Kinins--key mediators in inflammatory arthritis?

Recent evidence suggests an important role for kinins in the generation of pain, swelling and the cellular damage associated with inflammatory joint disease. Kinins are considered to be pro-inflammatory peptides for a variety of reasons. They stimulate c fibres in the synovium to cause pain and increase extravasation of fluid to produce swelling. Kinins possess the capacity to release neurotransmitters (substance P, acetylcholine) and a second wave of mediators (interleukin-1, tumour necrosis factor, interleukin-8, prostaglandins, leukotrienes). The steady levels and turnover of kinins is regulated by formation (enzymic action of kininogenases on endogenous substrates called kininogens) and by metabolism (kininases, peptidases that hydrolyse kinins). These components of the kinin system can enter the synovial joint space either by transudation from the plasma or from degranulating neutrophils chemotactically attracted into the synovium from which they migrate into the synovial fluid. If kinins are involved, one would expect neutrophil derived mediators of the system to dominate in rheumatoid arthritis and psoriatic arthritis and plasma derived products to be more important in osteoarthritis and gout. But, the question whether any of the functions attributed to each component of the system can be considered to be a primary factor in the cellular pathology of inflamed joints remains to be established. Future investigations, including therapeutic trials with kinin antagonists and kallikrein inhibitors, will need to address the differential role of the kallikreins and kinins in the different types of synovitis, on symptoms of inflammation and on any remedial effects on the progression of tissue damage within the joint.