Cellular uptake of valine by lactating porcine mammary tissue

The cellular uptake of branched-chain amino acids in mammary tissue is impo rtant for understanding their role in milk synthesis in the sow. This study characterized the kinetic properties and substrate specificity of the vali ne uptake system in the porcine mammary gland. Mammary tissue was collected from lactating sows at slaughter and tissue explants were incubated in med ia containing isosmotic salt and amino acids of interest, plus [H-3]valine tracer. Valine uptake was time-dependent and was dependent on the presence of sodium, as indicated by a reduction in uptake when sodium in the medium was replaced by choline. The valine transport system in porcine mammary tis sue had a K-m of 0.64 mM, a V-max of 1.84 mmol.kg cell water(-1).30 min(-1) , and a K-d (diffusion constant) of 1.16 L.kg cell water(-1).30 min(-1). Va line uptake was inhibited by leucine and alpha -aminoisobutyric acid and by high concentrations of L-alanine, L-lysine, cycloleucine, L-glutamine, and L-methionine, but not by 2-(methyl-amino)-isobutyric acid. This transport system is the primary system responsible for uptake of valine, and probably other branched-chain amino acids, in lactating sow mammary tissue. Physiol ogical concentrations of valine in the blood are below the K-m of the speci fic valine transport system and well below the diffusion uptake capabilitie s. The kinetic parameters of this valine transport system should not be lim iting to valine uptake for milk protein synthesis. However, competition of valine uptake with branched-chain amino acids, as well as with other amino acids, may affect valine uptake in lactating tissue.