NA-A AMINO-ACID TRANSPORTER ACTIVITY IN PURIFIED MICROVILLOUS PLASMA-MEMBRANE ISOLATED FROM FIRST-TRIMESTER HUMAN PLACENTA - COMPARISON WITH THE TERM MICROVILLOUS MEMBRANE( TRANSPORT, H+ CONCENTRATION GRADIENTDISSIPATION, AND SYSTEM)

Our purpose was to isolate microvillous plasma membrane from first-tri mester placenta and to measure its transport properties with regard to Na+, H+, and a neutral amino acid. Microvillous membrane was isolated from first-trimester (10 to 13 weeks) and term (38 to 42 weeks) place nta and the purity determined. Uptake of Na-22(+) was measured in the presence of an outwardly directly Hf gradient in the presence or absen ce of amiloride (0.5 mmol/L). The rate of dissipation of an H+ concent ration gradient was determined with the H+-sensitive fluorescent probe 2',7'-bis(carboxyethyl)-5(6)-carboxyfluorescein. C-14-methylaminoisob utyric acid uptake was measured in the presence and absence of an inwa rdly directed Na+ gradient. Purity, vesicle volume, vesicle orientatio n, and electron micrographic appearance of the first-trimester membran es were similar to those obtained from term placenta, but vesicle prot ein recovery was lower. Amiloride-sensitive Na+ uptake and Na+-depende nt C-14-methylaminoisobutyric acid uptake was threefold to fourfold lo wer by first-trimester than by term membranes. The rate of H+ concentr ation gradient dissipation was greater in the former. The first-trimes ter microvillous membrane has similar general characteristics to that from term placenta, but its transport activity is quite different.