Time course of substance P-induced protein extravasation in the rat knee joint measured by micro-turbidimetry.

Intra-articular perfusion of the rat knee joint with substance P (SP) resulted in protein extravasation into the synovial cavity. This response was tested over a range of SP concentrations from 100 pM to 100 microM. The response was dose dependent from 10 nM to 10 microM, with each dose producing a steady rise of the protein content in the fluid aspirated from the synovial cavity for 12-16 min and then fell sharply again over an equivalent period despite continuing perfusion with SP. Concentrations below 10 nM gave rise to very brief increases in protein extravasation whose magnitude differed little over the range of 100 pM to 10 nM but differed significantly from control (saline) perfusion.