Metabolism of [³H]Sph in HUVECs. (Upper panel) HUVECs were incubated with [³H]Sph for various durations. Lipids were then extracted from cells or media and analyzed for [³H]sphingolipids by TLC autoradiography. Locations of standard lipids are indicated on the left. SM, sphingomyelin; O, origin. (Lower panel) HUVECs and human platelets were incubated with [³H]Sph for 4 hours and, after the autoradiography, silica gel areas containing radiolabeled sphingolipids were scraped off and counted using a liquid scintillation counter. Radioactivity was expressed as a percentage of the value of [³H]Sph at time 0. Columns and error bars represent the mean ± SD (n = 3).