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The article discusses the historical context and scientific understanding of diabetes, focusing on the interplay between pancreatic β cell function and insulin resistance, as elucidated by the hyperbolic law of glucose tolerance. Topics include the distinction between type 1 and type 2 diabetes, the debate over the primary pathogenetic defect in type 2 diabetes, and the application of the hyperbolic curve in studying glucose regulation and predicting disease onset.

Woolcott, Orison O., and Richard N. Bergman. Mortality attributed to COVID-19 in high-altitude populations. High Alt Med Biol. 21:409–416, 2020. Background: Since partial oxygen pressure decreases as altitude increases, environmental hypoxia could worsen Coronavirus Disease 2019 (COVID-19) patient's hypoxemia. We compared COVID-19 mortality at different altitudes. Methods: Retrospective analysis of population-level data on COVID-19 deaths was conducted in the United States (1,016 counties) and Mexico (567 municipalities). Mixed-model Poisson regression analysis of the association between altitude and COVID-19 mortality was conducted using individual-level data from 40,168 Mexican subjects with COVID-19, adjusting for multiple covariates. Results: Between January 20 and April 13, 2020, mortality rates were higher in U.S. counties located at ≥2,000 m elevation versus those located <1,500 m (12.3 vs. 3.2 per 100,000; p < 0.001). In Mexico, between March 13 and May 13, 2020, mortality rates were higher in municipalities located at ≥2,000 m versus those located <1,500 m (5.3 vs. 3.9 per 100,000; p < 0.001). Among Mexican subjects younger than 65 years, the risk of death was 36% higher in those living at ≥2,000 m versus those living at <1,500 m (adjusted incidence rate ratio [IRR]: 1.36; confidence interval [95% CI], 1.05–1.78; p = 0.022). Among Mexican men, the risk of death was 31% higher at ≥2,000 m versus that at <1,500 m (adjusted IRR: 1.31; 95% CI, 1.03–1.66; p = 0.025). No association between altitude and COVID-19 mortality was found among Mexican women or among Mexican subjects 65 years of age and older. Conclusions: Altitude is associated with COVID-19 mortality in men younger than 65 years. [ABSTRACT FROM AUTHOR]

Insulin clearance has recently been highlighted as a fundamental aspect of glucose metabolism, as it has been hypothesized that its impairment could be related to an increased risk of developing type 2 diabetes. This review focuses on methods used to calculate insulin clearance: from the early surrogate indices employing C-peptide:insulin molar ratio, to direct measurement methods used in animal models, to modeling-based techniques to estimate the components of insulin clearance (hepatic versus extrahepatic). The methods are explored and interpreted by critically highlighting advantages and limitations. [ABSTRACT FROM AUTHOR]

There is wide variance among individuals in the fraction of insulin cleared by the liver (20% to 80%). Hepatic insulin clearance is 67% lower in African Americans than European Americans. Clearance is also lower in African American children 7-13 years of age. Lower hepatic insulin clearance will result in peripheral hyperinsulinemia: this exacerbates insulin resistance, which stresses the β-cells, possibly resulting in their ultimate failure and onset of type 2 diabetes. We hypothesize that lower insulin clearance can be a primary cause of type 2 diabetes in at-risk individuals. [ABSTRACT FROM AUTHOR]

The article discusses the use of deep phenotyping in diabetes and the association between reduced hepatic insulin degradation and type 2 diabetes mellitus (T2DM) pathogenesis. Several factors that determine glucose tolerance are discussed including hepatic insulin clearance, variation in hepatic insulin extraction, and insulin signaling. Particular focus is also given to insulin resistance and &#946-cell function.

On the basis of studies that investigated the intraportal versus systemic insulin infusion and transendothelial transport of insulin, we proposed the "single gateway hypothesis," which supposes an indirect regulation of hepatic glucose production by insulin; the rate-limiting transport of insulin across the adipose tissue capillaries is responsible for the slow suppression of free fatty acids (FFAs), which in turn is responsible for delayed suppression of hepatic endogenous glucose production (EGP) during insulin infusion. Preventing the fall in plasma FFAs during insulin infusion either by administering intralipids or by inhibiting adipose tissue lipolysis led to failure in EGP suppression, thus supporting our hypothesis. More recently, mice lacking hepatic Foxo1 in addition to Akt1 and Akt2 (L-AktFoxo1TKO), all required for insulin signaling, surprisingly showed normal glycemia. Inhibiting the fall of plasma FFAs in these mice prevented the suppression of EGP during a clamp, reaffirming that the site of insulin action to control EGP is extrahepatic. Measuring whole-body turnover rates of glucose and FFAs in L-AktFoxo1TKO mice also confirmed that hepatic EGP was regulated by insulin-mediated control of FFAs. The knockout mouse model in combination with sophisticated molecular techniques confirmed our physiological findings and the single gateway hypothesis. [ABSTRACT FROM AUTHOR]

Activation of the sympathetic nervous system (SNS) constitutes a putative mechanism of obesity-induced insulin resistance. Thus, we hypothesized that inhibiting the SNS by using renal denervation (RDN) will improve insulin sensitivity (SI) in a nonhypertensive obese canine model. SI was measured using euglycemic-hyperinsulinemic clamp (EGC), before (week 0 [w0]) and after 6 weeks of high-fat diet (w6-HFD) feeding and after either RDN (HFD + RDN) or sham surgery (HFD + sham). As expected, HFD induced insulin resistance in the liver (sham 2.5 ± 0.6 vs. 0.7 ± 0.6 × 10-4 dL ⋅ kg-1 ⋅ min-1 ⋅ pmol/L-1 at w0 vs. w6-HFD [P < 0.05], respectively; HFD + RDN 1.6 ± 0.3 vs. 0.5 ± 0.3 × 10-4 dL ⋅ kg-1 ⋅ min-1 ⋅ pmol/L-1 at w0 vs. w6-HFD [P < 0.001], respectively). In sham animals, this insulin resistance persisted, yet RDN completely normalized hepatic SI in HFD-fed animals (1.8 ± 0.3 × 10-4 dL ⋅ kg-1 ⋅ min-1 ⋅ pmol/L-1 at HFD + RDN [P < 0.001] vs. w6-HFD, [P not significant] vs. w0) by reducing hepatic gluconeogenic genes, including G6Pase, PEPCK, and FOXO1. The data suggest that RDN downregulated hepatic gluconeogenesis primarily by upregulating liver X receptor α through the natriuretic peptide pathway. In conclusion, bilateral RDN completely normalizes hepatic SI in obese canines. These preclinical data implicate a novel mechanistic role for the renal nerves in the regulation of insulin action specifically at the level of the liver and show that the renal nerves constitute a new therapeutic target to counteract insulin resistance. [ABSTRACT FROM AUTHOR]

Insulin clearance is a highly variable and important factor that affects circulating insulin concentrations. We developed a novel model-based method to estimate both hepatic and extrahepatic insulin clearance using plasma insulin and C-peptide profiles obtained from the insulin-modified frequently sampled intravenous glucose tolerance test. Data from 100 African immigrants without diabetes (mean age 38 years, body weight 81.7 kg, fasting plasma glucose concentration 83 mg/dL, and fasting insulin concentration 37 pmol/L) were used. Endogenous insulin secretion (calculated by C-peptide deconvolution) and insulin infusion rates were used as inputs to a new two-compartment model of insulin kinetics and hepatic and extrahepatic clearance parameters were estimated. Good agreement between modeled and measured plasma insulin profiles was observed (mean normalized root mean square error 6.8%), and considerable intersubject variability in parameters of insulin clearance among individuals was identified (the mean [interquartile range] for hepatic extraction was 25.8% [32.7%], and for extrahepatic insulin clearance was 20.7 mL/kg/min [11.7 mL/kg/min]). Parameters of insulin clearance were correlated with measures of insulin sensitivity and acute insulin response to glucose. The method described appears promising for future research aimed at characterizing variability in insulin clearance and the mechanisms involved in the regulation of insulin clearance. [ABSTRACT FROM AUTHOR]

Prevalence of Type 2 diabetes has increased at an alarming rate, highlighting the need to correctly predict the development of this disease in order to allow intervention and thus, slow progression of the disease and resulting metabolic derangement. There have been many recent 'advances' geared toward the detection of pre-diabetes, including genome wide association studies and metabolomics. Although these approaches generate a large amount of data with a single blood sample, studies have indicated limited success using genetic and metabolomics information alone for identification of disease risk. Clinical assessment of the disposition index (DI), based on the hyperbolic law of glucose tolerance, is a powerful predictor of Type 2 diabetes, but is not easily assessed in the clinical setting. Thus, it is evident that combining genetic or metabolomic approaches for a more simple assessment of DI may provide a useful tool to identify those at highest risk for Type 2 diabetes, allowing for intervention and prevention. [ABSTRACT FROM AUTHOR]

In the 1950's, Dr. I. Arthur Mirsky first recognized the possible importance of insulin degradation changes to the pathogenesis of type 2 diabetes. While this mechanism was ignored for decades, insulin degradation is now being recognized as a possible factor in diabetes risk. After Mirsky, the relative importance of defects in insulin release and insulin resistance were recognized as risk factors. The hyperbolic relationship between secretion and sensitivity was introduced, as was the relationship between them, as expressed as the disposition index (DI). The DI was shown to be affected by environmental and genetic factors, and it was shown to be differentiated among ethnic groups. However, the importance of differences in insulin degradation (clearance) on the disposition index relationship remains to be clarified. Direct measure of insulin clearance revealed it to be highly variable among even normal individuals, and to be affected by fat feeding and other physiologic factors. Insulin clearance is relatively lower in ethnic groups at high risk for diabetes such as African Americans and Hispanic Americans, compared to European Americans. These differences exist even for young children. Two possible mechanisms have been proposed for the importance of insulin clearance for diabetes risk: in one concept, insulin resistance per se leads to reduced clearance and diabetes risk. In a second and new concept, reduced degradation is a primary factor leading to diabetes risk, such that lower clearance (resulting from genetics or environment) leads to systemic hyperinsulinemia, insulin resistance, and beta-cell stress. Recent data by Chang and colleagues appear to support this latter hypothesis in Native Americans. The importance of insulin clearance as a risk factor for metabolic disease is becoming recognized and may be treatable. [ABSTRACT FROM AUTHOR]

Review of the structure and function of the endothelium, its contribution to the function of hormones, and its involvement in disease. [ABSTRACT FROM AUTHOR]

The article presents the different approaches in order to address the increasing rates of type 2 diabetes in the U.S., in westernized countries, and in the third world. Understanding the pathogenesis of diabetes would lead to better approaches to diagnose, prevent and to treat existing cases. In the reductionist approach, knowledge of disease would result from describing events at increasingly more microscopic levels. In the systems approach, mathematical or computer models are used.

The minimal model was proposed over 25 years ago. Despite (or because of) its simplicity it continues to be used today – both as a clinical tool and an approach to understanding the composite effects of insulin secretion and insulin sensitivity on glucose tolerance and risk for type 2 diabetes mellitus. The original assumptions of the model have led to an understanding of the kinetics of insulin in vivo, as well as the relative importance of β-cell compensatory failure in the pathogenesis of diabetes. The disposition index (DI), a parameter emerging from the model, represents the ability of the pancreatic islets to compensate for insulin resistance. There is evidence that a locus on chromosome 11 codes for the DI, which has a significant heritability and can predict type 2 diabetes better than any known genetic locus. Even today, the model continues to be a subject of scientific discovery and discourse. Copyright © 2005 S. Karger AG, Basel [ABSTRACT FROM AUTHOR]

In an earlier paper (Cohen and Bergman, Am. J. Physiol. 268 (1995) E397), we explored the relationship between the exponents in the exponential curve fit to isotopic enrichment versus time and the fractional turnover rate of the largest metabolic pool in the pathway. Here we present the analysis on a more rigorous footing and apply it to questions of cerebral and cardiac metabolism. Our emphasis in this paper is to describe and justify mathematically an approach for analysis of metabolic dynamics, not with the intention of replacing the use of numerical software for estimation of flux rates but for giving the scientist the opportunity to examine the system in an approximate manner, and thereby to check not only that the results of the numerical solution are the correct solutions to the equations but also that the equations portray the correct simplification of the metabolic pathway.We introduce the “dominant rate constant” as a tool for deriving algebraic formulas relating rates of metabolic flux, sizes of metabolic pools, and the dynamics of isotopic enrichment. Illustrations of such algebraic formulas are provided for the rates of the citric acid cycle (CAC), glycolysis and glutamine synthesis in brain, as well as the rate of the CAC in heart. In addition, we prove that formulas for estimation of rates of glycolysis and of the CAC depend critically on the fractional turnover rates of lactate and glutamate, respectively.The justification for analysis of simulated data is that we are studying the effects of simplifications of metabolic models on the accuracy of estimation of metabolic pathways. Our use of the dominant rate constant is an analytical convenience that allows us to assess proposed simplifications of metabolic pathways. [Copyright &y& Elsevier]

Insulin resistance is an important risk factor for development of type 2 diabetes as well as other chronic conditions, including hypertension, cardiovascular disease, and colon cancer. To find genes for insulin resistance it is necessary to assess insulin action in large populations. We have previously measured insulin action in a large cohort of subjects (Insulin Resistance and Atherosclerosis Study [IRAS] Family Study) using the minimal model approach. In this study, we compare sensitivity from the minimal model (insulin sensitivity index [S(I)]) with the measure of insulin resistance emanating from the homeostasis model assessment (HOMA) approach. The former measure emerges from the glycemic response to endogenous and exogenous insulin; the latter is based solely on fasting measures of glucose and insulin. A total of 112 pedigrees were represented, including 1,362 individuals with full phenotypic assessment. Heritability of S(I) was significantly greater than that for HOMA (0.310 vs. 0.163) and for fasting insulin (0.171), adjusted for age, sex, ethnicity, and BMI. In addition, correlation between S(I) and either HOMA or fasting insulin was only approximately 50% accounted for by genetic factors, with the remainder accounted for by environment. Thus S(I), a direct measure of insulin sensitivity, is determined more by genetic factors rather than measures such as HOMA, which reflect fasting insulin. [ABSTRACT FROM AUTHOR]

PURPOSE: Insulin resistance and visceral adiposity are associated with increased risk of type 2 diabetes. In this report, we describe the methods of the IRAS Family Study, which was designed to identify the genetic and environmental risk factors for insulin resistance and visceral adiposity.METHODS: Families from two ethnic groups (African American and Hispanic) have been recruited from three clinical sites. Blood samples for DNA as well as other standard measures were collected. A CT scan (visceral adiposity) and a frequently sampled glucose tolerance test (insulin resistance) were performed. Preliminary estimates of heritability for indirect measures related to insulin resistance and visceral adiposity were obtained using a variance components approach in the first 93 families (approximately 1000 individuals).RESULTS: Estimates of heritability ranged from low (0.08) for fasting insulin and HOMA, to moderate (0.28) for fasting glucose, to high (0.54) for BMI. After adjustment for age, gender and ethnicity, all heritability estimates were significantly greater than zero (p < 0.05).CONCLUSIONS: These results are consistent with the expectation that intermediate measures of insulin resistance and visceral adiposity are heritable, and that the IRAS Family Study has statistical power to detect these intermediate phenotypes of type 2 diabetes and atherosclerosis. [Copyright &y& Elsevier]

Objective: The objective of this study was to compare insulin resistance relative to body fat and the associated compensatory responses in 57 healthy children living in Los Angeles, California (14 Caucasians, 15 African-Americans, and 28 Hispanics).Research Design and Methods: Insulin sensitivity and acute insulin response were determined by intravenous glucose tolerance test. Insulin secretion, hepatic insulin extraction, and insulin clearance were estimated by C-peptide and insulin modeling.Results: Insulin sensitivity was significantly lower in Hispanics and African-Americans compared with Caucasian children, and acute insulin response was significantly higher in African-American children. No ethnic differences were noted in the first-phase secretion, but second-phase insulin secretion was significantly higher in Hispanic children than in African-American children (200 +/- 53 vs. 289 +/- 41 nmol/min; P = 0.03). The greater acute insulin response in African-Americans, despite lower secretion, was explained by a lower hepatic insulin extraction in African-Americans compared with Hispanics (36.6 +/- 2.9 vs. 47.3 +/- 2.2%; P = 0.0006).Conclusions: In conclusion, Hispanic and African-American children are more insulin resistant than Caucasian children, but the associated compensatory responses are different across ethnic groups. [ABSTRACT FROM AUTHOR]

Discusses principles related to molecular evolution that may be discovered by the repetitive experimental testing of simple isomorphic computer or mathematical models of biological control systems. Mechanism of insulin; Importance of free fatty acids in controlling endogenous glucose production; Information on the hyperbolic law of glucose tolerance.

Objective: This study aimed to establish whether total fat or central fat was related to measures of insulin in obese Hispanic children with a family history of type 2 diabetes.Research Design and Methods: Subjects were 32 children aged 8-13 years. Visceral fat and subcutaneous abdominal fat were determined by magnetic resonance imaging at the umbilicus and total body fat was determined by dual-energy X-ray absorptiometry. Insulin sensitivity (S(i)) and acute insulin response (AIR) were determined by frequently sampled intravenous tolerance test with minimal modeling.Results: Mean fasting glucose and insulin, S(i), and AIR (+/- SD) were 5.3 +/- 0.3 mmol/l, 206 +/- 105 pmol/l, 11.8 +/- 5.7 [x 10(-4) min(-1)/(pmol/l)], and 17,175 +/- 9,695 (pmol/l x 10 min), respectively. In multivariate regression analysis, total fat mass was independently and positively related to fasting insulin (P < 0.01) and negatively related to S(i) (P < 0.05) but was not related to AIR. Visceral fat was independently and positively related to fasting insulin (P < 0.05) and AIR (P < 0.01) and negatively related to S(i) (P < 0.001).Conclusions: -These findings support the hypothesis that specific accumulation of visceral fat in addition to overall adiposity in Hispanic children increases the risk of type 2 diabetes. [ABSTRACT FROM AUTHOR]

Only in the last decade did modeling studies predict, and knockout experiments confirm, that type 2 diabetes is a "2-hit" disease in which insulin resistance is necessarily accompanied by beta-cell defect(s) preventing the compensatory upregulation of insulin secretion. This long- delayed insight was associated with the development of a constant, the "disposition index," describing the beta-cell sensitivity-secretion relationship as a rectangular hyperbola. Shifts in insulin sensitivity are accompanied by compensatory alterations in beta-cell sensitivity to glucose. Insulin-sensitive subjects do not require a massive insulin response to exogenous glucose to maintain a normal blood glucose. But if their insulin sensitivity decreases by 80%, as in late pregnancy, they need a fivefold greater insulin response to achieve an identical disposition index. Women with gestational diabetes have an insulin response similar to that of normal volunteers; at first glance, this suggests similar islet function, but the utility of the disposition index is to normalize this response to the amplitude of third trimester insulin resistance, revealing severe beta-cell deficiency. The index is a quantitative, convenient, and accurate tool in analyzing epidemiologic data and identifying incipient impaired glucose tolerance. Separate major issues remain, however: the causes of insulin resistance, the causes of the failure of adequate beta-cell compensation in type 2 diabetes, and the nature of the signal(s) from insulin-resistant tissues that fail to elicit the appropriate beta-cell increment in sensitivity to glucose and other stimuli. The disposition index is likely to remain the most accurate physiologic measure for testing hypotheses and solutions to these challenges in whole organisms. [ABSTRACT FROM AUTHOR]

Only in the last decade did modeling studies predict, and knockout experiments confirm, that type 2 diabetes is a “2-hit” disease in which insulin resistance is necessarily accompanied by β-cell defect(s) preventing the compensatory upregulation of insulin secretion. This long-delayed insight was associated with the development of a constant, the “disposition index,” describing the β-cell sensitivity-secretion relationship as a rectangular hyperbola. Shifts in insulin sensitivity are accompanied by compensatory alterations in β-cell sensitivity to glucose. Insulin-sensitive subjects do not require a massive insulin response to exogenous glucose to maintain a normal blood glucose. But if their insulin sensitivity decreases by 80%, as in late pregnancy, they need a fivefold greater insulin response to achieve an identical disposition index. Women with gestational diabetes have an insulin response similar to that of normal volunteers; at first glance, this suggests similar islet function, but the utility of the disposition index is to normalize this response to the amplitude of third trimester insulin resistance, revealing severe β-cell deficiency. The index is a quantitative, convenient, and accurate tool in analyzing epidemiologlc data and identifying incipient impaired glucose tolerance. Separate major issues remain, however: the causes of insulin resistance, the causes of the failure of adequate β-cell compensation in type 2 diabetes, and the nature of the signal(s) from insulin-resistant tissues that fail to elicit the appropriate β-cell increment in sensitivity to glucose and other stimuli. The disposition index is likely to remain the most accurate physiologic measure for testing hypotheses and solutions to these challenges in whole organisms. Diabetes 51 (Suppl. 1):S212–S220, 2002 [ABSTRACT FROM AUTHOR]

Our laboratory has previously demonstrated that hypoglycemic detection occurs in the portal vein, not the liver. To ascertain whether hypoglycemic detection may also occur in the hepatic artery, normoglycemia was established across the liver via a localized hepatic artery glucose infusion. Male mongrel dogs (n = 7) were infused with insulin (5.0 mU x kg(-1) x min(-1)) via the jugular vein to induce systemic hypoglycemia. Animals participated in two hyperinsulinemic-hypoglycemic clamp experiments distinguished by the site of glucose infusion. During the liver irrigation protocol, glucose was infused via the hepatic artery (HA protocol) to maintain liver normoglycemia as systemic glucose concentrations were systematically lowered over 260 min (nadir = 2.2 +/- 0.01 mmol/l). During control experiments, glucose was infused peripherally (PER protocol) to control reductions in blood glucose. Arterial glucose concentrations were not significantly different at any time between the two protocols (P = 0.73). Hepatic artery and liver glucose concentrations were significantly elevated in the HA versus PER protocol throughout the duration of the progressive hyperinsulinemic-hypoglycemic clamp. During the PER protocol, epinephrine and norepinephrine concentrations increased significantly above basal values (0.53 +/- 0.06 and 0.85 +/- 0.2 nmol/l, respectively) to plateaus of 4.4 +/- 0.86 (P = 0.0001) and 3.6 +/- 0.69 nmol/l (P = 0.001), respectively. There were no significant differences between the two protocols in the epinephrine (P = 0.81) and the norepinephrine (P = 0.68) response to hypoglycemia. The current findings indicate that glucosensors important to hypoglycemic detection do not reside in the hepatic artery. Furthermore, these data confirm our previous findings that glucosensors important to hypoglycemic detection are not present in the liver, but are in fact localized to the portal vein. [ABSTRACT FROM AUTHOR]

Reports the suppression of endogenous glucose production (EGP) by the inhibition of lipolysis under constant insulin in dogs. Difference in the sensitivity of free fatty acid and EGP suppression; Prevention of EGP reduction in lyposin control experiments; Increase of catecholamine and glucagon levels during N[sup 6]-cyclohexyladenosine infusion.

Presents information on a study which determined whether acute suppression of lipolysis will cause suppression of endogenous glucose production in vivo. Materials and methods; Results; Discussion.

The implication of beta-cell failure as an early defect in type 2 diabetes exacerbates the need for accurate but facile assessment of islet cell secretory rate, particularly in large group studies in which individual assessment of C-peptide kinetics is impractical. This study was designed to examine whether it is possible to obtain accurate secretory rates from the extended combined model, which provides insulin and C-peptide kinetics from plasma measurements of the two peptides. Equimolar intraportal infusions of insulin and C-peptide that are designed to simulate insulin secretion rates during both oral and intravenous glucose tolerance tests were used to generate plasma insulin and C-peptide data in conscious dogs that were examined under clamped glucose conditions. The plasma peptide kinetics were analyzed using the extended combined model to generate estimates of prehepatic insulin secretion that were then compared with the known intraportal infusion rates. The extended combined model was able to reproduce the known intraportal infusion profiles. The model-predicted rates were similar to those calculated with methods that require separate assessment of C-peptide kinetics. Simulation results supported lesser clearance of insulin during rapid changes of portal insulin (as measured by an intravenous glucose tolerance test) versus slow changes in portal insulin (as measured by an oral glucose tolerance test). The extended combined model accurately calculates prehepatic insulin appearance. It may be possible to apply this approach to large studies of beta-cell function designed to identify changes in islet function in subjects at risk for diabetes. Such an approach could strengthen epidemiological and genetic studies of the pathogenesis of diabetes. [ABSTRACT FROM AUTHOR]

We sought to elucidate the role of the portal vein afferents in the sympathetic response to hypoglycemia. Laparotomy was performed on 27 male Wistar rats. Portal veins were painted with either 90% phenol (denervation group [PDN]) or 0.9% saline solution (sham-operated group [SHAM]). Rats were chronically cannulated in the carotid artery (sampling), jugular vein (infusion), and portal vein (infusion). After a recovery period of 5 days, animals were exposed to a hyperinsulinemic-hypoglycemic clamp, with glucose infused either portally (POR) or peripherally (PER). In all animals, systemic hypoglycemia (2.48+/-0.09 mmol/l) was induced via jugular vein insulin infusion (50 mU x kg(-1) x min(-1)). Arterial plasma catecholamines were assessed at basal (-30 and 0 min) and during sustained hypoglycemia (60, 75, 90, and 105 min). By design, portal vein glucose concentrations were significantly elevated during POR versus PER (4.4+/-0.14 vs. 2.5+/-0.07 mmol/l; P<0.01, respectively) for both PDN and SHAM. There were no significant differences in arterial glucose or insulin concentration between the four experimental conditions at any point in time. When portal glycemia and systemic glycemia fell concomitantly (SHAM-PER), epinephrine increased 12-fold above basal (3.75+/-0.34 and 44.56+/-6.1 nmol/l; P<0.001). However, maintenance of portal normoglycemia (SHAM-POR) caused a 50% suppression of the epinephrine response, despite cerebral hypoglycemia (22.2+/-3.1 nmol/l, P<0.001). Portal denervation resulted in a significant blunting of the sympathoadrenal response to whole-body hypoglycemia (PDN-PER 27.6+/-3.8 nmol/l vs. SHAM-PER; P<0.002). In contrast to the sham experiments, there was no further suppression in arterial epinephrine concentrations observed during PDN-POR versus PDN-PER (P = 0.8). These findings indicate that portal vein afferent innervation is critical for hypoglycemic detection and normal sympathoadrenal counterregulation. [ABSTRACT FROM AUTHOR]

Sodium glucose cotransporter-2 (SGLT2) inhibitors promote urinary glucose excretion, consequently reducing blood glucose. Previously we showed that 6 weeks of Dapagliflozin (Dapa) treatment improved insulin sensitivity and prevented weight gain. In addition, Dapa promoted adipose tissue (AT) beiging, lipolysis and decreased adipocyte size primarily in the visceral (VIS) depot in a canine model of mild type 2 diabetes. Little is known about the effects of Dapa on AT inflammation and ceramide synthesis in both the subcutaneous (SC) and VIS fat depots. Twelve dogs were fed a high fat diet for 6 weeks, then received a single low dose of streptozotocin (18.5mg/kg) to induce mild type 2 diabetes. Animals were then exposed to Dapa (n=6, 1.25mg/kg) or placebo (n=6) once per day for 6 weeks, while remaining on the high fat diet. At the end of the treatment, fasting plasma parameters were measured, and SC and VIS fat biopsies were obtained for molecular studies. Dapa increased adiponectin, adiponectin receptors 1 and 2 gene expression by 90%, 74% and 69% (<0.001) in VIS depot respectively. Adiponectin gene expression was upregulated by 85% in the SC (P<0.05). Inflammation markers such as TNFα and IL-6 are downregulated by 74% (P<0.05) and 86% (P<0.001) in the VIS depot respectively. In addition, genes involved in ceramide synthesis, toll like receptor 4 (TLR4) and serine palmitoyl-CoA transferase (SPTLC2) decreased by 77% and 73 % respectively (P<0.05) in the VIS depot. In conclusion, Dapa promoted adiponectin and its receptors, decreased inflammation and enzymes involved in ceramide synthesis mainly in the VIS depot. These changes were independent of fasting glucose and insulin. Thus, for the first time we are beginning to identify mechanisms by which Dapa enhances AT function in regulating energy homeostasis and improving AT inflammation in mild type 2 diabetes canine model. Disclosure: M. Kabir: None. R. N. Bergman: None. V. Gopaul: None. H. Yang: Employee; Self; AstraZeneca. C. M. Kolka: Research Support; Self; AstraZeneca. Funding: National Institutes of Health [ABSTRACT FROM AUTHOR]

A Peripheral CB1R Antagonist Increases Lipolysis, Oxygen Consumption Rate, and Markers of Beiging in 3T3-L1 Adipocytes Similar to RIM, Suggesting That Central Effects Can Be Avoided. Graph: Figure 2 Peripheral cannabinoid receptor 1 (CB1R) antagonist increased oxygen consumption rate (OCR) inhibited by lipolysis blocker. Figures Graph: Figure 1 Peripheral cannabinoid receptor 1 (CB1R) antagonist increased oxygen consumption rate (OCR). [Extracted from the article]

In our study, we also performed a multivariate Poisson regression analysis using individual data to carefully examine the association between altitude and COVID-19-related mortality among Mexican subjects with COVID-19. We reaffirm our conclusions, among Mexican subjects with COVID-19, altitude is associated with COVID-19 mortality in men <65 years of age. Second, the authors suggest that case fatality rate (CFR) should decrease with altitude based on a report of a higher proportion of asymptomatic coronavirus disease 2019 (COVID-19) cases at higher altitude (Kong et al., [1]). [Extracted from the article]

Discusses the estimation of tricarboxylic acid (TCA) cycle flux, aminotransferase flux and anaplerosis in heart. Mathematical analysis of enrichment of carbons of glutamate; Formula for estimation of flux rates testing; Concentrations of metabolites in simulation of heart cells.

Presents a syntactic approach for the computer simulation of biochemical fluxes in the citric acid cycle in heart. Model of chemical kinetics; Simulation of heart cells; Isotopomers of glutamate during influx of (2-13C)acetate; Comparison between the predictions of the syntactic approach and those of a nuclear magnetic resonance-based model.

Examines transendothelial insulin transport (TET) under non-steady-state conditions. Determination of insulin action in vivo by TET across the capillary and insulin binding and postreceptor events; Intravenous glucose tolerance test; Model for minimal insulin sensitivity; TET as a contributor to insulin sensitivity in vivo; Insulin kinetics.

Looks at a study which examined the formulas for relative anaplerosis, while demonstrating that using isotopomer analysis conventional formulas may underestimate its value. Analysis of carbons or isotopomer; Methodology used to conduct the study; Results of the study.

Examines the assumptions of formulas for relative anaplerosis in heart using [13C]glutamate labeling by nuclear magnetic resonance spectroscopy. Estimates of relative anaplerosis and values of fractional enrichments of carbons C-i of glutamate and pyruvate; Effects of malate-aspartate shuttle on estimation of relative anaplerosis.

With the increased prevalence of obesity and related co-morbidities, such as type 2 diabetes (T2D), worldwide, improvements in pharmacological treatments are necessary. The brain- and peripheral-cannabinoid receptor 1 (CB1R) antagonist rimonabant (RIM) has been shown to induce weight loss and improve glucose homeostasis. We have previously demonstrated that RIM promotes adipose tissue beiging and decreased adipocyte cell size, even during maintenance on a high-fat diet. Given the adverse side-effects of brain-penetrance with RIM, in this study we aimed to determine the site of action for a non-brain-penetrating CB1R antagonist AM6545. By using in vitro assays, we demonstrated the direct effects of this non-brain-penetrating CB1R antagonist on cultured adipocytes. Specifically, we showed, for the first time, that AM6545 significantly increases markers of adipose tissue beiging, mitochondrial biogenesis, and lipolysis in 3T3-L1 adipocytes. In addition, the oxygen consumption rate (OCR), consisting of baseline respiratory rate, proton leak, maximal respiratory capacity, and ATP synthase activity, was greater for cells exposed to AM6545, demonstrating greater mitochondrial uncoupling. Using a lipolysis inhibitor during real-time OCR measurements, we determined that the impact of CB1R antagonism on adipocytes is driven by increased lipolysis. Thus, our data suggest the direct role of CB1R antagonism on adipocytes does not require brain penetrance, supporting the importance of focus on peripheral CB1R antagonism pharmacology for reducing the incidence of obesity and T2D. [ABSTRACT FROM AUTHOR]

Selective sodium glucose cotransporter-2 inhibition promotes urinary glucose excretion, thus reducing blood glucose. Previously we showed that 6 weeks of Dapagliflozin (Dapa) treatment improved insulin sensitivity and prevented weight gain in a canine model of mild type 2 diabetes. Little is known about the effects of Dapa on adipose tissue (AT) function. Eleven dogs were fed a high fat diet for 6 weeks, then received a single low dose of streptozotocin (18.5mg/kg) to induce mild type 2 diabetes. Animals were then exposed to Dapa (n=5, 1.25mg/kg) or placebo (n=6) once per day for 6 weeks, while remaining on the high fat diet. At the end of the study, subcutaneous (SC) and visceral (VIS) fat depots were obtained for adipocytes isolation and molecular studies and fasting plasma parameters were measured. Fasting plasma glucose and insulin levels were unaffected by Dapa treatment. Finite mixture modeling linear regression was used for adipocyte distributions. Dapa decreased adipocyte size and bimodal distributions in both SC and VIS by 10% (P<0.001). Dapa increased UCP1, PGC1α and Prdm16 gene expression by 2.68, 2.34 and 2.16 folds (<0.05) in SC and 2.5, 3.6 and 1.6 folds (<0.05) in VIS depots respectively. Immunostaining of UCP1 in SC and VIS tissues confirmed beiging process. Primary adipocytes were cultured for 4 and 24h. Lipolysis, indicated by glycerol release into the media increased by 40% in SC and 43% in vis after 24h (P<0.05). Adiponectin levels increased only in the SC adipocytes by 80% (P<0.05) after 4 and 24h cultures. Leptin levels increased in SC (by 3 folds P<0.05) and VIS by 2.6 P<0.05) after 24h adipocyte cultures. In conclusion, Dapa promotes AT beiging, lipolysis, adiponectin and leptin release by adipocytes and decreased adipocyte size and distributions. These changes were independent of glucose and insulin. Thus, for the first time we are beginning to identify mechanisms by which Dapa enhances AT function and improves energy homeostasis in mild type 2 diabetes canine model. Disclosure: M. Kabir: None. R.N. Bergman: None. F. Piccinini: Employee; Self; Medtronic. D. Stefanovski: None. V. Gopaul: None. H. Yang: Employee; Self; AstraZeneca. C.M. Kolka: Research Support; Self; AstraZeneca. Funding: AstraZeneca (D1690L00045-CSR212966) [ABSTRACT FROM AUTHOR]

A correction is presented to the article related to role of Reduced Hepatic Insulin Clearance in the Pathogenesis of Type 2 Diabetes that published in 2019 issue of the periodical.

We evaluated the ability of the Relative Fat Mass (RFM) to estimate whole-body fat percentage among children and adolescents who participated in the National Health and Nutrition Examination Survey from 1999 through 2006 (n = 10,390). The RFM equation for adults (64 − (20 × height/waist circumference) + (12 × sex)) may be used for adolescents 15 to 19 years of age. For children and adolescents 8 to 14 years of age, we suggest a modified RFM equation, named as the RFMp (RFM pediatric): 74 − (22 × height/waist circumference) + (5 × sex). In both equations, sex equals 0 for boys and 1 for girls. RFMp was more accurate than BMI to estimate whole-body fat percentage (measured by dual energy X-ray absorptiometry, DXA) among girls (percentage of estimates that were <20% of measured body fat percentage, 88.2% vs. 85.7%; P = 0.027) and boys 8 to 14 years of age (83.4% vs. 71.0%; P < 0.001). RFM was more accurate than BMI among boys 15 to 19 years of age (82.3% vs. 73.9%; P < 0.001) but slightly less accurate among girls (89.0% vs. 92.6%; P = 0.002). Compared with BMI-for-age percentiles, RFMp had lower misclassification error of overweight or obesity (defined as a DXA-measured body fat percentage at the 85th percentile or higher) among boys 8 to 14 years of age (6.5% vs. 7.9%; P = 0.018) but not girls (RFMp: 8.2%; BMI-for-age: 7.9%; P = 0.681). Misclassification error of overweight or obesity was similar for RFM and BMI-for-age percentiles among girls (RFM: 8.0%; BMI-for-age: 6.6%; P = 0.076) and boys 15 to 19 years of age (RFM: 6.9%; BMI-for-age: 7.8%; P = 0.11). RFMp for children and adolescents 8 to 14 years of age and RFM for adolescents 15 to 19 years of age were useful to estimate whole-body fat percentage and diagnose body fat-defined overweight or obesity. [ABSTRACT FROM AUTHOR]

We have previously demonstrated in dogs that following exposure to a high fat diet for 16 weeks, the brain and peripheral penetrating CB1R antagonist Rimonabant (RIM) promoted adipose tissue beiging and decreased adipocyte cell size. It is hoped that peripherally restricted CB1R antagonists, such as AM6545, will allow for the substantial beneficial metabolic effects of CB1R antagonism without the possible negative consequences associated with brain penetrance. Studies demonstrated beneficial metabolic effects of AM6545 in adipose tissue including increased energy expenditure and decreased adipocytes cell size. We thus compared the effects of RIM versus AM6545 on oxygen consumption and beiging in the well-characterized 3T3-L1 cell line model. Mature 3T3-L1s were treated for 4 or 48 hours with RIM, AM6545, vehicle or isoproterenol (ISO) (positive control). Baseline oxygen consumption and the proportional contribution of proton leak to maximal respiratory capacity was greater for cells exposed to RIM, ISO, and, in lesser extent, AM6545 after 48 hours, demonstrating greater mitochondrial uncoupling. Further, markers of adipose tissue beiging (UCP1, Prdm16, PGC1α) were also unregulated by approximately 2-fold (P<0.05) at 48 hours with RIM treatment and, in lesser extent, with AM6545. Hence, peripheral and central CB1R antagonists act directly on 3T3-L1 to increase mitochondrial function and adopt features of beige adipocytes. Thus, a direct role for CB1R antagonism on adipocytes does not require brain penetrance, supporting the importance of peripheral CB1R antagonism pharmacology for reducing the incidence of obesity and type 2 diabetes. Disclosure: R.K. Paszkiewicz: None. R.N. Bergman: Consultant; Self; Zafgen, Inc. A. Frank: None. O.O. Woolcott: None. R. de Souza Santos: None. M.S. Iyer: None. D. Clegg: None. M. Kabir: None. [ABSTRACT FROM AUTHOR]

ZGN-1061 is a second-generation methionine aminopeptidase 2 (MetAP2) inhibitor in development for treatment of T2D. Full results of a Phase 2 clinical trial of ZGN-1061 vs. placebo (PBO) administered SC for 12 weeks on change from baseline in A1C in subjects with T2D are reported elsewhere. Baseline (ITT population N=175) A1C 8.6±1.1%, BMI 37±7 kg/m2, T2D duration 8±6 y. ZGN-1061 was generally well tolerated; 93% completed the trial. LS mean±SE A1C change was 0.2±0.1% for PBO vs. ±0.4±0.1% and -0.9±0.2% for 0.9 and 1.8 mg ZGN-1061 (both p<0.001 vs. PBO). PBO weight change was -0.6±0.3 kg vs. -1.4±0.3 kg (p=ns) and ±2.8±0.5 kg (p<0.001) for 0.9 and 1.8 mg ZGN-1061. This analysis included a subset of subjects who participated in a mixed-meal tolerance test (MMTT) at baseline and week 12. ZGN-1061 produced dose-dependent improvements in fasting and postprandial glucose (p<0.05 vs. PBO). Minimal model analysis showed improvements in insulin sensitivity (SI) and disposition index (DI_OB; a measure of β-cell compensation) from baseline to week 12 with ZGN-1061 (Table). In an exploratory combined analysis of 0.9+1.8 mg ZGN-1061, SI and DI_OB increased (p=0.02 and p=0.07). SI and DI_OB were unchanged with PBO. In patients with T2D, improved glycemic control with ZGN-1061 may be driven by changes in insulin sensitivity and β-cell function. These results support future assessments using other definitive methods. Disclosure: R. Fabunmi: None. R.N. Bergman: Consultant; Self; Zafgen, Inc. F. Piccinini: None. T. Kim: Employee; Spouse/Partner; Celgene Corporation. Employee; Self; Zafgen, Inc. D. Zhuang: Employee; Self; Zafgen, Inc. K. Taylor: Employee; Self; Zafgen, Inc. D.D. Kim: Employee; Self; Zafgen, Inc. Funding: Zafgen, Inc. [ABSTRACT FROM AUTHOR]

CB1 receptor (CB1R) antagonism improves the deleterious effects of a high-fat diet (HFD) by reducing body fat mass and adipocyte cell size. Previous studies demonstrated that the beneficial effects of the CB1R antagonist rimonabant (RIM) in white adipose tissue (WAT) are partially due to an increase of mitochondria numbers and upregulation thermogenesis markers, suggesting an induction of WAT beiging. However, the molecular mechanism by which CB1R antagonism induces weight loss and WAT beiging is unclear. In this study, we probed for genes associated with beiging and explored longitudinal molecular mechanisms by which the beiging process occurs. HFD dogs received either RIM (HFD+RIM) or placebo (PL) (HFD+PL) for 16 wk. Several genes involved in beiging were increased in HFD+RIM compared with pre-fat, HFD, and HFD+PL. We evaluated lipolysis and its regulators including natriuretic peptide (NP) and its receptors (NPRs), β-1 and β-3 adrenergic receptor (β1R, β3R) genes. These genes were increased in WAT depots, accompanied by an increase in lipolysis in HFD+RIM. In addition, RIM decreased markers of inflammation and increased adiponectin receptors in WAT. We observed a small but significant increase in UCP1; therefore, we evaluated the newly discovered UCP1-independent thermogenesis pathway. We confirmed that SERCA2b and RYR2, the two key genes involved in this pathway, were upregulated in the WAT. Our data suggest that the upregulation of NPRs, β-1R and β-3R, lipolysis, and SERCA2b and RYR2 may be one of the mechanisms by which RIM promotes beiging and overall the improvement of metabolic homeostasis induced by RIM. [ABSTRACT FROM AUTHOR]

Context: Insulin resistance is associated with multiple complex diseases; however, precise measures of insulin resistance are invasive, expensive, and time-consuming. Objective: Develop estimation models for measures of insulin resistance, including insulin sensitivity index (SI) and homeostatic model assessment of insulin resistance (HOMA-IR) from metabolomics data. Design: Insulin Resistance Atherosclerosis Family Study (IRASFS). Setting: Community based. Participants: Mexican Americans (MA) and African Americans (AA). Main outcome: Estimation models for measures of insulin resistance, i.e. SI and HOMA-IR. Results: Least Absolute Shrinkage and Selection Operator (LASSO) and Elastic Net regression were used to build insulin resistance estimation models from 1274 metabolites combined with clinical data, e.g. age, sex, body mass index (BMI). Metabolite data were transformed using three approaches, i.e. inverse normal transformation, standardization, and Box Cox transformation. The analysis was performed in one MA recruitment site (San Luis Valley, Colorado (SLV); N = 450) and tested in another MA recruitment site (San Antonio, Texas (SA); N = 473). In addition, the two MA recruitment sites were combined and estimation models tested in the AA recruitment sample (Los Angeles, California; N = 495). Estimated and empiric SI were correlated in the SA (r2 = 0.77) and AA (r2 = 0.74) testing datasets. Further, estimated and empiric SI were consistently associated with BMI, low-density lipoprotein cholesterol (LDL), and triglycerides. We applied similar approaches to estimate HOMA-IR with similar results. Conclusions: We have developed a method for estimating insulin resistance with metabolomics data that has the potential for application to a wide range of biomedical studies and conditions. [ABSTRACT FROM AUTHOR]

A response by the author of the article "Estimating Hepatic Glucokinase Activity Using a Simple Model of Lactate Kinetics" which was published in a previous issue of thejournal "Diabetes Care" is presented.

An abstract of the article "Obesity, insulin resistance, and the pathway to type 2 diabetes," by Richard N. Bergman is presented.

It has been suggested that insulin resistance associated with normal aging is caused by a concomitant increase in visceral adipose mass. Previously, we presented data indicating that old animals may in fact have a greater susceptibility to abdominal adipose-induced insulin resistance as compared to young. We asked whether older animals are more at risk for insulin resistance due to central and/or peripheral adiposity by examining young and old rats under conditions of weight loss. Young (Y: 6 mos, n=34) high-fat-fed (3 weeks; 66% calories from fat) and old (O: 24 mos, n=35) obese male Fisher 344xBN rats were subjected to 0, 2, 4, or 6 weeks of caloric restriction (CR; 40% reduction in ad libitum calories). Insulin sensitivity (S[sub 1]) was assessed by euglycemic hyperinsulinemic (5mU/min per kg) clamp following a 6 hour fast. To determine body composition, a bolus of [sup 18]O-water (0.5g/kg BW) was administered to discern total body water and reveal lean body mass (LBM) and total body fat. Also, abdominal (AB) fat depots (epididymal, perirenal and mesenteric) were excised and weighed at necropsy to determine both AB and SubQ fat mass. Six weeks of CR caused a mild, but significant reduction in body weight in both Y and O animals (Y: ↓ 12.96±1.3%; O: ↓ 13.77±2.1%; p<0.05 by ANOVA), and was similar for both age groups (p=0.748). Surprisingly, this change in body weight was not due to a decline in SubQ fat (0 vs 6 wks for Y: p=0.629; O: p=0.370) or LBM (0 vs 6 wks for Y: p=0.641; O: p=0.257). In stark contrast, significant toss occurred in the AB fat depot (Y: ↓ 46.43±1.7%; O: ↓ 28.51±4.6%; p<0.05 by ANOVA). Most notably, mesenteric fat, the only true visceral fat depot in the rodent, was reduced equivalently in Y (↓ 52.70±2.53 %) and O (↓ 52.63±4.09%) rats with 6 weeks of CR (Y vs. O p=0.537). Despite strikingly similar losses in visceral fat in Y and O rats, S[sub I] improved to a lesser extent in O rats (↑ 32.76±9.80%) compared to Y (↑ 82.91±12.66%) when compared to 0 weeks. Comparison of regression analyses for these data further demonstrated that for a given loss of visceral fat mass, O rats exhibited less improvement in S[sub I] (p<0.0001). In conclusion, subcutaneous fat can not account for improvements in insulin sensitivity; rather these data demonstrate a preferential loss of fat in the abdominal depot with short-term diet restriction in both old and young animals. Furthermore, old rats have an increased susceptibility to abdominal-fat induced insulin resistance. The mechanism of this increased risk in older animals to obesity remains unclear. [ABSTRACT FROM AUTHOR]

Abstract: Visceral adiposity has been identified as an independent risk factor for cardiovascular disease and the so-called metabolic syndrome. The canine obesity model closely recapitulates the correlation between human visceral adiposity and insulin resistance. A recent canine study indicates that insulin expands the volume of distribution associated with skeletal muscle, and that its ability to enhance macromolecular distribution within this space is blunted in the fat-fed obese canine model. Our canine study supports the portal theory of insulin resistance, in which free fatty acids (FFAs) from visceral fat directly enter the liver and have a detrimental effect on insulin action. The role of adipokines in this condition remains less clear. Sympathetic nervous system hyperactivity in obesity may also contribute to excessive FFA release, hypertension, and insulin resistance. Pathologies interrelated with insulin resistance include ?-cell hypersecretion, reduced insulin clearance, and resultant hyperinsulinemia. An observed nocturnal increase in plasma FFA levels may account for both insulin resistance and compensatory hyperinsulinemia and warrants further investigation. The elucidation of these interrelated pathologies may help reveal points where medical intervention can reduce metabolic disease. [Copyright &y& Elsevier]

Recent studies in animal and human models have revealed that free tatty acid (FFA) release from adipose tissue is oscillatory. We have shown in our laboratory that these oscillations are controlled by the sympathetic nervous system (SNS). Although FFAs have been shown to directly stimulate glucose production [endogenous glucose production (EGP)] by the liver and to reduce peripheral glucose utilization, whether the specific pattern of FFA release affects glucose metabolism is unknown. The aim of this study was to examine the effects of pulsatile vs. constant infusion of FFA on glucose homeostasis in the canine model. Euglycemic clamps with basal insulin replacement (0.1 mU·kg-1·min-1 insulin) were performed in dogs (n = 8) during infusion of saline (SAL) or the medium-chain fatty acid octanoate, which was given by either pulsatile infusion (PUL: 10 mmol over 2 rain every 10 min) or continuous infusion (C-INF: 1 mmol/min) designed to achieve equivalent total FFA mass. Endogenous lipolytic pulses were suppressed with the β3-specific adrenergic receptor antagonist bupranolol. PUL infusion elicited a pulsatile pattern of FFA in circulation with average maximum pulse height of 0.82 ± 0.04 mM, whereas C-INF FFA levels reached 0.47 ± 0.03 mM (fasting levels) and were maintained throughout. Glucose uptake was not affected by PUL; however, C-INF significantly reduced glucose uptake compared with both SAL and PUL. Steady-state EGP increased by >90% from basal steady state during PUL but did not change during either SAL or C-INF. Thus, pulsatile FFA infusion led to an increase in EGP while preserving glucose disposal. These data suggest that the pattern of FFA may have a role in regulation of glucose homeostasis, which may have consequences in the obese or insulin-resistant state where the SNS is known to be altered. [ABSTRACT FROM AUTHOR]

OBJECTIVE--Age-associated insulin resistance may be caused by increased visceral adiposity and older animals appear to be more susceptible to obesity-related resistance than young animals. However, it is unclear to what extent the portally drained mesenteric fat depot influences this susceptibility. RESEARCH DESIGN AND METHODS--Young high-fat-fed and old obese rats were subjected to 0, 2, 4, or 6 weeks of caloric restriction. Insulin sensitivity (SI) was assessed by hyperinsulinemic clamp and lean body mass (LBM) and total body fat were assessed by 18O-water administration. RESULTS--Six weeks of caloric restriction caused a similar reduction in body weight in young and old animals (P = 0.748) that was not due to reduced subcutaneous fat or LBM, but rather preferential loss of abdominal fat (P < 0.05). Most notably, mesenteric fat was reduced equivalently in young and old rats after 6 weeks of caloric restriction (∼↓53%; P = 0.537). Despite similar visceral fat loss, SI improved less in old (↑ 32.76 ± 9.80%) than in young (↑ 82.91 ± 12.66%) rats versus week 0. In addition, there was significantly more reversal of fat accumulation in the liver in young (% reduction: 89 ± 2) versus old (64 ± 5) rats (P < 0.0001). Furthermore, in young rats, SI changed much more rapidly for a given change in mesenteric fat versus other abdominal depots (slope = 0.53 vs. ≤0.27 kg/min/mg per % fat). CONCLUSIONS--Improved SI during caloric restriction correlated with a preferential abdominal fat loss. This improvement was refractory in older animals, likely because of slower liberation of hepatic lipid. Furthermore, mesenteric fat was a better predictor of SI than other abdominal depots in young but not old rats. These results suggest a singular role for mesenteric fat to determine insulin resistance. This role may be related to delivery of lipid to liver, and associated accumulation of liver fat. Diabetes 59:1416-1423, 2010 [ABSTRACT FROM AUTHOR]