THE NATURE OF RHODOPSIN-TRIGGERED PHOTOCURRENTS IN CHLAMYDOMONAS .2. INFLUENCE OF MONOVALENT IONS

Chlamydomonas exhibits a sequence of a photoreceptor current and two f lagellar currents upon stimulation with bright green flashes. The curr ents are thought to be a prerequisite for the well-known photophobic r esponses. In the preceding paper, we analyzed the kinetics of these cu rrents and their dependence on extracellular divalent ions. Here, we s how 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+ i s able to evoke flagellar currents at low extracellular pH, This demon strates that signal transduction from the rhodopsin to the flagella is not inevitably dependent on extracellular Ca2+, Double-flash experime nts exclude a contribution of intra-rhodopsin charge movements to the photoreceptor current signal. Evidence will be provided for the existe nce of nonlocalized K+ outward currents, which counterbalance the loca lized Ca2+ influx and repolarize the cell after a light flash, A model is presented that explains the different pathways for direction chang es and phobic responses.