OSMOTICALLY INDUCIBLE UPTAKE OF BETAINE VIA AMINO-ACID-TRANSPORT SYSTEM-A IN SV-3T3 CELLS

The osmotically inducible uptake of betaine (NNN-trimethylglycine) by SV-3T3 cells has been studied and compared with the similar process in MDCK cells. Betaine uptake by SV-3T3 cells could be described in term s of a saturable, Na+-dependent, component plus a small non-saturable, Na+-independent, component. Transport was active, producing considera ble accumulation of betaine in the cells. After exposure of the cells to hypertonic conditions for 6 h, there was a marked increase in betai ne uptake. Kinetic analysis indicated that this increase resulted from an increase in the V-max. value of the saturable component, from abou t 88 to 185 nmol of betaine/ 5 min per mg of protein, the correspondin g K-m values of about 15 and 10 mM not being significantly different. This induction of transport activity was detectable only after about 2 h exposure of the cells to hypertonic medium, closely paralleling an induction of influx of N-methylaminoisobutyric acid, and was prevented by the presence of cycloheximide. Betaine influx was markedly inhibit ed by several neutral amino acids, particularly those transported by s ystem A, such as N-methylaminoisobutyric acid and the imino acid proli ne. A high concentration (25 mM) of betaine also significantly inhibit ed the uptake of proline by SV-3T3 cells. Although very similar result s were obtained with MDCK cells, prolonged exposure of cells to hypert onic conditions revealed distinct differences. When the hypertonic inc ubation was extended from 6 h to 24 h, betaine transport in SV-3T3 cel ls either remained the same or decreased, whereas it showed a further marked increase in MDCK cells, and also became sensitive to inhibition by gamma-aminobutyric acid. mRNA for the betaine transporter BGT-1 [Y amauchi, Uchida, Kwon, Preston, Brooks Robey, Garcia-Perez, Burg and H andler (1992) J. Biol. Chem. 267, 649-652] was detectable in MDCK cell s exposed to hypertonic medium for 24 h, but not in SV-3T3 cells under any conditions. It is concluded that SV-3T3 cells do not produce a sp ecific inducible transporter analogous to BGT-1, but they can accumula te betaine via the amino acid transport system A.