The purpose of the present study was to gain new insight regarding the role membrane permeabilization plays in the photosensitization-induced increase in intracellular calcium concentration. During continuous rose bengal photosensitization we monitored the contractile state (relaxed or hypercontracted) of isolated frog cardiac cells and assessed the photosensitization-induced membrane-leak conductance. We investigated the effects of irradiance, extracellular calcium concentration, intracellular chelation of calcium and substitution of tetraethylammonium (TEA) for extracellular sodium. We found that with 2 and 5 mM extracellular calcium cell hypercontracture occurred when leak conductance reached values on the order of 6–7 nS, independent of the illumination duration required to reach this conductance. With 0.5 mM calcium hypercontracture occurred when leak conductance reached values on the order of 11 nS. Chelation of intracellular calcium delayed the onset of cell hypercontracture and increased by two- to three-fold the leak conductance at the initiation of cell hypercontracture. Substitution of TEA for extracellular sodium did not affect the time to contracture onset but reduced leak conductance at contracture onset nearly three-fold. We discuss how our results support the conclusion that photosensitization induces an increase in intracellular calcium concentration via calcium influx through the transmembrane-permeability pathway opened by the photosensitization process.
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Vol. 73 • No. 4