Pergamon Prese

Lif~ Soisnaes Vol . 16, pp . 7-30 Printad in th~ U.S .A .

MIDiIREVI&i1 MECHANISMS OF TRANSPORT ACROSS THE PLACENTA :

AN IN VITRO APPROACH

Richard K. Miller and William 0. Berndt The Stein Research Center, Jefferson Medical College, Philadelphia, Pennsylvania 19107, and the Department of Pharniacology and Toxicology, Dartmouth Medical School, Hanover, New Hampshire 03755 The passage of nutrients, drugs, teratogens and carcinogens from the mother to the conceptus have been of interest for some time, yet only in the past decade has there been a critical evaluation of the movement for some of these compounds.

In 1960, Ernest Page posed the question : "Does compound X cross the

placenta in quantities (per unit of time) which have any nutritional or physiologic significance?" (1) . being investigated . quire explanation . processes involved?

Fourteen years later this crucial question is still

Besides the general question, more specific queries reHow do materials cross placentae?

Are active transport

Must substances be metabolized before passage?

Can other

agents or environmental influences alter the movement of nutrients, etc .?

Of

what importance is the visceral yolk sac (VYS) compared with the chorioallantoic placenta

(CAP)?

All of these questions deal with the problem of the phys-

iologic status of the placenta and associated membranes as involved in the transit of material

from the maternal

blood supply into the conceptus .

In this review, these questions are examined with data from in vitro technics, especially the slice technic,

and these observations are compared with

the results from in vivo experimentation (Tables 1 and 2) .

The further char

acterization of the transport mechanisms for a number of substances

(nutrients

and xenobiotic agents) are discussed applying the following in vitro technics . Morphology During gestation, two sites develop for the transfer of compounds, the visceral yolk sac (VYS) and the chorioallantoic placenta (CAP) . 7

Either one or

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Mechanisms of Transport Across the Placenta

Vol . 16, No . 1

both placentae may play an active role in the movement of substances . cant morphological differences exist between these as well of various species .

Signifi-

as between placentae

The human CAP consists of trophoblastic cells (syncytio-

trophoblast and cytotrophoblast), mesenchymal cells and endothelium .

The ro-

dent placenta has either two (rabbit) or three (rat, mouse) trophoblastic layers, while the term human placenta has one, i .e ., hemomonochorial .

Substances

have fewer cell layers to penetrate in the rodent visceral yolk sac than in the chorioallanotic placenta .

The absorptive surface for the visceral yolk sac is

a simple columnar epithelial layer resting on the basement membrane and mesenchyme, which separates the uterine space from the fetal vitelline vessels .

For

further information concerning placental morphology, the reader is referred to publications by Mossman (2) and Björkman (3) . Besides these two different types of placentae, there are numerous variations within a given gestational period .

In man, the function of the VYS is

minimal in maintaining the homeostatic environment of the conceptus during the last trimester.

The visceral yolk sac of the rodent is functional during this

period, however, and has differential permeabilities at term for certain proteins (4,5) .

Thus, when one considers the movement of a substance across these

membranes it nurst always be in relationship to gestational age and species under study .

In addition, the possible metabolism of the substance by the pla-

centa is another important consideration (6) . In Vitro Systems A variety of in vitro technlcs can be used to study the movement of substances through placental tissues from different stages of gestation without exposing either mother or child to experimental dangers .

At present, there are

three general categories of in vitro procedures being used : A) the perfusion of the isolated placenta, B) the Ussing chamber, and C) the incubation of tissue sections . A)

Isolated Perfused Placentae In general, three procedures are employed : I) the isolated human placenta

Vol . 16, No . 1

at term (7,8,9), rat yolk sac

Mechanisms of Transport Acrose the Placenta 2)

the isolated human cotelydon (10,11), and

3)

9

the isolated

In the first instance, there are no maternal vessels

(12,13,14) .

available for cannulation .

Only the umbilical vessels can be cannulated, while

the maternal side is bathed in a separate pool of medium .

Since the CAP has

two different blood supplies, cannulation of both maternal and fetal vessels have been achieved in situ using the guinea pig (15,16,17) .

These in vitro

technics have allowed the investigator to better control the circulation and movement of compounds both in the maternal and fetal directions . These perfusion systems will produce, metabolize and transport estrogens (18,19)

and catecholamines

(20)

as well as utilize oxygen and glucose

Directionality of transport can be determined by infusion of compounds

21) .

directly into either the maternal or fetal pools . (20)

(12,19,

and Gautieri and associates

(22,23,24)

Morgan, Sandier and Panigel

have studied a number of substances

(serotonin, bradykinin, catecholamines) which alter blood flow, and have found that these substances do alter the passage of materials .

It should be empha-

sized that one of the major difficulties with the isolated perfusion technic is the requirement for a near-perfectly delivered placenta with large fetal vessels and intact chorionic membranes, otherwise poor perfusion and leakage may occur . Nesbitt et al .

(21)

have noted that only 10~ of the placental material

available is usually suitable for investigative purposes because of tissue damage .

Also low fetal perfusion rates have presented problems .

Yet even with

these technical difficulties, many biochemical parameters can be varied, and measurements performed to give data about the unidirectionality and the rate of transfer of substances .

In addition, the influence of vascular supply as well

as biotransforniation of hormones, nutrients and drugs can be evaluated . Further refinements by using the isolated cotelydon instead of the whole placenta have eliminated many of the technical problems associated with the perfusion of the whole placenta (11) .

This reduction in area allows for the

increased opportunity of selecting a more viable intact tissue .

With these

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Mechanisms of Transport Acmse the Placenta

Vol . 16, No . 1

procedures different characteristics for the movement of certain related compounds were found,

e. g.,

L-amino

acids

transferred from the mother to the

conceptus across the human placenta more rapidly than were the D-isonmers (11) . With the third technic, a 12 day embryonic unit consisting of embryo and visceral yolk sac is ligated at the point in closest contact with the chorioallantoic placenta, thus eliminating the umbilical circulation .

This unit is

then placed in a bath of radioactive medium and the movement of substances is monitored both into the VYS and into the embryo .

The exposure of the embryonic

unit in vivo to xenobiotic agents, such as trypan blue, has been found to alter the in vitro movmient of ions (13) across the rat VYS .

In addition, after ex-

posure to chlorambucil, the concentration of L-valine within the rat embryo increased (14) ; however, the incorporation of L-valine into protein was not evaluated, so that the meaning of the amino acid concentrations within both yolk sac and embryo are in question .

In addition, sesame oil also appears to alter

the function of this isolated VYS-embryo unit (14) .

This technic may prove

useful for evaluating early embryonic function, yet the system requires further development and critical analysis . Except for the VYS studies just reported, little work has been done with placentae from the first and second trimester .

More information is needed to

evaluate the effects of environmental agents on placental function during this critical period of organogenesis .

For further inforniation concerning these

perfusion systems, the reader is referred to reviews by Dancis

(7) and Cedard

(9) . B)

Ussin9 Chamber Another technic which offers the investigator the opportunity to measure

transport characteristics, particularly the directionality of movement, is the Ussing chamber.

This chamber consists of two pools of bathing medium, each

bubbled with oxygen and separated by the tissue under study.

The tissue must

be a relatively sturdy, flat sheet that can be clamped between the two pools . Toad bladder and frog skin have been used extensively in this system (25) .

Of

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Machaniama of Transport Aoross the Plnoenta

Vol. 16, No . 1

the tissues relevant to this review the chorion laeve, yolk sac, and amnion have been tested . Using a modified Ussing chamber, Seeds and associates have studied the movement of water through the chorionic laeve in relation to osmotic pressure, as well as the penetration of many other substances (26,27,28,29) .

In part

these studies have been directed toward an understanding of intrauterine water accumulation, even though there is no difference in the total solute concentration across the placenta in vivo .

Seeds et al . have found that this phenomenon

may be explained by an unequal distribution of small solutes with different reflection coefficients on either side of the chorion .

Further, by measurement

of activation energies it was demonstrated that antipyrine appears to penetrate through the chorionic laeve by a combination of transcellular and extra-cellular diffusion, while the movement of p-aminohippurate, salicylate, and tritiated water were consistent with passage through mainly water filled extra-cellular channels (26) . With further modifications of the Ussing cell, McGaughey et al . (30) and Moore and Ward (31) have found that creatinine also crosses very slowly, and in fact, much slower than urea .

Furthermore, sugars also appear to be passively

transferred across these membranes (32) . Employing similar technics, Deren et al . (33) found that the VYS from rabbits transported amino acids.

L-valine was concentrated on the fetal side to

levels 76x higher than those on the maternal

side of the VYS, while the cells

of the VYS itself concentrated L-valine to 7.5 times the bathing solution on the endodermal side .

These studies have demonstrated the concentration of L-

valine within and across the membrane, specificity, and saturation of the transport process .

Thus it appears that both the VYS as well

as the CAP may be in-

volved in maintaining the homeostatic environment. The placenta may also play an important role in ion movanent, as alluded to by Seeds et al . (26,28) .

Sodium movement across the placenta in vivo ap-

pears to be regulated by pernieability characteristics of the tissue rather than

12

Mechanisms of Transport Across the Placenta

by limitations of blood flow (34) .

Vol . 16, No . 1

Crawford and McCance (35), using the iso-

lated chorioallantoic membranes of the swine, measured a small potential difference across the tissue in the direction that favored sodium transport from the fetus to the mother .

This observation could account for the maintenance

of fetal homeostasis by removing excess sodium, generating the so-called large sodium safety factor (36,37,38,39) .

More recently active sodium transport by

the chick chorioallantoic membrane was observed by Stewart and Terepka (40) . This tissue exhibits a membrane potential which changes as gestation progresses (at 6-7 days, -3 to -5 mv ; at 17 days, -40 mvr) .

Using human membrane, Knapowski,

Feliks and Adam (41) observed a potential difference as well as directional sodium transport from fetus to mother .

All of these in vitro studies indicate

the chorion laeve possesses an active polarized sodium transport system, which except with the chick, might indicate a direct influencé on the amniotic-mater nal exchange .

In addition, Brame (42) has found that sodium moves rapidly from

the amniotic fluid compartment to the maternal

compartment, when hypertonic sa-

line solutions are injected into the artmiotic side . this situation is across the amniochorion .

The sodium movement in

Since these membranes arise from

the same tissue as the frondosum, a transport system for sodium across the CAP must be entertained seriously. C)

Slice Technic A wide variety of animals is used for teratogenicity and carcinogenicity

studies, and it is desirable to have procedures for studying the movement of both nutrients and xenobiotic agents by placentae at different stages of gesta tion as well as in different species . perimental

Placental slices are usable for many ex-

situations, and obviate many of the technical problems associated

with perfusion studies, e.g ., perfusion of a day 12 rat placenta . The slice technic can be employed to estimate the ability of a membrane, whether CAP or VYS, to accumulate nutrients and to have this uptake influenced by environmental factors and drugs . for human therapy.

This may prove important in drug testing

For example, in a given animal species it will

be possible

Vol. 16, No . 1

Mechanisms of Transport Across the Placenta

13

to establish the relationship between slice uptake or runout and the in vivo behavior of a substance or group of substances .

This inforniation can be ex-

trapolated to the human situation on the basis of human placental slice studies. Thus in vitro , one can evaluate the movement of a substance in the human p1acents and its membranes without exposing either mother or child to the compound . The slice technic consists of incubating thin (less than 0.5 mm) tissue sections in a balanced salt medium (e .g., Krebs-Ringer phosphate or bicarbonate buffers) under carefully controlled conditions of temperature, atmosphere and pH .

After appropriate time intervals, the uptake of specific substances can be

monitored chemically or radiochemically in the slices .

In the same or parallel

experiments, tissue oxygen consumption, inulin space and tissue electrolytes can be measured .

With these technics a large number of variables can be stud-

ied with a single placenta .

Technical details are available from a number of

papers (43,44,45) . One major limitation to the slice technic is the Inability to determine the directionality of transport, i .e ., whether a substance is moved from mother to conceptus or vice versa .

However, since the bathing medium is exposed to a

much greater surface area on the maternal

side of the v111us than on the fetal

capillary side, a first approximation would be to suppose that the movement into the villus tissue slice would represent movement from mother to fetus .

For

absolute certainty, in vivo data must be used to define the directionality of transport; however, kinetics of transport, requiréménts for ions, energy sources, interaction of xenobiotic agents, specificity of transport processes, etc ., can certainly be easily evaluated by the in vitro technics . 1.

Active Transport Movement of a substance against its concentration Zor ideally, electro-

chemical) gradient is an important criterion

for

active transport.

This cri-

tenon necessarily excludes metabolism or tissue binding of the compound . general, such a process requires an input of cellular energy . ~A number of

In

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Mechanisms of Transport Across the Placenta

Vol . 16, No . 1

substances have been found to be concentrated by placental tissues, e .g., amino acids, acetylcholine, creatine, iron, vitamin B12 , and ions . a.

Amino Acids .

Slice technics have been used with various tissues, e .q.,

kidney (46,47,48,49), brain (50,51) and intestine (52,53), but only recently have these procedures been applied to the study of fetal membranes .

In 1967,

Litonjua et al . (54,55) and Dancis et al . (56) examined amino acid uptake by placental slices and found that a non-metabolizable, neutral amino acid, aaminolsobutyric acid (AIB), was accumulated by human and guinea pig CAP .

Sy-

bulski and Tremblay (57) examined transport and incorporation of glycine into proteins in the human term placenta .

They found that placentae concentrated

amino acids and that the accumulation depended on metabolic energy, both aerobic and anaerobic .

For example, high concentrations of sodium fluoride and

iodoacetic acid (inhibitors of glycolysis) reduced glycine uptake (57) .

Longo

et al . (43), in addition, found that arsenate reduced uptake of AIB, and that glycogen stores within human placentae were influential in maintaining the uptake process .

In this laboratory, oxidative inhibitors, e .g., nitrogen or di-

nitrophenol, and different metabolic substrates, e.g., acetate and glucose, were used .

As might be expected, AIB uptake proceeded in the oxygen atmosphere

when either acetate or glucose was present; however, in the presence of nitrogen or dinitrophenol, only glucose appeared to maintain uptake near normal levels (45) . Metabolizable amino acids (glycine) can be incorporated into proteins . The importance of this phenomenon was evaluated by Sybulski and Tremblay (57) . Puromycin, an inhibitor of protein synthesis, added at the time of incubation did not significantly reduce glycine uptake by the human term placenta, while the incorporation into proteins was decreased by 70~. The amino acid transport system of the CAP is dependent on both sodium and potassium ions ATPase .

(45), as well as a Mg-dependent, sodium + potassium-activated

Transport ATPase was identified in the human term placentae and found

to be sensitive to ouabain (58) .

Ouabain has also been found to inhibit amino

Vol . 16, No . 1

15

Mechanisms of Transport Across the Placenta

acid transport (57,45) .

In addition, Smith et al . (44) and Miller and Berndt

(45) have found that these transport processes conform to Michealis-Menten kinetics, 1f the diffusional or binding component is eliminated either mathematically (59) or by studying the differences between control and inhibited conditions (ouabain) . The specificity of the transport mechanism is emphasized by competition . studies .

For example, AIB uptake is blocked by glycine ând L-alanine, but not

by D-alanine, lysine (a basic amino acid), glutamic acid (an acidic amino ac id), creative or N-amidino alanine (45, unpublished observations) .

These last

two compounds were tested because of evidence that, at least in the rat, creative is concentrated in fetal blood as well as in embryonic membranes (60,61, 62) .

Both serine and alanine were found to inhibit glycine uptake by human

placentae ; however, phenylalanine, histidine, lysine and glutamic acid did not demonstrate such inhibition (57) .

These data indicate that there is a specific

L-neutral amino acid transport system in the human term placenta . Data derived from 13-15 week human placentae have demonstrated that this tissue will concentrate AIB to levels 2-3x as high as the term placentae (unpublished observations) . Other studies (63) failed to show amino acid accumulation by the VYS in the rat, although in this laboratory, AIB was concentrated by the VYS from both rat (64) and rabbit (65) .

The difference in these results may be attributed to

the time period for accumulation of valine (63) being too short in the negative study .

Longer incubations in the AIB studies did show accumulation against a

gradient which required energy and was specific for neutral amino acids (64) . Also a comparison of arAino acid to sugar uptake (63), where sugars move passively, demonstrates that the tissue amino acid levels were much larger than those of the sugar after 30 minutes of incubation in the rat YYS .

Perhaps some

of this increase could be accounted for by incorporation of valine into proteins .

These data are suggestive of a sluggish active transport process for

amino acids in the rat VYS .

These in vitro observations agree with in vivo

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Mechanisms of Transport Across the Placenta

Vol . 16, No . 1

accumulation of AIB in fetal blood as well as in the pregnant rat (B .M . Davis and T.R . Koszalka, personal communication) .

Butt and Wilson (66) using the

guinea pig VYS sections found that both lysine and valine were concentrated in the tissue and that dinitrophenol

(1 mM) reduced uptake by 45~ .

Also the guin-

ea pig VYS has also been found to concentrate lysine and valine to much higher levels early in gestation (66) . trates amino acids (67) .

In addition, the chicken yolk sac also concen-

Thus for both the CAP and VYS, there appear to be

transport processes for amino acids that are specific and are dependent on energy input . b.

Acetylcholine .

This substance has also been found in higher levels

within the villus of the human CAP, than in the medium . neural

There appears to be no

innervation to this organ which can account for these levels, although,

choline acetylase is present (68) .

Welsch (69) has most recently reported that

N-methyl-3H-acetylcholine was concentrated in the human CAP, when cholinesterase inhibitors were present . requiring energy input .

This uptake was temperature dependent as well as

Dinitrophenol, sodium cyanide and ouabain all depressed

uptake as did the replacement of sodium with lithium and rubidium .

Thus, there

does appear to be a concentrating mechanism for acetylcholine, although the physiological importance and the specificity of the carrier have not been determined . c.

Creatine .

(70) as well

14 C-Creatine was accumulated by human term placental slices

as rat. CAP and VYS (64), and the uptake process was found to have

similar energy and ion requirements as for AIB .

However, N-amidino alanine re

duced creatine uptake, while the neutral amino acids did not interact specifically .

Although AIB has been studied in the rabbit placenta and VYS, creatine

has not (65) .

Even though these transport processes appear to be present in

many different species, there are the variations within a gestational period . As seen with AIB, creatine transport by embryonic membranes was also more rapid earlier in gestation. greater .

Also, the total amount of material accwnulated was

For example, the 13-15 week human CAP, the 12-14 day rat placentae

Vol . 16, No . 1

17

Machaaiams of Transport Across the Placenta

and VYS, all accumulated creative to levels 2-3 times those seen at term (64, 70, unpublished observations) .

In vivo studies with other compounds such as

LSD (71) and tetrahydrocannabinol

(72) have shown that these compounds are more It appears, therefore, that creative

rapidly transferred early in gestation .

is concentrated by the placenta and the process is an energy dependent one, and specific for N-amidino compounds . d.

Iron and Transferrin .

59 Fe and

125

Laurell and Morgen (73) studied the uptake of

I_transferrin in the CAP of the 20 day pregnant rat.

uptake was initially rapid, and plateaued by 30 minutes .

Transferrin

The amount of trans

ferrin in the bathing medium did not influence the rate of uptake of iron . Only 3~ of the iron accumulated by the slices was converted to ferritin, as determined by precipitation by anti horse-ferritin serum.

In addition, more re-

cent work by Garrett et al . (74) has confirnied the Laurell and Morgen study in that arsenite, ethacrynic acid and rotenone did not reduce the uptake of iron by 16 day rat CAP, which reached levels many fold higher than in the medium Besides studying the CAP, Laurell and Morgen (73) also investigated iron

(69) .

and transferrin accumulation by the VYS .

The VYS bound twice the amount of

transferrin per gram of tissue than did the CAP, and the rate of binding of iron was 2-4 times that of the CAP.

Metabolic inhibitors (cyanide, fluoride

(73), ethacrynic acid, rotenone and arsenite (74)) reduced iron uptake to a much larger extent in the VYS than in the CAP .

In fact, dinitrophenol (5 mM)

did not reduce transferrin uptake by CAP, while it decreased accumulation by VYS to 60~ of control (74) .

These data may mean that the VYS has the more ac-

tive iron transport system of the two placentae . e.

Vitamin B12 .

Yltamin B12 and vitamin B12 -plus-intrinsic factor are

accumulated by both rat (63) and rabbit (75) placentae and VYS .

This accumula-

tion in vitro by rat placentae agree with the observed in vivo high concentra tions of vltanin B12 in the YYS (76) .

Vitamin B12 accumulation in the rabbit

YYS did not vary with the age of gestation from 5 to 30 days ; however, uptake in the rat (65) was decreased late in gestation.

If the YYS was incubated with

18

Mechanisms of Transport Across the Placenta

Vol . 16, No . 1

both vitamin B12 and intrinsic factor, there was a 6-7 fold increase in uptake of vitamin B12 in the rabbit with age of gestation (75), but not in the rat. Dinitrophenol (1 mM) inhibited uptake, while glucose abolished the dinitrophenol inhibition .

Also iodoacetate (1 mM) inhibited uptake (63,75) .

Even though

the uptake of vitamin B12 decreased toward term in vivo , the amount of vitamin B12 crossing into the fetus actually increased (76) .

This in vivo observation

can possibly be accounted for by the dramatic increase in surface area leading to a 7 fold Increase in transfer capacity for the vitamin BlZ-intrinsic factor complex (75) .

It appears, therefore, that vitamin Blp uptake by the VYS of

both rat and rabbit is stimulated significantly by the presence of intrinsic factor, and is dependent on both aerobic and anaerobic energy sources.

The

mechanisms of transport are not resolved, but do appear to have different relationships than found for iron and transferrin (73) . f.

Ions .

Berger and Yan Hornstein (77), and Widdowson and Spray (78) in

1951 reported sodium and potassium values for human fresh term placentae to be approximately 100 mEq/Kg and 40 mEq/Kg respectively .

These tissues were washed

in distilled water before the determinations were done, which may have caused some loss of the intracellular ions .

Amore recent study (79) measured 9 dif-

ferent can ons and found that sodüan, potassium and calcium content were altered in placentae from toxemic patients ; however, the sodium and potassium values in this study (79) do not agree with other data (77,78, unpublished observations) .

The sodium content was many fold higher, while the potassium content

was 2/3 lower .

Perhaps some of these differences can be attributed to the

samples being frozen with no attempt having been made to wash out the blood . It is doubtful, however, that these differences in procedure would account for the extremely large sodium values .

Studies in this laboratory revealed, as

might be expected, that dinitrophenol (10

-4 M), iodoacetamide (10-3 M) and oua-

-5 M) bain (10 altered the ion content of the human placental slices, as did alterations in pH below 7 and above 8 .

Similar pH changes depressed amino acid

transport for the human placental slices (45) .

Ouabain altered the ion levels

Vol. 16, No . 1

19

Machanismo o! Transport Across the Placenta

in rabbit placentae as well (unpublished observations) . These Garments refer to total tissue ion content, and give no inforniation about free versus bound ions, or the possibility of compartmentalization . Thus, the reason for the relatively high tissue sodium levels may reflect ei ther 1) that the placenta has a different ion distribution from other epithelial transporting tissues or 2) that the tissue was not homogenous and had other materials either within or without the villus which bound or compartmentalized the sodium thus increasing the apparent intracellular free sodium . Diffusional Processes

2.

There are many substances which do not have active, specific transport processes in the placenta .

The levels of these compounds inside the cell is

determined by virtual equilibration with the external medium .

There are at

least two different processes under the division of diffusion : 1) simple, where a substance is distributed according to Fick's law, i .e ., in the direction of its electrochemical gradient, and 2) facilitated diffusion, where the final distribution is achieved more rapidly than can be accounted for by simple diffusion .

In addition, for facilitated diffusion, there is a selectivity for the

movement, yet no movement against a gradient concentration of the substance is evident. Miong the compounds tested to date with these characteristics in the human and rat CAP, and rat VYS, are creatinine, urea, antipyrine and inulin (unpublished observations,

64) .

Inulin

(56,43,44,45)

was used as an extracellular

raster space marker, while creatinine, urea and antipyrine reflected the distribution of total tissue water. Although sugars are transferred across the placenta in vivo , they do not appear to be accumulated actively by placental slices (80) .

a-Methylglucoside

was not accumulated by the VYS from either rabbit (75), guinea pig

(66)

or rat

(63) ; however, some studies have demonstrated a specificity to sugar movement suggesting a facilitated diffusional process (Longo, personal communication) . Furthernare, since the glucose concentration intracellularly does not exceed

20

Mechanisms of Transport Across the Placenta

Vol . 1 6, No . 1

the bathing medium concentration (80), these data are consistent with sugar transfer by facilitated diffusion.

A possible exception to this generalization

comes from the work of Holdsworth and Wilson

(67), who observed active accumu-

lation of sugars by the VYS of the chicken . Since the placenta may be considered to act lüce a fetal kidney, one might anticipate the transport of organic anions and can ons .

p-Aminohippurate and

tetraethylammonium (substances known to be transported by renal organic anion and cation systems) were studied (81) ;~and neither was found to be accumulated against a gradient by human term placentae, rabbit placentae or VYS in the presence of acetate, glucose or succinate (65, unpublished observations) .

Mc-

Nay et al . (82) using in vivo perfusions of sheep placentae found that neither p-aminohippurate nor tetraethylammonium moved differently than antipyrine in either the maternal to fetal direction or vice versa . 3.

Effects of Hormones and Xenobiotic Agents Using the human term placenta, it has been observed that neither placental,

bovine or human growth hormone (56), lactogenic hornane (56), cortisone (54) nor human chorionicgonadotropin (54) altered amino acid movement . maining hormones studied, progesterone

Of the re-

(0 .03 mM) and testosterone (0 .03 mM) de-

pressed glycine uptake by 20% in the human placenta scattered effects on glycine accumulation (57) .

(57), whereas estradiol had

In low concentration estradiol

enhanced the incorporation of glycine into placental proteins, while testosterone and progesterone inhibited the incorporation (57) . reported no influence of estradiol ug/ml) .

Others (54,56) have

(3 ug/ml or 10 ug/ml) or estrone sulfate (3

Tseng et al . (83) found estrone, estrone sulfate, estradiol and pro-

gesterone to be metabolized by placental slices . The influence of insulin on amino acid or sugar transfer has also been studied in placental slices .

Litonjua et al . (54) have reported that the addi-

tion of either 0.1 or 0 .57 units/ml of insulin had no effect on AIB uptake by the human term placenta, while Dancis et al .

(56) did report an increase in up-

take although the insulin concentration was not given .

The guinea pig placental

Vol . 16, IQo . 1

llachanisms of Transport Across the Placenta

slices were unaffected by insulin (56) .

21

Most recently, Longo and Yuen (person-

al communication) have found that over an extremely wide range of insulin concentrations the uptake of AIB by the human term placenta was unaffected .

The

uptake of both sugars and amino acids in the fetal diaphragm of the rat was markedly increased by insulin (84) . Sugar movement and metabolism in the placenta were stimulated by insulin (85,86) ; however, Battaglia (80) and Szabo and Grimaldi effect of insulin .

(87) did not find an

One possible complication with all of these studies is the

high insulinase activity of placental tissue itself (88) .

It is not clear just

how much of an effect this has on the insulin concentration in the bathing solution, but this may account for some of the differences reported . Recent studies have shown that ethacrynic acid, a diuretic, which inhibits transport ATPase as well as Ca-ATPase in the placenta (58), reduced the uptake of AIB and creatine by term and early human placentae (45, unpublished observa tions), as well as iron transport in the VYS (74) .

Increased sodium and reduc-

ed potassium content of human term placentae have also been observed after ethacrynic acid addition to the bathing medium (unpublished observations) .

Thus

even though the specificity of action of ethacrynic acid is in question, this diuretic does have an effect on transport processes in the placental slice preparation . Some known teratogens have been studied in vitro .

Trypan blue did not re-

duce the accumulation of AIB, or alter the ion content by the human term placenta (unpublished observations), but did alter ion movement across the rat VYS in vitro after in vivo administration

(13) .

The teratogenic effects of serotonin have been reported by Robson and Sullivan (89) .

It is not clear, however, whether these effects were due to an al-

teration of placental permeability or to an action on the uterine vasculature . The slice system allows one to investigate one of these possibilities .

Sero-

tonin at 10 -5 to 10 -3 M did not alter the accumulation of AIB, creatine or the sodium and potassium content in the human term placenta (90, unpublished

Mechanisms of Transport Across the Placenta

22

observations) .

Vol . 16, No . 1

Thus it appears that serotonin does not have a direct effect on

placental uptake of these substances .

However, the constriction of the uterine

vessels due to serotonin may cause alterations in placental pern~eability by means of anoxia or pH changes.

For example, it has been noted in vitro that if

the pH is below 7.2 or above 8 .0, there is a significant depression of the above mentioned uptake processes (45) . Besides the Influence of xenobiotic agents on transport processes and placental

function, pathologic conditions may also alter these .

(39,91,92), as well

Sodium movement

as fetal amino acid levels (93), have been reported to be

altered under hypertenslve, pre-eclampsic conditions, or when the child is growth-retarded .

However, there are conflicting reports (94,95) .

Using in

vitro technics, Friedman et al . (96) have found a reduced oxygen consumption in placental slices from pre-eclampsic patients .

Tremblay et al . (97) found the

placentae in situations of retarded intrauterine growth showed an even lower oxygen consumption than did the larger category of toxemic patients . In a preliminary study from this laboratory consisting of only three placentae from pre-eclampsic patients, the oxygen consumption was reduced but was still within the lower limits of normal . and hypertension .

These patients had edema, albuminuria

Histologic examination of the incubated placental slices,

(which were grossly selected from regions not containing infarction or massive Ca deposition) demonstrated increased syncitial knots and a small amount of Ca deposition .

These are usual findings for pre-eclampsic placentae .

The inulin

space was not significantly increased ; however, the amino acid uptake by the slices was depressed significantly .

Placentae from patients with regulated

diabetes mellitus gave results similar to nornmls .

Thus, from these initial

experiments, there appears to be a reduction in the uptake of AIB in the preeclampsic placenta, but not in the placenta from regulated-diabetic patients . Sutmiar Using the in vitro approach, it appears certain aspects of placental function can be examined .

For example, studies of the mechanisms for transport of

Nechaniama of Transport Acroae the Placenta

Vol. 16, No . 1

both nutrients and xenobiotic agents have been possible .

23

Data presented here

and elsewhere have shown that sodium, amino acids, acetylcholine, creative, vitamin B12 , and iron are accumulated actively by embryonic membranes.

Certain

organic anions and cations, creatinine, urea, antipyrine, and tritiated water are passively transferred . chemical

gradient

by

Sugars also appear to be moved with their electro-

a specific facilitated diffusional process .

Almost

all of these observations do agree with the in vivo studies, wherein either transplacental movement or placental accumulation is the criterion studied . Tables 1 and 2 present these correlations for both actively and passively handled substances .

With the exception of a few situations where equivocal re-

sults were noted or no data are available, 1t appears that placental slice uptake to intracellular : extracellular distribution ratios > 1 .0 is an accurate representation of active transplacental movement of a variety of substances . When the net slice intracellular : extracellular ratio is 1 .0 or less, the data appear to indicate a passive transfer (Table 2) . Acknowledgements_ The authors gratefully acknowledge the assistance of Drs . T. Koszalka and R. Brent throughout this endeavor and in addition, express their appreciation to Drs . K. Chepenlk, B. Davis, and E . Johnson for their critical evaluations of early drafts . The work of the authors discussed in the manuscript was supported in part by the following grants from the National 06360 and HD 370.

Institutes of Health : AM 13020, HD

24

Mechanisms of Transport Across the Placenta

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25

Mechanisms of Transport Across the Placenta

References l . E . Page, Modern Med.

1(Feb.), 26-32(1960) .

2. H .W . Mossmnn, Carne9le Inst . Wash . 26, 130-239(1937) . 3. N . Bjorkman, An Atlas of Placental Fine Structure, the W1111ams b Wallons Co ., Baltlmore (1970) . 4. F.W .R . Bromhell, W.A . Hennings and M. Henderson, Antibodies and Embryos , the Athlone Press, London (1951) . -unq, 5. F.W .R . Brambell, The Transmission of Passive Innunlt~ from Mother to Yo , ortTi l~aPu~tilis~6~ng~ Frontiers of Biology , o . .o ., fer~am~70) 6. 0. Pelkonen and N .T . Karki, Life Sçi . 13, 1163-1180(1913) .

m

7. J . Dancis, Acta Endocrinol . 158 suppl ., 347-361(1972) . 8. K.E . Krantz, T.C . Panos and J . Evans, Amer . J . Obstet . 1227(1962) .

nec . 83, 1214-

9. L . Cedard, Acta Endocrinol . 158 suppl ., 331-343(1972) . 10 . M. Panigel, Amer . J . Obstet .

nec . 84, 1664-1682 (1962) .

11 . H. Schneider, M. Panigel and J . Dancis, Amer . J . Obstet . 828 (1972) .

ec . 114, 822-

12 . M .L . Netzloff, K.P . Chepenik, E.M . Johnson and S . Kaplan, Life Sçi . 401-405(1968) .

_-7,

13 . M .M . Kernas and E .M . Johnson, J . Embryol . ~. Morph . 22, 115-125(1968) . 14 . M .M . Kernas, Experientia

27, 1329-1331(1971) .

15 . W .L . Money and J . Dancis, Amer . J . Obstet .

ec .

80, 209-214(1960) .

16 . K. Thomsen, H. Schniewind, H . Schultz-Mosgan, M. Kramer and U. Jost, Arck . Gynak. 203, 226-233(1966) . 17 . K. Thamsen, H. Schniewind, and H . Schultz-Mosgan, Arck . Gynak. 58(1967) . 18 . E. A1sat and L . Cedard, Prosta9landins

204, 51-

3, 145-153(1973) .

19 . J. Varangot, L Cedard and S. Yannotti, Amer. J. Obstet . Gynea ~ 534-547 (1965) . 20 . C .D . Morgan, M. Sandler and M. Panigel, M~er . J . Obstet . 1075(1972) .

ec .

ls,

1068-

21, RrE .L . Nesbitt, Jr ., P .A, Rice, J,E, Rourke, V,F . Torresi, and A,M, Souchay, nec. Invest . 1, 185-203(1970) . 22 . C.O . Ward and R .F . Gautieri, J . Pharm. Sçi .

57, 287-292 (1968) .

23 . C .O . Ward and R .F . Gautieri, J . Pharm . Sçi .

55, 474-478(1966) .

24 . W .T . Sherman and R.F . Gautieri, J . Pharm . Sçi . ~, 878-883(1972) .

Z6

Mechanisms of Transport l~crosa the Placenta

Vol . 16, No . 1

25 .

H. H. Ussing, Açta P

siol . Scand . 17, 1-37 (1949) .

26 .

A . E. Seeds, J . J . Schruefer, J. A. Reinhardt and K. D. Garlid, Invest . 4, 31-37 (1973) .

27 .

S. J . Lloyd, K. D. Garlid, R. C. Rebe and A. E. Seeds, _J . Appl . P 26, 274-276 (1969) .

28 .

A. E . Seeds, Amer . J . Obstet .

29 .

A. E . Seeds, Amer . J . P

30 .

H. S. McGaughey, Jr ., E. L . Corey, W. A. Scoggin, C. H . Ficklen and W . N. Thornton, Jr ., Amer . J . Obstet . ec . 80, 108-113 (1960) .

31 .

W. M . 0. Moor e and B. S. Ward, Amer . J . Obstet .

32 .

W. M. 0 . Moore, A. E. Hellegers and F . C. Battaglia, Amer . _J . Obstet . nec . 96, 951-955 (1966) .

33 .

J . J . Deren, H. A. Padykula and T. H. Wilson, Devel . Biol . 13, 370-384 (1966) . -

34 .

F. C. Battaglia, R. E . Behrman, G. Meschia, A . E . Seeds and P . D. Bruns, Amer . J . Obstet . nec . 102, 1136 (1968) .

35 .

J . D. Crawford and R. A. McCance, J . P

36 .

L . B. Flexner and A. Gellhorn, Amer . J . Obstet .

37 .

L . B. Flexner, D. B. Cowie, L . M. Helluran, W. S. Wilde, and G . J . Vosburgh, Amer . J. Obstet . nec . 55, 469-480 (1948) .

38 .

L .W . Cox and T.A . Chalmers, J . Obstet .

nec . Brit . Emp. 60, 203-213 (1953

39 .

L .W . Cox and T .A . Chalmers, J . Obstet .

ec . Brit . ~. 60, 214-221(1953

40 .

M . E. Stewart and A. R. Terepka, Exptl . Cell . Res . 58, 93-106 (1969) .

41 .

J . Knapowski, M. Feliks and W . Adam, Ginekol . Polska 43, 283-297 (1972) .

42 .

R . G. Brame, Amer . J. Obstet .

43 .

L . D. Longo, P . Yuen and D. J . Gusseck, Nature 243, 531-533 (1973) .

44 .

C . H. Smith, E . W. Adcock III, F. Teasdale, G. Meschia and F. C . Battaglia, Amer . J. P siol . 224, 558-564 (1973) .

45 .

R . K. Miller and W . 0. Berndt, Amer . J . Ph.Ysiol . IN PRESS (1974) .

46 .

R . P. Forster, Science 108, 65-67 (1948) .

47 .

G . H. Mudge, Amer . J . Physiol . 165,

48 .

L . E . Rosenberg, A . Blair and S. Segel, Biothun. Bio (1961) .

49 .

W .O . Berndt and E. C . Beechwood, Amer . J . P

nec . sio1 .

ecol . 98, 568-571 (1967) .

siol . 219, 551-554 (1970) .

ec . 108, 635-637 (1970) .

siol . 151, 458-471 (1960) . ec . 43, 965-974 (1942) .

ec . 113, 1085-1089 (1972) .

113-127 (1951) . s . Acta _54, 479-488

siol . 208, 642-648 (1965) .

vol.

16, No . 1

a~

Mechanisms of Transport Across the Placxnta

50 . P. Battistin, F. Piccoli and A . Lajtha, Arch . Biochem . Biophys . 111(1972) .

151, 102_

51 . W.J . Coake and J .D . Robinson, Biochem. Phann. 20, 2355-2366(1971) . 52 . S .G . Schultz and P .F . Curran, P

sio1 . Rev . 50, 637-718(1970) .

53 . G .A . Kammich, Biochim. Biophys . Acta

300, 31-78(1973) .

54 . A .D . Litanjua, M . Canlas, J . Soliman and D .Q . Paulino, Amer . J . Obstet . ec . 99,242-246(1967) . 55 . A .G . Ranualdez, Jr . and A .D . Litanjua, Amer . J . Obstet . (1971) .

ec . 109, 1225

56 . J . Dancis, W .L . Money, D . Springer and M. Levltz, Amer . J . Obstet . 101820-829(1968) . 57 . S . Sybulski and P .C . Tremblay, Amer. J . Obstet .

ec .

ec .

97, 1111-1118(1967) .

58 . R.K . Mailer and W .O . Berndt, Proc . Soc. Ezptl . Bio1 . Med . i4 , 118-122(1973) . 59 . H . Akedo and H .N . Christensen, J . Biol . Chem . 237, 118-122(1962) . 60 . T.R . Koszalka, R.P . Jensh and R .L . Brent, Çam~. Biochem. P siol . 41B, 217229(1972) . 61 . B .M . Davis, T .R. Koszalka and R.K . Miller, Pharmacologist

15, 199(1973) .

62 . R .K . Miller, B .M . Dav1s, R.L . Brent and T .R . Koszalka, Fed. Proc . 33, 223 (1974) . 63 . H .A . Pa~rkula, J .J . Deren and T.H . Wilson, Devel . Biol .

1

311-348(1966) .

64 . R.K . Miller, T.R . Koszalka, B .M . Davis, C.L . Mdrew and R .L . Brent, Terat . IN PRESS(1974) . 65 . R.K . Miller and W .O . Berndt, Fed . Proc . 311 595(1972) . 66 . J .H . Butt II and T.H . Wilson, Amer . J . P sioi . 2~5, 1468-1477(1968) . 67 . C.D . Hol~worth and T .H . Wilson, Amer . J . Physiol . 68 . C .O . Hebb and D. Ratkovic, J. P siol .

212, 233-240(1967) .

1~ 307-314(1962) .

69 . F . Welsch, Fed. Proc . 33, 542(1974) . 70 . R .K . Miller, B .M . Davis, R.L . Brent and T. R. Koszalka, Pharmncologist , IN PRESS (1974) . 71 . J. Idänpâän-1~leikkila and J. Schoolar, Science 164, 1295-1297(1969) . 72 . J. Idänpätln-Kleikklla, 6 .E . Fritchie, L .F . Englert, B .T . Ho and W.M . McIsaac, New Engl . J . Med . 281, 330(1969) . 73 . C .B . Laurell and E. Morgan, Acta Physiol . Scand . 62 , 271-279(1964) . 74 . R.J,B . Garrett, N .E . Garrett and J .W . Archdeacon, Experientia 294, 463-464 (1973) . _

28

Mechanisms of Transport Across the Placenta

Vol. 16, No . 1

75 . J .J . Deren, H .A . Padykula and T.H . Wilson, Devel . Biol .

13, 349-369(1966) .

76 . S .E . Graber, U. Scheffel, B. Hokinson and P .A . McIntyre, _J . Clin . Invest . 50, 1000-1004(1971) . 77 . K. Diem and C . Lenter, Scientific Tables , 7th ed ., Ciba-Geigy Ltd ., Basle (1970) . 78 . E .M . Widdowson and C .M . Spray, Arch . Dis . Ch11dh . 26, 205-214(1951) . 79 . E .B . Dawson, H.A . Croft, R.R . Clark and W .J . McGanity, Amer . _J . Obstet . ec . 103, 1144-1147(1969) . 80 . F.C . Batty lia, G. Meschia, J . Blechner and D .H . Barron, Amer . _J . Physiol . 2=, 64-661961) . 81 . I .W . Weiner and G.H . Mudge, Amer . J . Med . 36, 743-762(1964) . 82 . J .L . McNay, E. Fuller, T . Kishlmoto, E. Malveaux and P .E . Dayton, Proc . Soc . Eue. Biol . Med . 131, 51-56(1969) . 83 . L . Tseng, A . Stolee and E . Gurpide, Endocrinol . 90, 405-414(1972) . 84 . R. Fricke and C .M . Clark, Jr . Amer . J . P

siol . 224, 117-121(1972) .

85 . C .A . V111ee, J . Biol . Chem . 205, 113-123(1953) . 86 . A .D . Litonjua, Acta Med . Philipp, 3, 247-254(1967) . 87 . A .J . Szabo and R .D . Grimaldi, Amer . J. Obstet , 88 . B .I . Posner, Bfabetes

nec . 106, 75-78(1970) .

22, 552-563(1973) .

89 . J .S . Robson and F.M . Sullivan, Proc . R. Soc. Med .

59, 744-748(1966) .

90 . R .K . Miller, W.O . .Berndt, T .R . Koszalka and R.L . Brent, Terat . 91 . T . Johnson and C .G . Clayton, Brit . Med . J . 1, 312-314(1957) . 92 . H . Quigley, L .L . Phillips and D .G . McKay, Meer . J . Obstet . 384(1965) .

1

25(1973) .

ec . 91, 377-

93 . B .S . Lindblad and R. Zetterstrom, Acta Paediat . Scand . 57, 195-204(1968) . 94 . M . Young and M.A . Prenton, J . Obstet . (1969) .

nec . Brit . Commonw. 76, 333-344

95 . H .S . McGaughey, H.C . Jones, L . Talbert and W .P . Anslow, Amer . _J . Obstet . ec . 75, 482-495(1958) . 96 . E .A . Friedrt~an, M .B . Koss, M .R . Sachtleben and E .M . St . John, Amer . _J . Obstet . nec . 107, 1-5(1970) . 97 . P .C . Trembla , S . Sybulski and G .B . Maughan, Amer . J . Obstet . 597-605(1965 .

ec . 91,

98 . M.B . Glendening, ~Transcri~t of the Third Rochester Trophoblast Conference edé, ~$6~95~ed . C .J . Lund and

Vol . 16, No . 1

29

[Mechanisms of Transport Across the Placenta

99 . W .H . Pearce and H . Sornson, Amer . J. Obstet .

nec . 105, 696-700(1969) .

17 5, 95-100(1948) . 100. H .N . Christensen and J .C . Streicher, J . Biol . Chem . = 101 . H .R . Crumpler, C.E . Dent and 0. Linden, Biochem . J . 47, 223-227(1950) . 102. P .M .M . Hill and M . Young, J . P siol . 2~, 409-422(1973) . 103. C . Dierks-Ventling, A.L . .Cone and R.A . Wapnlr, Biol . Neonate _17, 361-372 (1971) . 104. C .J . Hayter, E.A . Hutchinson, M.J . Karvonen and M. Young, _J . P siol . 175 11-13p(1964) . 105 . B.H . Feldman and H .N . Christensen, Proc . Soc . Expt . Biol . Med . 109, 700702(1962) . 106 . E . Knobil, ~Transcri _t of the Third Rochester Trophoblast Conference , ed . Lund, C.J . a~-Fr A . Th~de ~ljj~SS~~ 107 . M .M . Monsour, A.R . Schulert, and S .R . Classer, Amer . _J . P siol . 222, 1628-1633(1972) . 108. C .T . Wong and E.H . Morgen,

uat. J . Expl . P

siol . 58, 47-58(1973) .

109. C .B . Laurell, Acta P siol . Scand . 14 suppl . 46, 77-81(1947) . 110 . T .H . Bothwell, W .F . Pribilla, W . Mebust and C .A . Finch . Amer . J . Physiol . 193, 615-622(1958) . 111 . E . Baker and E .H . Morgen, Life Sçi . 9 part II, 765-772(1970) . 112 . G. Masthis, F.C . Battaglia and P.D . Bruns, J . Appl . P (1967) .

iol . 22, 1171-1181

113 . J .J . Stulc, W.J . Rietveld, D .W . Sveteman and A. Versprille, Bio1 . Neonate 21, 130-147(1972) . 114 . S. Ullberg, H. Kristoffersson, H . Flodh and A. Hanngren, Arch . int . Pharrtiacodyn. 167, 431-449(1967) . 115 . D .J . MacRae and D. Palayradji, J . Obstet . (1964) .

ec . Brit . Comm . 71,954-959

116 . F .P . Zuspan, W .H . Whal , G.H . Nelson and R.P . Ahlquist, Amer . J . Obstet . C~neç. 95j 284-289(1966. 117 . P . Patterson, L . Phillips and C. Wood, Amer . _J . Obstet . 945(1967) . 118. W .F . Widdas, J . P

ec . 98, 938-

siol . 1l8, 23-39(1953) .

119. J . Dancis, G. Olsen and 6 . Folkart, Amer . J. P

siol .

9~4, 44-46(1958) .

120. R.P . Maickel and W.R . Snodgrass, Tox. Appl . Pharm . 26, 218-230(1973) . 121. A.A . Hodari, F .C . Marions, R.T . Houlihan and J . Peralta, Obstet . 1 47-55(1973) .

nec .

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Mechanisms of Transport Across the Placenta

Vol . 16, No . 1

122 .

H . Ishida, Tonoku J . Expt . Med . 80, 205-210 (1963) .

123 .

Y. Shimidger, Amer . J . P

124 .

A . M. Rudolph and M. A. Heymann, Circul . Res . 21, 185-190 (1967) .

125 .

J . Cancis and W. L. Money, Amer . J . Obstet .

126.

E. A . Friedman, M . J . Gray, D. L . Hutchinson and A. A. Plentl, J . Clin . Invest . 38, 961-970 (1959) .

127.

D. L . Hutchinson, M. J . Gray, A. A . Plentl, H . Alvarez, R. CaldeyroBorcia, B. Kaplan and J . Lind, J . Clin . Invest . 38, 971-980 (1959) .

siol . 52, 377-394 (1920) .

nec . 80, 215-220 (1960) .

The present adress for Dr . Richard K. Miller is : Department of Obstetrics and Gynecology, University of Rochester School of Medicine and Dentistry, 260 The aresent address for Crittenden Boulevard, Rochester, New York 14620. Department of Pharmacology and Toxicology, UniDr . William 0. Berndt is : 39216 . versity of Mississippi Medical Center, Jackson, Mississippi

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