HYPERTONIC UP-REGULATION OF AMINO-ACID-TRANSPORT SYSTEM-A IN VASCULARSMOOTH-MUSCLE CELLS

The A10 line of vascular smooth muscle cells has Na+ dependent transpo rt systems for alanine, proline, and P-i, whereas uptake of leucine, m yo-inositol and D-glucose is Na+ independent. When A10 cells were incu bated for 4 h in medium made hypertonic by addition of sucrose, there was a marked increase in Na+-dependent transport of alanine and prolin e but no change in Na+-dependent P-i uptake or Na+-independent uptake of leucine and inositol. Intracellular alanine content was increased 6 1% by the hypertonic treatment. Other nonpenetrating solutes, such as cellobiose and mannitol, reproduced the effect of sucrose, but urea, a penetrating solute, did not. Studies with 2-(methylamino)-isobutyric acid revealed that the upregulation by hypertonicity involved only sys tem A. Increases in alanine and proline uptake also occurred after inc ubating the cells in isotonic medium containing 0.1 mM ouabain, sugges ting that an increase in intracellular Na+ may be part of the intracel lular signal for upregulation of system A. Hypertonic upregulation of Na+-dependent alanine transport occurred also in primary cultures of v ascular smooth muscle cells. The response was blocked by actinomycin D and cycloheximide, indicating that gene transcription and protein syn thesis play important roles in the mechanism leading to increased alan ine uptake. We conclude that vascular smooth muscle cells, during prol onged hypertonic stress, activate system A and accumulate specific neu tral amino acids which may act as organic osmolytes to help maintain n ormal cell volume.