ATP was measured by using the CellTiter-GLO® Luminescent Cell Viability kit (Promega), according to the manufacturer's recommendations. Briefly, cells were plated in a black opaque-walled 96 well plate (Packard Bioscience) at a density of 20000/well and incubated at 33°C overnight. The following day, CellTiter-GLO® Reagent was added to each well and cell lysis was induced by mixing 2 minutes. An ATP standard curve was prepared in the same plate. Before recording luminescence with a microplate luminometer (MicroLumat Plus LB 96V, Berthold Techonologies), the plate was dark adapted for 10 minutes at room temperature to stabilize the luminescence signal.