Starvation Ketosis and the Kidney

BACKGROUND

The remarkable ability of the body to adapt to long-term starvation has been critical for survival of primitive man. An appreciation of these processes can provide the clinician better insight into many clinical conditions characterized by ketoacidosis.

SUMMARY

The body adapts to long-term fasting by conserving nitrogen, as the brain increasingly utilizes keto acids, sparing the need for glucose. This shift in fuel utilization decreases the need for mobilization of amino acids from the muscle for purposes of gluconeogenesis. Loss of urinary nitrogen is initially in the form of urea when hepatic gluconeogenesis is dominant and later as ammonia reflecting increased glutamine uptake by the kidney. The carbon skeleton of glutamine is utilized for glucose production and regeneration of consumed HCO3-. The replacement of urea with NH4+ provides the osmoles needed for urine flow and waste product excretion. Over time, the urinary loss of nitrogen is minimized as kidney uptake of filtered ketone bodies becomes more complete. Adjustments in urine Na+ serve to minimize kidney K+ wasting and, along with changes in urine pH, minimize the likelihood of uric acid precipitation. There is a sexual dimorphism in response to starvation. Key Message: Ketoacidosis is a major feature of common clinical conditions to include diabetic ketoacidosis, alcoholic ketoacidosis, salicylate intoxication, SGLT2 inhibitor therapy, and calorie sufficient but carbohydrate-restricted diets. Familiarity with the pathophysiology and metabolic consequences of ketogenesis is critical, given the potential for the clinician to encounter one of these conditions.

Correction: Paszkiewicz et al. 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. Int. J. Mol. Sci. 2020, 21, 6639

The authors wish to make the following corrections to this paper [...].

Amenorrhea and Estrogen Disorders in Kidney Disease

Sexual and reproductive function are impacted negatively in individuals with chronic kidney disease and end-stage renal disease. Disruption of the hypothalamic-pituitary-gonadal axis plays a pivotal role in contributing to these manifestations as a result of decreasing kidney function and the development of uremia. Early menopause is encountered commonly in women with reduced kidney function, and treatment is problematic as a result of reduced kidney function changing the half-life of medications. Kidney transplantation corrects some of these abnormalities, but medications required after transplantation as well as the persistence of other comorbidities are barriers to normal restoration of gonadal dysfunction.

Clinical Management of Hyperkalemia

Hyperkalemia is an electrolyte abnormality with potentially life-threatening consequences. Despite various guidelines, no universally accepted consensus exists on best practices for hyperkalemia monitoring, with variations in precise potassium (K⁺) concentration thresholds or for the management of acute or chronic hyperkalemia. Based on the available evidence, this review identifies several critical issues and unmet needs with regard to the management of hyperkalemia. Real-world studies are needed for a better understanding of the prevalence of hyperkalemia outside the clinical trial setting. There is a need to improve effective management of hyperkalemia, including classification and K⁺ monitoring, when to reinitiate previously discontinued renin-angiotensin-aldosterone system inhibitor (RAASi) therapy, and when to use oral K⁺-binding agents. Monitoring serum K⁺ should be individualized; however, increased frequency of monitoring should be considered for patients with chronic kidney disease, diabetes, heart failure, or a history of hyperkalemia and for those receiving RAASi therapy. Recent clinical studies suggest that the newer K⁺ binders (patiromer sorbitex calcium and sodium zirconium cyclosilicate) may facilitate optimization of RAASi therapy. Enhancing the knowledge of primary care physicians and internists with respect to the safety profiles of these newer K⁺ binders may increase confidence in managing patients with hyperkalemia. Lastly, the availability of newer K⁺-binding agents requires further study to establish whether stringent dietary K⁺ restrictions are needed in patients receiving K⁺-binder therapy. Individualized monitoring of serum K⁺ among patients with an increased risk of hyperkalemia and the use of newer K⁺-binding agents may allow for optimization of RAASi therapy and more effective management of hyperkalemia.

Renal Tubular Acidosis and Management Strategies: A Narrative Review

Renal tubular acidosis (RTA) occurs when the kidneys are unable to maintain normal acid−base homeostasis because of tubular defects in acid excretion or bicarbonate ion reabsorption. Using illustrative clinical cases, this review describes the main types of RTA observed in clinical practice and provides an overview of their diagnosis and treatment. The three major forms of RTA are distal RTA (type 1; characterized by impaired acid excretion), proximal RTA (type 2; caused by defects in reabsorption of filtered bicarbonate), and hyperkalemic RTA (type 4; caused by abnormal excretion of acid and potassium in the collecting duct). Type 3 RTA is a rare form of the disease with features of both distal and proximal RTA. Accurate diagnosis of RTA plays an important role in optimal patient management. The diagnosis of distal versus proximal RTA involves assessment of urinary acid and bicarbonate secretion, while in hyperkalemic RTA, selective aldosterone deficiency or resistance to its effects is confirmed after exclusion of other causes of hyperkalemia. Treatment options include alkali therapy in patients with distal or proximal RTA and lowering of serum potassium concentrations through dietary modification and potential new pharmacotherapies in patients with hyperkalemic RTA including newer potassium binders.

Gastrointestinal potassium binding in hemodialysis

Hyperkalemia in patients on dialysis is associated with an increased mortality rate. Dietary restriction is often not effective and deprives patients of heart-healthy foods. Lowering the dialysate K⁺ concentration can potentially increase the risk of arrhythmias. In this commentary, we discuss the findings of Amdur et al., and the potential use for K⁺-binding drugs as a strategy to maintain plasma K⁺ concentrations within a narrow and normal range during the interdialytic and intradialytic intervals.

A Universally Accepted Definition of Gender Will Positively Impact Societal Understanding, Acceptance, and Appropriateness of Health Care

When individuals do not conform to stereotypes associated with "male" or "female," they are often ridiculed, bullied, and rejected, which leads to depression, psychological problems, and even suicide. The number of individuals who identify themselves as transgender, gender queer, or who do not conform to societal norms with respect to gender appears to be increasing. Despite this apparent increase, clinicians and health care facilities are ill-prepared to meet the needs of these individuals in a professional and appropriate manner. Unfortunately, there is an inherit distrust of the medical community by individuals who do not conform to societal norms with respect to gender because of a perception that the medical community does not understand the unique challenges of these individuals. Therefore, reducing the social stigma associated with gender nonconforming individuals is one way to begin to break down barriers of distrust and enhance communication within and outside the medical community. In this review, we discuss the scant amount of scientific data on the biological origins of gender identity. We highlight the fact that the biological definition of gender remains elusive in part because molecular and biological techniques have not been available to accurately probe the development of gender identity. We therefore advocate for the importance of enhancing our knowledge of the origins of gender identity with advanced scientific tools. Enhancing scientific understanding of the biological origins of gender identity may reduce stigma and barriers to care.

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

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.

Palmitic acid reduces the autophagic flux in hypothalamic neurons by impairing autophagosome-lysosome fusion and endolysosomal dynamics

High-fat diet (HFD)-induced obesity is associated with increased cancer risk. Long-term feeding with HFD increases the concentration of the saturated fatty acid palmitic acid (PA) in the hypothalamus. We previously showed that, in hypothalamic neuronal cells, exposure to PA inhibits the autophagic flux, which is the whole autophagic process from the synthesis of the autophagosomes, up to their lysosomal fusion and degradation. However, the mechanism by which PA impairs autophagy in hypothalamic neurons remains unknown. Here, we show that PA-mediated reduction of the autophagic flux is not caused by lysosomal dysfunction, as PA treatment does not impair lysosomal pH or the activity of cathepsin B.Instead, PA dysregulates autophagy by reducing autophagosome-lysosome fusion, which correlates with the swelling of endolysosomal compartments that show areduction in their dynamics. Finally, because lysosomes undergo constant dynamic regulation by the small Rab7 GTPase, we investigated the effect of PA treatment on its activity. Interestingly, we found PA treatment altered the activity of Rab7. Altogether, these results unveil the cellular process by which PA exposure impairs the autophagic flux. As impaired autophagy in hypothalamic neurons promotes obesity, and balanced autophagy is required to inhibit malignant transformation, this could affect tumor initiation, progression, and/or response to therapy of obesity-related cancers.

Impact of Dietary Potassium Restrictions in CKD on Clinical Outcomes: Benefits of a Plant-Based Diet

In patients with advanced-stage chronic kidney disease (CKD), progressive kidney function decline leads to increased risk for hyperkalemia (serum potassium > 5.0 or >5.5 mEq/L). Medications such as renin-angiotensin-aldosterone system inhibitors pose an additional hyperkalemia risk, especially in patients with CKD. When hyperkalemia develops, clinicians often recommend a diet that is lower in potassium content. This review discusses the barriers to adherence to a low-potassium diet and the impact of dietary restrictions on adverse clinical outcomes. Accumulating evidence indicates that a diet that incorporates potassium-rich foods has multiple health benefits, which may also be attributable to the other vitamin, mineral, and fiber content of potassium-rich foods. These benefits include blood pressure reductions and reduced risks for cardiovascular disease and stroke. High-potassium foods may also prevent CKD progression and reduce mortality risk in patients with CKD. Adjunctive treatment with the newer potassium-binding agents, patiromer and sodium zirconium cyclosilicate, may allow for optimal renin-angiotensin-aldosterone system inhibitor therapy in patients with CKD and hyperkalemia, potentially making it possible for patients with CKD and hyperkalemia to liberalize their diet. This may allow them the health benefits of a high-potassium diet without the increased risk for hyperkalemia, although further studies are needed.

Effects of ERβ and ERα on OVX-induced changes in adiposity and insulin resistance

Loss of ovarian hormones leads to increased adiposity and insulin resistance (IR), increasing the risk for cardiovascular and metabolic diseases. The purpose of this study was to investigate whether the molecular mechanism behind the adverse systemic and adipose tissue-specific metabolic effects of ovariectomy requires loss of signaling through estrogen receptor alpha (ERα) or estrogen receptor β (ERβ). We examined ovariectomized (OVX) and ovary-intactwild-type (WT), ERα-null (αKO), and ERβ-null (βKO) female mice (age ~49 weeks; n = 7-12/group). All mice were fed a phytoestrogen-free diet (<15 mg/kg) and either remained ovary-intact (INT) or were OVX and followed for 12 weeks. Body composition, energy expenditure, glucose tolerance, and adipose tissue gene and protein expression were analyzed. INT αKO were ~25% fatter with reduced energy expenditure compared to age-matched INT WT controls and βKO mice (all P < 0.001). Following OVX, αKO mice did not increase adiposity or experience a further increase in IR, unlike WT and βKO, suggesting that loss of signaling through ERα mediates OVX-induced metabolic dysfunction. In fact, OVX in αKO mice (i.e., signaling through ERβ in the absence of ERα) resulted in reduced adiposity, adipocyte size, and IR (P < 0.05 for all). βKO mice responded adversely to OVX in terms of increased adiposity and development of IR. Together, these findings challenge the paradigm that ERα mediates metabolic protection over ERβ in all settings. These findings lead us to suggest that, following ovarian hormone loss, ERβ may mediate protective metabolic benefits.

Potassium Homeostasis, Chronic Kidney Disease, and the Plant-Enriched Diets

There are data demonstrating that ingestion of potassium-rich foods reduces the incidence of stroke, hypertension, nephrolithiasis, and osteoporosis. Dietary-consumption data indicate Westernized diets are high in processed foods, high in sodium content, and low in potassium. In fact, there are data suggesting individuals are not consuming enough potassium in their diet. Although consumption of diets high in plant proteins, fruits, and vegetables—which are excellent sources of potassium—is recognized as healthy and beneficial, individuals with decrements in their kidney function have been advised to avoid these foods. In reviewing the literature that provides the rationale for potassium restriction in patients with reductions in kidney function, it appears there is little direct evidence to support the levels of restriction which are now prescribed. Additionally, there are two new potassium-binding agents which are well tolerated and have been documented to be effective in controlling serum potassium. Therefore, with the new binding agents and the lack of empirical evidence supporting the stringent dietary potassium restrictions, the authors conclude by indicating the pressing need for further research focusing on dietary liberalization of potassium in patients with reductions in kidney function to enhance overall health and well being, to provide them cardiovascular benefits, and to reduce overall risk of mortality through the incorporation of potassium-enriched foods.

Estradiol stimulates adipogenesis and Slc2a4/GLUT4 expression via ESR1-mediated activation of CEBPA

The ability of adipose tissue to expand is dependent on adipocyte differentiation and adipose tissue glucose disposal. The CCAAT/enhancer-binding protein alpha (CEBPA) enhances the expression of the Slc2a4 gene and GLUT4 protein, which are markers of adipocyte differentiation/glucose disposal. We hypothesized estradiol (E2) facilitates adipocyte differentiation/glucose disposal by an estrogen receptor 1 (ESR1)-dependent and CEBPA-mediated mechanism. Our results suggest that E2 (10 nM) has a positive effect on 3T3-L1 adipocyte differentiation (days 2-8), lipid accumulation, Slc2a4 and Cebpa mRNA expression, total GLUT4 and nuclear CEBPA contents, and CEBP/Slc2a4-binding activity. Esr1 silencing (∼50%) in mature adipocytes abrogates the 24-h E2 effects on nuclear CEBPA content, Slc2a4/GLUT4 expression and GLUT4 translocation to the cell membrane. Thus, E2 stimulates adipocyte differentiation and Slc2a4/GLUT4 expression in an ESR1/CEBPA-mediated pathway. Our data provide mechanistic insight demonstrating E2 participates in adipose-tissue differentiation and glucose transporter expression which ultimately can improve adipose tissue expandability and glycemic control.

Imaging Metabolically Active Fat: A Literature Review and Mechanistic Insights

Currently, obesity is one of the leading causes death in the world. Shortly before 2000, researchers began describing metabolically active adipose tissue on cancer-surveillance ¹⁸F-fluorodeoxyglucose (FDG) positron emission tomography/computed tomography (PET/CT) in adult humans. This tissue generates heat through mitochondrial uncoupling and functions similar to classical brown and beige adipose tissue in mice. Despite extensive research, human brown/beige fat's role in resistance to obesity in humans has not yet been fully delineated. FDG uptake is the de facto gold standard imaging technique when studying brown adipose tissue, although it has not been rigorously compared to other techniques. We, therefore, present a concise review of established and emerging methods to image brown adipose tissue activity in humans. Reviewed modalities include anatomic imaging with CT and magnetic resonance imaging (MRI); molecular imaging with FDG, fatty acids, and acetate; and emerging techniques. FDG-PET/CT is the most commonly used modality because of its widespread use in cancer imaging, but there are mechanistic reasons to believe other radiotracers may be more sensitive and accurate at detecting brown adipose tissue activity. Radiation-free modalities may help the longitudinal study of brown adipose tissue activity in the future.

Female Sex and Gender in Lung/Sleep Health and Disease. Increased Understanding of Basic Biological, Pathophysiological, and Behavioral Mechanisms Leading to Better Health for Female Patients with Lung Disease

Female sex/gender is an undercharacterized variable in studies related to lung development and disease. Notwithstanding, many aspects of lung and sleep biology and pathobiology are impacted by female sex and female reproductive transitions. These may manifest as differential gene expression or peculiar organ development. Some conditions are more prevalent in women, such as asthma and insomnia, or, in the case of lymphangioleiomyomatosis, are seen almost exclusively in women. In other diseases, presentation differs, such as the higher frequency of exacerbations experienced by women with chronic obstructive pulmonary disease or greater cardiac morbidity among women with sleep-disordered breathing. Recent advances in -omics and behavioral science provide an opportunity to specifically address sex-based differences and explore research needs and opportunities that will elucidate biochemical pathways, thus enabling more targeted/personalized therapies. To explore the status of and opportunities for research in this area, the NHLBI, in partnership with the NIH Office of Research on Women's Health and the Office of Rare Diseases Research, convened a workshop of investigators in Bethesda, Maryland on September 18 and 19, 2017. At the workshop, the participants reviewed the current understanding of the biological, behavioral, and clinical implications of female sex and gender on lung and sleep health and disease, and formulated recommendations that address research gaps, with a view to achieving better health outcomes through more precise management of female patients with nonneoplastic lung disease. This report summarizes those discussions.

Activation of NPRs and UCP1-independent pathway following CB1R antagonist treatment is associated with adipose tissue beiging in fat-fed male dogs

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.

Hyperchloremic normal gap metabolic acidosis

Metabolic acidosis is defined as a pathologic process that, when unopposed, increases the concentration of hydrogen ions (H+) in the body and reduces the bicarbonate (HCO3-) concentration. Metabolic acidosis can be of a kidney origin or an extrarenal cause. Assessment of urinary ammonium excretion by calculating the urine anion gap or osmolal gap is a useful method to distinguish between these two causes. Extrarenal processes include increased endogenous acid production and accelerated loss of bicarbonate from the body. Metabolic acidosis of renal origin is due to a primary defect in renal acidification with no increase in extrarenal hydrogen ion production. This situation can occur because either the renal input of new bicarbonate is insufficient to regenerate the bicarbonate lost in buffering endogenous acid as with distal renal tubular acidosis (RTA) or the RTA of renal insufficiency, or the filtered bicarbonate is lost by kidney wasting as in proximal RTA. In either condition, because of loss of either NaHCO3 (proximal RTA) or NaA (distal RTA), effective extracellular volume is reduced and as a result the avidity for chloride reabsorption derived from the diet is increased and results in a hyperchloremic normal gap metabolic acidosis. The RTA of renal insufficiency is also characterized by a normal gap acidosis, however, with severe reductions in the glomerular filtration rate an anion gap metabolic acidosis eventually develops.

Physiology and Pathophysiology of Potassium Homeostasis: Core Curriculum 2019

Total-body potassium (K⁺) content and appropriate distribution of K⁺ across the cell membrane is vitally important for normal cellular function. Total-body K⁺ content is determined by changes in excretion of K⁺ by the kidneys in response to intake levels. Under normal conditions, insulin and β-adrenergic tone also make important contributions in maintaining internal distribution of K⁺. However, despite these homeostatic pathways, disorders of altered K⁺ homeostasis are common. Appreciating the pathophysiology and regulatory influences that determine the internal distribution and external balance of K⁺ is critical in designing effective treatments to restore K⁺ homeostasis. We provide an up-to-date review of the regulatory aspects of normal K⁺ physiology as a preface to highlighting common disorders in K⁺ homeostasis and their treatment. This review of K⁺ homeostasis is designed as a resource for clinicians and a tool for educators who are teaching trainees to understand the pivotal factors involved in K⁺ balance.