The Electrogenic Na-K Pump in the Sheep Cardiac Purkinje Fibre
It is known that the Na-K pump in a wide variety of tissue can be electrogenic (see 13 for a review). The experiments described here examine the electrogenic Na-K pump in the sheep cardiac Purkinje fibre under conditions where the pump is stimulated by an elevation of internal Na. Recent reports have suggested that it is possible to measure directly the electrogenic Na pump current in this tissue using the voltage clamp technique (5, 7). Such measurements were originally made by Thomas (12) in snail neurones. He stimulated the Na-K pump with an intracellular iontophoretic injection of Na and measured an outward current under voltage-clamp conditions. By simultaneously monitoring the intracellular Na activity, a1 Na, using a glass Na+-sensitive microelectrode he observed that as the excess Na was pumped out of the cell both the outward current and a1 Na declined exponentially with the same time-constant confirming that the current was produced by activation of the Na-K pump.
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