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Nitric oxide (NO) is generated in immunocompetent cells by inducible NO-synthase (iNOS), and plays an important role in host defense. However, when produced in large amounts, NO can also exert damaging effects, a scenario that is observed during several inflammatory processes. In the study presented here, we investigated the impact of moderate endurance training (running volume: mean 53.1 km x week(-1), 95% confidence interval 41.2-65.1 km week(-1)) on the leukocyte expression of iNOS mRNA. Semi-quantitative reverse transcriptase-polymerase chain reaction (RT-PCR) analysis was used to examine iNOS mRNA expression in total leukocyte samples from 12 male trained subjects (TR) and a control group of 12 untrained men (UT) at rest. Relative iNOS mRNA levels (iNOS/beta-actin) were higher in UT (0.88, 0.73-1.03) when compared with TR (0.34, 0.09-0.58, P<0.001). iNOS mRNA was not detectable in 5 of the 12 TR subjects. These initial results show that the basal expression of iNOS mRNA is downregulated by moderate endurance training. Further research should clarify whether regular training also affects the responsiveness of leukocyte iNOS gene expression to stimulatory signals. It would be of interest to establish whether moderate training can exert a suppressive and therefore therapeutic effect on the elevated levels of expression of iNOS observed in, for example, several inflammatory disorders.

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