RESUMEN
The sites of methionine uptake by 10 day lactating mouse mammary gland were determined in vitro. Four modes of methionine entry characterised were: (i) A sodium-dependent, N-(methylamino) isobutyric acid (MeAIB)--sensitive system with a Vmax of 18.8 nmol/g cells/min (this mode of entry was similar to the A site in other tissues); (ii) A sodium-dependent, MeAIB--insensitive uptake system with a Vmax of 12.4 nmol/g cells/min); this mode of entry was inhibited by substrates preferred by ASC system); (iii) A sodium-independent, 2-amino-bicyclo heptane 2-carboxylic acid (BCH)-sensitive system L with a Vmax of 30 nmol/g cells/min; and (iv) A sodium-independent entry which was not inhibited by high concentrations of MeAIB or BCH. The Km value of each of the former three carrier mediated transport systems was 0.46 mM. Starvation of animals brought about important increase in the Vmax of the A system by 97% and that of ASC system by 1003% which was accompanied by similar increases in the Km values of these systems. These results show an adaptive regulation of these two sodium-dependent sites as a result of starvation.