Diffusion and partition of solutes in cartilage under static load

Abstract

We describe experimental apparatus, methodology and mathematical algorithms to measure diffusion and partition for typical small ionic solutes and inulin (a medium size solute) in statically loaded cartilage. The partition coefficient based on tissue water ( K H 2O ) of Na + increased from 1.8 to 4.5 and for SO 4 −2 decreased from 0.5 to 0.1, when the applied presure was raised from zero to 22 atm K H 2O of inulin decreased from 0.3 to 0.05, for an increase in pressure from zero to 11 atm. Our theoretical interpretation of the results is that the partition coefficient can be expressed as a function of fixed charge density (FCD) for both loaded and unloaded cartilage. The partition coefficient shows good agreement with the ideal Gibbs–Donnan equilibrium, particularly when FCD is based on extrafibrillar water (EFW). The diffusion coefficients, D̄ also decreased with an increase in applied pressure; raising the pressure from 0 to 22 atm resulted in the following changes in the values of D̄: for Na + from 2.86×10 −6 to 1.51×10 −6 cm 2/s, for SO 4 −2 from 1.58×10 −6 to 7.5×10 −7 cm 2/s, for leucine from 1.69×10 −6 to 8.30×10 −7 cm 2/s and for inulin from 1.80×10 −7 to 3.30×10 −8 cm 2/s. For the three small solutes (two charged and one neutral) the diffusion coefficient D̄ is highly correlated with the fraction of fluid volume in the tissue. These experimental results show good agreement with the simple model of Mackie and Meares: hence solute charge does not affect the diffusion of small solutes under load. For inulin D̄ & K show some agreement with a modified Ogston model based on two major components, viz., glycosaminoglycans (GAG) and core protein. We conclude that the changes in the partition and diffusion coefficients of small and medium size solutes in statically loaded cartilage can be interpreted as being due to the reduction in hydration and increase in FCD. The change in the latter affects the partition of small ionic solutes and the partition and diffusion of larger molecules. Our results throw light on the ionic environment of chondrocytes in loaded cartilage as well as on the transport of solutes through the matrix.