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Serum procalcitonin (PCT) concentration was recently introduced as valuable diagnostic marker for systemic bacterial infection and sepsis. At present, the cellular sources and biological properties of PCT are unclear. During sepsis and septic shock, inducible nitric oxide synthase (iNOS) gene expression is stimulated followed by the release of large amounts of nitric oxide (NO). We investigated the possible association between PCT and iNOS gene expression in an in vitro cell culture model.Prospective, controlled in vitro cell culture study.University research laboratories.Confluent rat vascular smooth muscle cells (VSMC) were incubated for 24 hrs and 48 hrs with PCT (1 ng/mL, 10 ng/mL, 100 ng/mL, 1000 ng/mL, 5000 ng/mL) alone or with the combination of tumor necrosis factor-alpha (TNF-α, 500 U/mL) plus interferon-gamma (IFN-γ, 100 U/mL). iNOS gene expression was measured by qualitative as well as quantitative polymerase chain reaction analysis, NO release was estimated by the modified Griess method.PCT in increasing concentrations had no effect on iNOS gene expression and nitrite/nitrate release for 24 hrs and 48 hrs, respectively. However, PCT ameliorated TNF-α/IFN-γ–induced iNOS gene expression in a dose-dependent manner (maximal inhibition at PCT 100 ng/mL by −66% for 24 hrs and −80% for 48 hrs). This was accompanied by a significantly reduced release of nitrite/nitrate into the cell culture supernatant (maximal reduction at PCT 100 ng/mL by −56% and −45% for 24 hrs and 48 hrs, respectively).We conclude that recombinant PCT inhibits the iNOS-inducing effects of the proinflammatory cytokines TNF-α/IFN-γ in a dose-dependent manner. This might be a counter-regulatory mechanism directed against the large production of NO and the concomitant systemic hypotension in severe sepsis and septic shock.