Nitric oxide metabolism in wounds.

Arginine can be metabolized in wounds to nitric oxide and citrulline by nitric oxide synthase or to urea and ornithine by arginase. We investigated the expression of these arginine metabolic pathways over a 3-week period. Groups of 8-10 male Balb/C mice underwent a dorsal skin incision and subcutaneous polyvinyl alcohol sponge implantation. The animals were sacrificed at various times, and sponges were harvested to obtain wound fluid and wound cells. Cells or whole sponges were incubated with L-[2,3-(3)H]arginine, with or without N(G)-L-monomethyl-arginine (NMMA, a competitive inhibitor of nitric oxide synthase). Nitrite and nitrate (both stable end products of nitric oxide metabolism) and amino acids were measured in wound fluid and wound cell culture supernatants. Increasing concentrations of nitrite and nitrate were noted in wound fluid and in whole sponge cultures until the second week postwounding, indicating sustained wound nitric oxide synthesis. In wound fluid arginine levels were undetectable at all times, suggesting sustained utilization. Wound fluid citrulline levels showed an early peak and then a gradual decrease, suggesting that recycling for continued nitric oxide production may occur. Wound fluid ornithine levels increased until Day 10 and remained elevated, indicative of continued arginase activity. In vitro production of nitrite/nitrate and citrulline by cells and whole sponges was inhibitable by NMMA. Inducible nitric oxide synthase expression was confirmed by immunoblotting, while immunohistochemistry demonstrated that macrophages are a major source of wound nitric oxide. The data show that nitric oxide synthesis occurs for prolonged periods after injury and macrophages appear to be a major cellular source.