Objectives
Upon completion of this chapter, the student should be able to answer the following questions:
- •
What three processes are involved in the production of urine?
- •
What is the composition of “normal” urine?
- •
What transport mechanisms are responsible for sodium chloride (NaCl) reabsorption by the nephron? Where are they located along the nephron?
- •
How is water reabsorption “coupled” to NaCl reabsorption in the proximal tubule?
- •
Why are solutes, but not water, reabsorbed by the thick ascending limb of Henle’s loop?
- •
What transport mechanisms are involved in the secretion of organic anions and cations? What is the physiologic relevance of these transport processes?
- •
What transport proteins are drug targets?
- •
What is glomerulotubular balance, and what is its physiologic importance?
- •
What are the major hormones that regulate NaCl and water reabsorption by the kidneys? What is the nephron site of action of each hormone?
- •
What is the aldosterone paradox?
Key Terms
Ultrafiltration
Reabsorption
Secretion
Passive transport (diffusion)
Osmosis
Solvent drag
Facilitated diffusion
Uniport
Coupled transport
Symport
Antiport
Secondary active transport
Active transport
ABC transporters
Endocytosis
Tight junctions
Lateral intercellular spaces
Type 2 diabetes mellitus
Paracellular pathway
Transcellular pathway
Fanconi syndrome
Aquaporins
Multiligand endocytic receptors
Megalin
Cubilin
Proteinuria
Tamm-Horsfall glycoprotein
Diluting segment
Zonula occludens
Bartter syndrome
Gitelman syndrome
Principal cells
Intercalated cells
Arginine vasopressin (AVP)
Angiotensin II
Aldosterone
Aldosterone-sensitive distal nephron (ASDN)
Aldosterone paradox
Atrial natriuretic peptide (ANP)
Brain natriuretic peptide (BNP)
Sgk
Urodilatin
Liddle syndrome
Pseudohypoaldosteronism (PHA)
Angiotensin-converting enzyme
Uroguanylin
Guanylin
Catecholamines
Sympathetic nerves
Dopamine
Adrenomedullin
Starling forces
Filtration fraction
Glomerulotubular balance (G-T balance)
The formation of urine involves three basic processes: (1) ultrafiltration of plasma by the glomerulus, (2) reabsorption of water and solutes from the ultrafiltrate, and (3) secretion of select solutes into the tubular fluid. Although an average of 115 to 180 L of fluid for women and 130 to 200 L of fluid for men is filtered by the human glomeruli each day, a
a For simplicity, we assume throughout the remainder of this section that the glomerular filtration rate is 180 L/day.
less than 1% of the filtered water and sodium chloride (NaCl) and variable amounts of other solutes are typically excreted in the urine ( Table 4.1 ). By the processes of reabsorption and secretion, the renal tubules determine the volume and composition of urine ( Table 4.2 ), which in turn allows the kidneys to precisely control the volume, osmolality, composition, and pH of the intracellular and extracellular fluid compartments. Transport proteins in cell membranes of the nephron mediate the reabsorption and secretion of solutes and water reabsorption in the kidneys. Approximately 5% to 10% of all human genes code for transport proteins, and genetic and acquired defects in transport proteins are the cause of many kidney diseases ( Table 4.3 ). Many of the transport proteins in the kidneys are important drug targets. Moreover, the kidneys are responsible for excreting numerous drugs and toxins. This chapter discusses NaCl and water reabsorption, organic anion and cation transport, the transport proteins involved in solute and water transport, and some of the factors and hormones that regulate NaCl transport. Details on acid-base transport and on K + , Ca ++ , and inorganic phosphate (P i ) transport and their regulation are provided in Chapter 7, Chapter 8, Chapter 9 .| Substance | Amount | Filtered ∗ | Excreted | Reabsorbed | % Filtered Amount Reabsorbed |
|---|---|---|---|---|---|
| Water | L/day | 180 | 1.5 | 178.5 | 99.2 |
| Na + | mEq/day | 25,200 | 150 | 25,050 | 99.4 |
| K + | mEq/day | 720 | 100 | 620 | 86.1 |
| Ca ++ | mEq/day | 540 | 10 | 530 | 98.2 |
| <SPAN role=presentation tabIndex=0 id=MathJax-Element-1-Frame class=MathJax style="POSITION: relative" data-mathml='HCO3−’>HC?−3HCO3− HC O 3 − | mEq/day | 4320 | 2 | 4318 | 99.9+ |
| Cl – | mEq/day | 18,000 | 150 | 17,850 | 99.2 |
| Glucose | mmol/day | 800 | 0 | 800 | 100.0 |
| Urea | g/day | 56 | 28 | 25 | 50.0 |
∗ The filtered amount of any substance is calculated by multiplying the concentration of that substance in the ultrafiltrate by the glomerular filtration rate (GFR); for example, the filtered amount of Na + is calculated as [Na + ] ultrafiltrate (140 mEq/L) × GFR (180 L/day) = 25,200 mEq/day.
| Substance | Concentration |
|---|---|
| Na + | 50–130 mEq/L |
| K + | 20–70 mEq/L |
| Ammonium | 30–50 mEq/L |
| Ca ++ | 5–12 mEq/L |
| Mg ++ | 2–18 mEq/L |
| Cl – | 50–130 mEq/L |
| Inorganic phosphate | 20–40 mEq/L |
| Urea | 200–400 mmol/L |
| Creatinine | 6–20 mmol/L |
| pH | 5.0–7.0 |
| Osmolality | 500–800 mOsm/kg H 2 O |
| Glucose | 0 |
| Amino acids | 0 |
| Protein | 0 |
| Blood | 0 |
| Ketones | 0 |
| Leukocytes | 0 |
| Bilirubin | 0 |
∗ The composition and volume of the urine can vary widely in the healthy state. These values represent average ranges. Water excretion ranges between 0.5 and 1.5 L/day.
| Diseases | Mode of Inheritance | Gene(s) | Transport Protein | Nephron Segment | Phenotype |
|---|---|---|---|---|---|
| Cystinuria, type I | AR | SLC3A1, SLC7A9 | Amino acid symporters | Proximal tubule | Increased excretion of basic amino acids, nephrolithiasis (kidney stones) |
| Proximal renal tubular acidosis | AR | SLC4A4 | Na + -HCO 3 – symporter | Proximal tubule | Hyperchloremic metabolic acidosis |
| X-linked nephrolithiasis (Dent disease) | XLR | CLCN, OCRL1 | Chloride channel | Distal tubule | Hypercalciuria, nephrolithiasis |
| Bartter syndrome | AR type I | SLC12A1 | Na + /K + /2Cl – symporter | TAL | Hypokalemia, metabolic alkalosis, hyperaldosteronism |
| AR type II | KCNJ1 | ROMK channel | TAL | Hypokalemia, metabolic alkalosis, hyperaldosteronism | |
| AR type III | CLCNKB | Chloride channel (basolateral membrane) | TAL | Hypokalemia, metabolic alkalosis, hyperaldosteronism | |
| AR type IV | BSND, CLCNKA, CLCNKB | Subunit of chloride channel, chloride channels | TAL | Hypokalemia, metabolic alkalosis, hyperaldosteronism | |
| Hypomagnesemia-hypercalciuria syndrome | AR | CLDN16 | Claudin-16, also known as paracellin 1 | TAL | Hypomagnesemia-hypercalciuria, nephrolithiasis |
| Gitelman syndrome | AR | SLC12A3 | Thiazide-sensitive symporter | Distal tubule | Hypomagnesemia, hypokalemic metabolic alkalosis, hypocalciuria, hypotension |
| Pseudohypoaldosteronism type I | AR | SCNN1A, SCNN1B, SCNN1G | α, β, and γ subunits of ENaC | Collecting duct | Increased excretion of Na + , hyperkalemia, hypotension |
| Pseudohypoaldosteronism type I | AD | MLR | Mineralocorticoid receptor | Collecting duct | Increased excretion of Na + , hyperkalemia, hypotension |
| Liddle syndrome | AD | SCNN1B, SCNN1G | β and γ subunits of ENaC | Collecting duct | Decreased excretion of Na + , hypertension |
| Nephrogenic diabetes insipidus type 2 | AR/AD | AQP2 | Aquaporin 2 water channel | Collecting duct | Polyuria, polydipsia, plasma hyperosmolality |
| Distal renal tubular acidosis | AD/AR | SLC4A1 | Cl – /HCO 3 – antiporter | Collecting duct | Metabolic acidosis, hypokalemia, hypercalciuria, nephrolithiasis |
| Distal renal tubular acidosis | AR | ATP6N1B | Subunit of H + ATPase | Collecting duct | Metabolic acidosis, hypokalemia, hypercalciuria, nephrolithiasis |
General Principles of Membrane Transport
Solutes may be transported across cell membranes by passive mechanisms, active transport mechanisms, or endocytosis. In mammals, solute movement occurs by both passive and active mechanisms, whereas all water movement is passive. The movement of a solute across a membrane is passive if it develops spontaneously and does not require direct expenditure of metabolic energy. Passive transport (diffusion) of uncharged solutes occurs from an area of higher concentration to one of lower concentration (i.e., down its chemical concentration gradient). In addition to concentration gradients, the passive diffusion of ions (but not uncharged solutes, such as glucose and urea) is affected by the electrical potential difference (i.e., electrical gradient) across cell membranes and the renal tubules. Cations (e.g., Na + and K + ) move to the negative side of the membrane, whereas anions (e.g., Cl – and bicarbonate [HCO 3 – ]) move to the positive side of the membrane. Diffusion of water ( osmosis ) occurs through aquaporin (AQP) water channels in the cell membrane and is driven by osmotic pressure gradients. When water is reabsorbed across tubule segments, the solutes dissolved in the water also are carried along with the water. This process is called solvent drag and can account for a substantial amount of solute reabsorption across the proximal tubule. Traditionally it was thought that the biologically important gases oxygen (O 2 ), carbon dioxide (CO 2 ), and ammonia (NH 3 ) diffused across the lipid bilayer of plasma membranes. It is now known that these gases also move across the membrane via specific membrane transport proteins (e.g., CO 2 and NH 3 have been found to cross the membrane via the AQP1 water channel).
In facilitated diffusion , transport depends on the interaction of the solute with a specific protein in the membrane that facilitates its movement across the membrane. If defined broadly, the term facilitated diffusion can be used to describe several different types of membrane transporters. For example, one form of facilitated diffusion is the diffusion of ions, such as Na + and K + , through aqueous-filled channels created by proteins that span the plasma membrane. Also, the movement of a single molecule across the membrane by a transport protein ( uniport ), such as occurs with urea and glucose, is a form of facilitated diffusion. b
b Some authors restrict the term facilitated diffusion to this type of transport and use as a classical example the glucose uniporter that brings glucose into cells.
Another form of facilitated diffusion is coupled transport, in which the movement of two or more solutes across a membrane depends on their interaction with a specific transport protein. Coupled transport of two or more solutes in the same direction is mediated by a symport mechanism. Examples of symport mechanisms in the kidneys include Na + -glucose, Na + –amino acid, and Na + -P i symporters in the proximal tubule and the Na + -K + -2Cl – symporter in the thick ascending limb of Henle’s loop. Coupled transport of two or more solutes in opposite directions is mediated by an antiport mechanism. An Na + -H + antiporter in the proximal tubule mediates Na + reabsorption and H + secretion. With coupled transporters, at least one of the solutes usually is transported against its electrochemical gradient. The energy for this uphill movement is derived from the passive downhill movement of at least one of the other solutes into the cell. For example, in the proximal tubule, operation of the Na + -H + antiporter in the apical membrane of the cell results in the movement of H + against its electrochemical gradient out of the cell into the tubular fluid. The movement of Na + from the tubular fluid into the cell, down its electrochemical gradient, drives the uphill movement of H + . The uphill movement of H + is termed secondary active transport , to reflect the fact that the movement of H + is not directly coupled to the hydrolysis of adenosine triphosphate (ATP) (see next). Instead, the energy is derived from the gradient of the other coupled ion (in this example, Na + ).
Active transport occurs when transport is coupled directly to energy derived from metabolic processes (i.e., it consumes ATP). Active transport of solutes usually takes place from an area of lower concentration to an area of higher concentration. In the kidneys the most prevalent active transport mechanism is sodium-potassium adenosine triphosphatase (Na + -K + -ATPase) (or the sodium pump), located in the basolateral membrane of the tubular cells. The Na + -K + -ATPase is made up of several proteins that together actively move Na + out of the cell and K + into the cell. Other active transport mechanisms in the kidneys include the H + -ATPase and H + -K + -ATPase, which are responsible for H + secretion in the collecting duct (see Chapter 8 ), and the Ca ++ -ATPase mechanism, which is responsible for Ca ++ movement from the cell cytoplasm into the blood (see Chapter 9 ). In addition to these transport ATPases, another large group of ATP-dependent transporters exists that is called A TP- b inding c assette, or ABC transporters . To date, 7 subfamilies and more than 48 specific ABC transporters have been identified in humans. They transport a diverse group of solutes, including Cl – , cholesterol, bile acids, drugs, iron, and organic anions and cations.
Endocytosis is the movement of a substance across the plasma membrane by a process involving the invagination of a piece of membrane until it completely pinches off and forms a vesicle in the cytoplasm. This mechanism is important for the reabsorption of small proteins and macromolecules by the proximal tubule. Because endocytosis requires ATP, it is a form of active transport.
General Principles of Transepithelial Solute and Water Transport
As illustrated in Fig. 4.1 , tight junctions hold renal cells together. Below the tight junctions, the cells are separated by lateral intercellular spaces . The tight junctions separate the apical membranes from the basolateral membranes.
In the nephron a substance can be reabsorbed or secreted through cells, the transcellular pathway, or between cells, the paracellular pathway (see Fig. 4.1 ). Na + reabsorption by the proximal tubule is a good example of transport by the transcellular pathway. Na + reabsorption in this nephron segment depends on the operation of the Na + -K + -ATPase (see Fig. 4.1 ). The Na + -K + -ATPase, which is located exclusively in the basolateral membrane, moves Na + out of the cell into the blood and K + into the cell. Thus the operation of the Na + -K + -ATPase lowers intracellular [Na + ] and increases intracellular [K + ]. Because intracellular [Na + ] is low (12 mEq/L) and the [Na + ] in tubular fluid is high (140 mEq/L), Na + can move across the apical membrane into the cell down this chemical gradient. This movement of Na + into the cell is coupled to the movement of other ions and molecules, either by an antiporter (e.g., Na + /H + antiporter) or symport (e.g., Na + -glucose) (see Figs. 4.2 to 4.4 ). The Na + -K + -ATPase senses the addition of Na + to the cell and is stimulated to increase its rate of Na + extrusion into the blood, thereby returning intracellular Na + to normal levels. Thus transcellular Na + reabsorption by the proximal tubule is a two-step process:
- 1.
Movement across the apical membrane into the cell, down a chemical concentration gradient established by the Na + -K + -ATPase
- 2.
Movement across the basolateral membrane against an electrochemical gradient through the Na + -K + -ATPase
The reabsorption of Ca ++ and K + across the proximal tubule is a good example of paracellular transport. Some of the water reabsorbed across the proximal tubule traverses the paracellular pathway (see Fig. 4.1 ). Some solutes dissolved in this water, particularly Ca ++ and K + , are entrained in the reabsorbed fluid and thereby reabsorbed by the process of solvent drag.
The tight junction in renal epithelial cells is a specialized membrane domain that creates a barrier that regulates the paracellular diffusion of solutes across the epithelia. Tight junctions are composed of linear arrays of several integral membrane proteins, including occludins, claudins, and several members of the immunoglobulin superfamily. The tight junction complex of proteins has biophysical properties of ion channels, including the ability to allow ions to diffuse selectively across the complex based on size and charge.
The tight junction in renal epithelial cells is a specialized membrane domain that creates a barrier that regulates the paracellular diffusion of solutes across the epithelia. Tight junctions are composed of linear arrays of several integral membrane proteins, including occludins, claudins, and several members of the immunoglobulin superfamily. The tight junction complex of proteins has biophysical properties of ion channels, including the ability to allow ions to diffuse selectively across the complex based on size and charge.
NaCl, Solute, and Water Reabsorption Along the Nephron
Quantitatively the reabsorption of NaCl and water represents the major function of nephrons. Approximately 25,050 mEq/day of Na + and 178.5 L/day of water are reabsorbed by the renal tubules (see Table 4.1 ). In addition, renal transport of many other important solutes is linked either directly or indirectly to Na + reabsorption. In the following sections the NaCl and water transport processes of each nephron segment and its regulation by hormones, along with other factors, are presented.
Proximal Tubule
The proximal tubule reabsorbs approximately 67% of filtered water, Na + , Cl – , K + , and other solutes. In addition, the proximal tubule reabsorbs virtually all the glucose and amino acids filtered by the glomerulus, as well as most of the HCO 3 – . The key element in proximal tubule reabsorption is the Na + -K + -ATPase in the basolateral membrane. The reabsorption of every substance, including water, is linked in some manner to the operation of the Na + -K + -ATPase.
Na + Reabsorption
Na + is reabsorbed by different mechanisms in the first and the second halves of the proximal tubule. In the first half of the proximal tubule, Na + is reabsorbed primarily with HCO 3 – and, to a lesser degree, with glucose, amino acids, P i , and lactate. In contrast, in the second half of the proximal tubule, Na + is reabsorbed mainly with Cl – . This disparity is mediated by differences in the Na + transport systems in the first and second halves of the proximal tubule and by differences in the composition of tubular fluid at these sites. In absolute terms, the first half of the proximal tubule reabsorbs significantly more Na + than the second half of the proximal tubule.
In the first half of the proximal tubule, Na + uptake into the cell is coupled with either H + or organic solutes, including glucose ( Fig. 4.2 ). Specific transport proteins mediate Na + entry into the cell across the apical membrane. For example, the Na + -H + antiporter, NHE3 ( Fig. 4.2A ), couples Na + entry with H + extrusion from the cell. H + secretion results in sodium bicarbonate (NaHCO 3 ) reabsorption (see Chapter 8 ). Na + also enters proximal tubule cells by several symporter mechanisms, including Na + -glucose (SGLT2), Na + –amino acid, Na + -P i , and Na + -lactate (see Fig. 4.2B ). The glucose and other organic solutes that enter the cell with Na + leave the cell across the basolateral membrane by passive transport mechanisms. Any Na + that enters the cell across the apical membrane leaves the cell and enters the blood by the Na + -K + -ATPase. Thus reabsorption of Na + in the first half of the proximal tubule is coupled to that of HCO 3 – , P i , and a number of organic molecules, and this generates a negative transepithelial voltage across the proximal tubule that provides the driving force for the paracellular reabsorption of Cl – . The reabsorption of many organic molecules, including glucose and lactate, is so avid that they are almost completely removed from the tubular fluid in the first half of the proximal tubule ( Fig. 4.3 ). The reabsorption of NaHCO 3 , Na + -P i , and Na + –organic solutes across the proximal tubule establishes a transtubular osmotic gradient (i.e., the osmolality of the interstitial fluid bathing the basolateral side of the cells is a few mOsm/L higher than the osmolality of tubule fluid) that provides the driving force for the passive reabsorption of water by osmosis. Because more water than Cl – is reabsorbed in the first half of the proximal tubule, the Cl – concentration in tubular fluid rises along the length of the proximal tubule (see Fig. 4.3 ).
In the second half of the proximal tubule, Na + reabsorption is primarily accompanied by Cl – via both transcellular and paracellular pathways ( Fig. 4.4 ). Na + is primarily reabsorbed with Cl – rather than organic solutes or HCO 3 – as the accompanying anion, because the Na + transport mechanisms in the second half of the proximal tubule differ from those in the first half, and because the tubular fluid that enters the second half contains very little glucose or amino acids (see Fig. 4.3 ). In addition, the high concentration of Cl – (140 mEq/L) in tubule fluid (see Fig. 4.3 ), which is due to the preferential reabsorption of Na + with HCO 3 – and organic solutes in the first half of the proximal tubule, facilitates the reabsorption of Cl – with Na + .
The mechanism of transcellular Na + reabsorption in the second half of the proximal tubule is shown in Fig. 4.4 . Na + enters the cell across the luminal membrane primarily through the parallel operation of a Na + -H + antiporter (NHE3) and one or more Cl – -base antiporters (e.g., CFEX). Because the secreted H + and base combine in the tubular fluid and reenter the cell, the operation of the Na + -H + and Cl – -base antiporters is equivalent to NaCl uptake from tubular fluid into the cell. Na + leaves the cell through the Na + -K + -ATPase, and Cl – leaves the cell and enters the blood through a K + -Cl – symporter (KCC) and a Cl – channel in the basolateral membrane.
Worldwide more than 350 million individuals have type 2 diabetes mellitus (T2DM), one of the most common causes of end-stage kidney disease. Patients with T2DM have hyperglycemia, decreased GFR, persistent albuminuria, and hypertension. Current treatment involves management of hyperglycemia with metformin, insulin, or pioglitazone and with inhibitors of the renin-angiotensin-aldosterone system (RAAS) (e.g., inhibitors of both angiotensin-converting enzyme I and the angiotensin II receptor). However, this does not adequately control hyperglycemia. Because SGLT2, located in the second half of the proximal tubule, reabsorbs ∼90% of the glucose filtered by the glomerulus, SGLT2 inhibitors, including canagliflozin, empagliflozin, and dapagliflozin, are also given to patients with T2DM to inhibit Na + -glucose reabsorption by the proximal tubule. In combination with antihyperglycemic agents and RAAS inhibitors, SGLT2 inhibitors reduce plasma glucose and reduce both body weight and blood pressure.
Some NaCl also is reabsorbed across the second half of the proximal tubule by the paracellular pathway . Paracellular NaCl reabsorption occurs because the rise in the Cl – concentration in the tubule fluid in the first half of the proximal tubule creates a Cl – concentration gradient (140 mEq/L in the tubule lumen and 105 mEq/L in the interstitium). This concentration gradient favors the diffusion of Cl – from the tubular lumen across the tight junctions into the lateral intercellular space. Movement of the negatively charged Cl – causes the tubular fluid to become positively charged relative to the blood. This positive transepithelial voltage causes the diffusion of positively charged Na + out of the tubular fluid across the tight junction into the blood. Thus in the second half of the proximal tubule some Na + and Cl – is reabsorbed across the tight junctions (the paracellular pathway) by passive diffusion. The reabsorption of NaCl by the proximal tubule establishes a small transtubular osmotic gradient that provides the driving force for the passive reabsorption of water by osmosis.
In summary, the reabsorption of Na + and Cl – in the proximal tubule occurs via both paracellular and transcellular pathways . Approximately 67% of the NaCl filtered each day is reabsorbed in the proximal tubule ( Table 4.4 ). Of this amount, two-thirds move across the transcellular pathway and the remaining one-third moves across the paracellular pathway.
Fanconi syndrome , a renal disease that is either hereditary or acquired, results from an impaired ability of the proximal tubule to reabsorb HCO 3 – , amino acids, glucose, and low-molecular-weight proteins. Because other downstream segments cannot reabsorb these solutes and protein, Fanconi syndrome results in increased urinary excretion of HCO 3 – , amino acids, glucose, inorganic phosphate, and low-molecular-weight proteins and often is associated with glucosuria, osteomalacia, acidosis, and hypokalemia.
| Segment | Percentage of Filtered NaCl Reabsorbed | Mechanism of Na + Entry Across Apical Membrane | Major Regulatory Hormones |
|---|---|---|---|
| Proximal tubule | 67 | Na + -H + antiporter (NHE3), Na + -glucose symporter (SGLT2), Na + -symporter with amino acids, Cl – /base antiporter (CFEX), paracellular | Angiotensin II, norepinephrine, epinephrine, dopamine |
| Loop of Henle | 25 | Na + -K + -2Cl – symporter (NKCC2), Na + -H + antiporter (NHE3) | Aldosterone, angiotensin II |
| Distal tubule | ∼5 | NaCl symporter (NCC) | Aldosterone, angiotensin II |
| Late distal tubule and collecting duct | ∼3 | Na + channel (α, β, γ-ENaC) | Aldosterone, ANP, BNP, urodilatin, uroguanylin, guanylin, angiotensin II |
Water Reabsorption
The proximal tubule reabsorbs 67% of the filtered water ( Table 4.5 ). The driving force for water reabsorption is a transtubular osmotic gradient established by solute reabsorption. The reabsorption of Na + along with organic solutes, HCO 3 – , P i , and Cl – from the tubular fluid into the lateral intercellular spaces reduces the osmolality of the tubular fluid and increases the osmolality of the lateral intercellular space ( Fig. 4.5 ). The osmotic gradient across the proximal tubule established by these transport processes is only a few mOsm/L (see Fig. 4.5 ). Because the proximal tubule is highly permeable to water, primarily because of the expression of aquaporin water channels (AQP1) in the apical and basolateral cell membranes, water is reabsorbed across cells by osmosis. In addition, the tight junctions in the proximal tubule are also water permeable, so some water is also reabsorbed across the paracellular pathway between proximal tubular cells. The accumulation of fluid and solutes within the lateral intercellular space increases the hydrostatic pressure in this compartment. This increased hydrostatic pressure forces fluid and solutes into the capillaries. c
c In addition, protein oncotic pressure in the peritubular capillaries (πpc) is elevated because of the process of glomerular filtration (see Chapter 3 ). The elevated πpc facilitates uptake of fluid and solute from the tubule lumen into the capillary.e
Thus water reabsorption follows solute reabsorption in the proximal tubule. The reabsorbed fluid is slightly hyperosmotic to plasma. However, this difference in osmolality is so small that it is commonly said that proximal tubule reabsorption is isosmotic (i.e., ∼67% of both the filtered solute and water are reabsorbed). Indeed, little difference is seen in the osmolality of tubular fluid at the start and end of the proximal tubule. An important consequence of osmotic water flow across the proximal tubule is that some solutes, especially K + and Ca ++ , are entrained in the reabsorbed fluid and thereby are reabsorbed by the process of solvent drag (see Fig. 4.5 ). Reabsorption of all organic solutes, HCO 3 – , Cl – , P i , and other ions—and water is coupled to Na + reabsorption. Therefore changes in Na + reabsorption influence the reabsorption of water and other solutes by the proximal tubule. This point will be discussed later, notably in Chapter 6 , and is especially relevant during volume depletion when increased Na + reabsorption by the proximal tubule is accompanied by a parallel increase in HCO 3 – reabsorption, which can contribute to metabolic alkalosis (i.e., volume contraction alkalosis).| Segment | Percentage of Filtered Reabsorbed | Mechanism of Water Reabsorption | Hormones That Regulate Water Permeability |
|---|---|---|---|
| Proximal tubule | 67 | Passive | None |
| Loop of Henle | 15 | Descending thin limb only; passive | None |
| Distal tubule | 0 | No water reabsorption | None |
| Late distal tubule and collecting duct | ∼8-17 | Passive | AVP, ANP ∗ , BNP ∗ |
∗ Atrial natriuretic peptide (ANP) and brain natriuretic peptide (BNP) inhibit arginine vasopressin (AVP)–stimulated water permeability.
Protein Reabsorption
Proteins filtered by the glomerulus are reabsorbed in the proximal tubule. As mentioned previously, peptide hormones, small proteins, and small amounts of large proteins such as albumin are filtered by the glomerulus. Overall, only a small percentage of proteins cross the glomerular filtration barrier and enter Bowman’s space (i.e., the concentration of proteins in the glomerular ultrafiltrate is only ∼40 mg/L). However, the total amount of protein filtered per day is significant because the glomerular filtration rate (GFR) is so high:
Filteredprotein=GFR×[Protein]intheultrafiltrateFilteredprotein=180L/day×40mg/L=7200mg/day,or7.2g/day
Water channels called aquaporins (AQPs) mediate the transcellular reabsorption of water across many nephron segments. To date, 14 aquaporins have been identified. The AQP family is divided into two groups based on their permeability characteristics. One group (AQPs) is permeable to water (AQP0, AQP1, AQP2, AQP4, AQP5, AQP6, AQP8, AQP11, and AQP12). The other group (aquaglyceroporins) is permeable to water and small solutes, especially glycerol (AQP3, AQP5, AQP7, AQP9, AQP10). AQPs form tetramers in the plasma membrane of cells, with each subunit forming a water channel. In the kidneys, AQP1 is expressed in the apical and basolateral membranes of proximal tubule cells and in portions of the descending thin limb of the loop of Henle (see Chapter 5 ). The importance of AQP1 in renal water reabsorption is underscored by studies in which the AQP1 gene was “knocked out” in mice. These mice exhibit increased urine output (polyuria) and a reduced ability to concentrate the urine. In addition, the osmotic water permeability of the proximal tubule is fivefold less in mice lacking APQ1 than in normal mice. AQP7 and AQP8 also are expressed in the proximal tubule. AQP2 is expressed in the apical plasma membrane of principal cells in the collecting duct, and its abundance in the membrane is regulated by arginine vasopressin (see Chapter 5 ). AQP3 and AQP4 are expressed in the basolateral membrane of principal cells in the collecting duct, and mice deficient in these AQPs (i.e., AQP3 and AQP4 knockout mice) have defects in the ability to concentrate urine (see Chapter 5 ). AQPs also are expressed in many other organs in the body, including the lung, eye, skin, secretory glands, and brain, where they play key physiologic roles. For example, AQP4 is expressed in cells that form the blood-brain barrier. Knockout of the AQP4 gene in mice affects the water permeability of the blood-brain barrier such that brain edema is reduced in AQP4-deficient mice after acute water loading and the subsequent development of hyponatremia.
The endocytosis of proteins and peptides by the proximal tubule is mediated by apical membrane receptors that specifically bind luminal proteins and peptides in tubule fluid. These multiligand endocytic receptors can bind a wide range of peptides and proteins and thereby mediate their endocytosis. Megalin and cubilin mediate protein and peptide endocytosis in the proximal tubule. Both are glycoproteins, with megalin being a member of the low-density lipoprotein receptor gene family.
Filtered proteins are reabsorbed in the proximal tubule by endocytosis, either as intact proteins or after being partially degraded by enzymes on the surface of the proximal tubule cells. Once the proteins and peptides are inside the cell, enzymes digest them into their constituent amino acids, which then leave the cell across the basolateral membrane by transport proteins and are returned to the blood. Normally this mechanism reabsorbs virtually all proteins filtered, and hence the urine is essentially protein free. However, because the mechanism is easily saturated, an increase in filtered proteins can result in proteinuria (i.e., appearance of protein in the urine). Disruption of the glomerular filtration barrier to proteins increases the filtration of proteins and results in proteinuria, which is often found in individuals with kidney disease.
Organic Anion and Organic Cation Secretion
Cells of the proximal tubule also secrete organic anions and organic cations into the tubule fluid. Secretion of organic anions and cations by the proximal tubule plays a key role in regulating the plasma levels of xenobiotics (e.g., a variety of antibiotics, diuretics, statins, antivirals, antineoplastics, immunosuppressants, neurotransmitters, and nonsteroidal antiinflammatory agents) and toxic compounds derived from endogenous and exogenous sources. Many of the organic anions and cations ( Boxes 4.1 and 4.2 ) secreted by the proximal tubule are end products of metabolism that circulate in plasma. Many of these organic compounds are bound to plasma proteins and thus are not readily filtered. Therefore only a small fraction of these potentially toxic substances is eliminated from the body by excretion resulting from filtration alone. Thus secretion of organic anions and cations, including many toxins from the peritubular capillary, into the tubular fluid promotes the elimination of these compounds from the plasma entering the kidneys. Hence these substances are removed from the plasma by secretion and to a lesser degree filtration. It is important to note that when kidney function is reduced by disease, the urinary excretion of organic anions and cations is severely reduced,
Urinalysis is an important and routine tool for the detection of kidney disease. A thorough analysis of the urine includes macroscopic, microscopic, and biochemical assessments. This is performed by visual assessment of the urine, microscopic examination of urine sediment, and biochemical evaluation of urinary composition using dipstick reagents strips. The dipstick test is both inexpensive and fast (i.e., it can be performed in less than 5 minutes) and tests the urine for both pH and the presence of many substances, including bilirubin, blood, glucose, ketones, and protein. It is normal to find trace amounts of protein in the urine, particularly concentrated urine. Urinary proteins are derived from two principal sources: (1) filtration exceeding the resorptive capacity of the proximal tubule and (2) the synthesis and secretion of Tamm-Horsfall glycoprotein by the thick ascending limb of Henle’s loop. Because the mechanism for protein reabsorption is “upstream” of the thick ascending limb (i.e., in the proximal tubule), the secreted Tamm-Horsfall glycoprotein appears in the urine. However, proteinuria in greater than trace amounts is often indicative of renal disease.
Endogenous Anions
cAMP, cGMP
Bile salts
Hippurates
Oxalate
Prostaglandins: PGE 2 , PGF 2 α
Urate
Vitamins: ascorbate, folate
Drugs
Acetazolamide
Acyclovir
Amoxicillin
Captopril
Chlorothiazide
Furosemide
Losartan
Penicillin
Probenecid
Salicylate (aspirin)
Hydrochlorothiazide
Simvastatin
Bumetanide
Nonsteroidal antiinflammatory drugs: indomethacin
cAMP, Cyclic adenosine monophosphate; cGMP, cyclic guanosine monophosphate.
Fig. 4.6 illustrates the mechanisms of organic anion (OA – ) secretion across the proximal tubule. These secretory pathways have maximum transport rates, low specificity (i.e., they transport many organic anions) and are responsible for the secretion of all organic anions listed in Box 4.1 . OA – s are taken up into the cell, across the basolateral membrane, and against their chemical gradient in exchange for α-ketoglutarate (α-KG) by several OA – –α-KG antiport mechanisms, including OAT1, OAT2, and OAT3. α-KG accumulates inside the cells by metabolism of glutamate and by an Na + -α-KG symporter (i.e., the Na + -dicarboxylate transporter [NaDC3]) also present in the basolateral membrane. Thus uptake of OA – into the cell against an electrochemical gradient is coupled to the exit of α-KG out of the cell, down its chemical gradient generated by the Na + –α-KG symporter mechanism. The exit of OA – s across the luminal membrane into the tubular fluid are mediated by multidrug resistance proteins 2 and 4 (MRP2 and 4) and breast cancer resistance protein 1 (BCRP), which require ATP for their operation (i.e., ABC transporters). Recent studies reveal that OAT4 mediates the reabsorption of the organic anion urate, the end product of purine catabolism, by the proximal tubule (see Fig. 4.6 ).
Fig. 4.7 illustrates the mechanism of organic cation (OC + ) secretion across the proximal tubule. Organic cations, including xenobiotics such as the antidiabetic agent metformin, the antiviral agent lamivudine, and the anticancer drug oxaliplatin, and many important monoamine neurotransmitters, including dopamine, epinephrine, histamine, and norepinephrine, are secreted by the proximal tubule. Organic cations are taken up into the cell, across the basolateral membrane, primarily by the organic cation transporter 2 (OCT2). Uptake of organic cations is driven by the magnitude of the cell-negative potential difference across the basolateral membrane. Organic cation transport across the luminal membrane into the tubular fluid, which is the rate-limiting step in secretion, is mediated primarily by electroneutral multidrug and toxin extrusion transporters (MATEs) and multidrug resistance protein (MDR1, also known as P-glycoprotein), which requires ATP for its operation (i.e., ABC transporters). These transport mechanisms are nonspecific, and several organic cations usually compete for secretion via a given transport pathway.
Because many organic anions compete for the same secretory pathways, elevated plasma levels of one transported anion often inhibit the secretion of the others. For example, infusing para-amino hippurate (PAH) can reduce penicillin secretion by the proximal tubule. Because the kidneys are responsible for eliminating penicillin, the infusion of PAH into individuals receiving penicillin reduces penicillin excretion and thereby extends its biologic half-life. In World War II, when penicillin was in short supply, hippurates were given with the penicillin to extend the drug’s therapeutic effect. Similar competition is observed for organic cation secretion by the proximal tubule, and elevated plasma levels of one transported cation species can inhibit the secretion of the other competing organic cations. For example, the H 2 antagonist cimetidine, which is used to treat gastric ulcers, is secreted via organic cation transport mechanisms in the proximal tubule. If cimetidine is given to patients who also receive procainamide, a drug used to treat cardiac arrhythmias, cimetidine reduces the urinary excretion of procainamide (also an organic cation) by direct competition for a common secretory pathway. Therefore coadministration of cationic drugs competing for the same pathway can increase the plasma concentrations of both drugs to levels much higher than those observed when the drugs are given alone. This effect can lead to drug toxicity.
Henle’s Loop
Henle’s loop reabsorbs approximately 25% of the filtered NaCl and 15% of the filtered water. Reabsorption of NaCl in the loop of Henle occurs in both the thin ascending and thick ascending limbs, whereas the descending thin limb does not reabsorb NaCl. In contrast, water reabsorption mediated by AQP1 water channels is restricted to the descending thin limb, whereas the ascending limb is impermeable to water (see Chapter 5 ). In addition, divalent cations (e.g., Ca ++ and Mg ++ ) and HCO 3 – are reabsorbed in the loop of Henle (see Chapter 8, Chapter 9 for details).
The thin ascending limb reabsorbs NaCl by a passive mechanism. Reabsorption of water, but not NaCl, in the descending thin limb increases the NaCl concentration in tubule fluid entering the ascending thin limb. As the NaCl – rich fluid moves toward the cortex, NaCl diffuses out of tubule lumen across the ascending thin limb and into the medullary interstitial fluid, down a concentration gradient directed from tubule fluid to interstitium (see Chapter 5 for details).
The key element in solute reabsorption by the thick ascending limb is Na + -K + -ATPase in the basolateral membrane ( Fig. 4.8 ). As with reabsorption in the proximal tubule, the reabsorption of every solute by the thick ascending limb is linked to Na + -K + -ATPase. This transporter maintains a low intracellular [Na + ], which provides a favorable chemical gradient for the movement of Na + from the tubular fluid into the cell. This movement of Na + across the apical membrane into the cell is mediated by the Na + -K + -2Cl – symporter (NKCC2), which couples the movement of Na + with K + and 2Cl – . Using the potential energy released by the downhill movement of Na + and Cl – , this symporter drives the uphill movement of K + into the cell. A K + channel (renal outer medullary potassium [ROMK]) in the apical plasma membrane plays an important role in NaCl reabsorption by the thick ascending limb. This K + channel allows the K + transported into the cell by NKCC2 to recycle back into tubule fluid. Because the [K + ] in tubule fluid is relatively low, K + recycling is required for the continued operation of NKCC2. A Na + -H + antiporter (NHE3) in the apical cell membrane also mediates Na + reabsorption as well as H + secretion (HCO 3 – reabsorption) in the thick ascending limb (see Chapter 8 ). The operation of the Na + -H + antiporter in the apical membrane results in the cellular uptake of Na + in exchange for H + . The production of H + inside cells generates HCO 3 – , which exits the cell across the basolateral membrane via a Cl – HCO 3 – antiporter (AE2). Na + leaves the cell across the basolateral membrane via the Na + -K + -ATPase, whereas K + , Cl – , and HCO 3 – leave the cell across the basolateral membrane by separate pathways in the basolateral membrane (i.e., K + and Cl – channels and the K + -Cl – symporter).
