Extracellular CySS is consumed more rapidly by fibroblasts from young mice than by fibroblasts from old mice. DMEM containing 45 μM CySS (a) or 100 μM Cys (b) was added to lung fibroblasts from young and old mice. Media were removed at different time points for analysis of Cys and CySS concentrations by HPLC. Concentrations were transformed as the natural logarithm. The line through the data represents the best fit via linear regression.