Amino acid transport and protein synthesis in lactating mammary tissue
L-Glutamate extraction from plasma by the mammary gland is high during lactation. The high extractions of L-glutamate are thought to reflect the high demand of lactating mammary tissue to satisfy milk protein synthesis. Accordingly, the transport of L-glutamate and D-aspartate was investigated. Experiments in rat mammary tissue explants revealed that L-glutamate and D-aspartate uptake was via a sodium dependent pathway with high affinity and was inhibited by L-and D-aspartate plus L-glutamate, but not D-glutamate, neutral or cationic amino acids. This suggested that the mammary anionic amino acid carrier was a member of system X-AG. Transport studies in the perfused rate mammary gland indicated that the anionic amino acid carrier was situated basolaterally. The pharmacology of inhibition of mammary anionic amino acid uptake indicated that the carrier may be a novel variant of the cloned glutamate transporters. D-Aspartate uptake by rat mammary tissue explants varied with the stage of lactation in line with milk yield and was reduced by bromocryptine treatment and milk stasis. The characteristics of anionic amino acid uptake by ovine and bovine mammary tissue were similar to those of the rat. The effects of cell volume changes on mammary protein synthesis were investigated. Hyposmotic and hyperosmotic conditions caused mammary tissue explants to swell and shrink respectively. Cell swelling increased total mammary protein synthesis and casein synthesis. Cell shrinkage reduced total protein and casein synthesis. Cell swelling induced by increasing the transmembrane sodium gradient and doubling the amino acid concentration of the incubation medium resulted in increased mammary protein synthesis. Thus, it is conceivable that sodium-dependent amino acid uptake, a process to which the mammary anionic amino acid carrier would contribute, caused mammary cells to swell, increasing the rate of mammary protein synthesis.