Ion fluxes inAcetabularia: Vesicular shuttle

Summary

Ion flux relations in the unicellular marine algaAcetabularia have been investigated by uptake and washout kinetics of radioactive tracers (²²Na⁺,⁴²K⁺,³⁶Cl⁻ and⁸⁶Rb⁺) in normal cells and in cell segments with altered compartmentation (depleted of vacuole or of cytoplasm). Some flux experiments were supplemented by simultaneous electrophysiological recordings. The main results and conclusions about the steady-state relations are: the plasmalemma is the dominating barrier for translocation of K⁺ with influx and efflux of about 100 nmol·m⁻²·sec⁻¹×K⁺ passes three- to sevenfold more easily than Rb⁺ does. Under normal conditions, Cl⁻ (the substrate of the electrogenic pump, which dominates the electrical properties of the plasmalemma in the resting state) shows two efflux components of about 17 and 2 μmol·m⁻²·sec⁻¹, and a cytoplasmic as well as vacuolar [Cl⁻] of about 420mm ([Cl⁻] _o =529mm). At 4°C, when the pump is inhibited, both influx and efflux, as well as the cellular [Cl⁻], are significantly reduced. Na⁺ ([Na⁺] _i : about 70mm, [Na⁺] _o : 461mm), which is of minor electrophysiological relevance compared to K⁺, exhibits rapid and virtually temperature-insensitive (electroneutral) exchange (two components with about 2 and 0.2 μmol·m⁻²·sec⁻¹ for influx and efflux). Some results with Na⁺ and Cl⁻ are inconsistent with conventional (noncyclic) compartmentation models: (i) equilibration of the vacuole (with the external medium) can be faster than equilibration of the cytoplasm, (ii) absurd concentration values result when calculated by conventional compartmental analysis, and (iii) large amounts of ions can be released from the cell without changes in the electrical potential of the cytoplasm. These observations can be explained by the particular compartmentation of normalAcetabularia cells (as known by electron micrographs) with about 1 part cytoplasm, 5 parts central vacuole, and 5 parts vacuolar vesicles. These vesicles communicate directly with the central vacuole, with the cytoplasm and with the external medium.