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

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 Naf-dependent system for L-glutamate transport in tissue explants was saturable (K-m = 112.5 +/- 19.7 mu 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. a-Aspartate inhibited L-glutamate uptake in a competitive fashion. L-Glutamate uptake by the perfused mammary gland was (a) Naf-dependent (b) saturable (K-m = 18.1 +/- 4.9 mu 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.