Intracellular free Mg2+ in Balanus photoreceptor measured with eriochrome blue

Intracellular free Mg2+ (Mgf) in darkness and during illumination was measured in the Balanus photoreceptor using the Mg2+ sensitive dye Eriochrome Blue SE (EB). The EB was pressure injected in the cell and Mgf determined from measurements of absorption changes at 570 nm and 600 nm. Mgf is 1.7 mM in the dark-adapted photoreceptor. There was no indication of a change in Mgf when the cell was illuminated with bright light, indicating that Mgf is not elevated during the phototransduction. Changes less than 5 X 10(-5) M could have occurred and remained undetected. The spectrophotometric properties of the indicator were studied at 10(-4) and 10(-3) M, at ionic conditions relevant to the intracellular measurements. Both the differential molar absorption coefficient for the EB-Mg complex at 600 nm (delta epsilon 600EBMg) and the apparent dissociation constant (KD) for the complex formation were different at 10(-4) and 10(-3) M EB.