Cell volume regulation in chondrocytes

Chondrocytes are the cells within cartilage which produce and maintain the extracellular matrix. Volume regulation in these cells is vital to their function and occurs in several different physiological and pathological contexts. Firstly, chondrocytes exist within an environment of changing osmolarity and compressive loads. Secondly, in osteoarthritic joint failure, cartilage water content changes and there is a notable increase in chondrocyte apoptosis. Thirdly, endochondral ossification requires chondrocyte swelling in association with hypertrophy. Regulatory volume decrease (RVD) and regulatory volume increase (RVI) have both been observed in articular chondrocytes and this review focuses on the mechanisms identified to account for these. There has been evidence so far to suggest TRPV4 is central to RVD; however other elements of the pathway have not yet been identified. Unlike RVD, RVI appears less robust in articular chondrocytes and there have been fewer mechanistic studies; the primary focus being on the Na(+)-K(+)-2Cl(-) co-transporter. The clinical significance of chondrocyte volume regulation remains unproven. Importantly however, transcript abundances of several ion channels implicated in volume control are changed in chondrocytes from osteoarthritic cartilage. A critical question is whether disturbances of volume regulation mechanisms lead to, result from or are simply coincidental to cartilage damage.