Chloride transport and the membrane potential in the marine alga,Halicystis parvula

Summary The relationship between the rate of Cl^– transport and the electrical properties ofHalicystis parvula was investigated. Three metabolic inhibitors-darkness, cyanide (2mm), and low temperature (4°C)-all rapidly and reversibly reduce both the short circuit current (SCC), which is a measure of net Cl^– transport, and the vacuole electrical potential (PD). Plotting thePD vs. SCC for inhibited cells yields a linear regression with ay-intercept of zero. ThePD is also greatly reduced when the Cl^–] of the external medium is lowered. Raising the external K⁺] produces an appreciable, but less than Nernstian, depolarization, while increasing the external H⁺] tenfold has no net effect on thePD. Decreasing the external Na⁺] by tenfold produces only a slight depolarization. Thus, the outer plasma membrane appears to be moderately selective for K⁺ over Na⁺ or H⁺. The effects of ion substitutions in the vacuolar perfusing solutions on thePD reveal that the vacuolar membrane does not discriminate electrically between Cl^– and the much larger anions, isethionate and benzenesulfonate, or between Na⁺ and K⁺. The data suggest that in internally perfused cells ofH. parvula generation of thePD of –50 to –60 mV by a transport system involving only electroneutral pumps is unlikely and that most of thisPD is generated by an electrogenic Cl^– pump.