AJP - Cell Physiology, Vol 254, Issue 6 C735-C743, Copyright © 1988 by American Physiological Society
Characterization of folate uptake in guinea pig placenta
J. H. Sweiry and D. L. Yudilevich
Department of Physiology, King's College London, University of London, United Kingdom.
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)
