Activities of the sodium pump in cat pyramidal tract cells investigated with intracellular injection of sodium ions

Summary Sodium ions were injected into cat pyramidal tract (PT) cells electrophoretically through an intracellular NaCl or Na glutamate-filled microelectrode. Following an injection there were decreases in the maximum rates of rise and fall of the spike potential and there was displacement of the inhibitory postsynaptic potential in a depolarizing direction. These changes recovered with an exponential time course, indicating concomitant changes in the internal sodium, potassium and chloride concentrations under the operation of the sodium pump in extruding excess sodium. From the exponential recovery curve, the rate constant of active sodium extrusion was estimated as about 60 hr^–1 in fast PT cells and about 90 hr^–1 in slow PT cells. It was suggested that the sodium pump was at least partly electrogenic, since the resting membrane was hyperpolarized by the sodium injection to the degree which depended on the amount of sodium-injecting current. Further support for this possibility was obtained by the experiment of high-frequency activation of PT cells, in which the sodium entry through the active membrane developed a slow post-tetanic hyperpolarization.