Bicarbonate-dependent pHi regulation by Chondrocytes within the superficial zone of bovine articular cartilage

Control of chondrocyte pH (pHi) determines articular cartilage matrix metabolism. However, the transporters of chondrocytes in situ throughout cartilage zones are unclear, and we tested the hypothesis that chonclocytes within the superficial zone (SZ) utilise a HCO3- dependent system absent from other zones. Imaging of single BCECF-labelled cells was used to monitor the pHi of in situ chondrocytes within the cartilage zones, and also that of cells isolated from the SZ or full depth (FD) explants. Resting pHi and intrinsic buffering power in HEPES-buffered saline was not different between SZ and DZ cells, however the pHi of SZ chondrocytes was lower in HCO3- saline. Ammonium pre-pulse was used to acid-load cells and pHi recovery by in situ or isolated SZ chondrocytes shown to be totally dependent on HCO3-. pHi recovery rate was significantly (P < 0.05) greater for in situ cells, suggesting that isolation damaged the HCO3- -dependent system. Recovery of pHi by in situ cells was blocked by the anion transport inhibitor DIDS, and partially inhibited by EIPA probably nonspecifically. Recovery of pHi by acidified MZ or DZ cells or those isolated from FD explants was not affected by HCO3-(P > 0.05). Na+ dependent HCO3- -(NBC) transporters were identified in SZ chondrocytes by fluorescence immunohistochernistry suggesting that this system might account for the HCO3- -dependent recovery of pHi. Bovine articular cartilage chondrocytes possess a HCO3- -dependent transporter which plays a key role in pHi regulation in cells in the SZ, but not in chondrocytes within deeper cartilage zones.