The osmotic pressure of chondroitin sulphate solutions: Experimental measurements and theoretical analysis

We used equilibrium dialysis to measure the osmotic pressure of chondroitin sulphate (CS) solutions as a function of their concentration and fixed cha rge density (FCD) and the ionic strength and composition of the solution. O smotic pressure varied nonlinearly with the concentration of chondroitin su lphate and in 0.15 M NaCl at FCDs typical of uncompressed cartilage (approx imately 0.4 mmol/g extrafibrillar H2O) was similar to 3 atmospheres. Osmoti c pressure fell by 60% as solution ionic strength increased up to about 1 M , but remained relatively constant at higher ionic strengths. The ratio of Ca2+ to Na+ in the medium was a minor determinant of osmotic pressure. The data are compared with a theoretical model of the electrostatic contributio n to osmotic pressure calculated from the Poisson-Boltzmann equation using a rod-in-cell model for CS. The effective radius of the polyelectrolyte rod is taken as a free parameter. The model qualitatively reproduces the non-l inear concentration dependence, but underestimates the osmotic pressure by an amount that is independent of ionic strength. This difference, presumabl y arising from oncotic and entropic effects, is approximately 1/3 of the to tal osmotic pressure at physiological polymer concentrations and ionic stre ngth.