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Figure 2 Effects of light and heavy smokers’ serum on endothelial nitric oxide synthase (eNOS) protein expression and eNOS activity in vitro. Confluent (approximately 85%) human umbilical endothelial cells (HUVECs) were incubated with an equal volume of medium and serum from nonsmokers (n = 8), light smokers (n = 7) or heavy smokers (n = 8) in 24-well plates. After 12 h culture, supernatant was collected, and the HUVECs were lysed. (A) Endothelial nitric oxide synthase protein concentration of the cell lysates was determined by ELISA; (B) eNOS activity of the cell lysates was determined by detecting the conversion of [³H]L-arginine to [³H]L-citrulline. (A) Endothelial nitric oxide synthase protein expression; one-way analysis of variance (ANOVA): group, p < 0.0001. Post-hoc Fishers protected least significant difference (PLSD): *nonsmokers versus light smokers, p < 0.0003; nonsmokers versus heavy smokers, p < 0.0001; light smokers versus heavy smokers, p = 0.40. (B) Endothelial nitric oxide synthase activity; one-way ANOVA: group, p < 0.02. Post-hoc Fisher PLSD: *nonsmokers versus light smokers, p < 0.03; nonsmokers versus heavy smokers, p < 0.004; light smokers versus heavy smokers, p = 0.63. The box represents the interquartile range (between the 25th and 75th percentiles); the median is shown as a horizontal bar within each box. The bars outside each box show the range of 95% of all values.