Transporter of Small Molecules

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doi: 10.1152/advan.00027.200226:146-157, 2002. ;Advan in Physiol Edu

Barbara E. GoodmanUREA TRANSPORTERSCELL MEMBRANES: WATER CHANNELS ANDTRANSPORT OF SMALL MOLECULES ACROSS

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TRANSPORT OF SMALL MOLECULES ACROSS CELL MEMBRANES:

WATER CHANNELS AND UREA TRANSPORTERS

Barbara E. Goodman

University of South Dakota School of Medicine, Vermillion, South Dakota 57069

How do small hydrophilic nonelectrolytes cross cell membranes? Which

pathways are most important for small lipid insoluble molecules to cross

cell membranes? These are questions that have been basic to membrane

transport physiology for decades. More importantly, these are questions whose

answers have changed significantly within the last 10 years. This review discusses the

evidence that pathways other than the lipid bilayer itself exist for the transport across

cell membranes of specific small hydrophilic nonelectrolytes. The description beginswith briefly analyzing the relevance of well accepted basic mathematical models for

transport for understanding the permeability of representative physiologically impor-

tant molecules across actual cell membranes. Particular emphasis is placed on

describing recently discovered proteins that facilitate the transport of some of the

smallest physiologically important lipid-insoluble molecules, water, and urea. Evi-

dence also exists for transport proteins that selectively enhance the transmembrane

transport of other small lipid-insoluble molecules. Do nonselective pores for small

molecules exist in cell membranes?

ADV PHYSIOL EDUC 26: 146157, 2002.10.1152/advan.00027.2002.

Key words: aquaporins; urea transporters; nonselective pores; membrane transport

How do small hydrophilic nonelectrolytes cross cellmembranes? Which pathways are most important forsmall lipid-insoluble molecules to cross cell mem-branes? These are questions that have been basic tomembrane transport physiology for decades. Moreimportantly, these are questions whose answers havechanged significantly within the last 10 years. Twoof the most important small molecules that need to beable to cross cell membranes are water and urea.

Water is the main component of the human body, andurea is one of the main excretory products of themetabolism of nutrients for cellular energy. Formerly,physiologists taught that very small lipid-insolublemolecules crossed cell membranes primarily by slip-ping between the lipids in the lipid bilayer or by usingsmall, nonselective pores (or holes within the lipid

bilayer). Within the last 10 years, highly specific trans-port proteins have been identified that are vitallyimportant for the physiology of water or urea move-ment across certain cell membranes.

Water (in the various compartments of an organism)is one of the major variables controlled physiologi-cally by homeostasis. Thus there are sensors, like theosmoreceptors in the brain of mammals, that sense

osmotic changes in the extracellular fluid compart-ment, and effectors, like changes in water fluxesacross cell membranes and the production of eitherconcentrated or dilute urine by mammalian kidneys.To accomplish osmotic homeostasis, water must rap-idly equilibrate across cell membranes.

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Aquatic animals excrete nitrogenous wastes from pro-tein and nucleic acid metabolism as toxic ammonia

released into the water. Other terrestrial animals, likeinsects, reptiles, birds, and some amphibians excretenitrogenous wastes as uric acid in solid or semisolid

form. Mammals excrete urea (10 times less toxicthan ammonia and quite soluble in water), allowing

waste products to be excreted in a lower volume ofurine than ammonia would require.

Although urea is often thought of primarily as a waste

product to be excreted, the conservation of urea in

the medullary interstitium of the kidney is essentialfor the production of hyperosmotic urine. In varioussegments of the nephron, urea is reabsorbed out ofthe tubular fluid and back into the interstitium. Ureareabsorption, combined with the countercurrent mul-tiplier and exchange systems in the kidneys, creates

an osmotic gradient deep in the medullary intersti-

tium. This NaCl and urea concentration enhances theosmotic reabsorption of water out of the tubule when

the tubule epithelia have pathways for water trans-

port.

Basic water reabsorption in the early nephron of

mammalian kidneys is constitutive, based on the pres-

ence of appropriate water channels, the tightness of

the junctions between cells, and the simultaneous

reabsorption of nutrient solutes. The water perme-

ability of distal tubule and collecting duct epithelia is

regulated by vasopressin (antidiuretic hormone). Urea

transporters and water channels are regulated by va-

sopressin to work together to conserve water in the

body as shown (Fig. 1). Thus movements of urea and

water molecules across cell membranes are essential

for the maintenance of homeostasis in the body fluidcompartments.

FIG. 1.

Diagrammatic representation of the location of different aquaporins (AQPs) in the

nephrons of mammalian kidneys. ADH, antidiuretic hormone (vasopressin). Some trans-

port proteins are capable of being constitutively active, and some must be regulated by

vasopressin. Urea transporters (UT) are involved in the development of the medullary

osmotic gradient, whereas aquaporins are involved in water permeability changes across

the epithelial cells. UT-A2 is found in the descending limb of the loop of Henle, and UT-A1

is found in the inner medullary collecting duct (IMCD) [from Nielsen et al. (18)].



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DIFFUSION ACROSS BIOLOGICAL MEMBRANES

Diffusive fluxes of uncharged molecules across lipidbilayer membranes are usually represented by Ficks

First Law of Diffusion

J dn/dt DA(dC/dx)

where D diffusion coefficient for molecule in thebarrier (cm2/sec), A surface area of the barrier

(cm2), dC/dx change in concentration over dis-tance in the barrier (mol cm3 cm1), and J dn/dt change in number of moles per unit time(mol/s).

The diffusion coefficient (D) is derived from theStokes-Einstein equation that models the diffusion of

spherical particles in a continuous fluid medium andrepresents the mobility of the solute in the mem-brane. The diffusion coefficient

D kT/6r

where k Boltzmanns constant, T absolute tem-perature (K), r molecular radius, and viscosityof solution.

The partition coefficient (solubility of the solute inthe membrane) is the ratio of the concentration ofsolute in the membrane to the concentration of solute

in water. It is difficult to determine the partitioncoefficient and the diffusion coefficient for a givensolute in cell membranes and the thickness of individ-ual living membranes. Thus these three variables arefrequently lumped together into the term permeabil-ity, where P (Dmem Bmem)/(membrane thick-ness) such that flux J PAC. Thus molecule size andsolubility taken together should completely accountfor the basal permeability (in cm/s) of solute mole-

cules across cell membranes.

Basal permeability across lipid bilayer membranes

takes advantage of the free van der Waals volume (orholes) in the hydrocarbon interior (25). Van der Waalsforces between lipids lead to many different smalltransient holes of various sizes that continuously form

and reform within the lipid bilayer. Elongated mole-cules have the highest diffusion rates across lipid

bilayers due to the greater probability of finding adja-

cent holes and thus being able to snake through themembrane. In many cells, the cell membrane treatedas only a lipid bilayer explains the basal permeabilities

for small nonelectrolytes. Thus small lipid-insoluble

molecules are capable of crossing lipid bilayers bysnaking or random walking between the lipids and donot require continuous aqueous channels or pores.

This pathway for crossing the lipid bilayer does notobey the Stokes-Einstein equation and thus has been

called non-Stokesian (25). For cell membranes, it isknown that lipid bilayers are predominantlyfluid lat-erally in the plane of one monolayer of the mem-brane. However, in the direction in which transmem-brane transport occurs, lipid hydrocarbon chains may

be partly immobilized by cholesterol and may notflow easily past the diffusing molecules. It has been

known since the early 1970s that water moves acrossmammalian red blood cell membranes much faster

under an osmotic gradient than by simple diffusion(25). This observation implies that osmotic flow ofwater may be laminar through aqueous channels (allwater molecules moving in the same direction) and

therefore faster than the random walking of individualwater molecules diffusing across a lipid bilayer.

Determination of the true basal permeability of smalllipid-insoluble molecules across cell membranes re-

quired the development of accurate experimentalmethods of measurement and the recognition of all

possible pathways for transport (25). Measurementsof the basal permeability of red blood cell membranes

to water are made with and without inhibitors of thetransport routes other than the lipid bilayer. When

appropriate measurements were made, the true basalpermeability of mammalian red blood cell membranesto water was 1.2 103 cm/s (25). In comparison,

measurements were made of the true basal permeabil-ity of mammalian red blood cell membranes to mole-

cules of small, relatively lipid-insoluble urea. Ureabasal permeability measured with inhibitors (phlor-

etin, organic mercurials, nitrophenols, and urea ana-

logs like thiourea) was 7.7

10

7

cm/s (25). Thebasal permeability of urea molecules through the lipidbilayer alone is considerably lower than the urea per-

meabilities that have been measured across manycells in the body. For example, urea permeabilityacross human red blood cell membranes is 1.2 103

cm/s (16).



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WATER TRANSPORT

Water molecules are very polar and are thus unlikelyto use hydrophobic pathways for crossing mem-

branes. If water molecules do not traverse cell mem-branes through the lipid bilayer, what other routes

are there for the transport of water molecules?

Evidence for Water Channels

In the 1950s, Goldstein and Solomon (5a) demon-strated that the high permeability of mammalian red

blood cells to water was strongly inhibited by mer-curial sulfhydryl compounds like mercuric chloride(HgCl2). Subsequently, specific nephron segments inmammalian kidneys were also shown to have highwater permeabilities. Water permeability in the col-

lecting duct was low in an unstimulated state andhigh in the presence of antidiuretic hormone (vaso-

pressin). This led to the membrane shuttle hypothe-sis for vasopressin-regulated water permeability,whereby vasopressin induces insertion of water trans-porters in apical membranes of collecting duct epi-

thelial cells (25). It was unclear whether these trans-porting pores consisted of protein, lipid, or acombination of protein and lipid components.

As the quest continued for complete understanding of

how water molecules cross cell membranes, analysisemerged of the biophysical characteristics of water

transport via various pathways. The Kedem-Katchal-sky nonequilibrium thermodynamic equations for

coupled water and solute movement model watermovement across membranes in response to either

osmotic or hydrostatic pressure gradients (27). Theosmotic (hydraulic) permeability coefficient (Pf) de-scribes water movement as the amount of volume

(water) flowing across the membrane in response todriving forces of osmotic and hydrostatic pressures.

For cell membranes, Pf 0.005 cm/s when watermoves by solubility/diffusion across the lipid bilayer,

and Pf 0.01 cm/s when water can also move

through continuous aqueous channels (27). On theother hand, the diffusional permeability of water (Pd)was modeled by Ficks First Law as the movement oftracer-labeled water molecules across cell membranesin the absence of other driving forces. Pf should equalPd for a simple lipid bilayer membrane that does not

contain water channels or unstirred layers (27). For a

narrow channel in which single-file movement of wa-ter occurs, Pf /PdNwhere Nis the number of watermolecules in the channel at one time (27). Thus

measurement of osmotic vs. diffusional water perme-

ability across cell membranes (with and withoutosmotic and hydrostatic pressure gradients) predictswhether water channels are present in the cells

or not.

The activation energy (Ea) for water permeability isalso a useful parameter for predicting the presence of

water channels in cell membranes (27). Ea is deter-mined by the Arrhenius equation from the tempera-ture dependence of Pf. Ea is 10 kcal/mol if water is

moving by a channel-independent solubility/diffusionpathway and is 6 kcal/mol if water is moving

through aqueous pores. Higher activation energy forwater movement through lipid relates to the forma-

tion and breaking of hydrogen bonds between polarwater and lipid. Movement of water through aqueous

pores is likely to involve fewer bond formation andbreaking events between water and surrounding sub-

strate. Thus measurement of the activation energy forwater permeability across cell membranes can predictwater transport through channels.

Experimental measurements have been developed to

distinguish osmotic water permeability from diffusivewater permeability (27). Osmotic water permeability

can be determined from changes in cell volume inresponse to osmotic gradients by measurement of

light scattering orfluorescence quenching in cells andvesicles. Diffusional water permeability can be deter-

mined by the diffusional movement oflabeled watermolecules in the absence of other gradients. Deuter-ated water has been used as isotopically labeled water

molecules to determine isotope differences in fluoro-phore quantum yields or infrared absorbance. Mag-

netically labeled water molecules have also been usedfor analysis by nuclear magnetic resonance.

In the mid-1980s, determination by radiation inactiva-tion of the target size for water transporters (in redblood cells and apical membrane vesicles from kidney

proximal tubule cells) indicated a protein of 20 30kDa (27). In 1988, Agre and coworkers (4a, 18a)isolated and cloned a 28-kDa integral membrane pro-

tein from mammalian red blood cells that they called



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CHIP28 for channel-forming integral protein of 28

kDa. Subsequently in 1992, Agre and coworkers (4a,18a) showed that CHIP28 encoded a water channel.

Xenopus oocytes do not normally have water chan-nels; however, when the CHIP28 protein was cloned

into the oocytes, there was a 10-fold increase inwater permeability compared with control oocytes

without CHIP28. The increased water permeabilitywas strongly inhibited by HgCl2. Thus strong evi-dence for water channels in cell membranes would

include one or more of the following characteristics:Pf 0.01 cm/s, Ea 6 kcal/mol, Pf /Pd 3.0, and

strong inhibition of water permeability by mercurialcompounds (27).

Structure of Water Channels

Since the initial discovery that a protein acts as a

water channel in mammalian red blood cell mem-

branes, additional water channels have been discov-

ered in other tissues, with currently 10 (or 11) known

mammalian water channels. CHIP28 [also known as

aquaporin-1 (AQP1)] is now identified as a represen-tative of this new class of transport proteins known as

aquaporins. AQP1 has 269 amino acids forming two

tandem repeats of three membrane-spanning -heli-

ces plus two short helical loops (B and E loops) within

the lipid bilayer (31). Figure 2 shows the hourglassmodel of AQP1. The carboxy and amino termini are

both cytoplasmic. The B loop connects helices 2 and

3 and the E loop connects helices 5 and 6. The

connecting loops each contain an Asn-Pro-Ala (NPA)

motif that appears to be the site of the channel for

water. Sui et al. (26) conducted definitive analysis ofAQP1 by X-ray crystallography down to a resolution

of 2.2 and in so doing clarified the selectivity of thepore region for water molecules. The short helical B

FIG. 2.

The hourglass model of a monomer of AQP1, which includes the 6 transmembrane-

spanning -helices and the short helical B and E loops with their Asn-Pro-Ala (NPA)

motifs within the membrane. The hydropathy plot in the membrane is shown ontop, and

on the bottom is the 3-dimensional plot that shows the juxtaposed NPA motifs forming a

single aqueous pore [from Jung et al. (9a)].



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and E loops are two membrane-inserted non-mem-brane-spanning helices capped by Asn residues. Thecentrally located channel adjacent to these loops has

a constriction of 2.8 (water molecules have aradius of 2.8 ). Thus the pore itself consists of anextracellular and a cytoplasmic vestibule connectedby an extended narrow pore with a long hydrophobic

core and a minimal number of solute-binding sites.Residues in the region of the constriction (particularly

histidine-182, which is conserved in the aquaporinfamily) are critical for the selectivity of the channel

for water molecules. The hydrophilic face of the poreprovides the chemical groups for displacing waters ofhydration to establish a pathway for coordinating the

transport of water molecules. It appears that the for-mation of hydrogen bonds between water molecules

and the pore residues causes the specificity of thechannel for water. Water molecules permeate thechannel single file and break hydrogen bonds witheach other to form one and then the other hydrogen

bonds with residues of the B and E loops within thechannel.

Mammalian aquaporins have been broken down intotwo groups (9): the CHIP group, simply known as

aquaporins, and the GLP group, known as aquaglyc-eroporins (GLP). Primary structures of both groups

are similar in size (250 290 amino acids). For bothgroups, the NH2 terminus is variable in length, there

are six putative transmembrane -helices, the COOHterminus is variable in length and hydrophilic, both

termini are found in the cytoplasm, and there are twohighly conserved NPA boxes. The CHIP group pro-

teins (aquaporins) have generalized water channelactivity, whereas the GLP group proteins are function-ally specialized for the transport of glycerol across cell

membranes. The groups are divided on the basis ofthe length of the extracellular sequences between the

NPA boxes with the GLP group having 1321 addi-tional amino acids between helices 3 and 4 and 1113additional amino acids between helices 5 and 6. Aqua-

glyceroporins are less selective than aquaporins andmay carry glycerol, water, and/or urea. Glycerol isused as an organic osmolyte as well as an energy

source in bacteria and some yeasts. For a structuralcomparison, the GLPF glycerol facilitator (an aqua-porin homolog) of Escherichia coli has the same six

major-helices and two shorter ones, but the selec-tivity filter leads to a larger 3.4- to 3.8--wide amphi-

pathic channel. The mammalian family of aquaporinsincludes representatives of both the CHIP and theGLP groups, with numbers chosen primarily by chro-

nology of discovery.

Physiology and Pathophysiology ofWater Channels

Aquaporins have been identified in cells of the eyesand blood-brain barrier, salivary glands, lungs, heart,spleen, pancreas, colon, and red blood cells and var-

ious parts of the nephron. Functional roles of aqua-porins include facilitating water reabsorption by thekidneys, assisting in fluid balance in various systemsvia rapid transepithelial transport, and minimizing theosmotic gradient and volume changes between cells

and interstitium in the kidney medulla (9, 18). AQP0appears to be a gap junction protein limited to lens

fiber cell membranes of the eye. AQP1 is widelydistributed in endothelia and in apical and basolateral

membranes of epithelia in multiple organs and isconstitutively active in red blood cells. AQP2 is found

in apical membranes and intracellular vesicles of col-lecting duct and principal cells of the kidney andparticipates in the vasopressin-regulated membrane

shuttle hypothesis for facilitating the production ofconcentrated or dilute urine. AQP3 is found in the

basolateral membranes of epithelia in multiple organs.AQP4 (previously known as mercurial-insensitive wa-

ter channel, or MIWC) is also found in basolateralmembranes of epithelia in multiple organs and is

unique for its insensitivity to inhibition by mercurials.AQP5 is found in apical membranes of epithelia in

salivary glands, lungs, and eyes. AQP6 is found inintracellular vesicles in intercalated cells of the col-lecting ducts of the kidneys and may be involved in

acid-base balance. AQP7 is found in apical membranesof testes and kidney proximal tubules and in adipo-

cytes. AQP8 is found in intracellular vesicles in mul-tiple organs. AQP9 is found in apical membranes in

multiple organs. Recently, preliminary data have iden-

tified a new protein of the GLP group, called AQP10,which is abundantly expressed in human duodenumand jejunum and may function as an absorptive path-

way for water (but not glycerol or urea) (7). Studiesto correlate tissue-specific expression of aquaporinswith their functions have shown that expression does

not necessarily imply major physiological importance.Thus aquaporin function needs to be evaluated on a



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tissue-by-tissue basis (28). The CHIP group of aqua-porins includes the generalized water channels AQP0,AQP1, AQP2, AQP4, AQP5, AQP6, and AQP8. The

GLP group of aquaglyceroporins contains the less

selective channels that transport glycerol, water,and/or urea, AQP3, AQP7, AQP9, and AQP10.

Elucidation of the physiological and pathophysiologi-cal significance of the various aquaporins in mammalshas been enhanced by numerous studies in knockoutmice and by studies in humans with known muta-

tions. In some cases, differences in the severity of thepathophysiological defect have been found betweenaquaporin mutations or in the absence of aquaporins

in humans and in mice. Different aquaporins arepresent in apical and basolateral membranes of epi-

thelia in various segments of the kidneys. Water per-meability across individual nephron segments is

highly correlated with expression of aquaporins (13).Thus specific aquaporin blockers are likely to becomeuseful as novel diuretic agents (28). Diuresis resultingfrom a major defect in urinary concentrating ability

(primarily due to reduced fluid absorption in the col-lecting ducts) has been found in AQP1 knockout mice(28). Discovery of the Colton blood group antigen on

AQP1 has allowed identification of humans who arerare Colton-null individuals and lack AQP1. Overtly,

these individuals have no obvious clinical phenotype.However, recent studies of AQP1-null humans show

that, although they do not have polyuria similar to theAQP1 knockout mice, they do have impaired ability to

concentrate urine maximally when deprived of water(11). In addition, these individuals have evidence of

decreased pulmonary vascular permeability in re-sponse to fluid challenge (12). AQP2 deficiency pro-duces nephrogenic diabetes insipidus with renal re-

sistance to vasopressin and the excretion of largevolumes of dilute urine. Four different aquaporins

have been identified in the respiratory tract and areinvolved in appropriate handling of water in the vas-

cular, interstitial, and airspace compartments of the

lungs. AQP1 is found mostly in pulmonary microvas-cular endothelia, AQP4 in airway epithelia, and AQP5in apical membranes of type 1 alveolar epithelial cells

(15). In AQP1 and AQP4 knockout mice, AQP1 wasshown to facilitate hydrostatically driven lung edemabut not be required for active absorption of alveolar

fluid (1). In AQP5 knockout mice, alveolar fluid clear-ance was unimpaired, suggesting that, although AQP5

is important for osmotically driven water movementout of alveolar spaces, it does not facilitate hydrostat-ically driven lung edema or active alveolar fluid ab-sorption (15). AQP1 is colocalized in the choroid

plexus with Na-K-ATPases and is likely involved incerebrospinal fluid production. Mice lacking AQP4have reduced brain swelling after acute hyponatremia

and ischemic stroke (28). Mutations in AQP0 (whichfunctions as a low-capacity water channel in lens fibercells of the eye) have been implicated in congenitalcataracts. AQP1 and AQP5 are likely involved in re-

ducing the water content of corneal and lens epitheliaof the eye, as has already been shown for the corneain knockout mice (13). AQP5 is abundant in secretory

cells of salivary and lacrimal glands and thus may beinvolved in Sjogrens disease, which presents with

immunologically dry eyes and mouth and desiccationof tracheobronchial secretions (13). Thus aquaporins

have been shown to be extremely important for main-taining water balance in many tissues and appear to

be the primary pathways for the movement of watermolecules across cell membranes. If the ubiquitous

and polar water molecules need protein-mediatedtransport to cross cell membranes, is it likely that ureamolecules also have proteins that mediate their trans-

port across the cell membranes of the compartmentsin which they are found.

UREA TRANSPORTUrea is found in the blood, tubular fluid, and intersti-tium of mammalian kidneys as well as in the liver(where ureagenesis occurs) and in various tissues that

synthesize polyamines in physiological or pathophys-iological conditions like the heart muscle during car-diac hypertrophy, the testis during spermatogenesis,

and the brain. Thus numerous cells in an organismmust be able to clear toxic urea.

Filtered urea moves out of the proximal tubule of the

kidneys and into the interstitium; however, much of

the rest of the tubule is relatively impermeable tourea. Thus urea is increasingly concentrated in thetubularfluid as water leaves in the loop of Henle anddistal tubule. In the inner medullary portion of thecollecting duct (IMCD), urea moves into the intersti-tium, adding to the hyperosmolality there. The move-

ment of urea out of the IMCD is regulated by vaso-pressin. Conversely, when dilute tubular fluid (no



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vasopressin) reaches the IMCD, urea moves from theinterstitium into the tubular lumen, and the osmoticgradient in the interstitium is reduced. Urea move-

ment in and out of the tubule leads to a mechanism

that allows the formation of either concentrated ordilute urine. Via the countercurrent multiplier andexchange systems in the kidneys, an osmotic gradient

develops deep into the medullary tissue that is com-posed of NaCl (due to a variety of transport systems in

the tubular segments) and urea in the interstitium.The amount of urea in the urine and in the medullary

interstitium of mammalian kidneys varies with theamount of urea filtered at the glomerulus, which inturn varies with the dietary intake of protein. A low-

protein diet prevents an individual from producingconcentrated urine.

Evidence for Urea Transporters

Numerous students have conducted experiments on

red blood hemolysis in various osmotic solutions toinvestigate the principles of diffusion and osmosis.

Red blood cells lyse very rapidly (too fast to measurethe time) when placed in an isosmotic solution ofurea (300 mosM). Red blood cells lyse measurably less

rapidly when placed in an isosmotic solution (300mosM) of the urea analog thiourea. The explanation

for the lysis that was commonly taught was that ureaand thiourea were able to permeate the red blood cell

membranes through nonselective pores to cause os-motically driven swelling and rupture of cells. How-

ever, identification of transport proteins for selectsmall molecules has shown that red blood cells have

numerous aquaporins (AQP1) that facilitate rapid wa-ter movement under osmotic gradients and numerousurea transporters (UT-B) that facilitate rapid urea

movement by diffusion under concentration gradi-ents. Thiourea molecules utilize the same urea trans-

porters to cross red blood cell membranes as urea butpermeate more slowly.

In the early 1970s, evidence was first obtained thaturea transport in human red blood cells saturates andcan be inhibited (by phloretin) without significantlyaltering simultaneous water transport (29). Subse-quently, using a fast flow system, Mayrand and Levitt(17) confirmed that urea transport across human redblood cell membranes was a saturable facilitated dif-fusion process that was competitively inhibited by a

large number of urea analogs. Early studies foundspecies differences between human and chicken redblood cells in the kinds of facilitated diffusion trans-

porters likely available for the movement of urea and

water molecules (3). However, the mechanisms forurea permeability across red blood cell membraneswere not further investigated at that time. Even

though urea is a highly polar molecule that shouldhave low permeability across lipid bilayers, renal

physiologists and most textbooks continued until thelate 1990s to describe urea transport as primarily

simple diffusion through the lipid bilayer.

On the basis of evidence from mammalian red blood

cells, specific facilitated urea transport was first pro-posed in 1987 for the terminal IMCD of mammalian

kidneys (22). Measured urea permeability in the ab-sence of vasopressin was 85 times greater than pre-

dicted for simple lipid phase diffusion or for paracel-lular transport. Subsequent experiments verified thaturea transport in the perfused terminal IMCD showedsaturation kinetics (4), supporting the hypothesis of a

protein-mediated transport process.

These phenomenological studies set the stage for the

subsequent molecular biological investigations search-ing for urea transporters. Expression cloning in Xeno-

pus oocytes of the cDNA clone of a renal urea trans-porter from rabbit inner medulla was first reported in

1993 (30). The oocytes expressed functional trans-porters that facilitated the transport of urea. Urea

transport activity of Xenopus oocytes injected withmRNA from human or rat kidney papilla or rabbit

reticulocytes was enhanced about threefold comparedwith water-injected controls (6). This expressed ureatransport could be completely inhibited by phloretin

or p-chloromercuribenzene sulfonate (pCMBS).

Structure of Urea Transporters

Structural analysis of urea transporters is incomplete.

Some of the urea transporters are predicted to be43 45 kDa, whereas one is predicted to be 95kDa (21). Figure 3 shows a current model of urea

transporter UT-A1 found in apical membranes of theIMCD in rat kidneys. All isoforms contain severalsequences of amino acids that are known to encode

for sites for N-glycosylation or N-myristoylation or forprotein kinases, suggesting that the proteins are



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highly regulated. The family of urea transporters ap-pears to be remarkably well conserved among various

species and various isoforms. Proposed models forthe structure of urea transporters predict that the

number of transmembrane segments ranges between8 and 16, with intracellular carboxy termini, an extra-

cellular N-glycosylation site located in a relatively hy-drophilic segment in the middle of the protein, andseveral internal tandem repeats (24).

Physiology and Pathophysiology ofUrea Transporters

Early studies of urea transport in human red blood cellmembranes showed saturation, competition, and

asymmetry, suggesting that a transport protein waslikely to be involved (17). Because urea transport

could be inhibited by phloretin and pCMBS (known

inhibitors of facilitated diffusion transport), urea

transporters in red blood cells were likely facilitated

diffusion transporters. The turnover number for mol-

ecule transport via urea transporters was estimated to

be from 0.3 to 1.0 105 molecules/s, suggesting

that facilitated urea transporters are likely to act as

channels instead of carriers (14).

The major functional advantage for having facilitated

diffusion transporters for urea in red blood cells is

known as the Macey hypothesis. In modeling thepassage of red blood cells through the human renal

medulla, which may have up to 0.6 M urea, urea

transporters likely play two critical roles (16). In pass-

ing along the vasa recta, red blood cells shrink by

FIG. 3.

Hypothetical model of UT-A1 found in the apical membranes of the IMCD in rat kidney.

Several consensus sequences for putative phosphorylation and glycosylation sites are

shown in their approximate positions in the primary sequence. , Consensus protein

kinase A sites; , protein kinase C sites; , a tyrosine phosphorylation site; ands, anN-glycosylation site [from Sands et al. (27)].



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byproduct of ornithine synthesis from arginine viaarginase in the polyamine synthesis pathway (other

nonrenal tissues). This polyamine pathway is also

present in the heart. Polyamine production is known

to increase in conditions associated with cardiachypertrophy. Ornithine decarboxylase inhibitors pro-

tect the heart from becoming hypertrophic. Du-chesne et al. (5) found that urea transporter proteins

are upregulated in the hearts of uremic or hyperten-

sive rats and in dilated cardiomyopathy in humans.

This upregulation may be important for facilitating

urea exit from the cells in pathological conditions inwhich urea production is increased. Recently, Ranade

and colleagues (19) showed that single nucleotide

polymorphisms in human kidney urea transporter

UT-A2 appear to be associated with variation in blood

pressure in men, but, for as yet unknown reasons, notwomen.

CONCLUSION

Although physiological studies in the 1970s and 1980sconfirmed that water channels are present in humanred blood cell membranes, molecular biological stud-

ies in the early 1990s with expression cloning in

Xenopus oocytes were necessary to identify the pro-teins involved in water transport. Likewise, convinc-

ing evidence for urea transport proteins was made

possible by molecular biological techniques in theearly 1990s. It appears that many other small, hydro-

philic molecules of physiological importance alsohave selective transport pathways facilitated by spe-

cific proteins. Thus the concept of small, nonselectivepores in cell membranes has decreased in significancefor a complete understanding of membrane transport.

With the growing understanding of the consequencesof absence or mutations in specific transport proteins,the future will likely lead to specific targeting ofparticular channels with new drugs to correct general

or local disturbances in water homeostasis such as

renal, liver, or heart failure, glaucoma, acute hydro-

cephalus, or Sjogrens syndrome.

Address for reprint requests and other correspondence: B. E. Good-

man, Division of Basic Biomedical Sciences, Univ. of South Dakota

School of Medicine, 414 E. Clark St., Vermillion, SD 57069

([email protected]).

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