Desensitization and Dark Recovery of the Photoreceptor Current in Chlamydomonas reinhardtii.

Photoexcitation of rhodopsin in Chlamydomonas reinhardtii triggers a complex of rapid bioelectric processes in the cell membrane. Photoreceptor and flagellar currents are the major components of this cascade and are the basis for the phototaxis and photoshock response, respectively. Desensitization and dark recovery of the extracellularly recorded photoreceptor current were investigated in double-flash excitation experiments. The data obtained show that the desensitization is determined by membrane depolarization rather than by rhodopsin bleaching. At external K+ concentrations less than 0.6 mM, generation of the flagellar current triggers a transient, depolarization-activated K+ efflux that contributes to membrane repolarization after light excitation. Acceleration of the dark recovery at 5 to 10 mM Ca2+ can be partially attributed to a blockade of K+ influx, which is triggered at higher external K+ concentrations. K+ currents constitute a novel component of the rhodopsin-mediated signaling system in C. reinhardtii that is involved in the process of dark adaptation of the system.