Evidence of saturable uptake mechanisms at maternal and fetal sides of the perfused human placenta by rapid paired-tracer dilution: studies with calcium and choline.

Rapid uptake and efflux of 45Ca2+ and [3H]choline at the maternal and fetal interfaces of the syncytiotrophoblast in the dually-perfused human placenta was investigated by application of the single circulation paired-tracer dilution method (Yudilevich, Eaton, Short & Leichtweiss 1979). Cotyledons were perfused with Krebs-bicarbonate containing dextran (30 g/l; MW = 60-70,000) at 20 and 6 ml/min on maternal and fetal sides, respectively. The paired-tracer (test substrate and extracellular marker) technique consisted of an intra-arterial injection of a tracer bolus, followed by venous sampling over 5-6 min. There was a rapid (sec) uptake of 45Ca2+, followed by backflux (efflux into the ipsilateral circulation) which, over 5-6 min, was 59-100% on the fetal side. It was more variable but generally lower on the maternal interface. At 0.1 mM calcium, 45Ca2+ maximal uptake (Umax) was about 53% on the fetal side but on the maternal side it was variable and averaged 17%. At 2.4 mM calcium fetal side Umax was reduced to 40%. However, on the maternal side the effect was not consistent. Unidirectional influx (nmol/min per g) appeared to be not different on the two sides of the placenta. For [3H]choline (in choline-free perfusates) Umax was about 50% and 30% on fetal and maternal sides, respectively; tracer backflux was variable on the maternal side and averaged 50% on the fetal side. [3H]Choline uptake was highly inhibited by either 1.0 mM choline or the specific competitive inhibitor, hemicholinium-3 (0.1 mM). Specific transplacental transfer of 45Ca2+ (i.e. in excess of the extracellular marker) was not significant in either direction. For [3H]choline there was an apparent small excess (about 4%) preferential towards the fetal circulation. These findings in the human placenta are similar to those demonstrated previously in the guinea-pig placenta which suggested the existence of specific transport systems for choline and calcium on both sides of the syncytiotrophoblast.