Exogenously-added sterols feed back on cholesterol and 24,25EC synthesis. CHO-7 cells were incubated with either increasing concentrations (A) or 50 μM (E) of cholesterol complexed to methyl-β-cyclodextrin; or either increasing concentrations (B) or 10 μM (F) of 24,25EC in the presence of [1-14C]-acetate for 24 h or (E, F) labelled 2 h before the indicated time of harvest. Neutral lipid extracts were separated by thin-layer chromatography and bands corresponding to authentic cholesterol and 24,25EC were visualized by phosphorimager. These phosphorimages are representative of 2 separate experiments. Bands corresponding to cholesterol and 24,25EC were quantified by densitometry. For (C, D) data sets from 2 (C) or 3 (D) separate experiments represented by Figure 1A and Figure 1B were pooled. Equation for the lines of best fit (C) cholesterol (solid diamond; solid line) y = -1.35x + 84.79; R2 = 0.75; P < 0.01; (D) 24,25EC (empty square; dashed line) y = -5.68x + 76; R2 = 0.58; P < 0.01; cholesterol (solid diamond; solid line) y = -5.96x + 78; R2= 0.63; P < 0.01. For (E, F) data sets are representativeof n = 2 separate experiments.