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Our observations on the response of TPP+ uptake into K+-depleted cells on the extracellular K+ concentration and on the addition of ouabain and amino acid appear to provide suggestive evidence of, or are at least consistent with, the operation of a powerful, K+-activated electrogenic pump. They are also consistent with the assumption that in these K+-depleted cells the active uptake of amino acid energized by the electric PD presumably generated by this electrogenic pump. It follows that an energy source other than the electrochemical potential gradient of Na+ ions need not be invoked to explain active amino acid transport with inverted Na+ and K+ distribution. In the presence of an electrogenic cation pump one should expect that owing to the activity of this pump the electrical PD is raised under these conditions to a value outweighing the opposed chemical potential gradient of Na+. The experimental evidence so far available is clearly consistent with such an assumption.

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