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Mechanism of action of the sodium pump in vertebrate photoreceptors.

Membrane potential and photoresponse of rods in the isolated toad retina was recorded while changing the ionic composition of the extracellular medium. In the presence of 10 mM external Cs+ the bright flash response consisted of an initial fast component of about 35 mV followed by a much slower component as large as 50 mV. The slow component of the photoresponse was blocked by micromolar amounts of both ouabain and strophanthidin. The effects induced by the latter were almost completely irreversible. The effects induced by changing the external concentrations of Na+, K+ and Ca2+ were analysed assuming that the amplitude of the slow component of voltage photoresponse reflects the activity of Na+--K+ pump. It is shown that external Na+, K+ and Ca2+ have the same effect on the amplitude of the slow component as on the Na+-K+ pump activity of other tissues.