Scientists at the University of Bristol, the University of Cambridge and Imperial College London, all in the UK, have made real-time measurements of ATP levels in heart cells using luciferase. Their findings could lead to improved patient recovery from heart attacks and after heart surgery. As published in the Journal of Biological Chemistry on Sept. 22, they incubated heart cells from adult rats with adenoviruses encoding either cytosolically or mitochondrially targeted luciferase. At various cellular activity levels, the researchers measured light emission from luciferase. Images were taken at a rate of 60 fps using an Olympus inverted microscope in time-resolved imaging mode and a triply intensified CCD camera. Although calcium is known to stimulate ATP production, transient calcium signaling in the mitochondria of adult rat heart cells has not been studied. Therefore, the researchers also measured calcium levels using aequorin, another light-emitting reporter. During the contractile cycle, calcium levels increased in both the cytosol and mitochondria, while ATP levels remained constant. The researchers said that these findings support the parallel activation model, whereby cytosolic calcium stimulates ATP use and mitochondrial calcium triggers ATP production, ensuring that ATP production and use are well-matched. Under normal conditions, ATP levels increased rapidly to meet stress demands. However, when the heart cells were stimulated from rest, ATP levels more slowly returned to sufficient levels. The researchers said that such an ATP deficiency could prevent the heart from beating properly.