Irradiance dependence of the conduction block of an in vitro cardiomyocyte wire

Background To obtain therapeutic condition precisely by in vitro experiment, we studied the irradiance dependence of the electrical conduction blockage caused by a photodynamic reaction using a high extracellular concentration of talaporfin sodium on a novel in vitro cardiomyocyte electrical conduction wire. Methods The cardiomyocyte wires were constructed on patterned cultivation cover glass, which had cultivation areas 60 μm in width, and a maximum length of 10 mm. The talaporfin sodium concentration was set to 20 μg/mL. The photodynamic reaction with a high extracellular photosensitizer concentration was performed with a short time interval (approximately 15 min) between photosensitizer exposure and irradiation. A 663-nm laser was applied to the cardiomyocyte wire, and the irradiance was varied between 3 and 120 mW/cm². The cardiomyocyte electrical conduction was evaluated using the cross-correlation function of intracellular Ca²⁺ probe fluorescence brightness from an upper and lower section outside the laser irradiation area of a wire every 10 s, which lasted up to 600 s. Results The onset of electrical conduction blockage was defined by an 85% decrease in the cross-correlation function, compared with its initial value. The time for the electrical conduction blockage decreased from 600 to 300 s as the irradiance was increased. Also, the probability of electrical conduction blockage was found to increase with increasing irradiance. Conclusions We found a strong dependence on the irradiance for the time and probability of electrical conduction blockage.