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The membrane potential of mouse ascites-tumour cells studied with the fluorescent probe 3,3'-dipropyloxadicarbocyanine. Amplitude of the depolarization caused by amino acids.

1. The magnitude of the K+ gradient across the plasma membrane, which was in equilibrium with the membrane potential (E) of the tumour cells, was determined by the "null point" procedure of Hoffman & Laris (1974) [J. Physiol. (London) 239, 519--552] in which the fluorescence of the dye serves as an indicator of changes in the magnitude of E. 2. A mixture of oligomycin, 2,4-dinitrophenol and antimycin was used to stop the mitochondria from interfering with the fluorescence signal. Transport functions at the plasmalemma were maintained under these conditions in the presence of glucose. 3. Physiological circumstances were found in which incubation with glycine or with glucose changed the "null point" value of E within the range--20mV to--100mV. The fluorescence intensity at the "null point" was an approximately linear function of E over that range. The procedure enabled E to be inferred form the fluorescence intensity in circumstances where titration to the "null point" was not feasible. 4. The rapid depolarization caused by l-methionine or glycine was shown in this way to have a maximum amplitude of about 60mV. A mathematical model of this process was devised. 5. The electrogenic Na+ pump hyperpolarized the cells up to about --80mV when the cellular and extracellular concentrations of K+ were roughly equal. 6. The observations show that the factors generating the membrane potential represent a major source of energy available for the transport of amino acids in this system.