Role of Kidneys in Metabolism

The key functions of the kidney are filtration of plasma, and subsequent tubular excretion and reabsorption of ions, low-molecular-weight substances and water

The excretory function of the kidneys involves filtration of the plasma in the glomeruli, transport of water and solutes from the tubular lumen back to the blood (tubular reabsorption), and secretion of different substances from the tubular cells into the lumen.

The kidneys remove, in urine, products of metabolism such as urea, uric acid and creatinine, and retain substances such as glucose, amino acids, and proteins. They also metabolize and remove drugs and toxins. They play a critical role in the regulation of extracellular fluid volume and composition and in maintaining the acid–base balance (Chapters 24 and 25). These functions are regulated hormonally by vasopressin (antidiuretic hormone, ADH) produced in the posterior pituitary, and by the renin–angiotensin system. Kidney function is also controlled by norepinephrine and dopamine released from nerve terminals in the kidney itself.

The kidneys also produce calcitriol (1,25-dihydroxycholecalciferol, 1,25(OH)₂D₃), a vitamin involved in calcium homeostasis (Chapter 26) and erythropoietin, which controls the production of erythrocytes.

The functional unit of the kidney is the nephron

Each kidney consists of approximately 1 million nephrons composed of glomerulus and an excretory tubule (Fig. 23.1). Glomeruli, located in the kidney cortex, are biological filters, interfacing the plasma with the tubules that reabsorb and excrete substances. The segments of each tubule (starting from the glomerulus end) are known as the proximal tubule, the loop of Henle, the distal tubule, and the collecting duct.

The glomerular filtration barrier

Plasma from the glomerular capillaries is filtered into the interior of the glomerulus known as the Bowman's space. The filtration barrier includes a layer of endothelial cells that line glomerular blood vessels, the basement membrane, and the epithelial cells (podocytes) with characteristic foot processes (Fig. 23.2). Glomerular filtration depends on the filtration surface and on the permeability of the barrier.

The main component of the glomerular basement membrane is type IV collagen, which forms a network of filaments providing elasticity and resistance to hydrostatic pressure. The membrane also contains laminin, fibronectin and proteoglycans with negatively charged heparan sulfate groups, which form an electrostatic barrier for proteins filtered from plasma.

The podocytes and mesangial cells possess receptors for a range of vasoactive substances such as angiotensin II, vasopressin, bradykinin, ATP, endothelin, prostaglandins, dopamine, natriuretic peptides and adenine nucleotides.

The fenestrations in the endothelial layer and the spaces between foot processes of the podocytes form a sieve that filters water and small molecules. Filtration of larger molecules is limited by their size, shape, and electric charge. For instance, at pH 7.4, most plasma proteins are negatively charged, and so is the filtration barrier; this hinders filtration of even the smallest proteins, such as myoglobin (molecular mass 17 kDa), and almost completely prevents filtration of the larger (69 kDa) albumin.

Glomerular filtration is driven by the hydrostatic pressure in the glomerular capillaries, which is approximately 50 mmHg. The hydrostatic pressure is counteracted by the oncotic pressure of the plasma and the back-pressure (approximately 10 mmHg) of the filtrate in the glomerular capsule. Changes in the glomerular filtration rate alter the total amount of filtered water and solute, but not the composition of the filtrate. A decrease in the blood pressure in the afferent arteriole of the glomerulus is sensed by the group of cells known as the juxtaglomerular apparatus. This stimulates renin secretion and activates the renin–angiotensin system. An increase in the intracapillary blood pressure and/or hyperfiltration may induce glomerular cells' injury and may lead to severe kidney damage.

Kidneys consume large amounts of oxygen, mainly to support active sodium transport

Most of the metabolic processes in the kidneys are aerobic, and consequently their oxygen consumption is high: it approximately equals that of the cardiac muscle, and is three times greater than that of the brain. Such a high metabolic activity is required to maintain tubular reabsorption; about 70% of the oxygen consumed by the kidney is used to support active sodium transport, which in turn determines reabsorption of glucose and amino acids.

Renal gluconeogenesis

In humans, only the liver and kidney are capable of gluconeogenesis (Chapter 13). After an overnight fast, 75–80% of glucose released into the circulation derives from the liver and the remaining 20–25% derives from the kidney. Surprisingly, after a meal, renal gluconeogenesis increases approximately twofold and accounts for ~60% of endogenous glucose release in the postprandial period (hepatic gluconeogenesis decreases by ~80% at that time). Glutamine and lactate are preferential gluconeogenic precursors in the kidney.

The key enzymes of gluconeogenesis, pyruvate carboxylase, phosphoenol pyruvate carboxykinase, fructose-1,6-bisphosphatase, and glucose-6-phosphatase, are found mainly in the renal cortical cells. The rate of gluconeogenesis depends on the glucose concentration, substrate availability and hormonal control. Insulin suppresses, and epinephrine increases, glucose release by the kidney. Glucagon has no effect on renal glucose release. The produced glucose provides the brain and other organs with a needed energy substrate, particularly when liver gluconeogenesis is impaired: e.g. after liver transplantation, in hepatic failure, and during prolonged fasting, hypoglycemia and acidosis.

Renal glucose utilization

The metabolic fate of glucose is different in different regions of the kidney. Glucose is an essential energy substrate for the renal medulla, because of low oxygen tension and low levels of oxidative enzymes there. Consequently, in the medulla, the lactate is the main metabolic end product of glucose metabolism.

In contrast, the renal cortex contains high levels of oxidative enzymes. The main energy substrates in the renal cortex are thus fatty acids, lactate, glutamate, citrate, and ketone bodies. Of these, the fatty acids are the main source of energy.

Renal tubular cells with a high Na⁺/K⁺-ATPase activity possess multiple mitochondria situated close to the plasma membrane, so that the released ATP is easily accessible.

Neuronal control of kidney function

The kidneys communicate with the central nervous system via the sensory (afferent) nerves. An increase in renal afferent activity directly influences sympathetic outflow to the kidneys via efferent nerves. Norepinephrine released from the nerve terminals, together with co-transmitters (such as ATP, other adenine mono- and dinucleotides, and neuropeptide Y) modify kidney function (Fig. 23.6). Norepinephrine increases sodium reabsorption directly via tubular Na⁺/K⁺-ATPase activation, and indirectly via the stimulation of the renin–angiotensin system (Fig. 23.7). Increased renal sympathetic activity has been identified as a major contributor to the complex pathophysiology of hypertension. On the other hand, kidney denervation leads to blood pressure lowering.

Na⁺/K⁺-ATPase

Na⁺/K⁺-ATPase activity in the kidney is several thousand times higher than in other tissues. In the kidney, its major function is sodium reabsorption by catalyzing the extrusion of sodium to the interstitial fluid. There is a close relationship between the amount of Na⁺/K⁺-ATPase and sodium reabsorptive capacity of the different segments of the nephron. In the renal tubular cells, as in all sodium-reabsorbing epithelial cells, Na⁺/K⁺-ATPase is present in the basolateral membrane. Its activity in the nephron is controlled by a number of hormones, mainly by aldosterone, angiotensin II, and by neurotransmitters norephinephrine and dopamine (Chapter 24). In humans, the kidney reabsorbs about 18 moles of sodium per day and utilizes about 6 moles of ATP for this process. Thus, Na⁺/K⁺-ATPase is an energy transducer that converts metabolic energy into ionic gradients.

Sodium transport system in the renal tubules

Sodium reabsorption occurs along the nephron, with the exception of the descending limb of the loop of Henle. The driving force for the reabsorption of sodium and other solutes is the electrochemical gradient generated by the Na⁺/K⁺-ATPase in the basolateral membrane of tubular cells. The transport of sodium ions across cellular membrane, similarly to other polar molecules, requires the involvement of specific membrane proteins.

Understanding the renal transport systems elucidates the action of diuretics

In the proximal tubule the sodium ions enter into the cell from the luminal side via the sodium–hydrogen exchanger (NHE3), through ion channels and via co-transporters with glucose, phosphates and amino acids. The sodium–bicarbonate co-transporter (known as NBC1) is located in the basolateral membrane.

In the thin ascending limb of the loop of Henle the sodium ions move into the cell via sodium–potassium–chloride co-transporter (known as NKCC2), which is inhibited by furosemide. In this segment, potassium ions are secreted into the lumen by the ATP-sensitive rectifier potassium channel.

In the distal tubule the reabsorption of sodium ions involves sodium–chloride co-transporter (NCC), which is thiazide-sensitive.

In the collecting duct the sodium ions are reabsorbed by amiloride-sensitive epithelial sodium channel (ENaC). Aldosterone, through the mineralocorticoid receptor stimulates both ENaC expression and the activity of the Na⁺/K⁺-ATPase. The pharmacologic antagonist of aldosterone is the diuretic spironolactone.

Reabsorption of sodium in turn drives the movement of water. The inhibitors of sodium reabsorption, e.g. thiazide diuretics (hydrochlorothiazide), loop diuretics (e.g. furosemide, torasemide), amiloride and spironolactone, are extensively used in clinical practice to induce natriuresis and diuresis.

Glucose reabsorption

Normally, at the average plasma glucose concentration around 5 mmol/L (~100 mg/dL), approximately 200 g of glucose are filtered by the kidney each day. In healthy individuals, all filtered glucose is reabsorbed into the circulation and the urine is glucose-free. Reabsorption of glucose from the filtrate in the proximal convoluted tubules occurs with the participation of sodium–glucose co-transporters (SGLTs). There are two sodium-dependent–glucose co-transporter proteins: SGLT2 is a low-affinity, high-capacity transporter reabsorbing 90% of filtered glucose, and SGLT1 is a high-affinity low-capacity transporter. Glucose transport mediated by the SGLTs is an active process, utilizing energy derived from the sodium electrochemical gradient maintained by the Na⁺/K⁺-ATPase. Glucose reabsorbed into the tubular epithelial cells via SGLTs is released into the circulation by the specific glucose transporters (GLUT-1 and GLUT-2) located in the basolateral membranes.

Reabsorption of filtered glucose increases linearly until the maximal capacity of the tubules (T_max) is exceeded

The point of saturation of the transport system, the so-called renal threshold, is reached at the plasma glucose concentration of 11.0 mmol/L (198 mg/dL) in healthy adults. Above this concentration, the percentage of filtered glucose that is reabsorbed decreases and glucosuria appears. The threshold is decreased in individuals with a rare condition known as familial renal glucosuria, caused by a range of mutations to the SLC5A2 gene, which encodes the SGLT2. Depending on the nature of the mutations, affected individuals have varying degrees of glucosuria. In the most severe form they can lose more than 100 g of glucose per day in the urine.

Renal handling of amino acids

In humans, the total plasma concentration of amino acids is approximately 2.5 mmol/L (~25 mg/dL). Free plasma amino acids are filtered in the renal glomeruli (approximately 50 g per day). In parallel, amino acids are transported into the kidney cells and are metabolized. For example glutamine, transported through both luminal and basolateral membranes, is deaminated by glutaminase. The product of this reaction, ammonia, is secreted into the tubular lumen where it buffers the hydrogen ion (Chapter 25). In the kidney glutamine is also a substrate for gluconeogenesis.

The near-complete reabsorption of filtered amino acids is a fundamental transport function of the proximal tubule. Their transepithelial transport from the tubular lumen, across the cell and into the circulation, is driven by sodium electrochemical gradient. On the luminal membrane of tubular cells, there are three types of sodium-dependent amino acid co-transporters (belonging to solute carrier transporters family; SLC): one for neutral amino acids (glycine, alanine, proline), one for acidic amino acids (glutamate, aspartate) and one for basic amino acids (cystine, arginine, ornithine, lysine). The transport out of the cell involves both co-transporters and antiporters (the latter also belonging to the SLC family). Individual amino acids can be transported by more than one transporter, providing backup capacity if the transport is partially impaired. Elevated urine amino acid excretion may be caused by mutation in the gene(s) encoding for a particular amino acid transporter, as happens in cystinosis.

Only small amounts of amino acids (0.7 g/24 h) are normally present in the urine. Each substance, which is reabsorbed in the renal tubules, has its own renal transport maximum (T_max). T_max can be exceeded either when the amount of filtered substance becomes too large to handle or when the tubular cells do not function properly. Thus, aminoaciduria may result from the accumulation of amino acids such as phenylalanine, leucine, isoleucine, and valine in the plasma, or from an impaired tubular function. Aminoaciduria may also be symptom of proximal tubule damage (the Fanconi syndrome).

Renal handling of phosphate

Homeostasis of inorganic phosphate (Pi) depends mainly on the balance between the intestinal absorption of Pi and its urinary excretion. Adaptation of renal Pi handling to changing dietary Pi content is also well documented. The renal handling of Pi is regulated by hormonal and nonhormonal factors. Changes in urinary excretion of Pi are usually due to changes in the activity of the renal tubular transport system.

Approximately 90% of plasma phosphates are ultrafiltrable in the glomeruli (10% of Pi is bound to plasma albumin). Reabsorption of the filtered Pi occurs in the proximal, and also to some extent in the distal, tubules. The transport of Pi across tubular cells depends on the magnitude of sodium gradient formed by Na⁺/K⁺-ATPase in the basolateral membrane. However, the rate-limiting factor for Pi transport is the abundance of specific membrane transport proteins.

The transport of Pi across brush border membrane of the renal proximal tubules is mediated by the two Na⁺-dependent Pi co-transporter proteins, NaPi-IIa and NaPi-IIc (NaPi-IIb is also highly abundant in the brush border membrane of the small intestine). Na-Pi IIa is electrogenic, because the transport of one divalent Pi ion is coupled with three sodium ions (3Na⁺:HPO₄^2–). NaPi-IIc, in contrast, is electroneutral and couples two sodium ions with one divalent Pi ion (2Na⁺:HPO₄^2–). The expression of NaPi-IIa and NaPi-IIc adjusts the renal reabsorption of Pi to the organism's needs. For example, dietary restriction of Pi intake increases the quantity of NaPi-IIa and NaPi-IIc proteins in the luminal membrane. In contrast, high Pi diet leads to a decrease in the transporter expression. The membrane expression of NaPi-IIa is regulated by parathyroid hormone (PTH). PTH receptors are located on both the basolateral and luminal membranes of the tubular cells. PTH activates adenylate cyclase via receptors located on basolateral membrane and increases the intracellular cyclic AMP. cAMP, in turn, activates protein kinase A, which phosphorylates NaPi-IIa transporter, leading to its internalization (a shift into the cytosol). Also, through the binding to luminal receptors, PTH causes phosphorylation of NaPi-IIa by phospholipase C and protein kinase C, and, again, the subsequent internalization of NaPi-IIa transporter. All this leads to a decreased availability of transporters for the filtered Pi, and its urinary excretion increases.

Urine analysis provides clinically important information

Urine analysis (urinalysis) in clinical laboratories includes testing for the presence of protein, glucose, ketone bodies, bilirubin, and urobilinogen, and for traces of blood. Measuring urinary osmolality assesses the concentrating capacity of the kidney. The urine is also tested for the presence of leukocytes and various crystals and deposits. Specialist investigations include analysis of urinary amino acids, hormones and other metabolites.

Only traces of protein are normally detectable in the urine. This increases when the glomeruli are damaged: the presence of significant amounts of protein in urine is an important sign of renal disease. Even a minimal amount of albumin in the urine (microalbuminuria) predicts the development of diabetic nephropathy (Chapter 21). Larger proteins such as immunoglobulins appear in the urine when the damage is more extensive: the immunoglobulin light chains (the Bence Jones protein) are present in urine in multiple myeloma (Chapter 4). In a hemolytic anemia, urine may contain free hemoglobin and urobilinogen. The presence of myoglobin is a marker of muscle damage (rhabdomyolysis). The measurement of urine glucose and ketones is important in the assessment of glycemic control in diabetic patients (Chapter 21). Measurements of urobilinogen and bilirubin help to assess liver function (Chapter 29).

Glomerular filtration rate is the most important characteristic describing kidney function

The renal clearance is the volume of plasma (in milliliters) that the kidney clears of a given substance every minute. The glomerular filtration rate (GFR) is the most important characteristic describing kidney function. GFR could be estimated by measuring the clearance of a substance, such as the polysaccharide inulin, that is neither secreted nor reabsorbed in the renal tubules. The amount of inulin filtered from plasma (i.e. its plasma concentration, P_in, multiplied by GFR) equals the amount recovered in the urine (i.e. its urinary concentration, U_in, multiplied by the urine formation rate, V):

(1)

From this we calculate the GFR:

(2)

The average GFR is 120 mL/min in men and 100 mL/min in women. The renal clearance of inulin equals the GFR.

Serum creatinine and urea are first-line tests in the diagnosis of renal disease

To administer inulin intravenously every time one would want to assess the GFR is impractical. In clinical practice, we use the creatinine clearance instead.

Creatinine is derived from skeletal muscle phosphocreatine

The clearance of creatinine is similar to that of inulin. Although some creatinine is reabsorbed in the renal tubules, this is compensated by an equivalent tubular secretion. To calculate creatinine clearance, one needs a sample of blood, and urine collected over 24 hours. The concentrations of creatinine in serum (P_Cre) and urine (U_Cre) are measured first. Urine excretion rate is calculated by dividing the urine volume by the collection time (V, above). The creatinine clearance is then calculated according to the formula:

Serum concentration of creatinine is 20–80 mmol/L (0.28–0.90 mg/dL). An increase in serum creatinine concentration reflects the decrease in GFR: serum creatinine concentration doubles when the GFR decreases by 50%. Another test used to assess kidney function is measurement of serum urea concentration. However, because urea is the end product of protein catabolism, its level in plasma is also dependent on factors such as the dietary protein intake and the rate of tissue breakdown.

In clinical practice, serum urea and creatinine are first-line tests in the diagnosis of renal failure (Fig. 23.8). Renal failure leads to a decrease in urine volume and creatinine clearance, and to an increase in serum urea and creatinine. The refinements in laboratory testing include standardization of creatinine measurements by making methods used in clinical laboratories traceable to the reference method, which is isotope dilution mass spectroscopy.

Serum cystatin C concentration is another marker of the GFR

Cystatin C is a 122-amino acid, 13-kDa protein belonging to the family of cysteine proteinase inhibitors. It is a product of the housekeeping gene expressed in all nucleated cells, and is produced at a constant rate. Because of its small size and basic isoelectric point, cystatin C is freely filtered through the glomerulus. It is not secreted by the tubules and although it is reabsorbed, it is subsequently catabolized and therefore does not return to plasma. Its serum concentration is not significantly affected by age, and therefore it is a preferential marker of GFR in children. However, other factors, independent of the GFR, such as the inflammatory phenomena, may affect serum cystatin C concentration.

Neutrophil gelatinase-associated lipocalin is a novel biomarker of acute kidney injury

Neutrophil gelatinase-associated lipocalin (NGAL) is a 178 amino acid, 25-kDa protein belonging to lipocalin family (Greek lipos, ‘fat, grease’ and calyx, ‘cup’). This protein is produced in various epithelial cells (kidney, liver, lung, intestine) and in myelocytes, and is a component of innate immunity. It also plays a role in tissue remodeling, especially after heterodimerization with the metalloproteinase 9 (MMP-9, collagenase IV). NGAL synthesis and secretion into the bloodstream is immediately increased after tissue damage.

Serum NGAL is freely filtered through the glomeruli and is reabsorbed in the proximal tubules. The rising serum NGAL during acute kidney injury may reflect reduced GFR. Though the major source of the urinary NGAL is the distal nephron, a small fraction may come from the filtered pool, escaping tubular reabsorption, due to proximal injury.

Recent studies demonstrated a quantitative link between cellular stress, expression of NGAL in the kidney and excretion of NGAL in urine. The cells in the distal nephron are activated to express NGAL within a few hours following damage (e.g. by ischemia, hypoxia, drug toxicity), before another nephron segment may be affected. Urine NGAL is also being evaluated as a sign of the earliest stage of acute kidney injury, before there are evident clinical abnormalities. The limitations of urine NGAL measurements as biomarker of an acute renal injury are that it may be affected by coexisting chronic kidney disease, renal inflammation and urinary tract infections.

Estimated GFR (eGFR)

Creatinine clearance changes with age, body surface and gender, and also varies with race. Also, the relationship between GFR and creatinine concentration may differ between healthy population and patients with renal disease. In current practice, the GFR values are being adjusted using formulas that include, apart from serum creatinine concentration, factors such as age, gender, weight and race.

Different equations provide useful estimates of GFR: in adults, the equations used are the one developed by the Modification of Diet in Renal Disease Study Group (MDRD) and the Cockcroft–Gault equation; in children, the Schwartz and Counahan–Barratt equations are used. The so-called “abbreviated MDRD formula” includes four variables: serum creatinine, age, sex and race. The original MDRD six-variable equation included serum albumin and serum urea in addition to these. A detailed discussion of these formulas is beyond the scope of this text – the reader is referred to the Further Reading section.

The eGFR equations are useful in the detection of renal impairment. Calculation of the estimated GFR is recommended for the identification and classification, as well as for screening and monitoring, of chronic kidney disease. The severity of chronic kidney disease is classified into six stages (Table 23.2).

One should note though, that in individuals with exceptional dietary intake (vegan, vegetarian diet, creatinine supplements) or abnormal muscle mass (resulting from amputation, malnutrition, muscle wasting), the renal function should still be assessed by measuring creatinine clearance.