Membrane potential and conductance during transport of sodium, potassium and rubidium in frog muscle

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Abstract
Muscles with high intra-cellular Na concentrations can extrude Na into solutions which contain 10 m-equiv/1 of either K or Rb. K or Rb replaces the extruded intra-cellular Na. These cation movements take place equally well when the external anion is chloride or sulfate, though muscles deteriorate if left for long periods in sulfate solutions. Measurements of intracellular potentials during extrusion of Na into solutions containing K show: an internal potential more negative than the K equilibrium potential (EK); at 20[degree]C the difference is nearly 20 mV; a difference between the membrane potential and EK depends on temperature and is abolished by 10-5 [image] ouabain; an internal potential which becomes more negative in the presence of 0.2% cocaine, a concentration of cocaine which substantially increases the membrane resistance to K movement. In the absence of K or Rb no such hyperpolarization occurs. When muscles extrude into solutions which contain Rb they have internal potentials which are 10-20 mV more negative than when extruding Na into corresponding solutions containing K. Measurements of electrical conductance in the K solution suggest that the electrochemical potential difference for K ions may be large enough to account for the measured inward K movements during Na extrusion. Some part of the inward K movement is probably chemically linked to outward movement. Measurements of membrane conductance in solutions containing Rb, and of net movements of Rb in the presence and absence of ouabain, indicate that at least 90% of the inward Rb movement during Na extrusion must be chemically linked to the Na movement. The hyperpolarization during extrusion of Na may be due to a decrease of the K or Rb concentration in a region of the extracellular space immediately external to the membrane. The Na pump is capable of transferring electric charge across the membrane in which it is operating, but the net charge transferred in a given time, is less than the charge on the Na ions that the pump has transported, by an amount that corresponds to the charge on the K or Rb ions chemically transported.

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