Substrate specificity of the mammary tissue anionic amino acid carrier operating in the cotransport and exchange modes.

Abstract The substrate specificity of the rat mammary tissue high affinity, Na + -dependent anionic amino acid transport system has been investigated using explants and the perfused mammary gland. d -Aspartate appears to be transported via the high affinity, Na + - dependent l -glutamate carrier. Thus, d -aspartate transport by rat mammary tissue was Na + -dependent and saturable with respect to extracellular d -aspartate with a K m and V max of 32.4 μ M and 49.0 nmol/2 min per g of cells respectively. The uptake of d -aspartate by mammary explants was cis -inhibited by l -glutamate and l -aspartate, but not by d -glutamate. l -glutamate uptake by mammary tissue explants was cis -inhibited by β -glutamate, l -cysteate, l -cysteine sulfinate and dihydrokainate but not by dl - α -aminoadipate. In addition, dihydrokainate, but not dl - α -aminoadipate inhibited d -aspartate and l -glutamate uptake by the perfused gland. d -Aspartate efflux from mammary tissue explants was trans -accelerated by external l -glutamate in a dose-dependent fashion (50-500 μ 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 μ M) also markedly trans -stimulated d -aspartate efflux from mammary tissue explants. In contrast, l -cysteine, d -glutamate, l -leucine, dihydrokainate and dl - α -aminoadipate were either weak stimulators of d -aspartate efflux or were without effect. d -Aspartate efflux from the perfused mammary gland was trans -stimulated by l -glutamate but not by d -glutamate and only weakly by l -cysteine (all at 500 μ M). It appears that the mammary tissue high affinity anionic amino acid carrier can operate in the exchange mode with a similar substrate specificity to that of the co-transport mode.