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 (β 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. J. Cell. Physiol. 212:600–609, 2007. © 2007 Wiley‐Liss, Inc.