PLASMA-MEMBRANE DEPOLARIZATION AND DISTURBED NA-P-TRIFLUOROMETHOXYPHENYL-HYDRAZON IN ISOLATED NERVE-TERMINALS( HOMEOSTASIS INDUCED BY THE PROTONOPHORE CARBONYL CYANIDE)

The effect of the protonophore carbonyl cyanide-p-trifluoromethoxyphen yl-hydrazon (FCCP) was studied on the intracellular [Na+], pH, and pla sma membrane potential in isolated nerve terminals. FCCP induced a ris e of [Na+], at, and even below, the concentrations (0.025-1 mu M) in w hich it is usually used in intact cells to eliminate Ca2+ uptake by mi tochondria. The FCCP-induced increase of [Na+](i) correlates with a fa ll 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 mu M). Pa rallel 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, change and the fast depolarization of the plasm a membrane were proportional to the proton electrochemical gradient ac ross the plasma membrane; when this gradient was increased, greater de polarization was detected. The slower decrease of the membrane potenti al after the fast initial depolarization was abolished when the medium contained no Na+. It is concluded that FCCP (1) gives rise to a depol arization by setting the plasma membrane potential close to the proton equilibrium potential and (2) enhances the intracellular [Na+] as a c onsequence of an insufficient ATP level and ATP/ADP ratio to fuel the Na+,K+/ATPase. Because both disturbed Na+ homeostasis and plasma membr ane depolarization could profoundly interfere with Ca2+ homeostasis in the presence of protonophores, consideration given to these alteratio ns may help to clarify the cellular Ca2+ sequestration processes.