sP-selectin and sP-selectin-Fc have long half-lives in the circulation. (A) Plasma levels of sP-selectin in WT or Selp^−/− mice. (B) Relative plasma levels of sP-selectin or sP-selectin-Fc in Selp^−/− mice at the indicated times after injecting 10 to 20 μg of the indicated protein intravenously (i.v.) or intraperitoneally (i.p.). The circulating half-life was determined using a 2-phase linear regression fit. (C) Representative absolute plasma concentrations of sP-selectin or sP-selectin-Fc in Selp^−/− mice at the indicated times after injecting 10 μg of the indicated protein i.p. or i.v. The data in panels A and B represent the mean ± SEM from 3 to 5 experiments.