Comparison of synovial PO2 and sympathetic vasoconstrictor responses in normal and acutely inflamed rabbit knee joints.

Experiments were performed to assess the effect of acute inflammation of the rabbit knee joint on the partial pressure of oxygen in synovial fluid (Ps,O2) and nerve-mediated vasoconstrictor responses of articular blood vessels. With the hypodermic needle oxygen electrode sited within the synovial cavity in the posterior region of the knee joint, mean (+/- S.E.M.). Ps,O2 was 37.4 +/- 3.6 mmHg (n = 10) in the inflamed group, which differed significantly (P < 0.05) from that occurring in the normal group from a different series (48.2 +/- 3.1 mmHg; n = 18). Ps,O2 was found to decrease with increasing depth of penetration of the oxygen electrode into the joint cavity of the inflamed knee, as in the normal knee. The lowest values were observed close to articular cartilage. Absolute blood flow was measured using radiolabelled microspheres whilst relative changes in blood flow were assessed using laser Doppler flowmetry. The former technique showed that the inflamed joints had a significantly higher blood flow. Electrical stimulation of the posterior articular nerve (PAN) of the knee resulted in vasoconstriction of knee joint blood vessels, which was accompanied by a reduction in Ps,O2. The frequency-response and voltage-response profiles to electrical stimulation of the PAN, although differing in magnitude, showed a high degree of correlation between blood flow and Ps,O2. The frequency-response profile to electrical stimulation of the PAN shifted to the right in inflamed joints compared with normal joints, suggesting a reduction in the efficacy of the sympathetic nervous system in regulating blood flow to the inflamed joints. Although the inflamed joint had a higher blood flow, Ps,O2 was lower compared with the normal joint. The results of this study show significantly altered blood flow, Ps,O2 and nerve-mediated constrictor responses in the acutely inflamed joint. These are related to the inflammatory response and may contribute to the pathogenesis of arthritis.