Solute adsorption to surfaces of articular cartilage explants: apparent versus actual partition coefficients

Abstract

Solute transport is important to biological activities and contrast agent-based imaging of articular cartilage. Solute partitioning, which characterizes equilibrium concentration distributions within tissues, is typically measured assuming that solute is distributed within the tissue volume. However, solutes also adsorb to cartilage surfaces. Ignoring this adsorption results in measurements of apparent partition coefficients (K_a) significantly greater than actual ones (K). A theoretical model was developed to predict the resulting errors in partition coefficient measurements and to estimate K based on K_a data. These errors increase as explant surface/volume ratio (S/V) increases. A range of solutes including 3 or 4 kDa and 40 kDa dextrans, chondroitin sulfate, insulin and myoglobin conjugated to three different fluorophores (fluorescein isothiocyanate (FITC), tetramethylrhodamine isothiocyanate (TRITC) and carboxytetramethylrhodamine (TAMRA)) and fluorophores alone were studied. Experiments showed significant increases in K_a with S/V for FITC and TRITC and their conjugates. For TAMRA, while increased S/V increased K_a of dextrans, it did not affect K_a of chondroitin sulfate, insulin, myoglobin or TAMRA alone. S/V had the most significant effect on K_a for TRITC and its conjugates and smallest effects for TAMRA and its conjugates. Fluorescence microscopy confirmed solute accumulation at cartilage surfaces for all solutes, with strongest adsorption for TAMRA and weakest for FITC. Consistent with theory, under these extreme conditions K_a was less affected by adsorption; otherwise differences between K and K_a were significant. Differences in adsorption and transport properties of solutes labeled with different fluorophores highlighted the importance of molecular-level solute–matrix interactions on solute transport in cartilage.