Trophob la s t Research 13:343-361, 1999

M A T E R N O F E T A L A N D T R A N S P L A C E N T A L E L E C T R I C A L

P O T E N T I A L D I F F E R E N C E S

- A R e v i e w -

Stuart Ward *, Colin Sibley: and Robert Boyd ~

'Academic Units of Obstetrics and Gynaecology and Reproduct ive Health Care and

~'Child Health and the School of Biological Sciences Universi ty of Manchester, Saint Mary ' s Hospi ta l

Manchester M13 OJH, United Kingdom

Sain t George 's Hospi tal Medical School Universi ty of I ,ondon

I~ondon SW17 ORE, United Kingdom

I N T R O D U C T I O N

Electrical potential differences (PD's) have been recorded between sites in mother and fetus since Meschia et al. (1958) first repor ted the existence of a substantial potent ial difference of -25 to -133 mV (maternal site taken to be reference zero as throughout this review) between the materna l and fetal circulations in the goat. A satisfactory analysis of this observat ion requires knowledge of current flows and conductances between sites of measurement . With the exception of l imited studies in vitro of the pig placenta (e.g., Sibley et al., 1986) and of the guinea pig endomet r ium (McNaughton et al., 1991), such data are not available. Because of this lack of knowledge it has been a mat ter of controversy as to whether the existence of such a potential implies that there is a potential difference across the placental exchange area as Meschia et al. (1958) assumed.

While earlier workers (e.g., Mellor, 1969; Stulc et al., 1972, Weedon et al., 1978) made the same assumpt ion as Meschia et al. (1958), other groups (e.g., Binder et al., 1978; Thornburg et al., 1979; McNaughton et al., 1991) have disagreed. There are major species differences in recorded values of PD which complicate the discussion. Here we review more recent data from the human, before considering work on other species and present ing a model which may allow resolution of some conflicting perspectives.

Measurement and Nomenc la ture

Maternofetal potent ial differences between sites in the mother and fetus have general ly been measured using a high impedance voltmeter connected via half-cells with, as 'electrodes, agar br idges or saline-filled catheters or, occasionally, silver-silver chloride electrodes (Thomburg et al., 1979). The results with different electrodes have not been formally compared but appear to be broadly similar. The nomencla ture of potentials measured or assumed be tween different electrode sites is easily confused. Here we use the terminology indicated in Figure 1.

343 �9 of Rochester

344 Ward et al.

/ .,/ \

[ [ i ",,

', ,\ l ' e t u S

~\ ~ /' \ \ , //

x

v

\,, circulation ,

.Y I

/ 'x \ ,/ \ , \

\

Figure 1. Nomencla ture of potential differences. PDm~ - potent ial difference between maternal and fetal vasculature (or adjacent extracellular sites); P D , ............. - potential difference between uterine cavity and outside of uterus ( including maternal vasculature) in vivo or in vitro; PDe~,. ..... - potential difference across placental exchange area. See text for details. Not i l lustrated: PDm. ' - potential difference be tween arrmiotic cavity and maternal vasculature; P D - potential difference between extraembryonic coelom and maternal vasculature. In all cases maternal site is taken as reference zero. Most measurements of PDmf have been made between the circulations of mother and fetus. Other measurements of PDmf have been made from extravascular sites e.g., in tissue planes or in per i toneum and are not different from intravascular measurements (Mellor, 1969, 1970). PDp~ .......... has been inferred from PDm, or, in a few instances, been measured more directly. PD.o,~ ...... ~,, has been measured directly with or wi thout inclusion of the overlying uterine coixnective tissue and myometr ium. Recordings between extrafetal posi t ions in the conceptus e.g., between extraembryonic coelom and maternal circulation P D (not shown) or amniotic cavity (or allantoic cavity - not considered here) and maternal circulation PDm, ' (not shown) may be different from PDmf. PD's have also been repor ted be tween the lumen of stomach (Wright, 1962) or lung (Strang, 1991) in the fetus and its vasculature and are bel ieved to be generated by gastric or pu lmonary epi thel ium and are not considered in this review.

A PD~ ......... might be generated locally through electrogenic ion t ransport across the placenta or one of its components . Alternat ively, or in addit ion, a PD~ ........ might reflect the placenta being a non-electrogenic resistance element in series with an electrogenic source elsewhere in the mother or fetus (Duncan, 1976; Faber et al., 1987). Regardless of its site of generation, a PDp~ ...... of any magni tude will be an impor tan t dr iving force for t ransplacental diffusion of ions.

Electrical Potent ia l D i f f e r e n c e s 345

A -30

-20

t-, -10

0 ' - " "

B

"i

C D E

k'--t rain

microvillous membrane ~ 2

basal membrane ~

fetal capillary A [ ' ~

Figure 2. Measurement of transtrophoblastic potential difference (PD). Upper panel: example of PD profile recorded with electrode in bath solution (A), during intial impalement (B), after advance of electrode (C), after partial withdrawal (D), and after complete removal from tissue (E). Lower panel: schematic representation of human mature intermediate villus in cross section, showing hypothesized position of recording electrode during recording above. PD was initially set at 0 mV with respect to ground with the recording electrode in the bath solution (taken with permission from Greenwood et al., 1993).

H U M A N D A T A

Greenwood et al. (1993) measured PD across tile syncytiotrophoblast m vitro as a measure of PDf,~.,, ....... (Figure 2). To date it has only been technically possible to obtain this measurement using mature intermediate villi. It is not known if these vill i are the major site of ion exchange between maternal and fetal circulations. Whether the PDe,~ ...... at other sites along the villus has the same value will depend on the relative conductances and current flows across the syncytial membrane and laterally along the interior of the syncytium - both unknown quantities. The villous core recorded a negative potential with respect to a reference electrode in the intervillous space. The range of PD phil o~h~ values measured in this way was zero to -15mV, (villous core negative with regard to maternal bath), with a median value of -3inV. Oniy villi at term have been studied.

346 Ward et al.

.M

t

Figure 3. Measurement of PDm~. in woman (at 44 - 67 days gestat ion from first day of last menstrual per iod es t imated from crown rump length) under general anesthesia at termination of pregnancy for psychosocial reasons. Upper panel: Diagram of a gestational sac at 8 weeks of pregnancy and sites of measurement . One saline-filled catheter was inserted into a large per ipheral maternal vein, the other being passed into the exocoelomic cavity under direct u l t rasound vision via a needle. Uterus (U), placenta (P), embryo (E), exocoelomic cavity (ECC), amniotic membrane (AM), amniotic cavity (AC), yolk sac (YS) and umbil ical cord (UC). Lower panel: Tissue layers between exocoelomic cavity (ECC) and mtervil lous space (IVS) - t rophoblas t (T), mesenchyme (M) [reprinted from Jones and Jauniaux (1995) with permiss ion from Elsevier Science].

P D

In the first tr imester, it has been possible to obtain measurements dur ing terminat ion of pregnancy of P D using as an exploring electrode a saline-filled catheter inserted under ultrasonic guidance (Figure 3, Ward et al., 1998). As is shown in the lower half of Figure 3, the only substantial epithelial barrier at this stage of pregnancy between the exocoelomic cavity and intervillous space is trophoblast . We therefore assume that PDm, is a reasonable surrogate for PDr~., " ..... in early gestation. The mean _+ SEM value is - 8.7 _+ 1 mV, again fetal side negative.

Electrical Potential Differences 347

P D f

Two measurements ot P D , have been obtained dur ing humai~ pregnancy. Mellor et al. (1969) in an early s tudy in which the baby was exteriorized from the uterus before measurements were made between umbilical vessels and mother, found a range of values not statistically different from zero. It is not known whether this was because of the posit ion of the baby. Stulc et al. (1978) who made a similar measurement when the baby was still in utero at 15 - 22 weeks, found a small P D , of -2.7 _+ 0.4 mV fetal side again negative. Duncan et al. (1976) reported, in a brief abstract, a P I ) of -4.3 _+ 0.6 mV dur ing labor.

It would be very helpful to have measurements of P D , throughout gestat ion and microelectrode measurements of PDp, .......... in v i tro in early and mid-gesta t ion to add to the data a l ready obtained at term. It would also be useful to confirm the absence of a PD between a maternal per ipheral vein and mtervil lous space. Nevertheless, the human picture is relat ively consistent with there being a modes t PD, .......... fetal side negative, perhaps of greater absolute magni tude in early gestation than at term. P D , appears to be reasonably similar to PDrl ......... if the exteriorized value recorded by Mellor et al. (1969) i~ d isregarded.

Al though these values are numerical ly relatively low, the values of PDp,,,~o~ and PD,,,~. repor ted above are sufficiently large to be an impor tan t dr iv ing force for ion movement . If the values are substi tuted in the Ussing flux ratio equation as quoted in Weedon et al. (1978) and if, unjustifiably but to s implify the discussion, both concentration differences and volume flows across placenta are taken to be zero, the effect on the a symmet ry of ion for monovalent and divalent ion fluxes can be calculated as in Table 1. Because of the unkalown quanti tat ive influence of volume flows and because of the quant i ta t ively uncertain balance between paracel lular diffusion and the transcellular t ransport of ions (Sibley and Boyd, 1998) the values given in Table 1 can only give a general indication. These asymmetr ies m Table 1 are of a magni tude which might, for example, explain the observed higher fetal than maternal plasma concentrations of calcium (Schauberger and l ' i tkin, 1979) without invoking active transport , or contr ibute an important dr iv ing force for the extrusion from the fetal circulation of HCO~ or for the fetal accumulat ion of those amino acids which are cotransported electrogenically across, at least, the microvi l lous membrane of human syncyt io t ropboblas t (cf., Sibley and Boyd, 1998).

As there is no obvious alternative site for electrogenicity in the human it appears likely that PDp~,~,,,~ is genera ted locally within the placental epithel ium. For this to be so, electrogenic transporters , such as N a . K-ATPase , need to be asymmetr ica l ly dis t r ibuted at the microvi l lous and basal faces of the syncytium. ~lhis is general ly taken to be the case, based on isolation of the two membranes (Kelley et al., 1983), but this may not be absolutely secure reformat ion (Powell et al., 1995). Ouabain mhibitable oxygen consumpt ion is a substantial fraction of total oxygen consumpt ion in the isolated human villus and shows a gestat ional decline (Birdsey et al., 1997). This is consistent with a decline in PDp, .......... with gestat ional age implied in the difference between PD ..... in the first t r imester and PD ,.,,,,,., at term referred to above.

348 Ward et al.

Table 1

Human: transplacental ion diffusion, dependence of flux ratio on PI) j,,~,. .....

PD monovalent (e.g. Na*, CI) divalent (e.g. Ca ~*) (mY)

-3.0 (PDp~ ......... ) 1.1 1.3 -8.7 (PD,,~) 1.4 1.9

Estimate of the degree of asymmetry in monovalent and divalent diffusional ion fluxes across placenta in the context of a PD, ...... of the magnitude indicated for PD,,~.~,~.~,.~ or PDo,. (volume flow and ion concentration differences taken to be zero - see text).

PDmt and PDp~,~ in other species

There are two strands of e~ idence that in species other than the human, PD,,~ artd PD, ......... may not be identical. Firstly, the 'symmetry paradox' and secondly, data showing PD generation by the endometrium.

McNaughton and Power (1991) describe as the ' symmetry paradox' the observation, first highlighted by Binder et al. (1978) and Thornburg et al. (1979), that the numerically high measured values of PD,,,, found in both sheep and guinea pigs were unlikely to be identical in magnitude to PDr, ....... in those species. This is because both endogenous ions present in plasma and injected 'foreign' ions came to steady state concentration ratios between fetal and maternal plasma far different from those which would be predicted at equilibrium if the magnitude of PDp,~. ..... was identical to PDmr At the time of the studies, the authors believed that the exogenous ions they used (SO~ ~, Br , Rb- and Li ) were not handled by biological transport systems; an assumption that has not turned out to be correct [e.g., sulphate is transported by the placental Na*/SO~ ~ transporter (Cole, 1984), Rb* by K + transport proteins (Boyd, 1983)]. Nevertheless, this incorrect assumption does not substantially undermine a powerful general argument that it is a priori implausible on grounds of both complexity and energetics to propose that placental transport controls the steady state concentration ratios between maternal and fetal plasma for a whole range of solutes far from electrochemical equilibrium (e.g., Armentrout et al., 1977). Binder et al. (1978) and Thornburg et al. (1979) proposed that in these species, PD~ .......... must be small in comparison to PDm, and consequently suggested that PDm~ must be generated at a non-placental site.

Power and colleagues (Dale et al., 1990) subsequently provided elegant experimental evidence to support such a generation of PD~ at a non-placental site when, in the guinea pig, they found, m contrast to earlier reports by Mellor (1969), PD,,d ............... to be numerically close to PD,1,, after removal of the fetus and placenta (Figure 4). They demonstrated by analysis of ion fluxes and short circuit current that active net sodium transport towards the mother was responsible for the generation of PDo<~ ............ (McNaughton et al., 1991). The concordance between P D ~ ........ ~,,,, and PD,,,, supports their concept that both are generated by endometr ium and that PDr,~,~.o, ~ is likely to be zero.

Electrical Potent ia l Di f fe rences 349

4 0 - -

>

---- 3 0 - - G~

�9 ~ 2 0 -

. ,....~

1 0 - - (D

0 - -

0 6 0 1 2 0 1 8 0 2 4 0

T i m e ( m i n )

Figure 4. Measurement of PD in guinea pig between uterine lumen and the maternal carotid artery following removal of the fetus ( P D d .......... ). Time course of t ransuter ine potential difference. Time 0 was defined as 30 minutes after uterine pouch was filled with Earle 's solution. Uterine cavity was always electrically negat ive with respect to maternal abdomen. Decline in potent ia l difference with time became statistically significant at 60 minutes. Means and 95% confidence intervals of the mean are indicated (~1 = 11) (taken with permission from Dale et al., 1990).

However , it remains difficult to reconcile such a conclusion easily with observat ions of Stulc and SvLhovec (1973) using fetally perfused in sit~z guinea pig placenta, that either sod ium replacement in or addi t ion of cyanide to the perfusate both reduce PDmr substantially. Van Dijk and van Kreel (1982) and van Kreel and van Dijk (1983) also using guinea pig took a totally different approach to est imating transplacental PD and d id so from the equil ibr ium distr ibution of the charged molecules [3H] choline chloride and [HC] tetra ethyl ammonium bromide between maternal and fetal perfusate and placental tissue. They concluded that PD ,~,.,,,~ was approx imate ly 6 mV fetal side positive, i.e. of opposi te polar i ty to other investigators. This original and unconfirmed approach has not been pursued.

In vitro, Power 's group also repor ted a PDn d ......... ~,,,, in the sheep (McNaughton and Power, 1991) almost identical in magni tude and of the same polar i ty as the PD~ at the same gestational age repor ted by Weedon et al. (1980) in vivo. As with the guinea pig, the PD was amilor ide inhibitable (Figure 5). Earlier ion flux studies in the sheep in vi~o are, however , difficult to reconcile with the conclusion that in the sheep, PDp, ...... is close to zero or is not at least a function of PDmf for fetomaternal clearance of the anions b romide and iodide are both statistically related to PDo,~, being higher in those animals with greater fetal e lectronegat ivi ty (Boyd et al., 1981; Canning et al., 1986) (Figure 6). This would be the case if t ransport of these anions from fetus to mother across placenta was diffusional and dr iven by PD

pla< ent~"

350 Ward et al.

2>

~ 20

0 J - �9 �9 I

-2 {} 2 4 6 8 IO T ime ( � 9

Figure 5. Time course of potential difference ( P D , ....... ~,,,,) across the uterine wall of sheep mounted in Ussing chambers. Results (mean _+ SEM) are from eight samples of uterine wall, taken at approximate ly 140 days of pregnancy and s t r ipped of an ' � 9 and allantois. The uterus was bathed on both sides with ]{arle's solution. The N a channel blocker, amilor ide (5 x 105M), was added to the luminal side at t ime zero and washed out 3 minutes later (taken with permission from McNaughton and Power, 1991).

24 []

KI fin m l � 9 ~

12

[]

[] [] []

[]

m ! t

[] 6 [ ] [] []

[] []

[]

[ ] [ ]

[ ]

[ ]

1 I '1 I

20 40 60 80

PDmr (mV)

Figure 6. Radio iodide clearance to mother (K~) fol lowing injection to chronically ins t rumented fetal sheep.

K~,,, = 5 .7 + 0 .15 P D , ( m V ) r = 0 .51 , n = 27, p<0.01

K ~',,,= 084 + 0.06 PD,, (mV) r = 0.92, n = 7, p<0.005 (data not shown),

(taken with permiss ion from Canning et al., 1986).

Electr ical P o t e n t i a l D i f f e r e n c e s 351

15

>

10 r

r

"~ 5 G~ c3

0

T

TTITTTTYTTTTYT 1 I 1 I

0 5 10 15

Time (mm)

Figure 7. Effect of cooling rat placenta on the PD recorded between fetal blood vessel and exterior of conceptus following removal from the mother. This is taken to be P D t . Open circles are the PD in a control group kept at 37"C (n = 3), closed circles are the PD in a group whose placenta was cooled to 2-4~ (n = 5). Mean values and limits (SEM) are given (taken with permission from Stulc and Stulcova, 1996).

A recent study by Stulc and Stulcova (1996) in the rat was not concordant with Power and colleagues' guinea pig findings. PD~ ............. in pregnancy was found to be zero in contrast to earlier reports of such a PD, modifiable by adrenaline, in the non-pregnant rat uterus (Levin and Edwards, 1968; I~evin and Phillips, 1983). Stulc and Stulcova (1996) measured PD~ between maternal and fetal vasculature and found values comparable to those of Mellor (1969). They then removed the conceptus from the uterus and found that the PD was maintained. It was subsequently reduced to zero by cooling of the placenta alone (Figure 7) which led them to suppose that PDm~ was generated in the placenta in rat.

In the pig, I)DpE .......... in vitro is small h~ magni tude (in contrast to an earlier unconfirmed report, Crawford and McCance 1960) and the fetal side is positive (Sibley et al., 1986; Rice et al., 1991). PD .... in riP0 is of opposite sign and greater magnitude. However, both PDp~.,, ..... in vitro and PDmt in vivo are modified by ~3-catecholamine stimulation in the direction of the fetal side becoming more positive, hi vitro this increase in PDp, ...... appears to be caused by electrogenic net sodium transport to the fetus. PDp~,,~. ..... in vitro is also much higher if the chloride ion is excluded, suggesting the presence of N a - CI cotransport (Boyd et al., lc~85). Microelectrode studies indicate a complex electrical PD profile across the organ (Ward et al., 1998).

352 Ward et al.

> V-

~3 , - L

E

-40

~20

0

Adrenal ine

[ IllJ 11 .

Adrenal ine

-40 -

20

Figure 8. PDm~ and PDm. ' before and after intravascular injection of 20 Bg adrenal ine to the fetus. Maternal vascular catheter taken to be reference zero. The first marker (arrow) represents start of the adrenal ine injection, the second start of saline flush and third the finish of injection (taken with permission from Ward et al., 1998).

The difference in magni tude between I'D,~ .......... (5.9 _+ 0.4 rnV fetal side positive) (Sibley et al., 1986) and PDmf at approximate ly the same gestat ional age [-18 _+ 4 mV fetal side negative soon after surgery and -29 +_ 5mV fetal side negat ive the next day (Boyd et al., 1989)] would appear to imply that PD7 , ......... in vitro and PDm, in vivo are generated by different mechanisms. However PD,7 d ......... ~u,,, cannot be a separate site of generation in this species as its endomet r ium forms an integral layer in the p ig ' s epi thel iochorial placenta and is thus included in the tissue mounted in vitro. PDmf and the PD between amnion and maternal vasculature, PD ...... respond identically to fetally injected adrenal ine (Figure 8; Ward , 1995; Ward et al., 1998) which we take to imply that at least the catecholamine s t imulatable element of P D , is generated within the placenta as the placenta is the only substantial epithelial layer between arrmiotic and maternal electrodes. The quanti tat ive difference of response in ~qtro (Sibley et al., 1986) and m vivo (Boyd et al., 1989) may speculat ively reflect the absence of key solutes in vitro rather than differeilt sites of generation.

Electrical Potential Differences 353

The Symmetry Paradox- A Possible Synthesis of Experimental Data

If the PD across e n d o m e t r i u m and p lacenta were in s imple series in an ohmic circui t and were the only e lements cont r ibu t ing to the m a g n i t u d e of PD~ their va lues w o u l d be addi t ive .

PDn , = PDp~ ........ + PDo,~ . . . . . . . . . . . . . . .

Clear ly such a circui t is a gross overs impl i f i ca t ion but the present , albeit patchy, data sugges t that P D , is indeed probab ly some funct ion of PDp, . . . . . . . . . . and PDn d ............ wi th the balance b e t w e e n the two be ing very dif ferent in d i f fe ren t species. F igure 9 i l lustrates this point.

P t e PDmf (mV) I-r _29

p e [%* sheep I -28

p e

I" I ~ rat

P

1" human -2.7

p e , guinea-pig

Figure 9. Sugges t ed role of P D L ......... (p) and I 'D o,, ............ (e) in the genera t ion of P D , . P = placenta , e = e n d o m e t r i u m , * - quan t i t a t ive ly impor tan t , (*) - of uncer ta in quan t i t a t ive impor tance , t = t ropboblast . Sources of P D , va lues -p ig -Boyd et al., 1989

sheep - W e e d o n et al., 1980 rat -Mellor, 1060 h u m a n -S tud et al., 1978 gu inea pig -Mellor, 1969

354 W a r d et al.

For example, in the pig, endometr ium in the form of uterine epithelium and the trophoblastic chorionic membrane come together in close appositional interdigitation to form the placenta (Hunter, 1781; Patten, 1958). This may explain the tmusual microelectrode profile of PD across the placenta including a positive compartment (Ward et al. 1998). As maternofetal exchange has to take place across both these barriers albeit perhaps in specialized areolar areas (Firth et al. 1986) PD~ ........ in vivo is probably equal in both sign and magnitude to PDm,. The difference between the former value in vitro and the latter in vivo probably reflects experimental conditions. The extremely low permeability of pig placenta, which is at least an order of magni tude less permeant to sodium per unit weight than other species (Gellhom and Flexner, 1941; Flexner and Gelthorn, 1942), indicates that in this species the symmetry paradox is most likely resolved by diffusional ion flux being of very low quantitative importance.

In the sheep P D d , ~, i m and PDm~ are probably much larger than PD ,~ ~ PD �9 t m r u p c p l a c e n t a

is, however, unlikely to be zero but is likely to be a proportionate function of PD~ because of tile observed relationship between PD,. and anion clearance, The predominant site of electrogenesis is probably the endometrium. The symmetry paradox in this species is most easily resolved by assuming only a modest electrical driving force (I 'D, ......... ) coupled with an array of controlled ion pumps. The statistically reliable differences in ion concentrations between maternal and fetal plasma for Ca~'* and K (Armentrout et al., 1977) already, implies the existence of such controls for at least these ions. The relatively low permeability of sheep placenta to sodium and chloride (Faber et al., 1983) suggests that the energetic cost of maintaining this control should not be high. A reported relationship in sheep between fetal acidosis and increasing magnitude of PDm,. similar to that reported for blood brain barrier (Held et al., 1964) but in which equal H concentrations in maternal and fetal circulations were not associated with zero PD (Weedon el al., 1980) can be reinterpreted as the transplacental hydrogen ion gradient influencing PD,~, ...... but not PD,,,,~ ............ . The reported increase in the magnitude of P D , on fetal death (Weedon et al., 1980) can also be explained if PD, .......... is of opposite sign to [ 'D~ ............... and is selectively reduced on fetal death. This explanation, however, would imply that the anion eflux from the fetus (Figure 6) is electrogenic and not diffusional.

In the rat, whose transport characteristics have been reviewed by Sibley (1994), present evidence suggests that PDa .......... is equal in magnitude to PD f and that the magnitude of PD.nd ......... ,,m is close to zero. The fact that chloride transport across the rat placenta is 80% inhibitable by DIDS (Stulc et al., 1996) and thus predominantly transcellular, as is also transfer of calcium (Stulc and Stulcova, i986) suggests that the energetic cost of maintaining ionic equilibrium across placenta will be relatively modest but that control mechanisms again have to be invoked.

In the human, implantation is very deep and the residual endometr ium is not included in tile interface between conceptus and mother at the implantation site (Boyd and Hamilton, 1970). For PD.,1 . .............. to make a contribution to PDo,, it would be necessary to assume that any PD generated bv the endometr ium on the internal face of the uterus opposite the fetus was not nullified by an equal and opposite PD generated by the reflexed endometr ium overlying the fetus itself, a point not considered, incidentally, in the work oil guinea pig referred to above. For these reasons PDn d ....... ~,,n, in the human is presumed not to contribute to PDm~ or PDp, ......... although this cannot be altogether excluded with regard t o P l ) in earlv gestation, lt is striking that PD , is of much lower magnitude than for the other species discussed here [and comparable to the low values reported in rabbit (Mellor, 1969)J. This is in keeping with the observation that, at Yeast in the placenta

Electrical Potential Di f ferences 355

perfused at term, over 80%, of chloride (Doughty et al., 1996) and 68% of calcium (Stulc et al., 1994) flux across perfused human placenta appears to be paracel lular implying a major shunt pa thway across the exchange barrier. The low PD h~,,,,., in late gestation allows energetically attractive resolution of the symmet ry paradox but does indicate that some control led transcellular exchange of e.g., N a C I will be required. The fetal side electronegativi ty in the placenta could also help to contr ibute to the fetal accumulat ion of calcium and certain amino acids against a concentration gradient.

Modulat ion of PD placenta

It is str iking that PD,,,r declines with gestational age in most species for which the data is available (Table 2). The decline is indeed very obvious in the original measurements by Meschia et al. (1958) in the goat (plotted in Figure 10). Al though PDm~ is not PDp, .......... and there are no direct data on gestat ional change in PD~a ~ .... we may speculate that PDr~.~,.,,,. ' is likely to be another variable contr ibut ing to changes in placental transfer per unit surface area as gestation advances.

The best evidence of acute modula t ion of PD k.,,,.,,,. ' is seen in the pig. In this species physiological ly relevant concentrations of catecholamine st imulate PDpL ....... and maternofetal sod ium flux in vitro (Macdonald et al., 1984; Sibley et ai., 1986; Boyd et al., 1989). In the human, da ta is extremely limited. The observat ion that volume stresses on intact villi (Birdsey et al., 1999) alter membrane potent ial suggests that more detai led electrophysiological analysis of placental membrane potentials will provide impor tant clues as to control mechanisms of ion movement into and across placenta. Detai led analysis is long overdue to replace the uncertainties and speculat ions of PD~ measurements with a clearer quanti tat ive analysis.

Table 2

PDm~ and Gestation (mother reference zero)

Goat

Sheep

Rat

Guinea pig

79 days -133 mV; 131 days -25mV (Meschia et al., 1958)

PD = -164 + gestational age (days) [range 117 - 143 days (Weedon et al., 1980)]

15 - 20 days, +15mV 22 days, 0mV (Mellor, 1969)

PD decline with increasing fetal weight (Binder et al., 1978) probable decline (Stulc et al., 1972, Figure 1) no change - 18mV (Mellor et al., 1969)

3 5 6 W a r d e t al.

140 -

- 1 2 0 -

- I ( X )

PDmf (mY) - s0

4()

- 2 0 -

"... ~

[ ] "',, "....

"... [ ]

Linear regression ............. 95% Confidence interval "... []

% ",.%

| I I | l

50 70 9o l~o 130 150

Gestation age (days)

Figure 10. P D , measured between saline-filled catheters inserted in maternal jugular or femoral and umbil ical vessels and gestat ion in the goat (plotted from data in Meschia et al., 1958).

S U M M A R Y

A transplacental PD (PDf,~,,,~,~,,), which may be generated by local electrogenic transport , could be an impor tant dr iving force for diffusion of ions or uncharged molecules co- t ranspor ted with a charged solute (e.g., amino acids). For several mammal ian species, a PD has been repor ted to exist between major maternal and fetal blood vessels (PDm,) or other sites in vivo. There are mul t ip le species (and gestational) differences in PDo,,, and a P D t ......... is not a lways reflected by a PD, . For example, ha guinea pigs, the PDm~ does not have a placental component and is generated by endometr ia l epi thel ium. In the rat, PDm, appears to reflect a PDe, ......... ; in the pig, an adrenal ine-modif iable component of PDm, is probably a PD,.,~,~,. In the human, the PD, .......... is similar to the PDmf and should be considered in an analysis of fetal nutrition�9 In summary , the differences in potential across the placenta and the endomet r ium for different species al lows one to account for variations in the magni tudes of PD~.

A C K N O W L E D G E M E N T S

We would like to acknowledge the important contr ibut ion to our studies repor ted here of Jane Canning, Jo Glazier, Sue Greenwood, Eric Jauniaux, Terry Stacey, H u m p h r e y Ward and the late Paul Weedon.

Electrical Potential Di f ferences 357

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