Lysine and alanine transport in the perfused guinea-pig placenta

Abstract

The characteristics of l-lysine transport were investigated at brush-border (maternal) and basal (fetal) sides of the syncytiotrophoblast in the term guinea-pig placenta artificially perfused either through the umbilical vessels in situ or through both circulations simultaneously. Cellular uptake, efflux and transplacental transfer were determined using a single-circulation paired-tracer dilution technique. Unidirectional l-[ 3H]lysine uptake (%) (perfusate lysine 50 μM) was high on maternal ( M = 87 ± 1) and fetal ( F = 73 ± 2) sides. l-[ 3H]Lysine efflux back into the ipsilateral circulation was asymmetrical (F/M ratio = 2.3) and transplacental flux occurred in favour of the fetal circulation. Unidirectional lysine influx kinetics (0.05–8.00 mM) gave K m values of 1.75 ± 0.70 mM and 0.90 ± 0.25 mM at maternal and fetal sides, respectively; corresponding V max values were 1.95 ± 0.38 and 0.87 ± 0.10 μmol·min −1·g −1. At both sides, lysine influx (50 μM) could be inhibited (about 60–80%) by 4 mM l-lysine and l-ornithine and less effectively (about 10–40%) by l-citrulline, l-arginine, d-lysine and l-histidine. At the basal side: (i) lysine influx kinetics were greatly modified in the presence of 10 mM l-alanine ( K m = 6.25 ± 3.27 mM; V max = 2.62 ± 0.94 μmol·min −1 · g −1), but unchanged by equimolar l-phenylalanine or l-tryptophan; (ii) in the converse experiments, lysine (10 mM) did not affect the kinetic characteristics for either l-alanine or l-phenylalanine; (iii) l-lysine and l-alaninie influx kinetics were not dependent on the sodium gradient; (iv) the inhibition of l-[ 3H]lysine uptake by 4 mM l-homoserine was partially (60%) Na +-dependent. At the maternal side the kinetic characteristics for alanine influx were highly Na +-dependent, while lysine influx was partially Na +-dependent only at low concentrations (0.05–0.5 mM). Bilateral perfusion with 2,4-dinitrophenol (1 mM) reduced l-[ 3H]lysine uptake into the trophoblast and abolished transplacental transfer. It is suggested that lysine transport in the guinea-pig placenta is mediated by a specific transport system (y +) for cationic amino-acids. The asymmetry in the degree of sodium-dependency at both trophoblast membranes may in part explain the maternal-to-foetal polarity of placental amino-acid transfer in vivo.