The dependence of sodium pump current on internal Na concentration and membrane potential in cardioballs from sheep Purkinje fibres

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Abstract
The effect of various intracellular Na concentrations (c Na i ) and membrane potentials on the Na pump current (I P) was studied in isolated, cultured sheep cardiac Purkinje cells (‘cardioballs’).I P was identified as cardiac steroid sensitive current. The dependence ofI P onc Na i was investigated at a membrane potential of −40 mV by means of whote-cell recording from cardioballs internally perfused with media containing various Na concentrations. Internal perfusion with a Na free solution abolishedI P. The amplitude ofI P as a function ofc Na i displayed saturation kinetics. Half maximal activation ofI P occured at ac Na i of about 9 mM. The maximalI P density was estimated to be 1.1μA/cm2. The potential dependence ofI P was studied by conventional whole-cell recording under various ionic conditions. GenerallyI P displayed little voltage dependence at membrane potentials positive to −20 mV.I P declined at more negative potentials. The pump cycle probably includes only one voltage sensitive step. The potential dependence ofI P was more pronounced at lowerc Na i or lower concentrations of the external pump activator Cs+. The findings are in line with the idea that increasingly steeper ionic gradients against which the cations are pumped strengthen the voltage dependence ofI P in the potential range studied. Other factors probably affecting the pump current-voltage (I P-V) relation are discussed. The results suggest thatI P varies during electrical activity.