AJP - Cell Physiology, Vol 234, Issue 1 27-C35, Copyright © 1978 by American Physiological Society
Placental amino acid uptake. IV. Transport microvillous membrane vesicles
S. M. Ruzycki, L. K. Kelley and C. H. Smith
The recent characterization of a vesicular microvillous plasma membrane
preparation from human placenta has made possible study of the
characteristics of amino acid transport by the isolated membrane. Variation
of medium osmolality demonstrated that alpha-aminoisobutyric acid (AIB) is
taken up virtually entirely into a vesicular space. Vesicular volume was
determined from equilibrium uptake of three dissimilar nonamino acid
substrates, all of which gave a common value. Temperature dependence,
saturability, specificity, and similarity of kinetic constants all
indicated that AIB uptake by the vesicles resembles that by whole villous
tissue. AIB uptake was stimulated specifically by an inward sodium gradient
and unaffected by an outward potassium gradient. The persistence of
stimulation up to 2 h after the gradient was established could be
correlated with the slow equilibration of sodium across the membrane. AIB
exodus could be studied with minimal problems of substrate recapture by
using preloaded membrane vesicles bound tightly to cellulose ester filters
to facilitate transfer to fresh medium. The exodus process was saturable
with a Km considerably greater than that for entry.
