Membrane potential measurement of protoplasts isolated from vigna mungo hypocotyl using a fluorescent probe, diS-C₃-(5)

Membrane potentials of protoplasts isolated from Vigna mungo hypocotyl segments were measured using the fluorescent probe diS-C₃-(5). The fluorescence intensity changed in response to the external K⁺ concentration. Membrane potential was estimated to be inside negative (-85±8 mV at 0.1 mM KCl) from the Nernst equation for K₊. The membrane potential was not affected by DCCD (50 μM) or low temperature (5°C). Addition of 0.5 mM Ca₂₊ to the protoplast suspension markedly depolarized the membrane potential, and subsequent EDTA treatment repolarized it to the initial level. The Ca₂₊ effect on the membrane potential may be due to change in the permeability ratio of Cl^- to K₊.