CHARACTERISTICS OF ALPHA-AMINOISOBUTYRIC-ACID TRANSPORT BY LACTATING RAT MAMMARY-GLAND

The transport of alpha-aminoisobutyric acid (BIB) by lactating rat mam mary tissue has been examined. AIB uptake by mammary tissue was via bo th 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 phenylalan ine, tryptophan, leucine, isoleucine and methionine. AIB efflux from m ammary tissue slices was found to be both Na+-dependent and Naf-indepe ndent. Trans-stimulation of BIB efflux by other amino acids was not ob served; in contrast, external phenylalanine, tryptophan and leucine in hibited AIB efflux. The results are largely consistent with the presen ce 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.