Dependence of electrical conductivity on fixed charge density in articular cartilage

A new system for measuring the resistivity of articular cartilage is based on the four-point probe technique. The system measures resistivity as a function of depth from the articular surface. The measurements obtained showed that the conductivity, kappa, of articular cartilage equilibrated in Ringer's solution (pH, 7.4) is constant in all zones, with a value of 6-10 X 10(-3) mho/cm. Conversely, kappa of articular cartilage equilibrated in distilled water varies distinctly, being low in the superficial zone (about 0.7 X 10(-3) mho/cm) and high in the middle to deep zone (about 3 X 10(-3) mho/cm). This is almost the same variation shown by the fixed charge density, FCD, which was measured by the tracer cation method; a linear correlation between kappa and the FCD was found (correlation coefficient, 0.88). These observations were well understood based on the Donnan equilibrium; the mobility of Na+ in cartilage was thought to be independent of FCD.