Am. J. Respir. Cell Mol. Biol., Volume 19, Number 2, August, 1998 308-315

L-Arginine Uptake and Metabolism by Lung Macrophages and Neutrophils Following Intratracheal Instillation of Silica In Vivo

Ralph M. Schapira, John H. Wiessner, James F. Morrisey, Urias A. Almagro, and Leif D. Nelin

Division of Pulmonary and Critical Care Medicine, and Departments of Pathology and Pediatrics, Medical College of Wisconsin; and Sections of Pulmonary and Critical Care Medicine, and Pathology and Laboratory Medicine, and Research Service, Clement J. Zablocki Veterans Affairs Medical Center, Milwaukee, Wisconsin

Nitric oxide (NO) has been associated with lung inflammation following exposure to silica. L-arginine can be converted to NO and L-citrulline by nitric oxide synthase (NOS), or into urea and L-ornithine by arginase. We tested the hypothesis that after instillation of silica into rat lungs in vivo, lung inflammatory cells increase L-arginine metabolism by both NOS and arginase, which is associated with an increase in L-arginine uptake. We isolated lung inflammatory cells 3 d after silica or saline (control) exposure. The uptake of [³H]L-arginine at 24 h by cells from silica-exposed lungs (73.9 ± 4.8%) was significantly greater than uptake by control cells (24.7 ± 2.2%; P < 0.05) and was a saturable process. The greater [³H]L-arginine uptake by cells from silica-exposed lungs was associated with greater NO and urea production than by control cells. The uptake of [³H]L-arginine by cells from control or silica-exposed lungs was blocked in a dose-dependent manner by L-ornithine (an inhibitor of L-arginine transport) and by N-nitro-L-arginine methyl ester (L-NAME) (an NOS inhibitor), but not by L-valine (an arginase inhibitor). The production of NO by cells from silica-exposed lungs was completely blocked by L-NAME. The addition of L-arginine to media resulted in dose-dependent production of NO and urea. The results show that lung inflammatory cells increase L-arginine uptake and metabolism by both NOS and arginase following in vivo silica exposure. The increase in L-arginine uptake may represent a mechanism to maintain an intracellular supply of this amino acid. NO can react to generate peroxynitrite, a potential mediator of lung injury following silica exposure.

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