Electrophysiological responses of rat myocardial cells to exogenous photosensitization reactions for a short period of incubation with two photosensitizers, talaporfin sodium or porfimer sodium, were measured in a subsecond time scale. The loading period of the photosensitizer when the photosensitizer might not be taken up by the cells was selected as 15 min, which was determined by the fluorescence microscopic observation. We measured the intracellular Ca2+ concentration ([Ca2+]in) by using a fluorescent Ca2+ indicator, Fluo-4 AM, under a high-speed confocal laser microscope to evaluate the acute electrophysiological cell response to the photosensitization reaction. The measured temporal change in Fluo-4 fluorescence intensity indicated that the response to the photosensitization reaction might be divided into two phases in both photosensitizers. The first phase is acute response: disappearance of Ca 2+ oscillation when irradiation starts, which might be caused by ion channel dysfunction. The second phase is slow response: [Ca2+] in elevation indicating influx of Ca2+ due to the concentration gradient. The continuous Ca2+ influx followed by changes in cell morphology suggested micropore formation on the surface of the cell membrane, resulting in necrotic cell death.
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