Detection of light-induced changes of intracellular ionized calcium concentration in Limulus ventral photoreceptors using arsenazo III.

1. The metallochromic indicator dye, arsenazo III, was injected intracellularly into Limulus ventral photoreceptor cells to concentrations greater than 1 mM.2. The absorption spectrum (450-750 nm) of the dye in single dark-adapted cells was measured by a scanning microspectrophotometer. When a cell was light-adapted, the absorption of the dye changed; the difference spectrum had two maxima at about 610 and 660 nm, a broad minimum at about 540 nm and an isosbestic point at about 585 nm.3. When intracellular calcium concentration was raised in dark-adapted cells previously injected with arsenazo III, the difference spectum had two maxima at about 610 and 660 nm, a broad minimum at about 530 nm and an isosbestic point at about 585 nm. The injection of Mg(2+) into dark-adapted cells previously injected with the dye induced a difference spectrum that had a single maximum at about 620 nm. Also, decreasing the intracellular pH of cells previously injected with the dye induced a difference spectrum that had a minimum at about 620 nm. The evidence suggests that there is a rise of intracellular ionized calcium when a Limulus ventral photoreceptor is light-adapted.4. The intracellular calcium concentration, Ca(2+), in light-adapted photoreceptors was estimated to reach at least 10(-4)M by compaing the light-induced difference spectra measured in ventral photoreceptors with a standard curve determined in microcuvettes containing 2mM arsenazo III in 400 mM-KCl, 1 mM-MgCl(2) and 25 mM MOPS at pH 7.0.5. In cells injected to less than 3 mM arsenazo III, light induced a transient decrease in optical transmission at 660 nm (T(660)). This decrease in T(660) indicates that illumination of a ventral photoreceptor normally causes a transient increase of Ca(2+).6. Arsenazo III was found to be sensitive, selective and rapid enough to measure light-induced changes of intracellular ionized calcium in Limulus ventral photoreceptor cells.