Changes to chondrocyte volume have a deleterious effect on the metabolism of the extracellular matrix of articular cartilage, possibly leading to the cartilage loss characteristic of osteoarthritis (OA; Urban et al. 1993). Using confocal laser scanning microscopy (CLSM) we have reported that chondrocytes from areas of degenerate tissue are larger than those from non-degenerate regions (Bush & Hall, 2003). It is possible that this cell swelling results from the inability of the cells to undergo regulatory volume decrease (RVD). Here, we have tested the RVD capacity of in situ human chondrocytes within non-degenerate and degenerate cartilage.

Cartilage from human tibial plateaux (with Ethical permission) was obtained at total knee replacement surgery and cultured in Dulbecco’s modified Eagle’s medium (280 mosmol l^-1). The fluorescent dye calcein was incorporated into chondrocytes (calcein-AM; 5 µM, 37 °C, 30 min). The resting volume of mid zone (MZ) chondrocytes, and their RVD capacity during a rapid (< 30 s) osmotic challenge (280 to 180 mosmol l^-1; 21 °C) were measured from CLSM images (Zeiss LSM 510, Axioskop, X 63 ceramic water dipping lens) using analysis software (Bitplane, Zurich). Chondrocyte volumes were reported as the percentage change compared to the value before osmotic challenge and plotted against time. Data are means ± S.E.M. (total number of joints [total number of cells]).

Paired samples of relatively non-degenerate articular cartilage (AC_ND, (3[21])) or degenerate articular cartilage (AC_D (3[20]); demonstrating surface fibrillation) were taken from three joints. The resting volumes of cells before hypo-osmotic challenge were larger in AC_D 792 ± 44 µm³ compared to 671 ± 41 µm³ for AC_ND (P < 0.002; Students’ paired t test). When exposed to a hypotonic challenge, there was no difference in the extent of cell swelling, 28 ± 4 % and 36 ± 3 % for AC_ND and AC_D, respectively (P > 0.05). After 20 min post-osmotic challenge, cell volume had recovered from the point of maximal swelling by 32 ± 12 % and 30 ± 17 % for AC_ND and AC_D, respectively (P > 0.05).

Human articular chondrocytes within degenerate cartilage (AC_D) are larger than those in non-degenerate tissue (AC_ND), but there was no difference in the response to the hypotonic challenge. The extent of cell swelling and the rate of volume recovery by RVD were similar. These data show that the RVD mechanism activated in response to an acute hypotonic challenge is not compromised in chondrocytes within degenerate cartilage.

This work was supported by the Arthritis Research Campaign (H0621). We thank Mr J. Aderinto for helping with the cartilage samples.