Use of Lipophilic Cations to Measure the Membrane Potential of Oat Leaf Protoplasts

Uptake of the lipophilic cation triphenylmethylphosphonium into mesophyll protoplasts of oat (Avena sativa L. cv. "Garry") approaches equilibrium at 3 to 4 hours. The resulting external and internal concentrations are then used with the Nernst equation to obtain a membrane potential of -62 millivolts, inside negative. Potentials calculated in this manner are depolarized by adding 2 mm sodium azide and 50 mum carbonyl cyanide m-chlorophenylhydrazone as well as by increasing the external proton and potassium concentrations. The depolarizations are qualitatively similar to those seen when oat mesoyphll cells are measured in situ with microelectrodes. It is concluded that due to the lack of turgor and fragility of protoplasts, estimations of their membrane potential may be made more reliably, under some conditions, with lipophilic cations than with microelectrodes.