Microvascular substance P binding to normal and inflamed rat and human synovium

The regulatory peptide substance P has been implicated in the development and persistence of inflammatory synovitis. The authors used quantitative in vitro receptor autoradiography to compare synovial binding of 125Iodine-Bolton Hunter-labeled substance P ([125I]BH-SP) in rats and humans and between uniflamed and persistently inflamed synovium. [125I]BH-SP binding to microvascular endothelium paralleled the distribution of substance P-immunoreactive nerves and had characteristics of the neurokinin (NK) 1 class of tachykinin receptor. Specific binding was inhibited by the selective NK1 receptor antagonist, FK888, and the dual NK1/NK2 receptor antagonist FK224, with Hill coefficients near unity. FK888 was > 1000 times and FK224 > 10 times more potent at inhibiting binding in human compared with rat synovium. Synovium from patients and rats with chronic arthritis contained heterogeneously distributed inflammatory cell infiltrates. For the 10 microvessels with the densest [125I]BH-SP binding in each section, no significant differences in binding density, affinity, or Ki values for substance P, FK888 or FK224 were found between synovium from naive and monoarthritic rats, nor between that from patients with rheumatoid arthritis or osteoarthritis. However, in both rat and human specimens, microscopic examination suggested that microvascular [125I]BH-SP binding in intensely infiltrated regions of synovium was less dense than in adjacent, less infiltrated areas. It was concluded that NK1 receptors are similarly distributed in rat and human synovium but show major differences in selectivity for antagonists such as FK888. NK1 receptors in synovium may mediate proinflammatory actions of locally released substance P; defective neurovascular regulation may contribute to the persistence of chronic arthritis.