Light adaptation of invertebrate photoreceptors: influence of intracellular pH buffering capacity.

1. The possible role of pH changes in mediating light adaptation in Limulus ventral photoreceptor cells was studied by intracellular injection of zwitterionic pH buffers. The intracellular concentration of buffer was estimated by inclusion of a radioactive marker in the injection solution. 2. The light-induced increase of intracellular Ca2+ concentration was monitored by intracellular aequorin. The light-induced increase of Ca2+ concentration was not markedly altered by injection of pH buffer to an intracellular concentration of about 200 mM. 3. The progressive decrease in responsiveness during intracellular ionophoretic injection of Ca2+ was not markedly altered by injection of pH buffer to an intracellular concentration of about 200 mM. 4. Photoreceptors of both Limulus and Balanus were impaled with two micropipettes and voltage clamped. Membrane current induced by a prolonged steady illumination declined from an early transient to a plateau. This delayed decline of current indicates a light-induced reduction of sensitivity (i.e. light adaptation). The wave forms were similar before and after injection of pH buffer to an intracellular concentration of about 200 mM. 5. We conclude that it is unlikely that a light-induced change of cytosolic pH mediates light adaptation in Limulus (and Balanus) photoreceptors.