Sepsis depletes intracellular stores of ATP and NAD+, leading to cellular energy failure. Liposome encapsulation improves intracellular delivery of bulky, charged molecules and substrates susceptible to extracellular enzyme degradation. We hypothesized that treatments with liposome encapsulated ATP or NAD+ would protect human endothelial cells exposed to endotoxin (LPS) and interferon-gamma (IFN-gamma) from energy failure. Liposomal ATP and NAD+ were prepared by a modification of the thin film method. Human endothelial cells were exposed to LPS 50 microg/ml and IFN-gamma 50 ng/ml for 72 hours, and liposomal ATP and NAD+ treatments were dosed at 0 and 24 hours. Energy state was determined by rate of mitochondrial respiration as measured by WST-1 assay. Mitochondrial respiration significantly decreased to 57% +/- 3 of control in LPS/IFN-gamma exposed cells after 72 hours. Liposomal ATP (200 microM) and NAD+ (100 microM) completely reversed this respiratory depression while empty liposomes, free ATP (200 microM). and free NAD+ (100 microM) did not. These results support the hypothesis that treatments with liposome encapsulated ATP or NAD+ protect human endothelial cells from energy failure in a cell culture model of sepsis and potentially may provide a novel therapy for use in clinical sepsis.