REGULATION OF SYSTEM-B-0 AMINO-ACID-TRANSPORT ACTIVITY IN THE RENAL EPITHELIAL-CELL LINE NBL-1 AND CONCOMITANT CHANGES IN SAAT1 HYBRIDIZINGTRANSCRIPTS

alpha-(Methylamino)isobutyric acid (MeAIB) insensitive Na+-dependent a lanine transport activity in the bovine kidney cell line NBL-1 was inc reased upon amino acid starvation (greater than or equal to 20% over c ontrol levels). When L-phenylalanine(3 mM) was included in the starvat ion medium the increase was further enhanced (greater than or equal to 85%, over control levels). In cells grown in control medium the V-max for MeAIB-insensitive Na+/alanine co-transport was found to be 6.0+/- 0.7 nmol/3 min per mg (K-m 41+/-12 mu M) and for L-phenylalanine-treat ed amino-acid-starved cells the V-max was 21+/-5 nmol/3 min per mg (K- m 92+/-40 mu M). The increase in V-max was prevented by cycloheximide. Substrate specificity analysis identified the L-phenylalanine-induced transport system as System B degrees. [S-35]Methionine labelling of c ells during the amino acid starvation/phenylalanine treatments resulte d in the differential labelling of a protein of 78 kDa. Northern-blot analysis using a SAAT1-specific probe revealed the presence of a new t ranscript (3.2 kb) in RNA extracted from cells incubated in amino acid starvation medium with L-phenylalanine included. The present findings suggest a novel means of control for System-B degrees by the use of p hysiological stress. It is also proposed that SAAT1 and System B degre es transcripts have considerable sequence similarity.