HYPEROSMOTIC STRESS UP-REGULATES AMINO-ACID-TRANSPORT IN VASCULAR ENDOTHELIAL-CELLS

Cultured vascular endothelial cells take up L-proline by sodium-depend ent transport. Cells incubated in medium made hyperosmotic by addition of sucrose showed a dose-dependent increase in Na+/proline cotranspor t. Studies with alpha-(methylamino)isobutyric acid revealed that the u p-regulation was specific for amino acid transport system A. Up-regula tion was blocked by actinomycin D and cycloheximide, indicating roles for gene transcription and protein synthesis. Upregulation was maximum after five to six hours of hyperosmotic treatment, but returned to co ntrol levels when osmotic stress was maintained for 24 hours. The decl ine at 24 hours was accompanied by a significant increase in Na+/gamma -aminobutyric acid cotransport. The activity of this system, which als o transports betaine, remained unchanged after just five hours of hype rosmotic stress. Inclusion of betaine in the hyperosmotic medium reduc ed up-regulation of system A. Na/Pi cotransport also was upregulated b y five hours of hyperosmotic stress. Up-regulation of system A, but no t Na/Pi cotransport, was detected in isolated membrane fractions indic ating that increased activity of this membrane transport system may be one mechanism by which vascular endothelial cells accumulate amino ac ids. The amino acids may act as organic osmolytes to help maintain nor mal cell volume during the early phase of hyperosmotic stress.