Carbonic anhydrase localization in normal and osteochondrotic joint cartilage of growing pigs

Bicarbonate-dependent pH(i) regulation by chondrocytes within the superficial zone of bovine articular cartilage

Control of chondrocyte pH (pH(i)) determines articular cartilage matrix metabolism. However, the transporters of chondrocytes in situ throughout cartilage zones are unclear, and we tested the hypothesis that chondocytes within the superficial zone (SZ) utilise a HCO(3) (-)-dependent system absent from other zones. Imaging of single BCECF-labelled cells was used to monitor the pH(i) of in situ chondrocytes within the cartilage zones, and also that of cells isolated from the SZ or full depth (FD) explants. Resting pH(i) and intrinsic buffering power (beta(i)) in HEPES-buffered saline was not different between SZ and DZ cells, however the pH(i) of SZ chondrocytes was lower in HCO(3) (-) saline. Ammonium pre-pulse was used to acid-load cells and pH(i) recovery by in situ or isolated SZ chondrocytes shown to be totally dependent on HCO(3) (-). pH(i) recovery rate was significantly (P < 0.05) greater for in situ cells, suggesting that isolation damaged the HCO(3) (-)-dependent system. Recovery of pH(i) by in situ cells was blocked by the anion transport inhibitor DIDS, and partially inhibited by EIPA probably non-specifically. Recovery of pH(i) by acidified MZ or DZ cells or those isolated from FD explants was not affected by HCO(3) (-) (P > 0.05). Na(+)-dependent HCO(3) (-)-(NBC) transporters were identified in SZ chondrocytes by fluorescence immunohistochemistry suggesting that this system might account for the HCO(3) (-)-dependent recovery of pH(i). Bovine articular cartilage chondrocytes possess a HCO(3) (-)-dependent transporter which plays a key role in pH(i) regulation in cells in the SZ, but not in chondrocytes within deeper cartilage zones.

Comparison of proteoglycan and collagen in articular cartilage of horses with naturally developing osteochondrosis and healing osteochondral fragments of experimentally induced fractures

OBJECTIVE

To compare articular cartilage from horses with naturally developing osteochondrosis (OC) with normal articular cartilage and healing cartilage obtained from horses with experimentally induced osteochondral fractures.

SAMPLE POPULATION

109 specimens of articular cartilage from 78 horses.

PROCEDURE

Morphologic characteristics, proteoglycan (PG), and type II collagen were analyzed in articular cartilage of OC specimens (group 1), matched healing cartilage obtained 40 days after experimentally induced osteochondral fractures (group 2), and matched normal cartilage from the same sites (group 3).

RESULTS

79 specimens of OC cartilage were obtained from horses. Ex vivo PG synthesis was significantly greater in the femoral cartilage, compared with synthesis in the tibial cartilage, and significantly greater for groups 1 and 2, compared with group 3. For groups 1 and 2, femoral fragments had significantly greater PG content, compared with PG content in tibial fragments. Keratan sulfate content was significantly less in group 3, compared with groups 1 and 2. Cartilage from the OC specimens had loss of structural architecture. The OC tissue bed stained positive for chondroitin sulfate and type II collagen, but the fracture bed did not.

CONCLUSIONS AND CLINICAL RELEVANCE

Our analyses could not distinguish articular cartilage from horses with OC and a healing fracture. Both resembled an anabolic, reparative process. Immunohistochemical analysis suggested a chondromyxoid tissue in the OC bed that was morphologically similar to fibrous tissue but phenotypically resembled hyaline cartilage. Thus, tissue in the OC bed may be degenerative cartilage, whereas tissue in the fracture bed may be reparative fibrous callus.

Functional and molecular determination of carbonic anhydrase levels in bovine and cultured human chondrocytes

In this study, bovine articular and human chondrocytes from the C-20/A4 cell line were tested for the functional activity and molecular presence of the enzyme carbonic anhydrase. This enzyme is classically considered to be important in the maintenance of high cellular buffering capacity by catalysing the slow attainment of equilibrium between CO(2) and HCO(3)(-). The first functional assay measured the rate of pH equilibration after administration of a fixed dose of CO(2) solution to cell lysates. Compared to positive controls (human erythrocytes, murine M1 cells and purified carbonic anhydrase), chondrocyte lysates attained equilibrium at a significantly slower rate, similar to the rate obtained with a negative control (Xenopus oocytes). A second functional assay studied CO(2) hydration kinetics in intact C-20/A4 cells, using a pH-sensitive fluorescent dye, as the CO(2) content of the extracellular solution was changed. It was shown that C-20/A4 cells accelerate hydration only to a small degree. Hydration kinetics were reduced to the spontaneous rate in the presence of acetazolamide. Western immunoblotting with isoform-nonspecific antibodies to carbonic anhydrase demonstrated weak staining in both bovine and human chondrocytes.