Chronic Osmotic Challenge The responses of chondrocyte volume, the membrane transporters and ion channels,and cell metabolism to gradual changes in extracellularosmolarity are very different compared with those involved in acute osmotic change. This is mainly because the corresponding osmolyte movements mediated by thevolume-sensitive membrane transporters and ion channels keep pace with the volume changes through con-tinuous volume adjustment resulting in limited changes to cell volume. This is termed iso-volumetric regulation(IVR), and although it has been studied in detail in other cell types (e.g. [83]) and despite its potential importance, it has not yet received sufficient attention inchondrocyte volume regulation. An interesting study on freshly isolated bovine articular chondrocytes reducedosmolarity from 350 to 140 mOsm either rapidly(5 min) or gradually (over 180 min). The acute hypo-osmotic challenge caused rapid cell swelling followedby robust RVD, whereas the gradual change caused amuch smaller increase in cell volume, with only a weakRVD response [80]. Importantly, up- or downregulationof transporter expression may occur as well as changesto cell metabolism leading to enhanced synthesis ordegradation of intracellular osmolytes (e.g. sugars,polyols, amino acids) to compensate for the changes in volume. For example, transcription of the non-essentialamino acid taurine (2-aminoethanesulphonic acid) trans-porter gene in chondrocytic cells is upregulated by hypertonic conditions [84] leading to osmolyte accumulation. In response to changes in osmolarity, these osmo-protective molecules may moderate changes to cell volume under anisotonic conditions and potentially confercyto-protective and anti-inflammatory effects [85]. Inhuman intervertebral disc cells following hyperosmotic challenge, the gene expression profile identified 42genes that were significantly changed, including those involved in cytoskeletal remodelling and ion and osmolyte transport [86].