Papkovsky DB. Sensing intracellular oxygen using near-infrared phosphorescent probes and live-cell fluorescence imaging. Am J Physiol Regul Integr Comp Physiol 292: R1613-R1620, 2007. First published December 14, 2006; doi:10.1152/ajpregu.00707.2006.-The development and application of a methodology for measurement of oxygen within single mammalian cells are presented, which employ novel macromolecular near infrared (NIR) oxygen probes based on new metalloporphyrin dyes. The probes, which display optimal spectral characteristics and sensitivity to oxygen, excellent photostability, and low cytotoxicity and phototoxicity, are loaded into cells by simple transfection procedures and subsequently analyzed by high-resolution fluorescence microscopy. The methodology is demonstrated by sensing intracellular oxygen in different mammalian cell lines, including A549, Jurkat, and HeLa, and monitoring rapid and transient changes in response to mitochondrial uncoupling by valinomycin and inhibition by antimycin A. Furthermore, the effect of ryanodine receptormediated Ca 2ϩ influx on cellular oxygen uptake is shown by substantial changes in the level of intracellular oxygen. The results demonstrate the ability of this technique to measure small, rapid, and transient changes in intracellular oxygen in response to different biological effectors. Moreover, this technique has wide ranging applicability in cell biology and is particularly useful in the study of low oxygen environments (cellular hypoxia), mitochondrial and cellular (dys)function, and for therapeutic areas, such as cardiovascular and neurological research, metabolic diseases, and cancer. metalloporphyrin; mitochondrial function; uncoupling MOLECULAR OXYGEN IS A KEY substrate in aerobic biological systems, providing the terminal electron acceptor of the electron transport chain (7). The rate of oxygen consumption is closely regulated by the ATP/ADP ratio (24), while also being highly dependent on the concentration of available oxygen (31) and the action of signaling molecules such as NO (9, 37) and Ca 2ϩ (10, 23), which, in turn, are related to pathways of cell survival and death (22). Furthermore, the induction of cellular hypoxia results in the activation of a specific adaptive transcriptional response governed by the hypoxia inducible transcription factors (9). Oxygen consumption is, therefore, an informative marker of cellular metabolism, which is broadly applicable to various biological systems from mitochondria to cells to whole organisms. It may be used to elucidate biochemical pathways of the cell and alterations in metabolism caused by various stimuli or disease states. Accordingly, there is a requirement for methods that can effectively monitor cellular oxygen levels and oxygen consumption rates.Optical schemes, based on the quenching by molecular oxygen of long-decay fluorescent and phosphorescent dyes, such as metalloporphyrins and ruthenium(II) complexes, have been under active development in recent years (32). The group of Wilson and coworkers (8, 47) ha...