Induction of betaine-γ-aminobutyric acid transport activity in porcine chondrocytes exposed to hypertonicity

摘要

class="enumerated" style="list-style-type:decimal">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.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 γ-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.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.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.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.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.