Measurement of the charge properties of articular cartilage by an electrokinetic method

The charge structure of the surface of articular cartilage determines its i nteractions with the macromolecules and cells of synovial fluid. It may the reby be important to the physiological function and pathological degenerati on of the tissue. To determine whether the electrokinetic properties of the surface differ from those of the bulk tissue, we measured the streaming po tential generated by the flow of electrolyte over the surface of a cartilag e plug mounted in a chamber built for that purpose. We then calculated the effective surface charge density. In nonfibrillated cartilage from the huma n femoral head, the surface charge density, 0.037 +/- 0.004 Cm-2 (mean +/- SD), was approximately half that measured at the surface of slices cut from the middle and deep zones. In addition, the surface charge density fell re latively little at low pH; this is consistent with a higher proportion of s trongly acidic groups. The variations in surface charge density were found to be similar to those in total fixed charge density in the slices by the t racer cation method. Therefore, no evidence exists that the actual surface differs in composition from the immediately underlying matrix. The addition of synovial fluid (0.0025 ml/ml) to the superfusing solution reduced the s urface charge density by 25 +/- 9% (n = 5), and we attributed this to the b inding of synovial-fluid macromolecules.