. The responses were subjected to cluster analysis to identify significant peaks. Control experiments without islets and with a constant insulin concentration were run to detect false peaks. Both ob/ob and NMRI islets displayed large synchronous oscillations of [Ca(2+)](i) and insulin secretion in response to repetitive depolarizations with 30 mmol/l K(+) in the presence of 0.1 mmol/l diazoxide and 12 mmol/l glucose. Continuous depolarization with high K(+) steadily elevated [Ca(2+)](i) in all types of islets, with no significant oscillation, and caused a biphasic insulin response. In islets from young (4-month-old) NMRI mice and 7-month-old lean mice, the insulin profile did not show significant peaks when [Ca(2+)](i) was stable. In contrast, two or more peaks were detected over 20 min in the response of most ob/ob islets. Similar insulin peaks appeared in the insulin response of 10-month-old lean and NMRI mice. However, the size of the insulin peaks detected in the presence of stable [Ca(2+)](i) was small, so that no more than 10-13% of total insulin secretion occurred in a pulsatile manner. In conclusion, insulin secretion does not oscillate when [Ca(2+)](i) is stably elevated in beta-cells from young normal mice. Some oscillations are observed in aged mice and are seen more often in ob/ob islets. These fluctuations of the insulin secretion rate at stably elevated [Ca(2+)](i), however, are small compared with the large oscillations induced by [Ca(2+)](i) oscillations in beta-cells.

Atherosclerosis and pancreatic damage

The appearance of diabetes in advanced age may be considered as a part of the involutive processes of aging, and as such, it might have a pathogenesis completely different from that of type 2 diabetes of medium age. As a matter of fact, it has been observed that the pancreas undergoes numerous structural and functional alterations with advancing age, both in exocrine and endocrine parts. The present studies have been performed to reveal the quantity and quality of the pancreatic lesions, which may be attributed to atherosclerosis. We have already studied elderly subjects, therefore, we were now looking for further supports in a population of middle age people, died in complications of malignant hypertension. We investigated the pancreas, kidney and heart of 36 subjects (20 males and 16 females) with mean age of 48.6+/-8.9 years. Of this group, eight subjects (22.2%) became diabetic after the appearance of malignant hypertension. Arteriolar atherosclerosis damage (hyalinosis, thickening and stenosis) of the pancreatic arterioles were found in 92.8% of the non-diabetic, and in 87.5% of the diabetic subjects. Lesions of the pancreatic islets were observed in 32% of the non-diabetics, and in 50% of the diabetic subjects. The pancreas is an organ, which tends particularly to develop atherosclerotic damage. The vascular lesion of atherosclerotic origin, independently from the mechanism of its appearance, causes first only a decrease of the blood flux and hypoxia in the pancreatic islets with a consecutive functional decline of the beta-cells. This is then followed by structural modifications of the islets accompanied by the appearance of hyalinosis, loss of beta-cells, and a further decrease of insulin production.

Correlation among fasting plasma glucose, two-hour plasma glucose levels in OGTT and HbA1c

A study was made on the association among 2-h plasma glucose (PG) in oral glucose tolerance test (OGTT), fasting plasma glucose (FPG) using correlation and regression equation. Subjects were 13174 OGTT examinees tested between 1980 and 1998. Blood glucose was determined by the glucose oxidase method and glycated hemoglobin (HbA1c) by the HPLC method. As for correlation between 2-h PG and FPG, regression equation of the <60 year group was y=57.1+0.336x (r=0.866, P<0.0001) and that of the >==60 year group was y=61.5+0.286x (r=0.814, P<0. 0001). FPG was calculated at 124.3 in the <60 year group and 118.7 mg/dl in the >==60 year group for 2-h PG of 200 mg/dl, 2-h PG were calculated at 199.5 and 210.7 mg/dl for FPG of 126 mg/dl, respectively. In the <60 year group, FPG were calculated at 121.7 and 124.4 mg/dl and 2-h PG at 193.2 and 199.3 mg/dl for HbA1c of 6.0 and 6.1%, respectively. As for associations between HbA1c and FPG or 2-h PG being high correlation, it is possible to estimate a prevalence of DM in a group using HbA1c>==6.1%. High correlations were demonstrated among all the three measures; FPG, 2-h PG, HbA1c. If 2-h PG is used in diagnosing diabetes mellitus, an FPG of 126 mg/dl proposed by ADA and World Health Organization (WHO) as a diagnostic level of FPG is an acceptable value for the Japanese.

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.

Twenty-four-hour rhythms of plasma glucose and insulin secretion rate in regular night workers

To determine whether the ultradian and circadian rhythms of glucose and insulin secretion rate (ISR) are adapted to their permanent nocturnal schedule, eight night workers were studied during their usual 24-h cycle with continuous enteral nutrition and a 10-min blood sampling procedure and were compared with 8 day-active subjects studied once with nocturnal sleep and once with an acute 8-h-shifted sleep. The mean 24-h glucose and ISR levels were similar in the three experiments. The duration and the number of the ultradian oscillations were influenced neither by the time of day nor by the sleep condition or its shift, but their mean amplitude increased during sleep whenever it occurred. In day-active subjects, glucose and ISR levels were high during nighttime sleep and then decreased to a minimum in the afternoon. After the acute sleep shift, the glucose and ISR rhythms were split in a biphasic pattern with a slight increase during the night of deprivation and another during daytime sleep. In night workers, the glucose and ISR peak levels exhibited an 8-h shift in accordance with the sleep shift, but the onset of the glucose rise underwent a shift of only 6 h and the sleep-related amplification of the glucose and ISR oscillations did not occur simultaneously. These results demonstrate that despite a predominant influence of sleep, the 24-h glucose and ISR rhythms are only partially adapted in permanent night workers.

The effect of age and glycemic level on the response of the beta-cell to glucose-dependent insulinotropic polypeptide and peripheral tissue sensitivity to endogenously released insulin

Normal aging is characterized by a progressive impairment in glucose tolerance. An important mechanism underlying the glucose intolerance of aging is an impairment in glucose-induced insulin release. These studies were conducted to determine whether the age-related impairment in insulin release was caused by a decreased beta-cell sensitivity to glucose-dependent insulinotropic polypeptide (GIP). Thirty-one Caucasian men were divided into four groups: two young groups (age range: 19-26 yr, n = 15) and two old groups (age range: 67-79 yr, n = 16). Each volunteer participated in three studies (n = 93 clamps). Hyperglycemic clamps were conducted at two doses [basal plasma glucose (G) + 5.4 mmol/L and G + 12.8 mmol/L] for 120 min. In the initial hyperglycemic clamp, only glucose was infused. In subsequent studies, GIP was infused at a final rate of 2 or 4 pmol/ kg(-1) x min(-1) from 60-120 min. Basal plasma insulin and GIP levels were similar in the young (41 +/- 6 and 51 +/- 6 pmol/L) and the old subjects (42 +/- 6 and 66 +/- 12 pmol/L) in all studies. First- and second-phase insulin responses were similar during the control study and during the first 60 min of each GIP infusion study in both groups. The 90-120 min GIP values were similar between groups and between hyperglycemic plateaus during the 2 and 4 pmol/kg(-1) x min(-1) GIP infusion (young: 342 +/- 28 and 601 +/- 44 pmol/L, old: 387 +/- 45 and 568 +/- 49 pmol/L). In response to the GIP infusions, significant increases in insulin occurred in young and old at both glucose levels (P < 0.01). The potentiation of the insulin response caused by GIP was greater in the young subjects than in the old, in the G + 5.4 mmol/L studies (P < 0.05). However, the insulin response to GIP was similar in both young and old during the G + 12.8 mmol/L clamps. The insulinotropic effect of the incretin was higher in the young and in the old, in the G + 12.8 mmol clamps, than in the G + 5.4 mmol/L clamps. We conclude that normal aging is characterized by a decreased beta-cell sensitivity to GIP during modest hyperglycemia, which may explain, in part, the age-related impairment in glucose-induced insulin release. We also find that the insulinotropic effect of GIP is increased with increasing levels of hyperglycemia.

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.

Non-insulin-dependent diabetes mellitus in the elderly

The prevalence of non-insulin-dependent diabetes mellitus dramatically increases with age. Older diabetic subjects have an increased frequency of complications from diabetes compared with their younger counterparts and higher morbidity and mortality rates compared with age-matched non-diabetic controls. Elderly patients with diabetes are generally treated following the same approach as in younger patients: dietary therapy first, followed by oral hypoglycaemic agents and ultimately insulin. However, several specificities should be pointed out. Changes associated with ageing may affect the pharmacokinetics and pharmacodynamics of both sulphonylureas (increasing the risk of severe hypoglycaemia) and biguanides (increasing the risk of lactic acidosis). The best insulin regimen in old age is not known, but a twice-daily injection of a pre-mixed insulin preparation is usually recommended. Goals of therapy must be realistic and not cause disabling side-effects. The general practitioner plays a crucial role in the care of elderly diabetic patients, but access to a multidisciplinary specialized team may be necessary.