Characterization of folate uptake in guinea pig placenta

Trophoblast uptake of folate and methotrexate (MTX) was investigated in an in situ or dually perfused (maternal and fetal side) guinea pig placenta by using a single-circulation, paired-tracer technique. For [3H]folate, uptake into trophoblast was rapid (s), high (60-80%) and Na+ independent, and exhibited negligible efflux on both poles of placenta. [3H]folate uptake could be inhibited by folate or 5-methyltetrahydrofolate (CH3THF) but not by equimolar (0.1 microM) MTX, folinic acid, aminopterin, trimoprim, or adenine when these compounds were present in perfusate. Inhibitory effect of folate was time dependent, and its complete reversal by folate-free perfusion required up to 20 min. This suggests the presence of a high-affinity folate carrier that exhibits a slow rate of self exchange. A sudden (bolus) increase of 10 microM folate of CH3THF caused a 70-80% inhibition of [3H]folate uptake, whereas folinic acid, MTX, and trimoprim were two- to threefold less effective. [3H]folate uptake was insensitive to DIDS, SITS, nicotine, ethanol, or phenytoin. For [3H]MTX, uptake was high (60-80%) on both sides of trophoblast, however, as distinct from [3H]folate, rapid and complete efflux followed the initial uptake. [3H]MTX uptake was not inhibited by 0.1 microM MTX, but equimolar folate or CH3THF were highly effective (90%) inhibitors; higher concentration (1 microM) of MTX reduced [3H]MTX uptake by 58%. Transplacental transfer of [3H]folate or [3H]MTX in excess of the leak pathway marker in either direction was not observed. Inhibition obtained by highly concentrated substrate bolus injections indicates saturation (less than 2 microM) of membrane folate carrier.(ABSTRACT TRUNCATED AT 250 WORDS)