Characteristics of alpha-aminoisobutyric acid transport by lactating rat mammary gland.

The transport of alpha-aminoisobutyric acid (AIB) by lactating rat mammary tissue has been examined. AIB uptake by mammary tissue was via both Na(+)-dependent and Na(+)-independent pathways. AIB uptake via the Na(+)-dependent pathway was inhibited by (methylamino)isobutyric acid (MeAIB) whereas AIB uptake via the Na(+)-independent pathway was blocked by 2-aminobicyclo[2,2,1]-heptane-2-carboxylic acid (BCH). A small fraction of AIB influx persisted in the presence of both MeAIB and BCH. The Na(+)-independent moiety of AIB uptake was strongly inhibited by phenylalanine, tryptophan, leucine, isoleucine and methionine. AIB efflux from mammary tissue slices was found to be both Na(+)-dependent and Na(+)-independent. Trans-stimulation of AIB efflux by other amino acids was not observed; in contrast, external phenylalanine, tryptophan and leucine inhibited AIB efflux. The results are largely consistent with the presence of systems A and L in lactating rat mammary tissue. However, the Na(+)-independent fraction of AIB transport may represent transport via a tissue specific form of system L.