Induction of betaine-gamma-aminobutyric acid transport activity in porcinechondrocytes exposed to hypertonicity

1. We measured the rates of uptake of selected amino acids and betaine by p rimary cultures of chondrocytes from porcine articular cartilage after the cells had been incubated in 'isotonic' (0.3 osmol l(-1)) or hypertonic (0.5 osmol l(-1)) media. 2. Na+-dependent uptake of methylaminoisobutyric acid increased rapidly whe n the cells were exposed to hyper tonic conditions, reached a peak after 6- 9 h, and then gradually decreased so that after 24 h it was only slightly a bove the control value. Conversely, (Na+ + Cl-)-dependent influx of gamma-a minobutyric acid (GABA) remained low for the first 9 h of hyper tonic incub ation, but then increased markedly to reach a plateau value after 24-30 h. Betaine influx also increased in cells incubated in hypertonic medium, bein g mainly Na+ dependent after 6 h, but (Na+ + Cl-)-dependent after 24 h. 3. This pattern indicates that exposure of the chondrocytes to hypertonicit y induces first amino acid transport system A and then, as this decreases a gain, betaine-GABA transport activity. 4. Induction of betaine-GABA transport activity did not require continuous exposure of chondrocytes to hypertonicity; but the magnitude of the increas e measured st the end of a 24 h incubation period was proportional to the l ength of time the cells had been exposed to hypertonicity during the 24 h. 5. Isolation and culture of chondrocytes in 0.4 osmol l(-1) medium, instead of 0.3 osmol l(-1) significantly increased their betaine-GABA transport ac tivity but not their system A activity. 6. Induction of betaine-GABA transport activity was prevented by addition o f either actinomycin D or cycloheximide to the medium, but no mRNA for the betaine-GABA transporter known as BGT-1 was detected by Northern blot analy sis of extracts of chondrocytes.