The role of a swelling-activated taurine transport pathway in the regulation of articular chondrocyte volume

Abstract.

Swelling articular chondrocytes by reducing osmolarity stimulates a taurine transport pathway, which is implicated in regulatory volume decrease (RVD) in various cell types. The present study investigated factors controlling the activity of this pathway in chondrocytes, in particular (1) the effects of the acute (seconds) and chronic (hours) exposure of chondrocytes to anisotonic media, and (2) whether there is a role for metabolites from the arachidonic acid cascade in activating the taurine transport pathway. For in situ and isolated chondrocytes, the point at which swelling-activated [¹⁴C]taurine efflux was stimulated (the "set-point") corresponded closely to the osmolarity of the incubation medium (180, 280 or 380 mosmol/l). However, the volume of chondrocytes isolated into these media and measured by confocal microscopy was not different (≅645 µm³). Activity of the swelling-activated taurine transport pathway was inhibited by REV5901 (an inhibitor of steps of the arachidonic acid cascade; K _0.5 8±4 µM), NDGA (a general lipoxygenase inhibitor; K _0.5 28±5 µM), or MK886 (an inhibitor of the 5-lipoxygenase-activating protein; 91% inhibition at 10 µM), but weakly by the more potent 5-lipoxygenase inhibitor REV5901 para (K _0.5 350±100 µM). Addition of the leukotriene (LT) B₄ or D₄ receptor antagonists, CP-105,696 and L660,711 respectively, or of the leukotrienes LTB₄, LTC₄, LTD₄ and LTE₄ or lipoxins (hepoxylin A₃ or B₃) had no effect on the activity of the pathway in isotonic or hypotonic media. The role of the pathway in RVD was determined in isolated calcein-loaded chondrocytes using fluorescence imaging. RVD was observed and inhibited by REV5901 (50 µM) and by NDGA (75 µM). The data show that despite chronic exposure of chondrocytes to anisotonic media, the cells maintain a pre-determined volume that is the "set-point" for the activation of the taurine transport pathway following acute hypotonic challenge. This pathway appears to play a role in chondrocyte RVD, but its activation does not involve metabolites of the arachidonic acid cascade.