The effect of brief illumination on intracellular free calcium concentration in cells with 5-aminolevulinic acid-induced protoporphyrin IX synthesis

The effect of illumination on intracellular free calcium concentration, Ca2+]i, was studied in a cell line (WiDr cells) derived from a primary adenocarcinoma of the rectosigmoid colon. In these cells the biosynthesis of protoporphyrin IX was stimulated by 5-aminolevulinic acid to reach levels of 600-700 pmol of protoporphyrin IX per mg cell protein. A brief (1-min) exposure of the cells to light (70% of light energy at 340-380 nm) resulted in an increase in Ca2+]i. This increase was not reversible over a period of at least 20 min following illumination. Elevation of Ca2+]i most probably represented an influx of calcium ions from the medium to the cell, since it was completely abolished in the presence of extracellular EGTA. The increased Ca2+]i did not reflect general membrane damage, as determined by trypan blue staining as well as measurement of the intercalation of ethidium bromide into cellular DNA, and neither did the sustained elevation of Ca2+]i lead to any substantial loss of clonogenicity following illumination of protoporphyrin-containing cells. Together these results indicate that an increased Ca2+]i level is not per se a cause of cell death during photodynamic therapy.