1. The role of nitric oxide (NO) in leukocyte (polymorphonuclear cells, monocytes and lymphocytes) emigration was studied in a model of carrageenin-sponge implants in rats. 2. The subcutaneous implantation of 1\% (w/v) of lambda-carrageenin-soaked sponges elicited an inflammatory response that was characterized by a time-related increase in leukocyte infiltration in the sponges and increased levels of nitrite in the exudate. Total leukocyte infiltration and nitrite production were maximal at 24 h and decreased after 48 and 96 h. The mononuclear cell influx was maximal at 48 h (21\% of the total leukocytes). Therefore, this time point was used in the successive experiments. 3. Polymorphonuclear cell (PMN) and lymphocyte infiltration in the sponges significantly increased when rats were treated with the non-specific NO-synthase (NOS) inhibitor, NG-nitro-L-arginine methylester (L-NAME) (1 mg ml-1) in drinking water ad libitum). Monocyte emigration was not affected by L-NAME treatment. The nitrite levels in the exudate of L-NAME-treated rats were significantly reduced. The concomitant ingestion of L-arginine (30 mg ml-1) resulted in a reversion of the L-NAME effect, while D-arginine (30 mg ml-1) had no effect, indicating the involvement of the L-arginine: NO pathway. 4. Administration of L-NAME resulted also in an increased release of tumour necrosis factor-alpha (TNF-alpha) and prostacyclin (measured as the stable metabolite, 6-keto-PGF 1 alpha). L-NAME had no effect on monocyte chemoattractant protein-1 (MCP-1) release in the exudate. 5. Since L-NAME may have effects on the local blood flow, phenylephrine (0.034 mg ml-2) in drinking water) was used as it has an effect on the local blood flow similar to L-NAME. Phenylephrine had no effect on either leukocyte emigration, or on nitrite, TNF-alpha, prostacyclin or MCP-1 accumulation in the exudate. 6. In contrast, the more selective iNOS inhibitor S-methyl-isothiourea (SMT) (10 micrograms ml-1) in drinking water) significantly reduced PMNs and lymphocyte influx in the sponge having no effect on monocyte influx. Moreover, SMT decreased nitrite production in the exudate to a comparable extent as L-NAME. 7. Administration of SMT significantly reduced MCP-1 release in the exudate, without an effect on TNF-alpha or prostacyclin production. Moreover SMT did not produce any changes in local blood flow. 8. Our results show that a different outcome of the inflammatory process can be obtained depending on the types of NOS inhibitor used.
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