Two-photon (2P) ratiometric redox fluorometry and microscopy of pyridine nucleotide (NAD(P)H) and flavoprotein (FP) fluorescence, at 800-nm excitation, has been demonstrated as a function of mitochondrial metabolic states in isolated adult dog cardiomyocytes. We have measured the 2P-excitation spectra of NAD(P)H, flavin adenine dinucleotide (FAD), and lipoamide dehydrogenase (LipDH) over the wavelength range of 720-1000 nm, The 2P-excitation action cross sections (σ2P) increase rapidly at wavelengths below 800 nm, and the maximum σ2p of LipDH is ∼5 and 12 times larger than those of FAD and NAD(P)H, respectively, Only FAD and LipDH can be efficiently excited at wavelengths above 800 nm with a broad 2P-excitation band around 900 nm. Two autofluorescence spectral regions (i.e., ∼410-490 nm and ∼510-650 nm) of isolated cardiomyocytes were imaged using 2P-laser scanning microscopy, At 750-nm excitation, fluorescence of both regions is dominated by NAD(P)H emission, as indicated by fluorescence intensity changes induced by mitochondrial inhibitor NaCN and mitochondria uncoupler carbonyl cyanide p-(trifluoromethoxy) phenyl hydrazone (FCCP). In contrast, 2P-FP fluorescence dominates at 900-nm excitation, which is in agreement with the σ2p measurements. Finally, 2P-autofluorescence emission spectra of single cardiac cells have been obtained, with results suggesting potential for substantial improvement of the proposed 2P-ratiometric technique.
Bibliographical noteFunding Information:
This project was carried out in the Biomedical Technology Resource, Developmental Resource for Biophysical Imaging Opto-electronics, and the publication was made possible by grant P41-RR04224 from the National Center for Research Resources, National Institutes of Health (Bethesda, MD).