Quantitative analysis of the effects of hyaluronan and aggrecan concentration and hyaluronan size on the elasticity of hyaluronan-aggrecan solutions.

We determined elasticity (G') and viscosity (G'') of various aggrecan-hyaluronan solutions using a controlled-stress rheometer with high (10 Hz) to low (0.1 Hz) frequencies. Aggrecan solution (50 mg/ml) alone showed little elasticity at any frequency, but the addition of 3300 kDa hyaluronan at 0.001-0.1 mg/ml markedly increased the elasticity, but not the viscosity, at all frequencies. Increasing hyaluronan concentration at >0.1 mg/ml did not further increase the elasticity of the aggrecan solution, and the elasticity of the aggrecan-hyaluronan complex solution reached a plateau at a 500:1 (w/w) ratio. In studies with increasing concentrations of aggrecan and a constant concentration (0.5 mg/ml) of 3300 kDa hyaluronan, aggrecan induced elasticity only at >20 mg/ml, indicating the presence of a critical concentration for elasticity. In the presence of 50 mg/ml aggrecan, 1000 kDa hyaluronan had far less effect on the elasticity of the aggrecan solution than did 3300 kDa hyaluronan. These findings suggest that only approximately 50% reduction in aggrecan concentration (<20 mg/ml), or reduced hyaluronan size (<1000 kDa)--compared with their physiological levels in young cartilage--can abolish the elastic network of the aggrecan-hyaluronan complex.