Substrate specificity of the mammary tissue anionic amino acid carrier operating in the cotransport and exchange modes.
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 Km and Vmax of 32.4 microM 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 beta-glutamate, L-cysteate, L-cysteine sulfinate and dihydrokainate but not by DL-alpha-aminoadipate. In addition, dihydrokainate, but not DL-alpha-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 microM). 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 microM) also markedly trans-stimulated D-aspartate efflux from mammary tissue explants. In contrast, L-cysteine. D-glutamate, L-leucine, dihydrokainate and DL-alpha-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 microM). 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.
Millar
I D
ID
Hannah Research Institute, Ayr, UK.
Calvert
D T
DT
Lomax
M A
MA
Shennan
D B
DB
D027322:Amino Acid Transport System X-AG; D000818:Animals; D001224:Aspartic Acid / Q000378:metabolism; D002352:Carrier Proteins / Q000378:metabolism*; D004848:Epithelium / Q000378:metabolism; D005260:Female; D050491:Glutamate Plasma Membrane Transport Proteins; D018698:Glutamic Acid / Q000494:pharmacology; D066298:In Vitro Techniques; D017136:Ion Transport; D008321:Mammary Glands, Animal / Q000737:chemistry / Q000378:metabolism*; D010477:Perfusion; D051381:Rats; D017208:Rats, Wistar; D012964:Sodium / Q000494:pharmacology; D013379:Substrate Specificity; D027981:Symporters
D027322:Amino Acid Transport System X-AG; D002352:Carrier Proteins; D050491:Glutamate Plasma Membrane Transport Proteins; D027981:Symporters; D001224:Aspartic Acid; D018698:Glutamic Acid; D012964:Sodium