The mechanism of l-glutamate transport by lactating rat mammary tissue

Abstract The transport of l -glutamate by lactating rat mammary gland has been examined using both tissue explants and a perfused mammary preparation. l -Glutamate uptake by mammary tissue explants was predominantly via a Na + -dependent pathway: Li + , choline + and NMDG + could not substitute for Na + . l -Glutamate efflux from preloaded explants was also influenced by the transmembrane Na + -gradient. These results are consistent with (Na + -glutamate) cotransport. The Na + -dependent system for l -glutamate transport in tissue explants was saturable ( K m = 112.5 ± 19.7 μ M; V max = 71.3 ± 10.4 nmol/min per g cells) and selective for anionic amino acids. Thus, d - and l -aspartate were high affinity inhibitors of l -glutamate uptake whereas neutral amino acids were relatively ineffective. d -Aspartate inhibited l -glutamate uptake in a competitive fashion. l -Glutamate uptake by the perfused mammary gland was (a) Na + -dependent (b) saturable ( K m = 18.1 ± 4.9 μ M; V max = 40.3 ± 3.7 nmol/min per g tissue) and (c) selective for anionic amino acids. The results suggest that the (Na + -glutamate) cotransporter is situated in the blood-facing aspect of the mammary epithelium.