The role of nitric oxide (NO) as a mediator of colonic circular smooth muscle relaxation by human leucocytes was investigated. Granulocytes and mononuclear cells were obtained by gradient centrifugation of venous blood from healthy volunteers. Both cell types relaxed precontracted distal colonic circular smooth muscle in a concentration dependent manner. Muscle relaxation was inhibited by preincubation of cells with NG-monomethyl-L-arginine (100 microM/l) but not by preincubation with NG-monomethyl-D-arginine (100 microM/l). Muscle relaxation by cells was reduced by 200 nM oxyhaemoglobin and 10 microM methylene blue but was increased by 60 units/ml superoxide dismutase. Non-viable cells did not produce muscle relaxation. Activation of mononuclear cells by incubation with 100 nM/l FMet-Leu-Phe increased muscle relaxation, whereas activation of granulocytes did not. Granulocytes and mononuclear cells relax precontracted distal colonic circular smooth muscle in vitro by the release of NO that may contribute to motility disorders of the gut associated with inflammation.
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