Maternofetal Potential Differences: Studies Using the In Vitro Pig Placenta
A potential difference (pd) between maternal and fetal circulations created by active transport of electrolytes by the placenta would be important not only for the actively transported species themselves, but also for the transfer of passively distributed charged molecules. In several species, it is possible to measure a pd between catheters placed in maternal and fetal vessels. It is possible that this maternofetal pd is a result of active electrolyte transport by the placenta. However, the recorded pd’s have a wide range, for example in the sheep it is 34 mV, fetus negative relative to mother (Weedon et al., 1978), whereas in the rat it is 14 mV, fetus positive (Mellor, 1969). Considerable doubt is shed on the placental basis of this pd by at least two lines of evidence. Firstly, there appears to be no relationship between the magnitude and polarity of the pd and the type of placenta, e.g., the guinea pig which has a relatively permeable (Hedley and Bradbury, 1980) haemochorial placenta demonstrates a pd of 18 mV, fetus negative (Mellor, 1969) more similar to that of the sheep, which has a relatively impermeable (Boyd et al., 1976) epitheliochorial type of placenta than to the term human (O mV, Mellor et al., 1969). Further evidence against the placenta being the site of pd generation comes from Binder et al. (1978) and Thornburg et al. (1979) who found in the guinea pig and sheep, respectively, that the steady-state concentration ratio between maternal and fetal plasmas of exogenous ions injected into the mother was consistent with a near zero transplacental pd. They suggest that the maternofetal pd is generated at a site outside of the placental exchange area.
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