Voltage modulation of Na+/K+ transport in human erythrocytes

For a cell to perform certain functions it presumably must maintain a steady-state transmembrane potential. This potential differential can take several forms, e.g. a proton gradient, an ionic concentration gradient, or an asymmetric distribution of charges in membraneous proteins and lipids. We report here a study which indicates that an externally applied electric field can induce opening/closing of membrane channels, including Na+/K+ ATPase channels. The method utilizes a continuous AC stimulation of cell suspensions. Reversible channel opening/closing of human erythrocytes in an isotonic suspension can be demonstrated by directly measuring the conductivity of the bulk suspension. Channel opening occurs at a field intensity of 10 V/cm, which corresponds to a maximum transmembrane potential of about 6 mV, when the frequency of AC field is maintained below 1 kHZ. The apparent half-time of the channel opening decreases with an increase in the stimulating voltage, and reaches a plateau value of 2 sec beyond 24 V/cm (15 mV of transmembrane potential). When the AC field is removed, these channels close with a half-time of 10.2 sec. Of the channels opened, roughly 20% belongs to the Na+/K+ ATPase, as this fraction of conductance signals can be quantitatively blocked by a specific inhibitor ouabain in a low ionic medium. The AC field appears to stimulate active Na+/K+ transport against a concentration gradient in erythrocytes. At a field strength of 24 V/cm, the net transport against a controlled sample is about 1 mM 42K+ ion per hour under our experimental condition. This translates into a rate of 60 K+ ions per channel per second. The AC field also increases the K+/Na+ ratio of cytoplasmic concentration in the stimulated sample, indicating an active extrusion of Na+ ion from the cells. Higher consumption of ATP is not detected in the stimulated cells as compared with unstimulated cells. As expected, an addition of ouabain in the external medium abolishes the above effects. Experiments described here would demonstrate that the phenomena observed are due to the effect of the field-induced transmembrane potential. The significance of these observations are discussed in the light of the recent discovery that transmembrane potential is an important driving force for certain cellular functions.