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Published online by Cambridge University Press: 02 July 2020
Two-photon excitation microscopy provides attractive advantages over confocal microscopy for three-dimensionally resolved fluorescence imaging. Because of the intensity-squared dependence of the two-photon absorption, the excitation is limited to the focal volume. This inherent localization minimizes photobleaching and photodamage - the ultimate limiting factors in fluorescence microscopy of living cells. One of the most powerful applications of two-photon excitation microscopy is imaging from the naturally occurring reduced pyridine nucleotides (NAD(P)H). NAD(P)H is a useful indicator of cellular metabolism, but it is not a “good“ fluorophore (it has a small absorption cross-section and a low quantum yield). Two-photon excitation of NAD(P)H yields minimal photodamage, thus allowing time-resolved threedimensional metabolic mapping of cellular redox state. We have used two-photon excitation microscopy to examine glucose metabolism in pancreatic and muscle cells. As glucose is metabolized by these cells, intermediate metabolism results in an increase in the reduced-tooxidized NAD(P)H/NAD(P)+ ratio, and a concomitant increase in autofluorescence.