A Review of Glucose Transport in the Lens

A review of glucose transport in the lens

John W. Patterson

The transport of glucose into the lens is reviewed against a background of data on glucosetransport in muscle and the red blood cell. In muscle, sorbitol does not penetrate tlie cellmembrane and is limited to the extracellular space. On the other hand, 3-0-methijlglucose isdistributed in most of the tissue water. Similar results are obtained in lens. The glucosethat is found in muscle and lens can be accounted for on the basis of the glucose thatwould be expected to be in the extracelhdar space. Thus, transport is a limiting step inglucose metabolism and the rate of utilization of glucose may be equated with the rate ofinward transport. Mediated transport is considered as taking place at the fiber membrane.The kinetics of glucose transport in the lens are similar to those in muscle and the red bloodcell. All three have Km values of about 7.0 mM. and Qw values of 2.0. Glucose competeswith 3-0-methylglucose for the transport system in the lens. Glucose transport in the lens,as it is in muscle, is enhanced by respiratory poisons and inhibited by glycolytic poisons andphlorizin. It is concluded that the process of glucose transport in the lens is similar to thatin tissues such as muscle and the red blood cell.

T-Lh

-he nature of the process by whichsugars pass across the cell membrane hasbeen studied extensively in red bloodcells1'2 and muscle.3"6 The literature onthe subject of the entry of sugars into thelens is much smaller in volume and not aswell authenticated. Therefore, in thispaper, material on the transport of glucoseinto the lens will be presented against abackground of established observations onthe transport of sugars in other tissues.For the most part, comparisons will bemade with muscle, since the techniques ofstudy for lens and muscle are similar.

In the tissues that are under considera-tion, and under ordinary circumstances,the net passage of glucose is in the direc-tion of the concentration gradient. There-

From the University of Connecticut, Storrs, Conn.Supported in part by Research Grant AM-08344

from the National Institute of Arthritis andMetabolic Diseases, United States Public HealthService, Bethesda, Md.

fore, as an initial hypothesis the trans-port process might be considered as sim-ple diffusion through the pores of a mem-brane. Accumulated evidence, however, in-dicates that the process is more complex7'Si ° and that the membrane behaves in aselective manner. Therefore, the termsfacilitated diffusion and mediated transporthave been developed to indicate an inter-action between the membrane and thetransported substances. The term activetransport is reserved for phenomena inwhich an energy contribution is essential.30

The more general term transport is usedwithout prejudice as to the mechanismof permeation.

There are three structural components ofthe lens which must be considered in anydiscussion of the transport of glucose fromoutside the lens into the fibers—the cap-sule, the epithelium, and the fiber mem-branes.

The capsule of the lens is an acellularstructure made up of collagen-like ma-terial.11 It is permeable to small molecules

667

Downloaded From: http://iovs.arvojournals.org/pdfaccess.ashx?url=/data/journals/iovs/932951/ on 01/30/2018

668 Patterson Investigative OphthalmologyAugust 1965

but impermeable to large proteins.12 Re-cently, Becker13 has demonstrated thatcation transport is unaffected by the re-moval of the capsule with collagenase.There is no evidence that the capsule playsa role in glucose transport.

The lens epithelium is interposed be-tween the aqueous humor and the anteriorportion of the lens fibers. It has a specialrole in the active transport of cations andamino acids into the lens. Following ex-posure of the lens to radioactive rubidium,which behaves like potassium, the highestconcentrations are found at the epithelialsurface.14 Amino acids and cations15 areactively transported and concentrated inthe lens from fluids exposed to the anteriorepithelial surface of the lens. Passageacross the posterior surface is by passivediffusion.15 Removal of the epithelium bydecapsulation eliminates active transportof amino acids15'1G and cations.15'1T Glu-cose uptake is not affected in the same way.After decapsulation, even though there issome injury to the remaining fibers, glucoseuptake may be as high as 83 per cent ofthat in the intact lens.ls The methylatedglucose derivative, 3-0-methylglucose,which is not metabolized but transportedin a manner similar to that of glucose,19

enters the lens at the same rate from theposterior surface as it does from the ante-rior epithelial surface.20 Although theepithelium plays a role in the active trans-port of amino acids and cations, there isno evidence to indicate that it should beassigned a special function in the transportof sugars in the isolated lens.

Within the lens, individual fibers havemembranes21 which separate the lens intointracellular and extracellular compart-ments in the same manner as the cell mem-branes of other tissues. Measurements ofthe distribution of Na, K-ATPase in thelens,22 of electrical potentials across thelens surface,23 and of the distribution ofsodium within the lens24 support the con-cept that the lens fiber membranes, as wellas the epithelium, serve as sites of cationtransport.-5 Fiber membranes retain the

polyhydroxy alcohols that are produced bythe reduction of sugars,20 and they preventthe entry of substances such as sucrose intothe fibers.27 This is consistent with the be-havior of other cell membranes, and byanalogy and by the process of eliminationone may suggest that the fiber membranesare the site of glucose transport in the iso-lated lens.

The conclusion that the fiber membraneis the structure that is being studied whenglucose transport is measured in isolatedlenses has three important implications:(1) the lenses should be small so that thedistance substances must move to reachthe site will be minimal; (2) the lensesshould be young so that there will be ahomogeneous fiber mass; and (3) the ex-tracellular space should be taken intoconsideration when results are interpreted.

The extracellular space which potentiallyexists in the capsule between epitheliumand fibers and between individual fiberscan be measured by determining the con-tent within the lens of a substance whichdoes not penetrate through cell mem-branes. For purposes of comparison, it iscommon to determine the hypothetical"space" or volume of tissue fluid that wouldbe occupied by a substance if it were pres-ent in the same concentration that occursin the bathing medium. It is calculated bydividing the concentration of the substancein the tissue by its concentration in the me-dium and multiplying by 100. The "space"of the substance, therefore, represents avolume of lens fluid per unit of lens weight(microliters per 100 mg.), or approximatelythe per cent of the wet weight that wouldbe occupied by the substance if it werepresent in the same concentration thatexists in the surrounding fluid. Since a sub-stance that freely diffuses into the extra-cellular space can have the same concen-tration in the extracellular fluid that occursin the surrounding medium, a "space" fora substance that is less than or equal to theextracellular space indicates an extracellu-lar distribution of the substance. A "space"that is greater than the extracellular space


Volume 4Number 4

Glucose transport in lens 669

MUSCLE LENS

40 -

20-

60

INCUBATION TIME (min.)

120 24C

Fig. 1. The sorbitol space and 3-0-methylglucose space after different times of incubation.In muscle, sorbitol is limited to the extracellular space5 and 3-0-methylglucose is distributedin most of the tissue water.19 In lens33 the results are similar. In order of increasing incuba-tion time the points for sorbitol represent the mean values for 30, 20, 10, 30, and 10 lenses,respectively, and for 3-0-methylglucose, the mean values for 2, 6, 12, 13, and 9 lenses,respectively.

indicates that the substance is also in theintracellular compartment. The sodiumspace of rabbit lens determined by sacri-ficing the animals following the injectionof radioactive sodium was found to be 6.1yu.1 per 100 mg. by Langham and Davson.27

A similar study by Huggert2S showed a so-dium space of 9 to 12.6 /i\ per 100 mg. inthe lens cortex and 3.3 to 7.3 pi per 100mg. in the lens nucleus for rabbits rangingfrom 3 weeks to 3 years in age. On the ba-sis of their estimates of the distribution ofweight between cortex and nucleus, theaverage sodium space estimated for thewhole lens is 7 to 10 pi per 100 mg. Sincethe distribution of sodium is accepted asbeing extracellular, these values are ac-cepted as reasonable estimates of the ex-tracellular space in rabbit lenses. Deter-minations made following incubation oflens in artificial media are generally higher.Bromide and sodium spaces of 29 to 36 piper 100 mg. for the whole lenses,27 20 to 30p\ per 100 mg. for lens cortex,2<J and 12 to20 /J per 100 mg. for the lens nucleus havebeen reported. Following incubation undersimilar circumstances, the lenses of sheephad a sucrose space of 13.5 pi per 100 mg.

Several types of studies in muscle havecontributed to an understanding of themechanism of glucose transport. These in-clude:

1. Differences in permeability of glucoseand structurally related molecules.

2. The kinetics of glucose transport.3. Competition for transport between

glucose and related substances.4. Induced counterflow of sugars against

a concentration gradient.5. The effects of metabolic poisons.A consideration of each of these items

provides a basis for reviewing the transportof glucose in the lens.

The permeability of membranes to dif-ferent substances may be estimated bymeasuring the accumulation of the sub-stance in the tissue following a period ofincubation. The permeability is indicatedby the extent to which the "space" occu-pied by the substance exceeds the extra-cellular space. Sorbitol does not penetratecell membranes™'31 and is used to measureextracellular space in muscle.32 The sugarderivative, 3-0-methylglucose is not metab-olized by tissue and distributes itself in theintracellular and extracellular space.19 The


670 Patterson Inooxtigulive OphthalmologyAugust 1965

Table I. Glucose space and sorbitol space determined simultaneously in ratlenses at 37° C. and 15° C. after 2 hour incubation in Tyrode's solutioncontaining 100 mg. per 100 ml. of each of glucose and sorbitol114

No. of lenses

1212

Temperature(C.°)

37°il5°

Glucose space(fil/lOOmg.)

Range

5.6-11.913.6-27.6

Mean

7.720.2

Sorbitol space9

(id/100 mg.)

Range

5.8-14.47.3-21.4

Mean

9.613.5

"Corrected for intracellular sorbitol.

behavior of these substances in the rat lensis similar (Fig. I).33 The sorbitol spacewas determined with tritium-labeled sor-bitol in the presence of 1,000 mg. per 100ml. of glucose. The value of 13.5 /xl per 100mg. is a little higher than the in vivo re-sults of 6 to 10 /A per 100 mg. that waspresented earlier for the sodium space oflenses from rabbits. Inasmuch as in vitrodeterminations yield higher results, thefinding may be accepted as a measure ofthe extracellular space of rat lens and as ademonstration that sorbitol does not pene-trate through the fiber membrane. The 3-0-methylglucose space was determined byusing a radioactively labeled compound.This space increases with time and equals40 [xl per 100 mg. of lens in 4 hours. Thespecificity of the membranes of muscle andlens as indicated by the permeability ofthese two derivatives of glucose is repre-sentative of the behavior of other sub-stances. In muscle, D-galactose34 and D-glucose10 enter the cell and their L-formsare excluded. This selectivity by the cellmembrane demonstrates that the transportprocess is limited to substances with spe-cific molecular configurations, and thatthere is a barrier in the lens to the passageof certain derivatives of glucose.

The net rate of glucose transport is equalto the rate of utilization plus the rate ofintracellular accumulation. The net rate oftransport is equal to the difference be-tween inward transport and outward trans-port which in turn are related, respectively,to the concentration of glucose outside andinside the membrane. The lower the levelof intracellular glucose, the more closely

the net rate of transport equals the rate ofinward transport. Thus, if it can be shownthat the concentration of intracellular glu-cose is very low, the values for the rate ofinward transport, the rate of uptake, andthe rate of utilization become the same.

In lenses from normal rats, the rate ofutilization has been shown to be equal tothe rate of glucose uptake.35 The glucosespace of lenses from normal rats is whatwould be expected for the extracellularspace. Kuck,3G Kinoshita,-G and Lennan,37

respectively, found glucose concentrations of8.4, 9.0, and 9.1 mg. per 100 Gm. of normalrat lens. Kinoshita reported an averageblood glucose level of 100 mg. per 100 ml.and Kuck reported a blood glucose levelof 102 mg. per 100 ml. and an aqueous glu-cose level of 104 mg. per 100 ml. in the ani-mals from which the lenses were obtained.The glucose space is thus 8 to 9 /xl per 100mg. and in the same range as the 6 to 10[xl per 100 mg. found for the in vivo extra-cellular space of rabbits. The fact that in-tracellular glucose is very low under thesecircumstances is confirmed by a simul-taneous determination of the glucose spaceand sorbitol space following incubation ofrat lenses in Tyrode's medium containing100 mg. per 100 ml. each of glucose and sor-bitol (Tables I and II). Thus, in lensesfrom normal rats, there does not appear tobe an appreciable amount of intracellularglucose, and the rate of utilization may beaccepted as a measure of the rate of inwardtransport.

There is an apparent contradiction tothis conclusion which must be considered.Harris, Hauschildt, and Nordquistis have


Volume 4Number 4


Table II. Ratio of glucose space toextracellular space at different temperatures

Organ Temperature Ratio

Diaphragm"

Lens

3712

3715

0.91.6

0.81.5

"Calculated from glucose space data of Park, Bornstein,and Post10 and the extracellular space data of Kipnis.4

shown that glucose accumulates (morethan 10 per cent) in rabbit lenses incu-bated in media containing over 100 mg.per 100 ml. of glucose. This has also beendemonstrated for rat lenses by Pattersonand Bunting.83 In rat lenses, the criticalconcentration is just under 200 mg. per100 ml. of glucose. The glucose space, how-ever, does not exceed the sorbitol space ifthe incubation time is less than 30 minuteseven though the glucose concentrationsrange up to 1,000 mg. per 100 ml.

The accumulation of glucose in the lensin the presence of high concentrations ofglucose and prolonged incubation periodsmust be the result of an increase in therate of transport of glucose into the fiberor the result of a decrease in the rate ofutilization of glucose. The sorbitol spaceis not increased with prolonged incubationtimes in the presence of 1,000 mg. per 100ml. of glucose (Fig. 1) and the rate of ac-cumulation of 3-0-methylglucose is de-creased rather than increased in the pres-ence of 1,000 mg. per 100 ml. of glucose(Fig. 4). Therefore, it appears unlikely thatthe rate of glucose transport is increased.

On the other hand, when intact lenses areincubated in media containing 400 mg. per100 ml. of glucose, the utilization of glucosedecreases as the incubation time in-creases.33 This phenomenon is apparentlysecondary to a decrease in glycolysis, inas-much as glucose utilization in unfortifiedlens homogenates, after different times ofpreincubation of the intact lenses, also de-creases with increasing times of preincuba-tion (Table III). This observation, indicat-ing that high concentrations of glucose soonhave a deleterious effect on glucose utiliza-tion, helps to explain two earlier observa-tions. First, it explains the glucose "tox-icity" at concentrations above 200 mg. per100 ml. of glucose that was observed byHarris, Hauschildt, and Nordquist3S in theirstudies of the reversible cation shift, and,second, it explains the anomalous resultsfor glucose uptake that were obtained byFarkas Ivory, and Patterson39 when 25 mg.rat lenses were incubated for two hours inmedium containing over 200 mg. per 100ml. glucose.

On the basis of studies on glucose ac-cumulation, it may be concluded that glu-cose does not accumulate in rat lenses inthe presence of a glucose concentration ofless than 200 mg. per 100 ml. but will grad-ually accumulate at higher concentrationsof glucose. Therefore, if the rate of inwardglucose transport is to be determined bymeasuring the rate of glucose utilization,it is essential that the incubation periodsbe short.

In heart muscle, the rate of inward glu-

Table III. Glucose utilization by an unfortified homogenate of a rat lens (19 ±2 mg.) plus 10 /A of glucose solution, after preincubation of the intact lens inTyrode's solution containing 400 mg. per 100 ml. of glucose04

No. of experiments*

Length ofpreincubation

(hours)

Glucose concentrationin homogenate(mg./lOOml.)

Range Mean

Glucose utilization(mg./Gm./hr.)

Range Mean200-432301-378282-397319-410

315323341380

3.24-4.062.35-2.402.11-2.221.39-1.94

3.592.372.161.75

"Each experiment included 5 lenses for zero time and 5 lenses that were incubated for 30 minutes.



MUSCLE LENS

200 400 200MEDIUM GLUCOSE mg/100 ml.

4 0 0

Fig. 2. Glucose uptake at different concentrations of glucose for muscle32 and lens.33 Thecurve labeled no membrane for muscle was calculated on the basis of the levels of intra-cellular glucose that are found in the presence of insulin. The points on the curve for intactlenses represent, in order of increasing concentration, the mean value for 10, 19, 18, 32,and 32 lenses, respectively. The no membrane curve for lens was determined on homogenatesof a lens (about 20 mg.) plus 10 i"l of sugar solution. One lens of a pair was analyzed atzero time and the other after 30 minutes of incubation. Each point represents 20 or morepairs of lenses.

cose transport at different glucose con-centrations is measured by determining therate of glucose disappearance from themedium.3'- Appreciable amounts of freeglucose do not accumulate within the cellsexcept in the presence of anoxia or insulin.The fact that free intracellular glucose ac-cumulates under these conditions providesa basis for calculating the rate of glucoseuptake for different concentrations of in-tracellular glucose without worrying aboutthe limiting effects of a membrane. Theresults for glucose uptake in the presenceand absence of a membrane are shown inFig. 2. The difference between the twocurves indicates the degree to which thecell membrane limits glucose utilization un-der normal aerobic conditions.

The rate of inward transport of glucoseinto the lens fiber is measured in 19 ± 2mg. rat lenses by determining the disap-pearance of glucose from the total systemof lens plus medium during a 30 minuteincubation period.33 The glucose uptake forlenses without membranes is estimated byhomogenizing a series of lenses in 10 /xl

of water containing different amounts ofadded glucose. One lens of a pair is an-alyzed at zero time and the second after30 minutes of incubation at 37° C. Thedifference in the glucose levels in the twolenses along with the average sugar con-centration during the incubation periodprovides the data for the calculations.33

The estimates for glucose uptake in lenseswith and without membranes is shown inFig. 2. The results are similar to those inmuscle. In both instances, glucose transportthrough the membrane appears to be thelimiting factor in glucose utilization, andin both instances the transport system be-comes saturated at higher external glucoseconcentrations.

The transport of glucose into the lensconforms with Michaelis-Menten kineticsand gives a straight line in a Lineweaver-Burk plot.33 The apparent Km for trans-port is 6.7 raM. and the maximum rate oftransport is 4 mg. per gram of lens perhour. The Km for glucose utilization inconcentrated unfortified lens homogenatesis 0.7 mM. and the maximum uptake is


Volume 4Number 4


Table IV. Km of glucose transport in rats

Intact cellsHeart muscle*Lens

Without membraneHeart muscle"Lens

Km(mM.)

6.66.7

0.60.7

(mg./Gm./hr.)

13.74.0

14.53.9

"Data from Morgan, Henderson, Regen, and Park.32

3.9 mg. per gram per hour. These findingsare compared with those for rat heart mus-cle in Table IV. The values for the Km ofglucose transport in red blood cells fallbetween 4 and 10 mM.3

Glucose uptake has been measured atdifferent temperatures in rat muscle'10 andin rat lens. The latter data were suppliedby Dr. T. G. Farkas who made availablethe manuscript of a paper submitted forpublication. With a decrease in tempera-ture, in muscle and lens, the rate of glu-cose metabolism decreases more rapidlythan the rate of transport. Therefore, atlower temperatures, intracellular glucoseaccumulates (Table I). A plot of the log-arithm of the rate of glucose transport

MUSCLE

against the reciprocal of the absolute tem-perature using the method of least squarespermits an estimate of the Q10 for the trans-port of glucose in muscle and lens (Fig.3). The values are 2.0 and 1.8, respectively.They are in agreement with a Q10 of 2.0reported for red blood cells."11

On the basis of kinetic studies the Kmand Q10 of glucose for transport in muscle,red blood cells, and lens are the same. Thevalues are consistent with those of an en-zymelike system.

The measurement of 3-0-methylglucosespace with different times of incubation inthe presence and absence of glucose pro-vides a simple method of measuring thecompetition between 3-0-methylglucoseand glucose. The results for rat heart mus-cle19 and rat lens33 are shown in Fig. 4.The results are qualitatively similar anddemonstrate that these two substances in-terfere with each other's passage throughthe membrane.

Induced counterflow has been demon-strated in muscle19 (Fig. 5) and in redblood cells.42 The phenomena may be ex-plained as follows. If a tissue is loadedwith a sugar derivative that is not metab-olized, such as 3-0-methylglucose, the con-

LENS

120 •

35

Fig. 3. The logarithm of the rate of glucose uptake plotted against the reciprocal of theabsolute temperature for muscle40 and for lens. The latter is calculated from data in amanuscript that was made available by Dr. T. G. Farkas. The straight lines are calculatedby the method of least squares.



MUSCLE LENS

20-

3 0 120 240

iNCUBATION TIME (min.)

Fig. 4. The 3-0-methylglucose space after different times of incubation in the absence andpresence of glucose. The rate of penetration is much faster in heart muscle19 than it is inlens.33 The curve for lens without glucose is the same as in Fig. 1. The points on the curvein the presence of glucose each represent four determinations.

MUSCLE

a£

2 60-

30-

N0 GLUCOSE

GLUCOSE ADDED19 mM AT

10 min.

30-

15-

i

i

//// ,/ // // /t

1I

NO

*

f

LENS

GLUCOSE

s 'GLUCOSE ADDED

56 m M AT30 mm.

15 30INCUBATION TIME (min.)

30 60

Fig. 5. The 3-0-methylglucose space as it is changed by the addition of glucose after therehas been some intracellular loading. The dotted lines reproduce the data of Fig. 4. Theaddition of glucose is made vvitliout altering the concentration of 3-0-methylglucose in themedium. In muscle19 the 3-0-methylglucose moves out against a concentration gradient untilit reaches a new equilibrium. In lensa3 the same process seems to occur but the changedoes not demonstrate "counterflow."

centration of this substance in the cell isdetermined by the relative rates of inwardand outward transport. The concentrationinside the cell will approach that of theoutside but it will not surpass it. If a highconcentration of a competitive sugar thatis readily metabolized, such as glucose, is

then introduced on the outside withoutchanging the concentration of the 3-0-methylglucose, a competitive situation iscreated for inward transport. Because theglucose is metabolized and does not ac-cumulate in the cell, the situation for out-ward transport remains the same. Under


Volume 4Number 4


01

A

HR

C,M

ANOXIA

NaCN

DNP

M3F « DNP

GLUCOSEPERCENT

50

1.0

0.25

1 ••

TRANSPORTOF NORMAL100 150

i

11

mM

i

mM |

i

|rtiM*o.25 mMi

I

HEART

DNP

ONP

15 min.

30 min.

i

0.25 mM ]

i

3 :

L

b

S

ANOXIA

NdCN

DNP

Nap |

I A A |

1

11

i.O mM 1

|

1 10 JmM

150 imM

i

25 JmM

Fig. 6. The effects of poisons on the rate of glu-cose uptake in rat diaphragm,45 rat heart mus-cle,'1 G and rabbit lens.is

appropriate conditions the rate of influxof 3-0-methylglucose may be reduced be-low the rate of efflux with the net effectbeing a temporary (until a new equilib-rium is reached) efflux of 3-0-methyl-glucose against its concentration gradient.An attempt to demonstrate counterflow inrat lenses did result in the demonstrationof competition and did shift the uptake of3-0-methylglucose to a new equilibriumposition (Fig. 5) but, with conditions thatwere used, counterflow against a concen-tration gradient was not shown.33

These experiments demonstrate that inmuscle and the red blood cell, the processfor inward and the process for outwardtransport are saturated on an independentbasis.

There are three types of poisons that af-fect glucose transport. The first type is dis-

tributed in the extracellular space, acts onthe cell membrane, and decreases trans-port. Phlorizin, N-ethylmaleimide, and p-chloromercuribenzoate are examples of thistype.43 The second type of poison inhibitsthe respiratory enzymes, stimulates glycol-ysis secondarily through the Pasteur effect,and increases glucose transport. This typeincludes cyanide, salicylate, arsenite, anddinitrophenol.46"47 Anoxia has an effect sim-ilar to that of these poisons.44 The thirdtype of poison inhibits glycolysis and in-hibits glucose transport. It includes fluor-ide,45 iodoacetate,47 and acetoacetate.4S

The effects of phlorizin are shown inTables V and VI.

The effects on glucose transport of theinhibition of respiration by anoxia andcyanide and the inhibition of glycolysis byfluoride and iodoacetate are shown in Fig.6. The effect of dinitrophenol seems to de-pend on the conditions under which it isemployed. It has been suggested that it in-hibits glycolysis as a secondary effect.40

The inhibitory effect of iodoacetate on glu-cose transport into the lens was also shownby Miiller.49 The effects of fluoride andiodoacetate are minimal in red blood cells47

and fluoride has only a small effect onxylose transport into muscle.50

Phenylethylbiguanide (DBI) acts as arespiratory poison51 and increases glucoseuptake in muscle51 and rabbit lens.52

Randle and Morgan0 have been im-pressed by the effects of poisons on glu-cose transport and more particularly by thestimulating effect of anoxia and the inhibi-tory effect of fatty acid metabolites, suchas acetoacetate, on the rate of glycolysis.

Table V. Effect of phlorizin on glucoseutilization of intact rat lenses incubatedfor 30 minutes at 37° C. in Tyrode'ssolution containing 100 mg. per 100 ml.glucose04

No. oflenses

Phlorizinconcentration

(mM.)

Glucose utilization0

(mg./Gm./hr.)

Range Mean1010

1.86-2.350.95-1.74

2.031.34

"Uptake from lens plus medium.



Since the rate of glucose transport seemsto parallel the rate of some of the phos-phorylation steps in glycolysis, there is apossible mechanism for the control of glu-cose transport on the basis of metabolicneed.

The parallel behavior of glucose and itsderivatives in muscle and lens supports theview that the nature of the transport pro-cess is the same in these two tissues. Thesimilarities between muscle and red bloodcells are well known, and the demonstra-tion that another tissue behaves in thesame way gives further authenticity to thelimited observations in lens. The absenceof demonstrable intracellular glucose in thelens indicates that the rate of transport ofglucose is equal to the rate of glucose utili-zation. The qualitative and quantitativeagreement of the kinetics of glucose utili-zation in lens with the kinetics of glucosetransport in muscle and the red blood cellsupports the conclusion that the transportprocess is the same in the membranes of thethree tissues. It could be argued, however,that the Km and Qt0 of glucose utilizationin the lens are reflections of glucose me-tabolism rather than transport and that thequantitative agreement with the transportvalues in other tissues is fortuitous. Thelimitation of the distribution of sorbitol inthe lens does demonstrate the existence ofa barrier consistent with the fiber mem-branes, and the limitation in the distribu-tion of 3-0-methylglucose by the additionof glucose can be explained by competitionat the membrane level, whereas it cannotbe explained at a metabolic level. Similarly,the observation that the accumulation ofglucose, under conditions of glucose "tox-icity," is decreased18 in lenses bathed inmedia containing glycolytic poisons, whichcan also decrease transport, is readily ex-plained on the basis of an inhibition oftransport at the fiber membrane. It can-not be explained, solely, on the basis ofan inhibition of glucose metabolism. Thus,the complete data on the lens are best ex-plained by the concept that the fiber mem-brane limits the access of glucose to the

cell, and that the transport of glucose acrossthe membrane takes place by a processsimilar to that which occurs in muscle andred blood cells.

As a result of the types of evidence thathave been presented for the transport ofglucose in tissues such as muscle and redblood cells, there is little doubt that glu-cose interacts with the membrane duringthe transport process. The nature of theprocess of mediated transport, however, isstill being debated. A model involving amobile carrier for the transport of glucoseacross the membrane has gained great sup-port.7' Si 9> 53'54 Recently another model,based on the formation of a dimer of glu-cose which passes through the lipid layerof the membrane to reform the monomeron the other side, has been suggested.55

Each of these models is supported bymathematical treatments which explain thecomplex aspects of sugar transport that oc-cur under special circumstances. It isclaimed, however, that the observed phe-nomena can also be explained, on a mathe-matical basis, if a fixed membrane carrieris used as a model.11'5G Danielli10 has sug-gested that the problem will not be solveduntil more is known about the molecularbasis of transport.

Recently, Keston57 has suggested that theenzyme mutarotase may be involved in thetransport process: The Km of the enzymeis about the same as that for transport; theenzyme is inhibited by phlorizin in a man-ner similar to transport, and the rate ofglucose utilization in various organs paral-lels the activity of mutarotase. Conceivably,the formation of the open chain form ofthe sugar, as distinct from the pyranosering form, might be a necessary step intransport. Mutarotase has been demon-Table VI. Per cent inhibition of glucoseutilization by 3 raM. phlorizin in the rat

OrganPer cent

inhibition Reference

DiaphragmHeart muscleLens

504535

434664


Volume 4Number 4


strated in lenses from cattle, rabbits, dogs,and rats.58 The Km of glucose for rabbitlens is 14 mM. The enzyme is inhibited bygalactose and xylose and by sugar alcohols.Clarification of the role of mutarotase inglucose transport must, however, awaitfurther studies. Wilbrandt2 failed to findmutarotase in red blood cells and Craneand Krane50 seem to differ with KestonG0

on the interpretation of the results obtainedwith 1-deoxyglucose, a sugar derivativewhich is not capable of mutarotating.

In this presentation, the effects of insulinon glucose transport have not been con-sidered. The effect of insulin on glucosetransport in muscle has been studied ex-tensively.0 The data on lens, however, areless conclusive. Insulin labeled with radio-active iodine does not enter the aqueous,01

and insulin does not stimulate glucoseutilization in the intact lens.ls> G2 However,it is reported to have some effect in thedecapsulated lens.TS> G3 Insulin injected ina rat prior to sacrifice is reported to in-crease the glucose uptake in the isolatedlens.39 This work, however, has lacked con-firmation. Further studies, with differentapproaches, need to be made. In the mean-time, it may be assumed that, under phys-iological conditions, insulin does not havea direct effect on glucose transport in thelens.

In conclusion, it may be stated that theprocess of glucose transport in the lens issimilar to the process for the transport ofglucose in tissues such as muscle and thered blood cell. If advances are made in ourunderstanding of the transport process inany of these tissues, the results will prob-ably also apply to the transport of glucosein the lens.

REFERENCES

1. Lefevre, P. G.: The evidence for activetransport of monosaccharides across the redcell membrane, in Brown, R., and Danielli,J. F., editors: Active Transport and Secretion,New York, 1954, Academic Press, Inc., pp.118-135.

2. Wilbrandt, W.: The sugar transport across

the red cell membrane, in Kleinzeller, A., andKotyk, A., editors: Symposium on MembraneTransport and Metabolism, Praha, 1961,Publishing House of Czechoslovak Academyof Science, pp. 388-398.

3. Henderson, M. J.: The uptake of glucoseinto cells and the role of insulin in glucosetransport, Canad. J. Biochem. 42: 933, 1964.

4. Kipnis, D. M.: Regulation of glucose uptakeby muscle: Functional significance of perme-ability and phosphorylating activity, Ann. NewYork Acad. Sc. 82: 354, 1963.

5. Park, C. R., Morgan, H. E., Henderson, M.J., Regen, D. M., Cadenas, E., and Post,R. L.: The regulation of glucose uptake inmuscle as studied in the perfused rat heart,in Pincus, C , editor: Recent Progress in Hor-mone Research, New York, 1961, AcademicPress, Inc., pp. 493-529.

6. Randle, P. J., and Morgan, H. E.: Regulationof glucose uptake by muscle, in Harris, R.S., and Wool, I. C , editors: Vitamins andHormones, New York, 1962, Academic Press,Inc., vol. 20, pp. 199-249.

7. Lefevre, P. G., and McGinnis, G. F.: Tracerexchange vs. net uptake of glucose throughhuman red cell surface, J. Gen. Physiol. 44:87, 1960.

8. Rosenberg, T., and Wilbrandt, W.: Thekinetics of membrane transports involvingchemical reactions, Exper. Cell. Res. 9: 49,1955.

9. Widdas, W. F.: Facilitated transfer of hexosesacross the human erythrocyte membrane, J.Physiol. 125: 163, 1954.

10. Danielli, J. F.: Structure of the cell surface,Circulation 26: 1163, 1962.

11. Pirie, A.: Composition of the ox lens capsule,Biochem. J. 48: 368, 1951.

12. Friedenwald, J. S.: Permeability of the lenscapsule: With special reference to the etiologyof senile cataract, Arch. Ophth. 3: 182, 1930.

13. Becker, B.: Paper presented at IX Conferenceon Ophthalmic Biochemistry, Dedham, Mass.,Feb. 21-23, 1964.

14. Becker, B., and Cotlier, E.: Distribution ofrubidium-86 accumulated in the rabbit lens,INVEST. OPHTH. 1: 642, 1962. '

15. Kinsey, V. E., and Reddy, D. V. N.: Studieson the crystalline lens. XI. The relative roleof epithelium and capsule in transport, IN-VEST. OPHTH. 3: 243, 1964.

16. Kinsey, V. E., and Reddy, D. V. N.: Studieson the crystalline lens. X. Transport of aminoacids. INVEST. OPHTH. 2: 229, 1963.

17. Harris, J. E., and Cruber, L.: The electroylteand water balance of the lens, Exper. EyeRes. 1: 372, 1962.

18. Harris, J. E., Hauschildt, J. D., and Nord-quist, L. T.: Transport of glucose across the


67S Patterson Invcstigutioe OphthalmologyAugust 1965

lens surfaces, Am. J. Ophth. 39: (Part II)161, 1955.

19. Morgan, H. E., Regen, D. M., and Park, C.R.: Identification of a mobile carrier-mediatedsugar transport system in muscle, J. Biol.Chem. 239: 369, 1964.

20. Kinsey, V. E.: Personal communication.21. Cogan, D.: Anatomy of lens and pathology

of cataracts, Exper. Eye Res. 1: 291, 1962.22. Bonting, S. L., Caravaggio, L. L., and Hawk-

ins, N. M.: Studies on sodium-potassiumactivated adenosinetriphosphatase. VI. Its roledn cation transport in the lens of cat, calfand rabbit, Arch. Biochem. & Biophys. 101:47, 1963.

23. Sperelakis, N., and Potts, A. M.: Additionalobservations on the bioelectric potentials ofthe lens, Am. J. Ophth. 47: 395, 1959.

24. Amoore, J. E., Bartley, W., and van Heynin-gen, R.: Distribution of sodium and potassiumwithin cattle lens, Biochem. J. 72: 126, 1959.

25. Kinoshita, J. H.: Selected topics in ophthal-mic biochemistry, Arch. Ophth. 70: 558, 1963.

26. Kinoshita, J. H., Merola, L. O., and Dikmak,E.: Osmotic changes in experimental galactosecataracts, Exper. Eye Res. 1: 405, 1962.

27. Langham, M., and Davson, M.: Studies onthe lens, Biochem. J. 44: 467, 1949.

28. Huggert, A.: Studies on the water of thecrystalline lens. 4. The sodium space of rab-bit lens in vivo, Acta ophth. 37: 522, 1959.

29. Huggert, A.: Studies on the water of thecrystalline lens. 2. The extracellular spaceof the cattle lens measured in vitro, Actaophth. 37: 26, 1959.

30. Wick, A. N., and Drury, D. R.: Action ofinsulin on permeability of cells to sorbitol,Am. J. Physiol. 166: 421, 1951.

31. Fisher, R. B., and Lindsay, D. B.: The actionof insulin on the penetration of sugars intothe perfused heart, J. Physiol. 131: 526, 1956.

32. Morgan, H. E., Henderson, M. J.: Regen, D.M., and Park, C. R.: 1. The effects of insulinand anoxia on glucose transport and phos-phorylation in the isolated, perfused heart ofnormal rats, J. Biol. Chem. 236: 253, 1961.

33. Patterson, J. W., and Bunting, K. W.: Glu-cose transport in rat lens, Am. J. Physiol.In press.

34. Park, C. R., Reinwein, D., Henderson, M. J.,Cardenas, E., and Morgan, H. E.: The actionof insulin on the transport of glucose throughthe cell membrane, Am. J. Med. 26: 674,1959.

35. Farkas, T. C , and Patterson, J. W.: Insulinand the lens, Am. J. Ophth. 44: (Part II)341, 1957.

36. Kuck, J. F. R.: Sugar and sugar alcohollevels in the aging rat lens, INVEST. OPHTH.2: 607, 1963.

37. Lerman, S.: Metabolic pathways in experi-mental diabetic cataract, INVEST. OPHTH.1: 507, 1962.

38. Harris, J. E., Hauschildt, J. D., and Nord-quist, L. T.: Lens metabolism as studied withthe reversible cation shift. I. The role ofglucose, Am. J. Ophth. 38: (Part II) 141,1954.

39. Farkas, T. C , Ivory, R. F., and Patterson,J. W.: Glucose uptake and utilization of iso-lated lenses for normal and diabetic ratsfollowing insulin injection, Anal. Rec. 138:235, 1960.

40. Park, C. R., Bornstein, J., and Post, R. L.:Effect of insulin on free glucose content ofrat diaphragm in vitro, Am. J. Physiol. 182:12, 1955.

41. Britton, H. G.: Permeability of the humanred cell to labelled glucose, J. Physiol. 170:1, 1964.

42. Rosenberg, T., and Wilbrandt, W.: Uphilltransport induced by counterflow, J. Gen.Physiol. 41: 289, 1957.

43. Battaglia, F. C., and Randle, P. J.: Regula-tion of glucose uptake by muscle. 4. Thespecificity of monosaccharide transport sys-tems in rat-diaphragm muscle, Biochem. J.73: 408, 1960.

44. Randle, P. J., and Smith, G. H.: Regulationof the uptake of glucose by the isolated ratdiaphragm, Biochim. et biophys. acta 25:442, 1957.

45. Randle, P. J., and Smith, G. H.: Regulationof glucose by muscle. 2. The effects of in-sulin, anaerobiosis, and cell poisons on thepenetration of isolated rat diaphragms bysugars, Biochem. J. 70: 501, 1958.

46. Morgan, H. E., Randle, P. J., and Regen, D.M.: Regulation of glucose uptake by muscle.3. The effect of insulin, anoxia, salicylate and2, 4 dinitrophenol on membrane transportand intracellular phosphorylation of glucose•in the isolated rat heart, Biochem. J. 73:573, 1959.

47. Laris, P. C : Permeability and utilization ofglucose in mammalian erythrocytes, J. Cell.& Comp. Physiol. 51: 273, 1958.

48. Ottoway, J. H.: The effects of growth hor-mone and ketone bodies on carbohydratemetabolism in diaphragm from normal andhypophysectomized rats, J. Endocrinol. 21:443, 1961.

49. Miiller, H. K.: Ueber die resorption vonglucose und askorbinsaure in die linse, Arch,f. Ophth. 140: 258, 1939.

50. Forbath, N., and Clarke, D. W.: Factors in-fluencing the distribution of pentoses in theisolated rat diaphragm, Canad. J. Biochem.Physiol. 38: 13, 1960.

51. Steiner, D. F., and Williams, R. H.: Actions


Volume 4Ntimber 4


of phenylethylbiguanide and related com-pounds, Diabetes 8: 154, 1959.

52. Ciles, K. M., and Harris, J. E.: The accumu-lation of C u from uniformly labeled glucoseby the normal and diabetic rabbit lens, Am.J. Ophth. 48: (Part II) 508, 1959.

53. Lacko, L., and Burger, M.: Kinetic compari-son of exchange transport of sugars withnonexchange transport in human erythrocytes,J. Biol. Chem. 238: 3478, 1963.

54. Lefevre, P. G., and Habich, K. I.: Absenceof rapid exchange component in a low affinitycarrier transport, J. Gen. Physiol. 46: 721,1962.

55. Stein, W. D.: Dimer formation and glucosetransfer across the membrane of the red bloodcell, Nature 191: 1277, 1961.

56. Britton, H. G.: Induced uphill and downhilltransport: Relationship to the Ussing criterion,Nature 198: 190, 1963.

57. Keston, A. S.: Kinetics and distribution ofmutarotases and their relation to sugar trans-port, J. Biol. Chem. 239: 3241, 1964.

58. Keston, A. S.: Inhibition of action of lensmutarotase on glucose by cataractogenicsugars and corresponding polyols, Arch. Bio-chem. & Biophys. 102: 306, 1963.

59. Crane, R. K., and Krane, S. M.: On themechanism of intestinal absorption of sugars,Biochim. et biophys. acta 20: 568-569, 1956.

60. Keston, A. S.: Mutarotase inhibition by 1-deoxyglucose, Science 143: 698, 1964.

61. Giles, K. M., and Harris, J. E.: Radioelectro-phoretic patterns of aqueous and plasma:After intravenous injection of I131 labeledinsulin into rabbits, Am. J. Ophth. 46: (PartII) 196, 1958.

62. Macintyre, M. N., Polt, S. S., and Patterson,J. W.: Glucose uptake by isolated normal anddiabetic rat lenses, Am. J. Physiol. 186: 406,1956.

63. Ross, E. J.: Insulin and the permeability ofcell membranes to glucose, Nature 171: 125,1953.

64. Patterson, J. W., and Bunting, K. W.: Un-published data.


Documents

A Review of Glucose Transport in the Lens