The influence of pH on the cell membrane potential of primary cultured rat hepatocytes as measured with tetraphenylphosphonium and dimethyloxazolidine-2,4-dione

Abstract

The accumulation of tetraphenylphosphonium in cultured rat hepatocytes is increased upon alkalization of the extracellular pH. External acidification causes a decrease in the ratio of the intracellular to the extracellular concentration of the cation. The addition of bicarbonate to the incubation medium induces an increase in the tetraphenylphosphonium distribution ratio whereas the effect of NH 4 + is to decrease it. Concomitant measurements of the distribution of dimethyloxazolidine-2,4-dione show that the intracellular accumulation of tetraphenylphosphonium is a function of the pH difference across the plasma membrane, i.e. it depends on the magnitude and direction of the (normally outwardly directed) transmembrane proton concentration gradient. Since the distribution of the lipophilic cation qualitatively monitors changes of the electrical plasma membrane potential of the liver cells, it is concluded that the changes of the tetraphenylphosphonium distribution occurring with changes of the transmembrane pH difference reflect modulations of the cellular membrane potential. Taking into consideration the very low permeability of the liver cell membrane to passive proton movements, it is suggested that the plasma membrane of the liver cells contains an electrogenic proton-translocating mechanism which is accelerated by increasing and is slowed down by decreasing the transmembrane pH difference (pH i < pH c).