Urology

Renal Handling of Urea

July 22, 2017 Laboratory Medicine, Nephrology, Physiology and Pathophysiology, Urology No comments , , , , , , ,

Renal Handling of Urate

Urate, an anion that is the base form of uric acid, provides a fascinating example of the renal handling of organic anions that is particularly important for clinical medicine and is illustrative of renal pathology. An increase in the plasma concentration of urate can cause gout and is thought to be involved in some forms of heart disease and renal disease; therefore, its removal from the blood is important. However, instead of excreting all the urate it can, the kidneys actually reabsorb most of the filtered urate. Urate is freely filterable. Almost all the filtered rate is reabsorbed early in the proximal tubule, primarily via antiporters (URAT1) that exchange urate for another organic anion. Further on the proximal tubule urate undergoes active tubular secretion. Then, in the straight portion, some of the urate is once again reabsorbed. Because the total rate of reabsorption is normally much greater than the rate of secretion, only a small fraction of the filtered load is excreted.

Although urate reabsorption is greater than secretion, the secretory process is controlled to maintain relative constancy of plasma urate. In other words, if plasma urate begins to increase because of increased urate production, the active proximal secretion of urate is stimulated, thereby increasing urate excretion.

Given these mechanisms of renal urate handling, the reader should be able to deduce the 3 ways by which altered renal function can lead to decreased urate excretion and hence increased plasma urate, as in gout: 1) decreased filtration of urate secondary to decreased GFR, 2) excessive reabsorption of urate, and 3) diminished secretion of urate.

Urate, and some other organic solutes, although more membrane permeable in the neutral form, are less soluble in aqueous solution and tend to precipitate. The combination of excess plasma urate and low urinary pH, which converts urate to the neutral uric acid, often leads to the formation of uric acid kidney stones.

Renal Handling of Urea

Urea is a very special substance for the kidney. It is an end product of protein metabolism, waste to be excreted, and also an important component for the regulation of water excretion. Urea differs from all the other organic solutes in several significant ways. 1) There are no membrane transport mechanisms in the proximal tubule; instead, it easily permeates the tight junctions of the proximal tubule where it is reabsorbed paracellularly. 2) Tubular elements beyond the proximal tubule express urea transporters and handle urea in a complex, regulated manner.

Urea is derived from proteins, which form much of the functional and structural substance of body tissues. Proteins are also a source of metabolic fuel. Dietary protein is first digested into its constituent amino acids. These are then used as building blocks for tissue protein, converted to fat or oxidized immediately. During fasting, the body breaks down proteins into amino acids that are used as fuel, in essence consuming itself. The metabolism of amino acids yields a nitrogen moiety (ammonium) and a carbohydrate moiety. The carbohydrate goes on to further metabolic processing, but the ammonium cannot be further oxidized and is a waste product. Ammonium per se is rather toxic to most tissues and the liver immediately converts most ammonium to urea and a smaller, but crucial amount to glutamine. While normal levels of urea are not toxic, the large amounts produced on a daily basis, particularly on a high protein diet, represent a large osmotic load that must be excreted. Whether a person is well fed or fasting, urea production proceeds continuously and constitutes about half of the usual solute content of urine.

The normal level of urea in the blood is quite variable, reflecting variations in both protein intake and renal handling of urea. Over days to weeks, renal urea excretion must match hepatic production; otherwise plasma levels would increase into the pathological range producing a condition called uremia. On a short-term basis (hours to days), urea excretion rate may not exactly match production rate because urea excretion is also regulated for purposes other than keeping a stable plasma level.

The gist of the renal handling of urea is the following: it is freely filtered. About half is reabsorbed passively in the proximal tubule. Then an amount equal to that reabsorbed is secreted back into the loop of Henle. Finally, about half is reabsorbed a second time in the medullary collecting duct. The net result is that about half the filtered load is excreted.

pH Dependence of Passive Reabsorption or Secretion

Many of the organic solutes handled by the kidney are weak acids or bases and exist in both, neutral and ionized forms. The state of ionization affects both the aqueous solubility and membrane permeability of the substance. Neutral solutes are more permeable than ionized solutes. As water is reabsorbed from the tubule, any substance remaining in the tubule becomes progressively more concentrated. And the luminal pH may change substantially during flow through the tubules. Therefore, both the progressive concentration of organic solutes and change in pH strongly influence the degree to which they are reabsorbed by passive diffusion through regions of tubule beyond the proximal tubule.

At low pH weak acids are predominantly neutral, while at high pH they dissociate into an anion and a proton. Imagine the case in which the tubular fluid becomes acidified relative to the plasma, which it does on a typical Western diet. For a weak acid in the tubular fluid, acidification converts much of the acid to the neutral form and therefore, increases its permeability. This favors diffusion out of the lumen (reabsorption). Highly acidic urine tends to increase passive reabsorption of weak acids (and promote less excretion). For many weak bases, the pH dependence is just opposite. At low pH they are protonated cations. As the urine becomes acidified, more is converted to the impermeable charged form and is trapped in the lumen. Less is reabsorbed passively, and more is excreted.

Basic Mechanisms of Renal Transepithelial Transport

October 8, 2015 Nephrology, Physiology and Pathophysiology, Urology No comments , , , , ,

CaduceusA similar thread is available at http://www.tomhsiung.com/wordpress/2015/11/proximal-tubule-reabsorption-and-secretion/

There are basically three mechanisms for the transepithelial transport of substances, including water. First, two basic formulas should be introduced here for the understanding of transepithelial mechanisms, for solutes and water, respectively.

For solutes, there is Pick’s first law of diffusion, Xd = D*A*(Δ[X]/ΔL), whereas,

D: diffusion coefficient

A: surface area

Δ[X]: concentration difference

ΔL: diffusion distance

Permeability: A + D + 1/ΔL

For water, Fluid movement = Kf[(Pc – Pi) – (πc – πi)], whereas,

Kf = hydraulic permeability × surface area

References:  Transcapillary Transport http://www.tomhsiung.com/wordpress/2015/07/transcapillary-transport/

Histology of Transport Barrier

The basic process of moving these substances (secretion and reabsorption) requires that solutes and water cross 2 cell layers: 1.the epithelium that makes up the walls of the tubules and 2.the endothelium that makes up the vascular walls. In the cortex, where the fluxes of many filtered substances are enormous, the vascular endothelium (peritubular capillaries) is fenestrated. The fenestrae and the loose underlying basement membrane offer virtually no resistance to the passive movement of water and small solutes. This facile permeation has 2 consequences. First, the rate of transport is governed almost exclusively by events in the tubular epithelium rather than the vascular endothelium; second, the cortical interstitium, which is the medium faced by the basolateral membranes of the tubular epithelia, has an osmolality and concentration of small solutes very close to those in plasma.

In contrast, both blood flow and transport events are less rapid in the medulla. Only some regions of the medullary vasculature are fenestrated, so that 1.overall transport depends on both the properties of the vascular endothelium and tubular epithelium, and 2.the medullary interstitial is most definitely not plasma-like in its composition.


Movement By Diffusion

Diffusion is the frenzied random movement of free molecules in solution (like the Ping-Pong balls in a lottery drawing). Net diffusion occurs across a barrier if there is driving force (i.e., a concentration gradient or, for charged molecules, a potential gradient) and if the barrier is permeable. This applies to almost all substances crossing the endothelial barrier lining the peritubular capillaries. It applies to substances taking the paracellular route around the tubular epithelium and to some substances taking the transcellular route through membranes.

Movement Through Channels

Most substances of biological importance cannot penetrate lipid membranes fast enough to meet cellular needs. To speed up the process their transmembrane flux is mediated by integral membrane proteins, which are divided into categories of channels and transporters. Channels are small pores (proteins with a "hole" through the interior of the protein) that permit, depending on their structure, water or specific solutes to diffuse through them. Channels typically flicker open and closed like camera shutters, so that the permeability of a membrane containing many channels is proportional to the number of channels and the probability of their being open. Movement through channels is passive, that is, no external energy is required. The energy to drive the diffusion is inherent in the concentration gradient or, strictly speaking, the electrochemical gradient, because charged ions are driven through channels and around cells via the paracellular route not only by gradients of concentration but also by gradients of voltage. Channels represent a mechanism for rapid movement of specific substances across membranes, which would otherwise diffuse slowly or not at all.

Regulation of Channel Permeability

A characteristic of channels critical for renal function is the regulation of their permeability by a number of environmental factors and signaling cascades. First, many channels types can be gated, meaning that the probability that the channel is open can be increased or decreased. The topic of channel gating is a whole story by itself, but several ways of gating channels include reversible binding of small molecules that are components of signaling cascades (ligand-gated channels/chemosensitive channels), changes in membrane potential (voltage-gated channels/voltage-sensitive channels), and mechanical distortion (stretch-gated channels/mechanosensitive channels). Second, many channel types have phosphorylation sites such that phosphorylation either locks the channel shut or allows it to be gated by one the mechanisms above. Third, some channel species can be moved back and forth between the surface membrane and intracellular vesicles, thereby regulating the number of the existing channels actually functioning as permeability pathways. Finally, and on a slower time scale, the genomic expression of channels is regulated so that the total number of channels, whether in the membrane or sequestered in vesicles, is altered up or down.

PS: Channel and transporter proteins are not permanent fixtures in the membrane. Their lifetimes in the membrane are generally in the range of a few hours.

Movement by Transporters

  • Uniporters
  • Symporters
  • Antiporters
  • Primary Active Transporters

Our genome codes for a large array of proteins that function as transporters, all with names and acronyms that suffuse the physiological literature. Transporters, like channels, permit the transmembrane flux of a solute that is otherwise impermeable in the lipid bilayer. Channels can move large amounts of materials across membranes in a short period of time, but most transporters have a lower rate of transport because the transported solutes bind much more strongly to the transport protein. Furthermore, the protein must undergo a more elaborate cycle of conformational change to move the solute from one side of the membrane to the other.

However, overall flux rate depends not only on the kinetics of individual transporters, but also on the density of transporters in the membrane. Total flux via transporters can be very high if the transporter density is high. As is the case for channels, the amount of substance moved via transporters is highly regulated, including changes in phosphorylation of the transporter (thereby turning its activity on or off), sequestration into vesicles, and of course, changes in genomic expression.

Transporters can be divided into uniporters, symporters and antiporters, and primary active transporters.

Uniporters

Uniporters permit movement of a single solute species through the membrane. The basic difference between a channel and a uniporter is that a channel is a tiny hole, whereas a uniporter requires that the solute bind to a site that is alternately available to one side and then the other side of the membrane. Movement through a uniporter is often called facilitated diffusion because, like diffusion, it is driven by concentration gradients, but the transported solute moves through the uniporter protein rather than the membrane lipid. A set of uriporters crucial for all cells includes those that facilitate the movement of glucose across cell membranes. These are members of the GLUT (glucose transporter) family of proteins that permit, in the kidney's proximal tubule epithelial cells, glucose to move from the cytosol across the basolateral membrane into the interstitium.

Symporters and Antiporters

Multiporters move 2 or more solute species across a membrane simultaneously. Symporters move them together in the same direction. Antiporters move them in opposite directions. In the literature, symporters are sometimes called cotransporters, and antiporters are called exchangers. Important symporters for the handling of glucose move sodium and glucose together into cell (members of the SGLT protein family). Each transport cycle moves 1 glucose molecule and either 1 or 2 sodium ions depending on the particular species of SGLT. Important anti porters in the kidneys and many other organs move sodium into a cell and protons out of a cell, member of the NHE protein family

All molecular transport requires energy. In the case of diffusion through a channel or movement via a uniporter, the energy is inherent in the electrochemical gradient of the solute. With symporters and antiporters, at least 1 of the solutes moves down its electrochemical gradient and provides the energy to move 1 or more of the other solutes up its electrochemical gradient. In the case of symporters and antiporters that do not hydrolyze ATP, the active transport is called secondary active transport because the energy is provided indirectly from the transport of another solute rather than directly from a chemical reaction.

Primary Active Transporters

Primary active transporters are membrane proteins that move 1 or more solutes up their electrochemical gradients using the energy obtained from the hydrolysis of ATP. All transporters that move solutes in this manner are ATPases, that is, their structure is both that of an enzyme that splits ATP, and a transporter that has binding sites that alternately are open to one side and then the other side of the membrane.

Update on Jan 7th 2016

Some important characteristics of transporters (may also include channels):

The system is carrier-mediated, so it is saturable.

The system is carrier-mediated, so it is structurally selective.

The system is carrier-mediated, so it shows competition kinetics.

The Theories and the Risk Factors for Urinary Stones

July 6, 2015 Urology No comments , , ,

 

 

I would like to dedicate this thread to my dad, Cheney Hsiung, who devote his life to my mom, myself, and his many students over the years at 49th Middle School in Chengdu, Sichuan.

 

 

Mineralization in all biologic systems has a common theme in that the crystals and matrix are intertwined. Urinary stones are no exception; they are polycrystalline aggregates composed of varying amounts of crystalloid and organic matrix. But, theories to explain urinary stone disease are incomplete.

Stone formation requires supersaturated urine. Supersaturation depends on urinary pH, ionic strength, solute concentration, and complexation. Urinary constituents may change dramatically during different physiologic states from a relatively acid urine in a first morning void to an alkaline tide noted after meals. Ionic strength is determined primarily by the relative concentration of monovalent ions. As ionic strength increases, the activity coefficient decreases. The activity coefficient reflects the availability to a particular ion.

The calculation of activity product from urinary stone risk factors is briefly described here, illustrated for calcium oxalate. It should be noted that the urinary saturation of calcium oxalate is defined by the activity product (alpha Ca2+ times alpha Ox2-), not by the product of total or ionic calcium and oxalate concentrations.

 

where alpha Ca2+ is the activity of calcium ion, alpha Ox2- is activity of oxalate ion, [Ca2+] the concentration of calcium ion, [Ox2-] concentration of oxalate ion, and gamma is the activity coefficient. [Ca2+] is the total calcium concentration minus the concentration of soluble complexes principally of calcium-oxalate and calcium-citrate. [Ox2-] is the total oxalate concentration minus the concentration of soluble calcium-oxalate complexes. gamma is an inverse function of ionic strength that is determined by the amount of cations and anions. Alpha Ca2+ is defined by the product of [Ca2+] and gamma. Alpha Ox2- is obtained as the product of [Ox2-] and gamma1.

The role of solute concentrations is clear: The greater the concentration of two ions, the more likely they are to precipitate. Low ion concentrations result in undersaturation and increased solubility. As ion concentrations increases, their activity product reaches a specific point termed the solubility product (Ksp).  Concentrations above this point are metastable and are capable of initiating crystal growth and heterogeneous nucleation. As solutions become more concentrated, the activity product eventually reaches the formation product (Kfp). Supersaturation levels beyond this point are unstable, and spontaneous homogeneous nucleation may occur.

Multiplying two ion concentrations reveals the concentration product. The concentration products of most ions are greater than established solubility products (Ksp). Other factors must play major roles in the development of urinary calculi, including complexation. Complexation influences the availability of specific ions. For instance, sodium complexes with oxalate and decrease it free ionic form, while sulfates can complex with calcium. Crystal formation is modified by a variety of other substances found in the urinary tract, including magnesium, citrate, pyrophosphate, and a variety of trace metals. These inhibitors may act at the active crystal growth sites or as inhibitors in solution (as with citrate).

The nucleation theory suggests that urinary stones originate from crystals or foreign bodies immersed in supersaturated urine. This theory is challenged by the same arguments that support it. Stones do not always form in patients who are hyperexcretors or who are at risk for dehydration. Additionally, up to a third of stone formers’s 24-hour urine collections are completely normal with respect to stone-forming ion concentrations.

The crystal inhibitor theory claims that calculi form due to the absence or low concentration of natural stone inhibitors including magnesium, citrate, pyrophosphate, and a variety of trace metals.

Crystal Component

Stones are composed primarily of a crystalline component. Crystals of adequate size and transparency are easily identified under a polarizing microscope. X-ray diffraction is preferred to assess the geometry and architecture of calculi. A group of stones from the same geographic location or the same historical time period typically have crystalline constituents that are common.

Multiple steps are involved in crystal formation, including nucleation, growth, and aggregation. Nucleation initiates the stone process and may be induced by a variety of substances, including proteinaceous matrix, crystals, foreign bodies, and other particulate tissues. Heterogeneous nucleation, which requires less energy and may occur in less saturated urine, is a common theme in stone formation. It should be suspected whenever an oriented conglomerate is found. A crystal of one type thereby serves as a nidus for the nucleation of another type with a similar crystal lattice. This is frequently seen with uric acid crystals initiating calcium oxalate formation. It takes time for these early nidi to grow or aggregate to form a stone incapable of passing with ease through the urinary tract.

How these early crystalline structures are retained in the upper urinary tract without uneventful passage down the ureter is unknown. The theory of mass precipitation or intranephronic calculus’s suggests that the distal tubules or collecting ducts, or both, become plugged with crystals, thereby establishing an environment of stasis, ripe for further stone growth. This explanation is unsatisfactory; tubules are conical in shape and enlarge as they enter the papilla (ducts of Bellini), thereby reducing the possibility of ductal obstruction. In addition, urine transit time from the glomerulus into the renal pelvis is only a few minutes, making crystal aggregation and growth within the uriniferous tubules unlikely.

The fixed particle theory postulates that formed crystals are somehow retained within cells or beneath tubular epithelium. Alexander Randall noted whitish-yellow precipitations of crystalline substances occurring on the tips of renal papillae as submucosal plaques. These plaques are associated with both the vasa recta and the urinary collecting ducts and grow deep within the papilla. The tips of the plaques can be appreciated during endoscopy of the upper urinary tract. Carr hypothesized that calculi form in obstructed lymphatics and then rupture into adjacent fornices of a calyx. Arguing against Carr’s theory are the grossly visible early stone elements in areas remote from fornices.

Matrix Component

The amount of the noncrystalline, matrix component of urinary stones varies with stone type, commonly ranging from 2% to 10% by weight. It is composed predominantly of protein, with small amounts of hexose and hexosamine. An unusual type to stone called a matrix calculus can be associated with previous kidney surgery or chronic urinary tract infections and has a gelatinous texture. Histologic inspection reveals laminations with scant calcifications. On plain abdominal radiographs, matrix calculi are usually radiolucent and can be confused with other filling defects, including blood clots, upper-tract tumors, and fungal bezoars. Noncontrast computed tomography reveals calcifications and can help to confirm the diagnosis.

The role of matrix in the initiation of ordinary urinary stones as well as matrix stones is unknown. It may serve as a nidus for crystal aggregation or as a naturally occurring glue to adhere small crystal components and thereby hinder uneventful passage down the urinary tract. Alternatively, matrix may have an inhibitory role in stone formation or may be an innocent bystander, playing no active role in stone formation.


Risk Factors

Crystalluria

Crystalluria is a risk factor for stones. Stone formers, especially those with calcium oxalate stones, frequently excrete more calcium oxalate crystals, and those crystals are larger than normal (>12 μm). The rate of stone formation is proportional to the percentage of large crystals and crystal aggregates. Crystal production is determined by the saturation of each salt and the urinary concentration of inhibitors and promoters.

Socioeconomic Factors

Renal stones are more common in affluent, industrialized countries. Immigrants from less industrialized nations gradually increase their stone incidence and eventually match that of the indigenous population. Use of soft water does not decrease the incidence of urinary stone.

Diet

Diet may have a significant impact no the incidence of urinary stones. As per capita income increases, the average diet changes, with an increase in saturated and unsaturated fatty acids; an increase in animal protein and sugar; and a decrease in dietary fiber, vegetable protein, and unrefined carbohydrates. A less energy-dense diet may decrease the incidence of stones. This fact has been documented during war years when diets containing minimal fat and protein resulted in a decreased incidence of stones. Vegetarians may have a decreased incidence of urinary stones. High sodium intake is associated with increased urinary sodium, calcium, and pH and a decreased excretion of citrate; this increases the likelihood of calcium salt crystallization because the urinary saturation of monosodium urate and calcium phosphate (brushite) is increased. Fluid intake and urine output may have an effect on urinary stone disease. The average daily urinary output in stone former is 1.6 L/d.

Occupation

Occupation can have an impact on the incidence of urinary stones. Physicians and other white-collar workers have an increased incidence of stones compared with manual laborers. This finding may be related to differences in diet but also may be related to physical activity; physical activity may agitate urine and dislodge crystal aggregates. Individuals exposed to high temperatures may develop higher concentrations of solutes owing to dehydration, which may have an impact on the incidence of stones.

Climate

Individuals living in hot climates are prone to dehydration, which results in an increased incidence of urinary stones, especially uric acid calculi. Although heat may cause a higher fluid intake, sweat loss results in lowered voided volumes. Hot climates usually expose people to more ultraviolet light, increasing vitamin D3 production. Increased calcium and oxalate excretion has been correlated with increased exposure time to sunlight. This factor has more impact on light-skinned people and may help explain why African Americans in the United States have a decreased stone incidence. Global warming may increase the incidence of urinary stone disease.

Family history

A family history of urinary stones is associated with an increased incidence of renal calculi. A patient with stones is twice as likely as a stone-free cohort to have at least one first-degree relative with renal stones (30% vs 15%). Those with a family history of stones have an increased incidence of multiple and early recurrences. Spouses of patients with calcium oxalate stones have an increased incidence of stones; this may be related to envi- ronmental or dietary factors. Large studies of identical twins have found that >50% of stones have a significant genetic component. New evidence is finding a significant association between urinary stones and cardiovascular disease.

Medications

A thorough history of medications taken may provide valuable insight into the cause of urinary calculi. The antihypertensive medication triamterene is found as a component of several medications, including Dyazide, and has been associated with urinary calculi with increasing frequency. Long-term use of antacids containing silica has been associated with the development of silicate stones. Car- bonic anhydrase inhibitors may be associated with urinary stone disease (10–20% incidence). The long-term effect of sodium- and calcium-containing medications on the devel- opment of renal calculi is not known. Protease inhibitors in immunocompromised patients are associated with radiolu- cent calculi.

References:

1.Charles Y C Pak1, Clarita V Odvina1, Margaret S Pearle, et. al. Effect of dietary modification on urinary stone risk factors. Kidney International (2005) 68, 2264–2273.