DETERMINATION OF ION ACTIVITY-COEFFICIENTS AND FIXED CHARGE-DENSITY IN CARTILAGE WITH NA-23 MAGNETIC-RESONANCE MICROSCOPY

The sodium concentration and the physicochemical state of sodium ions in biological polyelectrolytes may be an indirect measure of the macro molecular matrix integrity. Nuclear magnetic resonance (NMR) microscop y was used to measure the sodium concentration and to estimate the car tilage matrix fixed charge density (FCD) in control, acid-neutralized, and enzyme-digested bovine nasal cartilage (BNC), a model for human c artilage. The mean ion activity coefficient of sodium in cartilage was calculated 6 om the measured tissue sodium concentration as a functio n of external NaCl concentration and compared with the predictions of the Debye-Huckel and the modified Manning theories. Our results show t hat (1) the FCD in bovine nasal cartilage (at physiological NaCl conce ntrations) is in the range of 0.25-0.35 m, (2) the sodium content meas ured by NMR microscopy reflects indirectly the changes of FCD induced by changes in pH and enzymatic digestion, and (3) the measured activit y coefficient for NaCl in the bovine nasal cartilage matrix is signifi cantly lower than that of an ionic solution of the same salt concentra tion. An extension of the model developed by Manning for polyelectroly tes is in good qualitative agreement with our experimental results whi ch showed a decrease in the mean activity coefficient with decreased r eservoir concentration.