[Fluorescence imaging of the living heart for understanding the basis of arrhythmias].

Recent outstanding progress in microscopic imaging technology and the advent of fluorescent probes have enabled us to visualize high spatiotemporal dynamics of intracellular molecules in living tissues. Here I introduce our research outcomes on functional fluorescence imaging of the heart especially for understanding the pathogenesis of cardiac arrhythmias. On the in situ Ca imaging of perfused rat heart by rapid-scanning confocal microscopy, we found that burst emergence of intracellular Ca waves evokes arrhythmogenic triggered activity and subsequent oscillatory depolarizations via the Na-Ca exchanger. Besides, impairment of Ca release from the sarcoplasmic reticulum leads to emergence of Ca waves and spatiotemporally inhomogeneous Ca dynamics on systole, resulting in beat-to-beat Ca alternans. Such alternating behaviors of Ca dynamics are partly due to poor development of the transverse tubules, which are identified in murine atria and failing ventricular myocytes. In addition, impairment of the gap junctional communication via connexin 43 induced by dominant negative inhibition of neonatal rat ventricular myocyte monolayers results in generation of spiral wave reentry, suggesting the pivotal role of intercellular communications in genesis of arrhythmias. Furthermore, alterations in atrial histoanatomy, e.g., density and arrangements of myocytes and distribution of Cx43, could provide intrinsic arrhythmogenic bases of atrial fibrillation, which was revealed by combined optical imaging of the atria and precise histoanatomical examinations. In combination, fluorescence imaging of the living organisms provides indispensable information for unveiling functions and disease states.