Abstract

The effect of the protonophore carbonyl cyanide-p-trifluoromethoxyphenyl-hydrazon (FCCP) was studied on the intracellular [Na⁺], pH, and plasma membrane potential in isolated nerve terminals. FCCP induced a rise of [Na⁺]_i at, and even below, the concentrations (0.025–1 μm) in which it is usually used in intact cells to eliminate Ca²⁺ uptake by mitochondria. The FCCP-induced increase of [Na⁺]_i correlates with a fall in both the ATP level and the ATP/ADP ratio. In addition, a sudden rise of the intracellular proton concentration ([H⁺]_i) from 83 ± 0.4 to 124 ± 0.7 nm was observed on the addition of FCCP (1 μm). Parallel with the rise in [H⁺]_i, an abrupt depolarization was detected, followed by a slower decrease in the plasma membrane potential. Both the extent of the pH_i change and the fast depolarization of the plasma membrane were proportional to the proton electrochemical gradient across the plasma membrane; when this gradient was increased, greater depolarization was detected. The slower decrease of the membrane potential after the fast initial depolarization was abolished when the medium contained no Na⁺. It is concluded that FCCP (1) gives rise to a depolarization by setting the plasma membrane potential close to the proton equilibrium potential and (2) enhances the intracellular [Na⁺] as a consequence of an insufficient ATP level and ATP/ADP ratio to fuel the Na⁺,K⁺/ATPase. Because both disturbed Na⁺ homeostasis and plasma membrane depolarization could profoundly interfere with Ca²⁺ homeostasis in the presence of protonophores, consideration given to these alterations may help to clarify the cellular Ca²⁺ sequestration processes.