The nature of rhodopsin-triggered photocurrents in Chlamydomonas. II. Influence of monovalent ions.

Chlamydomonas exhibits a sequence of a photoreceptor current and two flagellar currents upon stimulation with bright green flashes. The currents are thought to be a prerequisite for the well-known photophobic responses. In the preceding paper, we analyzed the kinetics of these currents and their dependence on extracellular divalent ions. Here, we show that the photoreceptor current can be carried by monovalent ions (K+ > NH4+ > Na+), provided that the driving force is high enough. The small residual photoreceptor current observed in the absence of Ca2+ is able to evoke flagellar currents at low extracellular pH. This demonstrates that signal transduction from the rhodopsin to the flagella is not inevitably dependent on extracellular Ca2+. Double-flash experiments exclude a contribution of intra-rhodopsin charge movements to the photoreceptor current signal. Evidence will be provided for the existence of nonlocalized K+ outward currents, which counterbalance the localized Ca2+ influx and repolarize the cell after a light flash. A model is presented that explains the different pathways for direction changes and phobic responses.