The mechanism of PDT-induced electrical blockade: The dependence of time-lapse localization of talaporfin sodium on the cell death phenotypes in rat cardiac myocytes

We have proposed a new type of atrial fibrillation treatment with the early state photodynamic therapy (PDT), in which the interval time between the photosensitizer injection and irradiation is shorter than that in conventional way. We had demonstrated the acute electrical blockade by the PDT with talaporfin sodium and a red (670 nm) diodelaser in ex vivo and in vivo experiment using rat normal myocardial tissue. The previous study of intracellular Ca²⁺ concentration measurement in rat cardiac myocytes during the PDT indicated that Ca²⁺ influx induced by the plasma membrane damage might be the main cause of the acute reaction of myocardial tissue. We found that the cell damage of cardiac myocytes triggered by the PDT was mainly influenced by the site where the photosensitizer exists. In this study, we examined the relationship between the sites of talaporfin sodium existing and cell death phenotypes in response to the PDT, in order to clarify the mechanism of the acute electrical blockade induced by the PDT in myocardial tissue. The talaporfin sodium fluorescence was observed after the various incubation times to visualize the time-lapse intracellular photosensitizer localization. The distribution of the photosensitizer was dependent on the incubation time. The change in intracellular Ca²⁺ concentration during the PDT was examined with a fluorescent Ca²⁺ indicator by a high-speed Nipkow confocal laser microscope (CSU-X1, Yokogawa Electric Company). We obtained the Ca²⁺ dynamics during the PDT which can explain the PDT-induced cell death pathways. We concluded that the Ca²⁺ influx induced by plasma membrane damage is the possible mechanism of the electrical blockade by the early state PDT.