Immunoreactive (ir)-endothelin (ET)-1 concentrations in serum samples and synovial fluids from patients with rheumatoid arthritis were higher than concentrations in sera obtained from healthy volunteers. No significant difference in ir-ET-1 concentrations in synovial fluid was observed between rheumatoid arthritis patients and osteoarthritis patients. Cultured fluids of synovial cells collected from synovial tissues and leucocytes from synovial fluids of rheumatoid arthritis patients were studied to determine the origin of ir-ET-1 in synovial fluids. Ir-ET-1 was detected in the cultured fluids of synovial macrophage-like type A cells, but not in those of fibroblast-like type B cells from the synovial tissues or leucocytes from the synovial fluids. Longitudinal studies showed that the ir-ET-1 concentration in the cultured fluid reached a peak around 24 h after starting the culture. ET-1 secreted from macrophage-like synoviocytes may be involved in the pathogenesis of inflammatory arthritis.

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