title: Induction of betaine-gamma-aminobutyric acid transport activity in porcine chondrocytes exposed to hypertonicity.
abstract: 1. We measured the rates of uptake of selected amino acids and betaine by primary 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 when the cells were exposed to hypertonic conditions, reached a peak after 6-9 h, and then gradually decreased so that after 24 h it was only slightly above the control value. Conversely, (Na+ + Cl-)-dependent influx of gamma-aminobutyric acid (GABA) remained low for the first 9 h of hypertonic incubation, but then increased markedly to reach a plateau value after 24-30 h. Betaine influx also increased in cells incubated in hypertonic medium, being mainly Na+ dependent after 6 h, but (Na+ + Cl-)-dependent after 24 h. 3. This pattern indicates that exposure of the chondrocytes to hypertonicity induces first amino acid transport system A and then, as this decreases again, 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 increase measured at the end of a 24 h incubation period was proportional to the length 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 activity, but not their system A activity. 6. Induction of betaine-GABA transport activity was prevented by addition of 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 analysis of extracts of chondrocytes.
authors.affiliation: Istituto di Anatomia Patologica Veterinaria, Università degli Studi di Parma, 43100 Parma, Italy.
authors.forename: E
authors.initials: E
authors.lastname: de Angelis
authors.forename: P G
authors.initials: PG
authors.lastname: Petronini
authors.forename: P
authors.initials: P
authors.lastname: Borghetti
authors.forename: A F
authors.initials: AF
authors.lastname: Borghetti
authors.forename: K P
authors.initials: KP
authors.lastname: Wheeler
mesh_terms: D000596:Amino Acids / Q000378:metabolism; D000818:Animals; D001622:Betaine / Q000378:metabolism*; D001692:Biological Transport; D015152:Blotting, Northern; D002470:Cell Survival; D002478:Cells, Cultured; D019902:Chondrocytes / Q000187:drug effects / Q000378:metabolism*; D003513:Cycloheximide / Q000494:pharmacology; D003609:Dactinomycin / Q000494:pharmacology; D005765:Gastrointestinal Agents / Q000378:metabolism*; D006982:Hypertonic Solutions; D010641:Phenotype; D011500:Protein Synthesis Inhibitors / Q000494:pharmacology; D012333:RNA, Messenger / Q000096:biosynthesis; D013552:Swine; D005680:gamma-Aminobutyric Acid / Q000378:metabolism*
chemical_list: D000596:Amino Acids; D005765:Gastrointestinal Agents; D006982:Hypertonic Solutions; D011500:Protein Synthesis Inhibitors; D012333:RNA, Messenger; D003609:Dactinomycin; D001622:Betaine; D005680:gamma-Aminobutyric Acid; D003513:Cycloheximide