Expression of constitutive but not inducible cyclooxygenase maintains articular perfusion in the rat knee

Experiments were performed in the normal rat knee joint to investigate the role of different isoforms of cyclooxygenase (COX) in the regulation of bas al joint blood flow. Laser Doppler imaging (LDI) was used to measure articu lar perfusion, and reverse transcriptase polymerase chain reaction (RT-PCR) for the detection of COX-1 and COX-2 mRNA in joint tissue. Intravenous inf usion of indomethacin (a non-selective inhibitor of COX; 0.34 nmol min(-1)) over 40 min produced a time dependent increase in articular vascular resis tance (maximum 22.5 % at 40 min; P < 0.0001, one-way ANOVA) whereas vehicle over a similar time period had no effect in a control group. An equimolar concentration of a highly selective inhibitor for COX-2, SC-236, was admini stered in a further group of rats but this did not increase articular vascu lar resistance. While there was no significant difference between the respo nse to vehicle and SC-236 (two-way ANOVA; P = 0.686, n = 6) the response to indomethacin was significantly greater than vehicle or SC-236 (two-way Ano va; P < 0.0001, n = 6), COX-1, but not COX-2, was detectable by RT-PCR in a ll joint tissue samples examined (n = 4). The results of this study indicat e that prostaglandins (PGs) play an important role in the maintenance of ba sal perfusion in the rat knee joint, with COX-1 being the physiologically r elevant isoform.