Id. Millar et al., SUBSTRATE-SPECIFICITY OF THE MAMMARY TISSUE ANIONIC AMINO-ACID CARRIER OPERATING IN THE COTRANSPORT AND EXCHANGE MODES, Biochimica et biophysica acta. Biomembranes, 1326(1), 1997, pp. 92-102
The substrate specificity of the rat mammary tissue high affinity, Naf
-dependent anionic amino acid transport system has been investigated u
sing explants and the perfused mammary gland. D-Aspartate appears to b
e transported via the high affinity, Na+-dependent L-glutamate carrier
. Thus, D-aspartate transport by rat mammary tissue was Nas-dependent
and saturable with respect to extracellular D-aspartate with a K-m and
V-max of 32.4 mu M and 49.0 nmol/2 min per g of cells respectively. T
he uptake of D-aspartate by mammary explants was cis-inhibited by L-gl
utamate and L-aspartate, but not by D-glutamate. L-glutamate uptake by
mammary tissue explants was cis-inhibited by P-glutamate, L-cysteate,
L-cysteine sulfinate and dihydrokainate but not by DL-alpha-aminoadip
ate. In addition, dihydrokainate, but not DL-alpha-aminoadipate inhibi
ted D-aspartate and L-glutamate uptake by the perfused gland. D-Aspart
ate efflux from mammary tissue explants was a-ans-accelerated by exter
nal L-glutamate in a dose-dependent fashion (50-500 mu M). The effect
of L-glutamate on D-aspartate efflux was dependent on the presence of
extracellular Na+. D-Aspartate, L-aspartate and L-cysteine sulfinate (
at 500 mu M) also markedly trans-stimulated D-aspartate efflux from ma
mmary tissue explants, In contrast, L-cysteine, D-glutamate, L-leucine
, dihydrokainate and DL-alpha-aminoadipate were either weak stimulator
s of D-aspartate efflux or were without effect. D-Aspartate efflux fro
m the perfused mammary gland was trans-stimulated by L-glutamate but n
ot by D-glutamate and only weakly by L-cysteine (all at 500 mu M). It
appears that the mammary tissue high affinity anionic amino acid carri
er can operate in the exchange mode with a similar substrate specifici
ty to that of the co-transport mode.