Pfliigers Archiv
PflO.gers Arch. 381, 79-81 (1979)
EuropeanJournal of Physiobgy 9 by Springer-Verlag 1979
Lack of Effect of Ouabain on Sodium Transport across the Rat Placenta J. Stulc and B. Stulcov/t Department of Pharmacology, Faculty of Pediatrics, and Department of Physiology, Faculty of General Medicine, Charles University, 12800 Prague, Albertov 4, Czechoslovakia
Abstract. The umbilical vascular bed of the rat placenta was perfused in situ. Ouabain ( I 0 - ~ ) in the perfusion fluid had no effect on the unidirectional flux of Na + from the maternal (electrically negative) to the foetal (electrically positive) side of the placenta, or on the transplacental potential difference. This was taken to indicate that there is no significant active transport of Na + across the placenta of the rat.
Key words: Placental transport, potential difference,
sodium,
ouabain.
INTRODUCTION There is a maternal-foetal
potential
difference (p.d.) in many species (4,5, 6). If these p.d.s exist across the exchange barrier in the placenta, they would cause passive fluxes of ions between mother and foetus. Mineral homeostasis of the foetus would then require ionic pumps in the placenta to compensate for any difference between the ionic fluxes driven by the p.d. and the quantities of ions required by the tissues of the growing foetus. We have examined the validity of this postulate in the guinea-pig with respect to the transport of Na +, but the evidence obtained was not conclusive
(7).
M~ny Na + transporting epithelial barriers are similar in that the transcellular active transport of Na + proceeds as a result of a passive entry of Na + into the epithelial cells on one side followed by active extrusion on the other side. The active extrusion is (at least partly) sensitive to ouabain (from numerous publications, e.g. 3). In the present study we have assumed that active transport of Na + across the placenta, if such a transport exists, conforms to the same general pattern. Since this transport would be directed from the negative to the positive side of the barrier the active step of transport has been assumed to take place across the border of the barrier facing the positive side of the placenta. To test the validity of this concept we have administered ouabain to the positive side of the placenta and followed the effect of this treatment on the unidirectional flux of Na + from the negative to the positive side. The experiments have been performed in rats because in the rat - unlike all other species in which the transplacental p.d. has been measured - the foetus is positive with respect to the mother (4). By means of umbilical perfusicn of the rat placenta the foetal (positive) side of the barrier can be exposed to high concentrations of o u ~ b a i n w i t h o u t
affecting the mater-
nal heart adversely.
0031-6768/79/0381/0079/$01.00
80 METHODS The experiments were performed in rats on the 19 th - 20 th day of pregnancy, under pentcbarbital anaesthesia. The average foetal weight was 3.75 ~ 0.64 g (S.E., n = 8). The umbilical vascular bed of the placenta was perfused, and the p.d. recorded as described in detail for the guinea-pig (7). Krebs-bicarbonate fluid containing 6% of dextrau 70000 was used as the perfusion medium. Concentration of C1- in the perfusate was adjusted to I00 mM by replacing part of NaC1 with an equivalent quantity
comparable size. Since these authors measured the flux in intact animals exposed to m~nimum stress the correlation indicates that the placental transport of Na + in the present experiments was not grossly affected by perfusion. The p.d. in the present experiments is lower than the value of 15 mV reported for the rat by Mellor (4), which could be due to the relatively long duration of the experiment. Neither Jmf nor the p.d. were significantly influenced by ouabain, the respective values being 1.98 ~ 0.38/~vl/min
of Na-lactate. The perfusion rate was 0.25 ml/min. Unidirectional flux of Na + from the maternal to the foetal side of
and 9.4 ~ 4.0 mV. This may indicate that either there is no active transport of Na + across the placenta, or the trans-
the placenta (Jmf) was estimated by means of 22Na administered intravenously
port system is insensitive or inaccessible to ouabain.
to the mother, as described previously (7). The placenta was perfused without any treatment for at least 40 minutes, the control values of p.d. and Jmf were taken during the last 10 minutes of this period. Then the perfusion was continued with fluid containing ouabain (Strophauthin-g Spofa) at a concentration of 10 -4 M for 30 minutes. The values of p.d. and Jmf during the last 10 minutes of the ouabain treatment were compared with the control values, and the statistical significance of the differences was tested using the t-test for paired values. The experiments were performed in 8 placentae, the data are presented as means ~ S . E . . RE~I~TS AND DISCUSSION The control values of Jmf a n d p . d , are 2.15 ! 0.37 /uM/min and 9.5 ~ 3.8 mV (foetal side positive), respectively. Jmf correlates well v~th the value of 2.5 /uM/min calculated from the data of Plexner and Pohl (2) representing influx of Na + into the rat foetus of a
Binder and coworkers (I) measured the equilibrium distribution between materhal and foetal plasma in the guinea-pig of several ionic species which do not naturally occur in body fluids and, therefore, are extremlyunlikely to be actively transported by the placenta. The Nernst equilibrium potential of each of these ions was within I mY of zero, while the average p.d. between mother and foetus was over 30 mV. The authors therefore conclude that the maternalfoetal p.d. is not dissipated across the placental exchange barrier. Looking at the results of the present experiments in light of the evidence of Binder and coworkers we lean to the conclusion that there is no significant active transport of Na + across the placenta of the rat.
REFERENCES I. Binder, N.D., Paber, J.J., Thornburg, K.L.: The transplacental potential difference as distinguished from the
81 maternal-fetal potential difference of the guinea-pig. J.Physiel. 282, 561-570 (1978) 2. ~lexner, L.B., Pohl, H.A.: The transfer of radioactive sodium across the placenta of the white rat. J.cell.comp.Physiol. 18, 49-59 (1941) 3. Katz, A.I., Epstein, ~.H.: The physiological role of sodium-potassium activated adenosine triphosphatase in the active transport of cations across biological membranes. Israel J.Med.Sci. ~, 155-166 (1967) 4. Mellor, D.J.: Potential differences between mother and foetus at different gestational ages in the rat, rabbit and guinea-pig. J.Physiol. 204, 395-405 (1969)
5. Mellor, D.J.: Distribution of ions and electrical potential differences between mother and foetus at different gestational ages in goats and sheep. J.~hysiol. 207, 133-150 (1970) 6. Meschia, G., u A.S., Barren, D.H.: Difference in electric potential across the placenta of goats. Proc.natn.Aoad.Sci.U.S.A. 44, 483-485 (1958) 7. ~tulc, J., ~vihovec, J.: Placental transport of sodium in the g~ineapig. J.Physiol. 265, 691-703 (1977)
Received February 6~Accepted May 2,1979