The fluorescence signals of single cells were measured using alternating excitation at 340 and 380 nm and emission was detected at 510 nm. The ratio of fluorescence (340/380) was plotted against time. The measurements were performed using a Photon Technology International (PTI) (Birmingham, NJ) photomultiplier based system mounted on an Olympus IX71 microscope, equipped with a DeltaRAM excitation light source, or with a Ratiomaster 5 Imaging System (PTI) equipped with a Cool-snap HQ2 (Roper) Camera.