Fluorescent recovery from photobleaching coupled with confocal microscopy was explored as a potential high-resolution method of imaging the distribution of enzyme activity in single living cardiac myocytes without relying on steady state measurements of fluorescence. On a fundamental level, much remains to be determined regarding how local conditions within a cell affect metabolism. Many studies have suggested that energy metabolism in muscle cells cannot be accurately described assuming a homogenous system of enzymatic reactions [1,2]. The autofluorescence of NADH has been used in many studies as a quantitative assay of mitochondrial energy metabolism [3,4], but studies of steady state fluorescence cannot distinguish between changes in energy production or utilization.

In this study conditions were created where the fluorescent recovery of a probe would be solely dependent on cellular enzymatic activity (Enzyme Dependent Fluorescence Recovery after Photobleaching (ED-FRAP)). Experiments examining the inherent fluorescence of NADH (351nm excitation, 450 nm emission) were conducted on small droplets (less than 325 μm diameter) containing NADH alone, in droplets containing an enzyme system capable of synthesis of NADH (Figure 1A) and on isolated rabbit cardiac myocytes (Figure 1D). Photobleaching of the entire cell or droplet eliminated diffusion or bulk transport of NADH from non-bleached regions. Droplets containing NADH alone did not recover, while droplets containing enzyme were shown to recover exponentially (Figure 1B) with a rate constant of fluorescent recovery (kf) that was proportional to enzyme concentration (Figure 1C).