Measurement of the Membrane Potential of Isolated Nerve Terminals by the Lipophilic Cation [3H]Triphenylmethylphosphonium Bromide

The lipophilic cation [3H]triphenylrnethylphosphonium bromide ([3H]TPMP+) was investigated as a measure of the membrane potential of synaptosomes. Conditions under which [3H]TPMP+ achieved an equilibrium distribution were tested. The toxicity of TPMP has been studied relative to its inhibitory effects on [3H]y‐aminobutyric acid ([3H]GABA) transport. In some experiments the distribution of 86RbZ+ and [3H]TPMP+ was changed upon incubation in the presence of elevated levels of K+, ouabain, or KCN, or at 0°C in a way that would be expected from the membrane potential. In normal incubation conditions a membrane potential of ∼−60 mv was calculated.