Effects of physiological hyperinsulinemia on systemic, renal, and hepatic substrate metabolism

Liver or kidney: Who has the oar in the gluconeogenesis boat and when?

Gluconeogenesis is an endogenous process of glucose production from non-carbohydrate carbon substrates. Both the liver and kidneys express the key enzymes necessary for endogenous glucose production and its export into circulation. We would be remiss to add that more recently gluconeogenesis has been described in the small intestine, especially under high-protein, low-carbohydrate diets. The contribution of the liver glucose release, the net glucose flux, towards systemic glucose is already well known. The liver is, in most instances, the primary bulk contributor due to the sheer size of the organ (on average, over 1 kg). The contribution of the kidney (at just over 100 g each) to endogenous glucose production is often under-appreciated, especially on a weight basis. Glucose is released from the liver through the process of glycogenolysis and gluconeogenesis. Renal glucose release is almost exclusively due to gluconeogenesis, which occurs in only a fraction of the cells in that organ (proximal tubule cells). Thus, the efficiency of glucose production from other carbon sources may be superior in the kidney relative to the liver or at least on the level. In both these tissues, gluconeogenesis regulation is under tight hormonal control and depends on the availability of substrates. Liver and renal gluconeogenesis are differentially regulated under various pathological conditions. The impact of one source vs the other changes, based on post-prandial state, acid-base balance, hormonal status, and other less understood factors. Which organ has the oar (is more influential) in driving systemic glucose homeostasis is still in-conclusive and likely changes with the daily rhythms of life. We reviewed the literature on the differences in gluconeogenesis regulation between the kidneys and the liver to gain an insight into who drives the systemic glucose levels under various physiological and pathological conditions.

Visceral fat correlates with insulin secretion and sensitivity independent of BMI and subcutaneous fat in Chinese with type 2 diabetes

AIM

Clinical heterogeneity exists in overall obesity and abdominal obesity in terms of insulin secretion and sensitivity. Further, the impact of visceral fat (VF) on the first- and second-phase insulin secretion (FPIS and SPIS) is controversial. We aim to investigate insulin secretion and sensitivity in Chinese patients with T2DM according to different BMI and VF levels.

METHODS

This study enrolled 300 participants. A dual bioelectrical impedance analyzer was used to assess the visceral and subcutaneous fat area (VFA and SFA). VF levels were categorized as normal or high, with the cutoff value of 100 cm². FPIS and SPIS were evaluated by arginine stimulation test and standardized steamed bread meal tolerance test, respectively. β-cell function (HOMA2-β), insulin resistance (HOMA2-IR), and Gutt's insulin sensitivity index (Gutt-ISI) were also calculated. Spearman's correlation analysis and multivariate linear regression analysis were adopted for statistical analysis.

RESULTS

Participants were categorized into four groups: normal weight-normal VF, normal weight-high VF, overweight/obese-normal VF and overweight/obese-high VF. Multivariate linear regression showed that both VFA and SFA were correlated with FPIS, HOMA2-IR and Gutt-ISI after controlling for gender and diabetes duration. After further adjustment for BMI and VFA, some associations of SFA with insulin secretion and sensitivity disappeared. After adjustment for gender, diabetes duration, BMI and SFA, VFA was positively correlated with FPIS, SPIS and HOMA2-IR. Subjects with overweight/obese-high VF were more likely to have higher FPIS, HOMA2-IR and lower Gutt-ISI (all p < 0.05).

CONCLUSION

VF affects both FPIS and SPIS, and worsens insulin sensitivity independent of BMI and subcutaneous fat in Chinese patients with T2DM.

CLINICAL TRIAL REGISTRATION

http://www.chictr.org.cn, identifier ChiCTR2200062884.

The Contribution of Lipotoxicity to Diabetic Kidney Disease

Lipotoxicity is a fundamental pathophysiologic mechanism in diabetes and non-alcoholic fatty liver disease and is now increasingly recognized in diabetic kidney disease (DKD) pathogenesis. This review highlights lipotoxicity pathways in the podocyte and proximal tubule cell, which are arguably the two most critical sites in the nephron for DKD. The discussion focuses on membrane transporters and lipid droplets, which represent potential therapeutic targets, as well as current and developing pharmacologic approaches to reduce renal lipotoxicity.

Tracing the lactate shuttle to the mitochondrial reticulum

Isotope tracer infusion studies employing lactate, glucose, glycerol, and fatty acid isotope tracers were central to the deduction and demonstration of the Lactate Shuttle at the whole-body level. In concert with the ability to perform tissue metabolite concentration measurements, as well as determinations of unidirectional and net metabolite exchanges by means of arterial-venous difference (a-v) and blood flow measurements across tissue beds including skeletal muscle, the heart and the brain, lactate shuttling within organs and tissues was made evident. From an extensive body of work on men and women, resting or exercising, before or after endurance training, at sea level or high altitude, we now know that Organ-Organ, Cell-Cell, and Intracellular Lactate Shuttles operate continuously. By means of lactate shuttling, fuel-energy substrates can be exchanged between producer (driver) cells, such as those in skeletal muscle, and consumer (recipient) cells, such as those in the brain, heart, muscle, liver and kidneys. Within tissues, lactate can be exchanged between white and red fibers within a muscle bed and between astrocytes and neurons in the brain. Within cells, lactate can be exchanged between the cytosol and mitochondria and between the cytosol and peroxisomes. Lactate shuttling between driver and recipient cells depends on concentration gradients created by the mitochondrial respiratory apparatus in recipient cells for oxidative disposal of lactate.

Reduced insulin signaling and high glucagon in early insulin resistance impaired fast-fed regulation of renal gluconeogenesis via insulin receptor substrate

Gluconeogenesis is one of the key processes through which the kidney contributes to glucose homeostasis. Urinary exosomes (uE) have been used to study renal gene regulation noninvasively in humans and rodents. Recently, we demonstrated fast-fed regulation of phosphoenolpyruvate carboxykinase (PEPCK), the rate-limiting enzyme for gluconeogenesis, in human uE. The regulation was impaired in subjects with early insulin resistance. Here, we studied primary human proximal tubule cells (hPT) and human uE to elucidate a potential link between insulin resistance and fast-fed regulation of renal PEPCK. We demonstrate that fasted hPTs had higher PEPCK and insulin receptor substrate-2 (IRS2) mRNA and protein levels, relative to fed cells. The fast-fed regulation was, however, attenuated in insulin receptor knockdown (IRKO) hPTs. The IRKO was confirmed by the blunted insulin-induced response on PEPCK, PGC1α, p-IR, and p-AKT expression in IRKO cells. Exosomes secreted by the wild-type or IRKO hPT showed similar regulation to the respective hPT. Similarly, in human uE, the relative abundance of IRS-2 mRNA (to IRS1) was higher in the fasted state relative to the fed condition. However, the fast-fed difference was absent in subjects with early insulin resistance. These subjects had higher circulating glucagon levels relative to subjects with optimal insulin sensitivity. Furthermore, in hPT cells, glucagon significantly induced PEPCK and IRS2 gene, and gluconeogenesis. IR knockdown in hPT cells further increased the gene expression levels. Together the data suggest that reduced insulin sensitivity and high glucagon in early insulin resistance may impair renal gluconeogenesis via IRS2 regulation.

Tubular Cell Glucose Metabolism Shift During Acute and Chronic Injuries

Acute and chronic kidney disease are responsible for large healthcare costs worldwide. During injury, kidney metabolism undergoes profound modifications in order to adapt to oxygen and nutrient shortage. Several studies highlighted recently the importance of these metabolic adaptations in acute as well as in chronic phases of renal disease, with a potential deleterious effect on fibrosis progression. Until recently, glucose metabolism in the kidney has been poorly studied, even though the kidney has the capacity to use and produce glucose, depending on the segment of the nephron. During physiology, renal proximal tubular cells use the beta-oxidation of fatty acid to generate large amounts of energy, and can also produce glucose through gluconeogenesis. In acute kidney injury, proximal tubular cells metabolism undergo a metabolic shift, shifting away from beta-oxidation of fatty acids and gluconeogenesis toward glycolysis. In chronic kidney disease, the loss of fatty acid oxidation is also well-described, and data about glucose metabolism are emerging. We here review the modifications of proximal tubular cells glucose metabolism during acute and chronic kidney disease and their potential consequences, as well as the potential therapeutic implications.

New Pandemic: Obesity and Associated Nephropathy

Incidence of obesity related renal disorders have increased 10-folds in recent years. One of the consequences of obesity is an increased glomerular filtration rate (GFR) that leads to the enlargement of the renal glomerulus, i.e., glomerulomegaly. This heightened hyper-filtration in the setting of type 2 diabetes irreparably damages the kidney and leads to progression of end stage renal disease (ESRD). The patients suffering from type 2 diabetes have progressive proteinuria, and eventually one third of them develop chronic kidney disease (CKD) and ESRD. For ameliorating the progression of CKD, inhibitors of renin angiotensin aldosterone system (RAAS) seemed to be effective, but on a short-term basis only. Long term and stable treatment strategies like weight loss via restricted or hypo-caloric diet or bariatric surgery have yielded better promising results in terms of amelioration of proteinuria and maintenance of normal GFR. Body mass index (BMI) is considered as a traditional marker for the onset of obesity, but apparently, it is not a reliable indicator, and thus there is a need for more precise evaluation of regional fat distribution and amount of muscle mass. With respect to the pathogenesis, recent investigations have suggested perturbation in fatty acid and cholesterol metabolism as the critical mediators in ectopic renal lipid accumulation associated with inflammation, increased generation of ROS, RAAS activation and consequential tubulo-interstitial injury. This review summarizes the renewed approaches for the obesity assessment and evaluation of the pathogenesis of CKD, altered renal hemodynamics and potential therapeutic targets.

Renal gluconeogenesis in insulin resistance: A culprit for hyperglycemia in diabetes

Renal gluconeogenesis is one of the major pathways for endogenous glucose production. Impairment in this process may contribute to hyperglycemia in cases with insulin resistance and diabetes. We reviewed pertinent studies to elucidate the role of renal gluconeogenesis regulation in insulin resistance and diabetes. A consensus on the suppressive effect of insulin on kidney gluconeogenesis has started to build up. Insulin-resistant models exhibit reduced insulin receptor (IR) expression and/or post-receptor signaling in their kidney tissue. Reduced IR expression or post-receptor signaling can cause impairment in insulin’s action on kidneys, which may increase renal gluconeogenesis in the state of insulin resistance. It is now established that the kidney contributes up to 20% of all glucose production via gluconeogenesis in the post-absorptive phase. However, the rate of renal glucose release excessively increases in diabetes. The rise in renal glucose release in diabetes may contribute to fasting hyperglycemia and increased postprandial glucose levels. Enhanced glucose release by the kidneys and renal expression of the gluconeogenic-enzyme in diabetic rodents and humans further point towards the significance of renal gluconeogenesis. Overall, the available literature suggests that impairment in renal gluconeogenesis in an insulin-resistant state may contribute to hyperglycemia in type 2 diabetes.

Lactate in contemporary biology: a phoenix risen

After a century, it's time to turn the page on understanding of lactate metabolism and appreciate that lactate shuttling is an important component of intermediary metabolism in vivo. Cell‐cell and intracellular lactate shuttles fulfil purposes of energy substrate production and distribution, as well as cell signalling under fully aerobic conditions. Recognition of lactate shuttling came first in studies of physical exercise where the roles of driver (producer) and recipient (consumer) cells and tissues were obvious. Moreover, the presence of lactate shuttling as part of postprandial glucose disposal and satiety signalling has been recognized. Mitochondrial respiration creates the physiological sink for lactate disposal in vivo. Repeated lactate exposure from regular exercise results in adaptive processes such as mitochondrial biogenesis and other healthful circulatory and neurological characteristics such as improved physical work capacity, metabolic flexibility, learning, and memory. The importance of lactate and lactate shuttling in healthful living is further emphasized when lactate signalling and shuttling are dysregulated as occurs in particular illnesses and injuries. Like a phoenix, lactate has risen to major importance in 21st century biology.

A Journey in Diabetes: From Clinical Physiology to Novel Therapeutics: The 2020 Banting Medal for Scientific Achievement Lecture

Insulin resistance and β-cell dysfunction are the core pathophysiological mechanisms of all hyperglycemic syndromes. Advances in in vivo investigative techniques have made it possible to quantify insulin resistance in multiple sites (skeletal and myocardial muscle, subcutaneous and visceral fat depots, liver, kidney, vascular tissues, brain and intestine), to clarify its consequences for tissue substrate selection, and to establish its relation to tissue perfusion. Physiological modeling of β-cell function has provided a uniform tool to measure β-cell glucose sensitivity and potentiation in response to a variety of secretory stimuli, thereby allowing us to establish feedbacks with insulin resistance, to delineate the biphasic time course of conversion to diabetes, to gauge incretin effects, and to identify primary insulin hypersecretion. As insulin resistance also characterizes several of the comorbidities of diabetes (e.g., obesity, hypertension, dyslipidemia), with shared genetic and acquired influences, the concept is put forward that diabetes is a systemic disease from the outset, actually from the prediabetic stage. In fact, early multifactorial therapy, particularly with newer antihyperglycemic agents, has shown that the burden of micro- and macrovascular complications can be favorably modified despite the rising pressure imposed by protracted obesity.

Monitoring and management of hyperglycemia in patients with advanced diabetic kidney disease

Diabetes mellitus is the leading cause of end-stage renal disease, and uncontrolled hyperglycemia is directly related to the increased mortality in this setting. As kidney function decreases, it becomes more challenging to control blood glucose since the risk of hypoglycemia increases. Decreased appetite, changes in glycaemia homeostasis, along with reduced renal excretion of anti-hyperglycemic drugs tend to facilitate the occurrence of hypoglycemia, despite the paradoxical occurrence of insulin resistance in advanced kidney disease. Thus, in patients using insulin and/or oral anti-hyperglycemic agents, dynamic adjustments with drug dose reduction or drug switching are often necessary. Furthermore, in addition to consider these pharmacokinetics alterations, it is of utmost importance to choose drugs with proven cardio-renal benefits in this setting, such as sodium-glucose co-transporter 2 inhibitors and glucagon-like peptide 1 receptor agonists. In this review, we summarize the indications and contraindications, titration of doses and side effects of the available anti-hyperglycemic agents in the presence of advanced diabetic kidney disease (DKD) and dialysis, highlighting the risks and benefits of the different agents. Additionally, basic renal function assessment and monitoring of glycemic control in DKD will be evaluated in order to guide the use of drugs and define the glycemic targets to be achieved.

Regulation of Postabsorptive and Postprandial Glucose Metabolism by Insulin-Dependent and Insulin-Independent Mechanisms: An Integrative Approach

Glucose levels in blood must be constantly maintained within a tight physiological range to sustain anabolism. Insulin regulates glucose homeostasis via its effects on glucose production from the liver and kidneys and glucose disposal in peripheral tissues (mainly skeletal muscle). Blood levels of glucose are regulated simultaneously by insulin-mediated rates of glucose production from the liver (and kidneys) and removal from muscle; adipose tissue is a key partner in this scenario, providing nonesterified fatty acids (NEFA) as an alternative fuel for skeletal muscle and liver when blood glucose levels are depleted. During sleep at night, the gradual development of insulin resistance, due to growth hormone and cortisol surges, ensures that blood glucose levels will be maintained within normal levels by: (a) switching from glucose to NEFA oxidation in muscle; (b) modulating glucose production from the liver/kidneys. After meals, several mechanisms (sequence/composition of meals, gastric emptying/intestinal glucose absorption, gastrointestinal hormones, hyperglycemia mass action effects, insulin/glucagon secretion/action, de novo lipogenesis and glucose disposal) operate in concert for optimal regulation of postprandial glucose fluctuations. The contribution of the liver in postprandial glucose homeostasis is critical. The liver is preferentially used to dispose over 50% of the ingested glucose and restrict the acute increases of glucose and insulin in the bloodstream after meals, thus protecting the circulation and tissues from the adverse effects of marked hyperglycemia and hyperinsulinemia.

Phosphoenolpyruvate carboxykinase in urine exosomes reflect impairment in renal gluconeogenesis in early insulin resistance and diabetes

Impaired insulin-induced suppression of renal gluconeogenesis could be a risk for hyperglycemia. Diabetes is associated with elevated renal gluconeogenesis; however, its regulation in early insulin resistance is unclear in humans. A noninvasive marker of renal gluconeogenesis would be helpful. Here, we show that human urine exosomes (uE) contain three gluconeogenic enzymes: phosphoenolpyruvate carboxykinase (PEPCK), fructose 1,6-bisphosphatase, and glucose 6-phosphatase. Their protein levels were positively associated with whole body insulin sensitivity. PEPCK protein in uE exhibited a meal-induced suppression. However, subjects with lower insulin sensitivity had blunted meal-induced suppression. Also, uE from subjects with prediabetes and diabetic rats had higher PEPCK relative to nondiabetic controls. Moreover, uE-PEPCK was higher in drug-naïve subjects with diabetes relative to drug-treated subjects with diabetes. To determine whether increased renal gluconeogenesis is associated with hyperglycemia or PEPCK expression in uE, acidosis was induced in rats by 0.28 M NH₄Cl with 0.5% sucrose in drinking water. Control rats were maintained on 0.5% sucrose. At the seventh day posttreatment, gluconeogenic enzyme activity in the kidneys, but not in the liver, was higher in acidotic rats. These rats had elevated PEPCK in their uE and a significant rise in blood glucose relative to controls. The induction of gluconeogenesis in human proximal tubule cells increased PEPCK expression in both human proximal tubules and human proximal tubule-secreted exosomes in the media. Overall, gluconeogenic enzymes are detectable in human uE. Elevated PEPCK and its blunted meal-induced suppression in human urine exosomes are associated with diabetes and early insulin resistance.

Lactate as a fulcrum of metabolism

Mistakenly thought to be the consequence of oxygen lack in contracting skeletal muscle we now know that the L-enantiomer of the lactate anion is formed under fully aerobic conditions and is utilized continuously in diverse cells, tissues, organs and at the whole-body level. By shuttling between producer (driver) and consumer (recipient) cells lactate fulfills at least three purposes: 1] a major energy source for mitochondrial respiration; 2] the major gluconeogenic precursor; and 3] a signaling molecule. Working by mass action, cell redox regulation, allosteric binding, and reprogramming of chromatin by lactylation of lysine residues on histones, lactate has major influences in energy substrate partitioning. The physiological range of tissue [lactate] is 0.5–20 mM and the cellular Lactate/Pyruvate ratio (L/P) can range from 10 to >500; these changes during exercise and other stress-strain responses dwarf other metabolic signals in magnitude and span. Hence, lactate dynamics have rapid and major short- and long-term effects on cell redox and other control systems. By inhibiting lipolysis in adipose via HCAR-1, and muscle mitochondrial fatty acid uptake via malonyl-CoA and CPT1, lactate controls energy substrate partitioning. Repeated lactate exposure from regular exercise results in major effects on the expression of regulatory enzymes of glycolysis and mitochondrial respiration. Lactate is the fulcrum of metabolic regulation in vivo.

Effect of Insulin on Proximal Tubules Handling of Glucose: A Systematic Review

Renal proximal tubules reabsorb glucose from the glomerular filtrate and release it back into the circulation. Modulation of glomerular filtration and renal glucose disposal are some of the insulin actions, but little is known about a possible insulin effect on tubular glucose reabsorption. This review is aimed at synthesizing the current knowledge about insulin action on glucose handling by proximal tubules. Method. A systematic article selection from Medline (PubMed) and Embase between 2008 and 2019. 180 selected articles were clustered into topics (renal insulin handling, proximal tubule glucose transport, renal gluconeogenesis, and renal insulin resistance). Summary of Results. Insulin upregulates its renal uptake and degradation, and there is probably a renal site-specific insulin action and resistance; studies in diabetic animal models suggest that insulin increases renal SGLT2 protein content; in vivo human studies on glucose transport are few, and results of glucose transporter protein and mRNA contents are conflicting in human kidney biopsies; maximum renal glucose reabsorptive capacity is higher in diabetic patients than in healthy subjects; glucose stimulates SGLT1, SGLT2, and GLUT2 in renal cell cultures while insulin raises SGLT2 protein availability and activity and seems to directly inhibit the SGLT1 activity despite it activating this transporter indirectly. Besides, insulin regulates SGLT2 inhibitor bioavailability, inhibits renal gluconeogenesis, and interferes with Na⁺K⁺ATPase activity impacting on glucose transport. Conclusion. Available data points to an important insulin participation in renal glucose handling, including tubular glucose transport, but human studies with reproducible and comparable method are still needed.

Exercise-Induced Improvements to Whole Body Glucose Metabolism in Type 2 Diabetes: The Essential Role of the Liver

Type 2 diabetes (T2D) is a metabolic disease characterized by obesity, insulin resistance, and the dysfunction of several key glucoregulatory organs. Among these organs, impaired liver function is recognized as one of the earliest contributors to impaired whole-body glucose homeostasis, with well-characterized hepatic insulin resistance resulting in elevated rates of hepatic glucose production (HGP) and fasting hyperglycemia. One portion of this review will provide an overview of how HGP is regulated during the fasted state in healthy humans and how this process becomes dysregulated in patients with T2D. Less well-appreciated is the liver's role in post-prandial glucose metabolism, where it takes up and metabolizes one-third of orally ingested glucose. An abundance of literature has shown that the process of hepatic glucose uptake is impaired in patients with T2D, thereby contributing to glucose intolerance. A second portion of this review will outline how hepatic glucose uptake is regulated during the post-prandial state, and how it becomes dysfunctional in patients with T2D. Finally, it is well-known that exercise training has an insulin-sensitizing effect on the liver, which contributes to improved whole-body glucose metabolism in patients with T2D, thereby making it a cornerstone in the management of the disease. To this end, the impact of exercise on hepatic glucose metabolism will be thoroughly discussed, referencing key findings in the literature. At the same time, sources of heterogeneity that contribute to inconsistent findings in the field will be pointed out, as will important topics for future investigation.

Inhibition of NF-κB Reduces Renal Inflammation and Expression of PEPCK in Type 2 Diabetic Mice

Renal gluconeogenesis is markedly promoted in patients with type 2 diabetes mellitus (T2DM); however, the underlying mechanism remains largely unknown. Renal gluconeogenesis is found to be negatively regulated by insulin. T2DM is characterized by chronic and subacute inflammation; however, inflammation has been well recognized to induce insulin resistance. Therefore, this study aimed to investigate whether the enhanced renal gluconeogenesis in T2DM was partially due to the renal inflammation-mediated insulin resistance. If so, whether inflammation inhibitor could partially reverse such change. Diabetic db/db mice and db/m mice were used in our study. Typically, diabetic db/db mice were intraperitoneally treated with 1 mg/kg NF-κB inhibitor parthenolide (PTN) or saline as control every other day. Twelve weeks after treatment, animal samples were collected for measurements. Our results suggested that the expression levels of the inflammatory factors and the gluconeogenic rate-limiting enzyme phosphoenolpyruvate carboxykinase (PEPCK) were up-regulated in renal cortex of both db/db mice and T2DM patients. Moreover, reduced insulin signaling, as well as up-regulated expression of downstream genes FOXO1 and PGC-1ɑ, could be detected in renal cortex of db/db mice compared with that of db/m mice. Consistent with our hypothesis, PTN treatment could alleviate renal inflammation and insulin resistance in db/db mice. Moreover, it could also down-regulate the renal expression of PEPCK, indicating that inflammation could be one of the triggers of insulin resistance and the enhanced renal gluconeogenesis in db/db mice. This study can shed light on the role of inflammation in the enhanced renal gluconeogenesis in T2DM, which may yield a novel target for hyperglycemia.

Insulin receptors in the kidneys in health and disease

Insulin is an important hormone that affects various metabolic processes, including kidney function. Impairment in insulin’s action leads to insulin resistance in the target tissue. Besides defects in post-receptor insulin signaling, impairment at the receptor level could significantly affect insulin sensitivity of the target tissue. The kidney is a known target of insulin; however, whether the kidney develops “insulin resistance” is debatable. Regulation of the insulin receptor (IR) expression and its function is very well studied in major metabolic tissues like liver, skeletal muscles, and adipose tissue. The physiological relevance of IRs in the kidney has recently begun to be clarified. The credit goes to studies that showed a wide distribution of IR throughout the nephron segments and their reduced expression in the insulin resistance state. Moreover, altered renal and systemic metabolism observed in mice with targeted deletion of the IR from various epithelial cells of the kidney has strengthened this proposition. In this review, we recapitulate the crucial findings from literature that have expanded our knowledge regarding the significance of the renal IR in normal- and insulin-resistance states.

Interactions between kidney disease and diabetes: dangerous liaisons

BACKGROUND

Type 2 diabetes mellitus (DM) globally affects 18-20 % of adults over the age of 65 years. Diabetic kidney disease (DKD) is one of the most frequent and dangerous complications of DM2, affecting about one-third of the patients with DM2. In addition to the pancreas, adipocytes, liver, and intestines, the kidneys also play an important role in glycemic control, particularly due to renal contribution to gluconeogenesis and tubular reabsorption of glucose.

METHODS

In this review article, based on a report of discussions from an interdisciplinary group of experts in the areas of endocrinology, diabetology and nephrology, we detail the relationship between diabetes and kidney disease, addressing the care in the diagnosis, the difficulties in achieving glycemic control and possible treatments that can be applied according to the different degrees of impairment.

DISCUSSION

Glucose homeostasis is extremely altered in patients with DKD, who are exposed to a high risk of both hyperglycemia and hypoglycemia. Both high and low glycemic levels are associated with increased morbidity and shortened survival in this group of patients. Factors that are associated with an increased risk of hypoglycemia in DKD patients include decreased renal gluconeogenesis, deranged metabolic pathways (including altered metabolism of medications) and decreased insulin clearance. On the other hand, decrease glucose filtration and excretion, and inflammation-induce insulin resistance are predisposing factors to hyperglycemic episodes.

CONCLUSION

Appropriate glycaemic monitoring and control tailored for diabetic patients is required to avoid hypoglycaemia and other glycaemic disarrays in patients with DM2 and kidney disease. Understanding the renal physiology and pathophysiology of DKD has become essential to all specialties treating diabetic patients. Disseminating this knowledge and detailing the evidence will be important to initiate breakthrough research and to encourage proper treatment of this group of patients.

Obesity-related glomerulopathy: clinical and pathologic characteristics and pathogenesis

The prevalence of obesity-related glomerulopathy is increasing in parallel with the worldwide obesity epidemic. Glomerular hypertrophy and adaptive focal segmental glomerulosclerosis define the condition pathologically. The glomerulus enlarges in response to obesity-induced increases in glomerular filtration rate, renal plasma flow, filtration fraction and tubular sodium reabsorption. Normal insulin/phosphatidylinositol 3-kinase/Akt and mTOR signalling are critical for podocyte hypertrophy and adaptation. Adipokines and ectopic lipid accumulation in the kidney promote insulin resistance of podocytes and maladaptive responses to cope with the mechanical forces of renal hyperfiltration. Although most patients have stable or slowly progressive proteinuria, up to one-third develop progressive renal failure and end-stage renal disease. Renin-angiotensin-aldosterone blockade is effective in the short-term but weight loss by hypocaloric diet or bariatric surgery has induced more consistent and dramatic antiproteinuric effects and reversal of hyperfiltration. Altered fatty acid and cholesterol metabolism are increasingly recognized as key mediators of renal lipid accumulation, inflammation, oxidative stress and fibrosis. Newer therapies directed to lipid metabolism, including SREBP antagonists, PPARα agonists, FXR and TGR5 agonists, and LXR agonists, hold therapeutic promise.

Insulin Therapy of Nondiabetic Septic Patients Is Predicted by para-Tyrosine/Phenylalanine Ratio and by Hydroxyl Radical-Derived Products of Phenylalanine

Hydroxyl radical converts Phe to para-, meta-, and ortho-Tyr (p-Tyr, m-Tyr, o-Tyr), while Phe is converted enzymatically to p-Tyr in the kidney and could serve as substrate for gluconeogenesis. Pathological isoforms m- and o-Tyr are supposed to be involved in development of hormone resistances. Role of Phe and the three Tyr isoforms in influencing insulin need was examined in 25 nondiabetic septic patients. Daily insulin dose (DID) and insulin-glucose product (IGP) were calculated. Serum and urinary levels of Phe and Tyr isoforms were determined using a rpHPLC-method. Urinary m-Tyr/p-Tyr ratio was higher in patients with DID and IGP over median compared to those below median (P = 0.005 and P = 0.01, resp.). Urinary m-Tyr and m-Tyr/p-Tyr ratio showed positive correlation with DID (P = 0.009 and P = 0.023, resp.) and with IGP (P = 0.004 and P = 0.008, resp.). Serum Phe was a negative predictor, while serum p-Tyr/Phe ratio was positive predictor of both DID and IGP. Urinary m-Tyr and urinary m-Tyr/p-Tyr, o-Tyr/p-Tyr, and (m-Tyr+o-Tyr)/p-Tyr ratios were positive predictors of both DID and IGP. Phe and Tyr isoforms have a predictive role in carbohydrate metabolism of nondiabetic septic patients. Phe may serve as substrate for renal gluconeogenesis via enzymatically produced p-Tyr, while hydroxyl radical derived Phe products may interfere with insulin action.

The mitochondrial isoform of phosphoenolpyruvate carboxykinase (PEPCK-M) and glucose homeostasis: has it been overlooked?

BACKGROUND

Plasma glucose levels are tightly regulated within a narrow physiologic range. Insulin-mediated glucose uptake by tissues must be balanced by the appearance of glucose from nutritional sources, glycogen stores, or gluconeogenesis. In this regard, a common pathway regulating both glucose clearance and appearance has not been described. The metabolism of glucose to produce ATP is generally considered to be the primary stimulus for insulin release from beta-cells. Similarly, gluconeogenesis from phosphoenolpyruvate (PEP) is believed to be the primarily pathway via the cytosolic isoform of phosphoenolpyruvate carboxykinase (PEPCK-C). These models cannot adequately explain the regulation of insulin secretion or gluconeogenesis.

SCOPE OF REVIEW

A metabolic sensing pathway involving mitochondrial GTP (mtGTP) and PEP synthesis by the mitochondrial isoform of PEPCK (PEPCK-M) is associated with glucose-stimulated insulin secretion from pancreatic beta-cells. Here we examine whether there is evidence for a similar mtGTP-dependent pathway involved in gluconeogenesis. In both islets and the liver, mtGTP is produced at the substrate level by the enzyme succinyl CoA synthetase (SCS-GTP) with a rate proportional to the TCA cycle. In the beta-cell PEPCK-M then hydrolyzes mtGTP in the production of PEP that, unlike mtGTP, can escape the mitochondria to generate a signal for insulin release. Similarly, PEPCK-M and mtGTP might also provide a significant source of PEP in gluconeogenic tissues for the production of glucose. This review will focus on the possibility that PEPCK-M, as a sensor for TCA cycle flux, is a key mechanism to regulate both insulin secretion and gluconeogenesis suggesting conservation of this biochemical mechanism in regulating multiple aspects of glucose homeostasis. Moreover, we propose that this mechanism may be important for regulating insulin secretion and gluconeogenesis compared to canonical nutrient sensing pathways.

MAJOR CONCLUSIONS

PEPCK-M, initially believed to be absent in islets, carries a substantial metabolic flux in beta-cells. This flux is intimately involved with the coupling of glucose-stimulated insulin secretion. PEPCK-M activity may have been similarly underestimated in glucose producing tissues and could potentially be an unappreciated but important source of gluconeogenesis.

GENERAL SIGNIFICANCE

The generation of PEP via PEPCK-M may occur via a metabolic sensing pathway important for regulating both insulin secretion and gluconeogenesis. This article is part of a Special Issue entitled Frontiers of Mitochondrial Research.

Insulin secretion and insulin sensitivity on the oral glucose tolerance test (OGTT) in middle-aged Japanese

The aim of this study was to assess the changes in insulin secretion and insulin sensitivity in relation to fasting and 2-hour plasma glucose (PG) levels and to assess the independent contributions of their impairments to non-diabetic hyperglycemia. A total of 2157 Japanese workers (mean age 52.6±7.3 years and mean BMI 23.9±3.2 kg/m(2)) underwent an oral glucose tolerance test (OGTT). Of these subjects, 1125 had normal glucose tolerance (NGT), 525 subjects had isolated impaired fasting glucose (IFG), 159 subjects had isolated impaired glucose tolerance (IGT), 263 subjects had combined IFG and IGT, and 85 subjects had newly diagnosed type 2 diabetes. Insulinogenic index and Matsuda insulin sensitivity index (ISI) were significantly attenuated in subjects with normal but slightly elevated fasting PG, or in subjects with normal but slightly elevated 2-hour PG. Whereas, InsAUC(120)/GluAUC(120) was not significantly decreased in those subjects, and significant decrease of it was observed exclusively in subjects with abnormal fasting PG (≥ 106 mg/dL) or abnormal 2-hour PG (≥ 221 mg/dL). Using multiple regression analyses, both Matsuda ISI and insulinogenic index were independently correlated with PG concentrations in subjects with IFG and/or IGT, while Matsuda ISI alone was independently correlated with fasting PG concentrations in normoglycemic subjects. In conclusion, both insulinogenic index and Matsuda ISI were significantly attenuated in subjects with normal but slightly elevated PG. Lowering of Matsuda ISI was likely to be a strong contributor to 'elevation of fasting PG within the normal range' in this population.

Role of the kidney in normal glucose homeostasis and in the hyperglycaemia of diabetes mellitus: therapeutic implications

Considerable data have accumulated over the past 20 years, indicating that the human kidney is involved in the regulation of glucose via gluconeogenesis, taking up glucose from the circulation, and by reabsorbing glucose from the glomerular filtrate. In light of the development of glucose-lowering drugs involving inhibition of renal glucose reabsorption, this review summarizes these data. Medline was searched from 1989 to present using the terms 'renal gluconeogenesis', 'renal glucose utilization', 'diabetes mellitus' and 'glucose transporters'. The human liver and kidneys release approximately equal amounts of glucose via gluconeogenesis in the post-absorptive state. In the postprandial state, although overall endogenous glucose release decreases substantially, renal gluconeogenesis increases by approximately twofold. Glucose utilization by the kidneys after an overnight fast accounts for approximately 10% of glucose utilized by the body. Following a meal, glucose utilization by the kidney increases. Normally each day, approximately 180 g of glucose is filtered by the kidneys; almost all of this is reabsorbed by means of sodium-glucose co-transporter 2 (SGLT2), expressed in the proximal tubules. However, the capacity of SGLT2 to reabsorb glucose from the renal tubules is finite and, when plasma glucose concentrations exceed a threshold, glucose appears in the urine. Handling of glucose by the kidney is altered in Type 2 diabetes mellitus (T2DM): renal gluconeogenesis and renal glucose uptake are increased in both the post-absorptive and postprandial states, and renal glucose reabsorption is increased. Specific SGLT2 inhibitors are being developed as a novel means of controlling hyperglycaemia in T2DM.

Portal glucose infusion-glucose clamp measures hepatic influence on postprandial systemic glucose appearance as well as whole body glucose disposal

The full impact of the liver, through both glucose production and uptake, on systemic glucose appearance cannot be readily studied in a classical glucose clamp because hepatic glucose metabolism is regulated not only by portal insulin and glucose levels but also portal glucose delivery (the portal signal). In the present study, we modified the classical glucose clamp by giving exogenous glucose through portal vein, the "portal glucose infusion (PoG)-glucose clamp", to determine the net hepatic effect on postprandial systemic glucose supply along with the measurement of whole body glucose disposal. By comparing systemic rate of glucose appearance (R(a)) with portal glucose infusion rate (PoG(inf)), we quantified "net hepatic glucose addition (NHGA)" in the place of endogenous glucose production determined in a regular clamp. When PoG-glucose clamps (n = 6) were performed in dogs at basal insulinemia and hyperglycemia ( approximately 150 mg/dl, portal and systemic), we measured consistently higher R(a) than PoG(inf) (4.2 +/- 0.6 vs. 2.9 +/- 0.6 mg x kg(-1) x min(-1) at steady state, P < 0.001) and thus positive NHGA at 1.3 +/- 0.1 mg x kg(-1) x min(-1), identifying net hepatic addition of glucose to portal exogenous glucose. In contrast, when PoG-glucose clamps (n = 6) were performed at hyperinsulinemia ( approximately 250 pmol/l) and systemic euglycemia (portal hyperglycemia due to portal glucose infusion), we measured consistently lower R(a) than PoG(inf) (13.1 +/- 2.4 vs. 14.3 +/- 2.4 mg x kg(-1) x min(-1), P < 0.001), and therefore negative NHGA at -1.1 +/- 0.1 mg x kg(-1) x min(-1), identifying a switch of the liver from net production to net uptake of portal exogenous glucose. Steady-state whole body glucose disposal was 4.1 +/- 0.5 and 13.0 +/- 2.4 mg x kg(-1) x min(-1), respectively, determined as in a classical glucose clamp. We conclude that the PoG-glucose clamp, simulating postprandial glucose entry and metabolism, enables simultaneous assessment of the net hepatic effect on postprandial systemic glucose supply as well as whole body glucose disposal in various animal models (rodents, dogs, and pigs) with established portal vein catheterization.

Insulin sensitivity and first-phase insulin secretion in obese Chinese with hyperglycemia in 30 and/or 60 min during glucose tolerance tests

The purpose of this study was to investigate insulin sensitivity and first-phase insulin secretion in obesity with hyperglycemia in 30 and/or 60 min during oral glucose tolerance (OGTT, glucose > or = 11.1 mmol/l, post-loading hyperglycemia, PLH) in Chinese population. A total of 196 nondiabetic subjects were included in the present study, among them 99 had normal glucose tolerance (NGT, subdivided into 32 lean NGT and 67 obese NGT), 74 had obesity with impaired glucose tolerance (IGT) and 23 had obesity with PLH. A standard 75-g oral glucose tolerance test was performed after fasting and at 30 min, 1, 2 and 3 h. Insulin sensitivity index (S(I)) was assessed by the Bergman's minimal model method with frequently sampled intravenous glucose tolerance test (FSIGTT), insulin secretion was determined by acute insulin response to glucose (AIRg). The disposition index (DI), the product of AIRg and S(I) was used to determine whether AIRg was adequate to compensate for insulin resistance. S(I) was significantly equally lower in three obese subgroups. AIRg was significantly increased in obese NGT as compared with lean NGT controls, and reduced to the same extent in IGT and PLH subjects. There was no significant difference among lean NGT, IGT and PLH subjects. DI value was reduced from obese NGT individuals, IGT and PLH subjects had a similar lower level of DI. In conclusion, our present results demonstrated that the pathophysiological basis of obese subjects with PLH were clearly insulin resistance and defective in first-phase insulin secretion as that in IGT subjects in Chinese population.

High circulating retinol-binding protein 4 is associated with elevated liver fat but not with total, subcutaneous, visceral, or intramyocellular fat in humans

OBJECTIVE

Retinol-binding protein 4 (RBP4) is an adipokine that induced insulin resistance in mice, and high plasma RBP4 levels were associated with insulin-resistant states in humans. To determine which fat compartments are associated with elevated RBP4 levels in humans, we measured circulating RBP4 in 75 healthy subjects and used state-of-the-art measurements of body fat distribution.

RESEARCH DESIGN AND METHODS

Total body, visceral, and subcutaneous abdominal fat were determined by magnetic resonance tomography and liver fat and intramyocellular fat by localized proton magnetic resonance spectroscopy. Insulin sensitivity was measured by the euglycemic-hyperinsulinemic clamp and, together with insulin clearance, estimated from the oral glucose tolerance test (OGTT).

RESULTS

Adjusted circulating RBP4 correlated negatively with insulin sensitivity (clamp: r = -0.33, P = 0.005; OGTT: r = -0.36, P = 0.002) and positively with parameters in the fasting state as insulin levels (r = 0.35, P = 0.003) and homeostasis model assessment of insulin resistance (r = 0.34, P = 0.004). In addition, circulating RBP4 correlated negatively with hepatic insulin clearance (r = -0.25, P = 0.04). Circulating RBP4 was not associated with total body, visceral, or subcutaneous abdominal fat (all P > or = 0.29). Plasma RBP4 levels were also not associated with intramyocellular fat or circulating adiponectin or leptin. In contrast, plasma RBP4 levels correlated positively with liver fat in cross-sectional (r = 0.27, P = 0.03) and longitudinal (r = 0.37, P = 0.04) analyses.

CONCLUSIONS

Circulating RBP4 is not associated with the amount of fat in the classical depots or in the ectopic depots in muscle. However, it correlates positively with liver fat. Furthermore, metabolic parameters support the close relationship between circulating RBP4 with liver fat and, presumably, hepatic insulin resistance.

Different mechanisms for impaired fasting glucose and impaired postprandial glucose tolerance in humans

OBJECTIVE

To compare the pathophysiology of impaired fasting glucose (IFG) and impaired glucose tolerance (IGT) in a more comprehensive and standardized fashion than has hitherto been done.

RESEARCH DESIGN AND METHODS

We studied 21 individuals with isolated IFG (IFG/normal glucose tolerance [NGT]), 61 individuals with isolated IGT (normal fasting glucose [NFG]/IGT), and 240 healthy control subjects (NFG/NGT) by hyperglycemic clamps to determine first- and second-phase insulin release and insulin sensitivity. Homeostasis model assessment (HOMA) indexes of beta-cell function (HOMA-%B) and insulin resistance (HOMA-IR) were calculated from fasting plasma insulin and glucose concentrations.

RESULTS

Compared with NFG/NGT, IFG/NGT had similar fasting insulin concentrations despite hyperglycemia; therefore, HOMA-IR was increased approximately 30% (P < 0.05), but clamp-determined insulin sensitivity was normal (P > 0.8). HOMA-%B and first-phase insulin responses were reduced approximately 35% (P < 0.002) and approximately 30% (P < 0.02), respectively, but second-phase insulin responses were normal (P > 0.5). NFG/IGT had normal HOMA-IR but approximately 15% decreased clamp-determined insulin sensitivity (P < 0.03). Furthermore, HOMA-%B was normal but both first-phase (P < 0.0003) and second-phase (P < 0.0001) insulin responses were reduced approximately 30%. IFG/NGT differed from NFG/IGT by having approximately 40% lower HOMA-%B (P < 0.012) and approximately 50% greater second-phase insulin responses (P < 0.005).

CONCLUSIONS

Since first-phase insulin responses were similarly reduced in IFG/NGT and NFG/IGT, we conclude that IFG is due to impaired basal insulin secretion and preferential resistance of glucose production to suppression by insulin, as reflected by fasting hyperglycemia despite normal plasma insulin concentrations and increased HOMA-IR, whereas IGT mainly results from reduced second-phase insulin release and peripheral insulin resistance, as reflected by reduced clamp-determined insulin sensitivity.

Characteristics of glutamine metabolism in human precision-cut kidney slices: a 13C-NMR study

The metabolism of glutamine, a physiological substrate of the human kidney, plays a major role in systemic acid-base homoeostasis. Not only because of the limited availability of human renal tissue but also in part due to the lack of adequate cellular models, the mechanisms regulating the renal metabolism of this amino acid in humans have been poorly characterized. Therefore given the renewed interest in their use, human precision-cut renal cortical slices were incubated in Krebs-Henseleit medium (118 mM NaCl, 4.7 mM KCl, 1.18 mM KH2PO4, 1.18 mM MgSO4*7H2O, 24.9 mM NaHCO3 and 2.5 mM CaCl2*2H2O) with 2 mM unlabelled or 13C-labelled glutamine residues. After incubation, substrate utilization and product formation were measured by enzymatic and NMR spectroscopic methods. Glutamate accumulation tended to plateau but glutamine removal and ammonia, alanine and lactate production as well as flux through GLDH (glutamate dehydrogenase) increased to various extents with time for up to 4 h of incubation indicating the metabolic viability of the slices. Valproate, a stimulator of renal glutamine metabolism, markedly and in a dose-dependent fashion increased ammonia production. With [3-13C]glutamine as a substrate, and in the absence and presence of valproate, [13C]glutamate, [13C]alanine and [13C]lactate accounted for 81 and 96%, 34 and 63%, 30 and 46% of the glutamate, alanine and lactate accumulations measured enzymatically respectively. The slices also metabolized glutamine and retained their reactivity to valproate during incubations lasting for up to 48 h. These results demonstrate that, although endogenous metabolism substantially operates in the presence of glutamine, human precision-cut renal cortical slices are metabolically viable and strongly respond to the ammoniagenic effect of valproate. Thus, this experimental model is suitable for metabolic and pharmaco-toxicological studies.

Abnormal renal, hepatic, and muscle glucose metabolism following glucose ingestion in type 2 diabetes

Recent studies indicate an important role of the kidney in postprandial glucose homeostasis in normal humans. To determine its role in the abnormal postprandial glucose metabolism in type 2 diabetes mellitus (T2DM), we used a combination of the dual-isotope technique and net balance measurements across kidney and skeletal muscle in 10 subjects with T2DM and 10 age-, weight-, and sex-matched nondiabetic volunteers after ingestion of 75 g of glucose. Over the 4.5-h postprandial period, diabetic subjects had increased mean blood glucose levels (14.1 +/- 1.1 vs. 6.2 +/- 0.2 mM, P < 0.001) and increased systemic glucose appearance (100.0 +/- 6.3 vs. 70.0 +/- 3.3 g, P < 0.001). The latter was mainly due to approximately 23 g greater endogenous glucose release (39.8 +/- 5.9 vs. 17.0 +/- 1.8 g, P < 0.002), since systemic appearance of the ingested glucose was increased by only approximately 7 g (60.2 +/- 1.4 vs. 53.0 +/- 2.2 g, P < 0.02). Approximately 40% of the diabetic subjects' increased endogenous glucose release was due to increased renal glucose release (19.6 +/- 3.1 vs. 10.6 +/- 2.4 g, P < 0.05). Postprandial systemic tissue glucose uptake was also increased in the diabetic subjects (82.3 +/- 4.7 vs. 69.8 +/- 3.5 g, P < 0.05), and its distribution was altered; renal glucose uptake was increased (21.0 +/- 3.5 vs. 9.8 +/- 2.3 g, P < 0.03), whereas muscle glucose uptake was normal (18.5 +/- 1.8 vs. 25.9 +/- 3.3 g, P = 0.16). We conclude that, in T2DM, 1) both liver and kidney contribute to postprandial overproduction of glucose, and 2) postprandial renal glucose uptake is increased, resulting in a shift in the relative importance of muscle and kidney for glucose disposal. The latter may provide an explanation for the renal glycogen accumulation characteristic of diabetes mellitus as well as a mechanism by which hyperglycemia may lead to diabetic nephropathy.

Parallel manifestation of insulin resistance and beta cell decompensation is compatible with a common defect in Type 2 diabetes

AIMS/HYPOTHESIS

The aim of the study was to evaluate the relationship between insulin sensitivity, beta cell function and glucose tolerance, and its dependence on variants in the newly identified Type 2 diabetes susceptibility gene, calpain-10 ( CAPN10).

METHODS

We studied 203 men of the same age but with varying degrees of glucose tolerance. These men participated in (i) an oral glucose tolerance test, (ii) a euglycaemic clamp combined with indirect calorimetry and infusion of [3-(3)H]-glucose and (iii) a stepwise assessment of acute insulin response to arginine (AIR) at three different glucose concentrations (fasting, 14 and 28 mmol/l).

RESULTS

There was a linear increase in NEFA levels ( p<0.0005) and WHR ( p<0.0005) and decrease in glucose uptake due to a reduction in glucose storage over the entire range of glucose tolerance ( r=-0.404; p<0.005). No increase in endogenous glucose production (EGP) was seen until patients had manifest diabetes. However, when EGP was expressed relative to fasting insulin concentrations, there was a linear deterioration of basal hepatic insulin sensitivity ( r=-0.514; p<0.005). The AIR followed a bell-shaped curve with an initial rise and subsequent decrease. However, AIR adjusted for insulin sensitivity (disposition index) showed a linear decrease with increasing glucose concentrations ( r=-0.563; p<0.001) starting already in subjects with normal glucose tolerance. There was an inverse correlation between increase in WHR and NEFA and peripheral as well as hepatic insulin sensitivity. Subjects with the genotype combination of CAPN10 consisting of SNP44 TT and SNP43 GG genotypes had significantly lower insulin-stimulated glucose uptake than carriers of the other genotype combinations (5.3+/-0.4 vs 7.2+/-0.4 mg.ffm kg(-1).min(-1).mU.l(-1); p<0.005).

CONCLUSIONS/INTERPRETATION

We conclude that the pre-diabetic state is characterised by a similar linear deterioration of peripheral and hepatic insulin sensitivity as beta cell function and that variants in the CAPN10 gene modify this relationship. These findings are compatible with a common defect in muscle, liver and beta cells in the pathogenesis of Type 2 diabetes.

Nonhepatic glucose production in humans

Extrahepatic glucose release was evaluated during the anhepatic phase of liver transplantation in 14 recipients for localized hepatocarcinoma with mild or absent cirrhosis, who received a bolus of [6,6-2H2]glucose and l-[3-13C]alanine or l-[1,2-13C2]glutamine to measure glucose kinetics and to prove whether gluconeogenesis occurred from alanine and glutamine. Twelve were studied again 7 mo thereafter along with seven healthy subjects. At the beginning of the anhepatic phase, plasma glucose was increased and then declined by 15%/h. The right kidney released glucose, with an arteriovenous gradient of -3.7 mg/dl. Arterial and portal glucose concentrations were similar. The glucose clearance was 25% reduced, but glucose uptake was similar to that of the control groups. Glucose production was 9.5 +/- 0.9 micromol.kg-1. min-1, 30% less than in controls. Glucose became enriched with 13C from alanine and especially glutamine, proving the extrahepatic gluconeogenesis. The gluconeogenic precursors alanine, glutamine, lactate, pyruvate, and glycerol, insulin, and the counterregulatory hormones epinephrine, cortisol, growth hormone, and glucagon were increased severalfold. Extrahepatic organs synthesize glucose at a rate similar to that of postabsorptive healthy subjects when hepatic production is absent, and gluconeogenic precursors and counterregulatory hormones are markedly increased. The kidney is the main, but possibly not the unique, source of extrahepatic glucose production.

Bench-to-bedside review: glucose production from the kidney

Data obtained from net organ balance studies of glucose production lead to the classic view according to which glucose homeostasis is mainly ensured by the liver, and renal glucose production only plays a significant role during acidosis and prolonged starvation. Renal glucose release and uptake, as well as the participation of gluconeogenic substrates in renal gluconeogenesis, were recently re-evaluated using systemic and renal arteriovenous balance of substrates in combination with deuterated glucose dilution. Data obtained using these methods lead one to reconsider the magnitude of renal glucose production as well as its role in various physiological and pathological circumstances. These findings now conduce one to consider that renal gluconeogenesis substantially participates in postabsorptive glucose production, and that its role in glucose homeostasis is of first importance.

Role of the kidney in hyperglycemia in type 2 diabetes

Overproduction of glucose is the major factor responsible for fasting hyperglycemia in type 2 diabetes. Formerly, this had been considered to be solely due to excessive hepatic glucose production because the human kidney was not regarded as an important source of glucose except during acidosis and after prolonged fasting. However, data accumulated over the last 60 years in animal and in vitro studies have provided considerable evidence that the kidney plays an important role in glucose homeostasis in conditions other than acidosis and prolonged fasting. This article summarizes early work in animals and humans, discusses methodologic issues in assessing renal glucose release in vivo, and provides evidence from recent human studies that the kidney substantially contributes to glucose overproduction in type 2 diabetes.

Role of human liver, kidney, and skeletal muscle in postprandial glucose homeostasis

Recent studies indicate a role for the kidney in postabsorptive glucose homeostasis. The present studies were undertaken to evaluate the role of the kidney in postprandial glucose homeostasis and to compare its contribution to that of liver and skeletal muscle. Accordingly, we used the double isotope technique along with forearm and renal balance measurements to assess systemic, renal, and hepatic glucose release as well as glucose uptake by kidney, skeletal muscle, and splanchnic tissues in 10 normal volunteers after ingestion of 75 g of glucose. We found that, during the 4.5-h postprandial period, 22 +/- 2 g (30 +/- 3% of the ingested glucose) were initially extracted by splanchnic tissues. Of the remaining 53 +/- 2 g that entered the systemic circulation, 19 +/- 3 g were calculated to have been taken up by skeletal muscle and 7.5 +/- 1.7 g by the kidney (26 +/- 3 and 10 +/- 2%, respectively, of the ingested glucose). Endogenous glucose release during the postprandial period (16 +/- 2 g), calculated as the difference between overall systemic glucose appearance and the appearance of ingested glucose in the systemic circulation, was suppressed 61 +/- 3%. Surprisingly, renal glucose release increased twofold (10.6 +/- 2.5 g) and accounted for ~60% of postprandial endogenous glucose release. Hepatic glucose release (6.7 +/- 2.2 g), the difference between endogenous and renal glucose release, was suppressed 82 +/- 6%. These results demonstrate a hitherto unappreciated contribution of the kidney to postprandial glucose homeostasis and indicate that postprandial suppression of hepatic glucose release is nearly twofold greater than had been calculated in previous studies (42 +/- 4%), which had assumed that there was no renal glucose release. We postulate that increases in postprandial renal glucose release may play a role in facilitating efficient liver glycogen repletion by permitting substantial suppression of hepatic glucose release.

Renal substrate exchange and gluconeogenesis in normal postabsorptive humans

Release of glucose by the kidney in postabsorptive normal humans is generally regarded as being wholly due to gluconeogenesis. Although lactate is the most important systemic gluconeogenic precursor and there is appreciable net renal lactate uptake, renal lactate gluconeogenesis has not yet been investigated. The present studies were therefore undertaken to quantitate the contribution of lactate to renal gluconeogenesis and the role of the kidney in lactate metabolism. We determined systemic and renal lactate conversion to glucose as well as renal lactate net balance, fractional extraction, uptake, and release in 24 postabsorptive humans by use of a combination of isotopic and renal balance techniques. For comparative purposes, accumulated similar data for glutamine, alanine, and glycerol are also reported. Systemic lactate gluconeogenesis (1.97 +/- 0.12 micromol x kg(-1) x min(-1)) was about threefold greater than that from glycerol, glutamine, and alanine. The sum of gluconeogenesis from these precursors, uncorrected for tricarboxylic acid (TCA) cycle carbon exchange, explained 34% of systemic glucose release. Renal lactate uptake (3.33 +/- 0.28 micromol x kg(-1) x min(-1)) accounted for nearly 30% of its systemic turnover. Renal gluconeogenesis from lactate (0.78 +/- 0.10 micromol x kg(-1) x min(-1)) was 3.5, 2.5, and 9.6-fold greater than that from glycerol, glutamine, and alanine. The sum of renal gluconeogenesis from these precursors equaled approximately 40% of the sum of their systemic gluconeogenesis. When the isotopically determined rates of systemic and renal gluconeogenesis were corrected for TCA cycle carbon exchange, gluconeogenesis from these precursors accounted for 43% of systemic glucose release and 89% of renal glucose release. We conclude that 1) in postabsorptive normal humans, lactate is the dominant precursor for both renal and systemic gluconeogenesis; 2) the kidney is an important organ for lactate disposal; 3) under these conditions, renal glucose release is predominantly, if not exclusively, due to gluconeogenesis; and 4) liver and kidney are similarly important for systemic gluconeogenesis.

Role of peroxisome proliferator-activated receptor-alpha in the mechanism underlying changes in renal pyruvate dehydrogenase kinase isoform 4 protein expression in starvation and after refeeding

The pyruvate dehydrogenase complex (PDC) occupies a strategic role in renal intermediary metabolism, via partitioning of pyruvate flux between oxidation and entry into the gluconeogenic pathway. Inactivation of PDC via activation of pyruvate dehydrogenase kinases (PDKs), which catalyze PDC phosphorylation, occurs secondary to increased fatty acid oxidation (FAO). In kidney, inactivation of PDC after prolonged starvation is mediated by up-regulation of the protein expression of two PDK isoforms, PDK2 and PDK4. The lipid-activated transcription factor, peroxisome proliferator-activated receptor-alpha (PPAR alpha), plays a pivotal role in the cellular metabolic response to fatty acids and is abundant in kidney. In the present study we used PPAR alpha null mice to examine the potential role of PPAR alpha in regulating renal PDK protein expression. In wild-type mice, fasting (24 h) induced marked up-regulation of the protein expression of PDK4, together with modest up-regulation of PDK2 protein expression. In striking contrast, renal protein expression of PDK4 was only marginally induced by fasting in PPAR alpha null mice. The present results define a critical role for PPAR alpha in renal adaptation to fasting, and identify PDK4 as a downstream target of PPAR alpha activation in the kidney. We propose that specific up-regulation of renal PDK4 protein expression in starvation, by maintaining PDC activity relatively low, facilitates pyruvate carboxylation to oxaloacetate and therefore entry of acetyl-CoA derived from FA beta-oxidation into the TCA cycle, allowing adequate ATP production for brisk rates of gluconeogenesis.