Label-free imaging of redox status and collagen deposition showing metabolic differences in the heart.

The heart has high metabolic demand to maintain function. The primary source of energy supply to support correct contractile muscle function differs between a fetus and an adult. In fetal life, ATP is primarily generated by glycolysis and lactate oxidation, whereas following birth, there is a shift towards a reliance on mitochondrial metabolism and fatty acid oxidation. This change in metabolic status is an adaptation to different fuel availability, oxygenation and growth patterns. In this study, we have employed 2-photon excitation fluorescence microscopy to define the relationship between two critical metabolic cofactors nicotinamide adenine dinucleotide(P)H and flavin adenine dinucleotide, effectively utilizing a redox ratio to differentiate between the metabolic status in fetal (proliferative) and adult (quiescent/hypertrophic) hearts. Two-photon imaging was also used to visually confirm the known increase in collagen deposition in the adult heart. The changes observed were consistent with a hypertrophic growth profile and greater availability of fatty acids in the adult heart, compared to the proliferative fetal heart. Two-photon excitation fluorescence microscopy is therefore a convenient imaging technology that enables the monitoring of striated muscle architecture and the metabolic status of heart tissue. This imaging technology can potentially be employed to visualize cardiac and other muscle pathologies.