The relationship between the intra- and extracellular sodium activity of sheep heart Purkinje fibres during inhibition of the Na−K pump
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Removal of external potassium (Ko), or addition of the cardioactive steroid strophanthidin (10−5 M) resulted in a reversible rise in aNai.
The relationship between the steady-state aNai and the [Na]o appeared to be linear. In the presence of strophanthidin this relationship was still linear but had an increased slope.
The levels of aNai in the presence of strophanthidin were much lower than those predicted from an electrochemical equilibrium.
When the Na−K pump had been fully inhibited by strophanthidin (at normal [Na]o), removal of Ko caused either no change or adecrease in aNai.
In low [Na]o (one tenth normal) the addition of strophanthidin caused a relatively small increase in aNai, but the removal of Ko usually resulted in adecrease. The extent and the direction of the change of aNai upon removal of Ko varied with the [Na]o.
We conclude (1) that the aNai can be regulated at levels much below those predicted from the Na electrochemical equilibrium even when the Na−K pump is completely inhibited, and (2) that removal of Ko affects the aNai not only via inhibition of the Na−K pump but also can decrease aNai by a mechanism which remains to be elucidated.
Key wordsIntracellular Na activity Purkinje fibres External Na and K Cardioactive steroids Na−K pump
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