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

Abstract

1. Muscles with high intracellular sodium concentrations can extrude sodium into solutions which contain 10 m-equiv/l. of either potassium or rubidium. Potassium or rubidium replaces the extruded intracellular sodium. These cation movements take place equally well when the external anion is chloride or sulphate, though muscles deteriorate if left for long periods in sulphate solutions.2. Measurements of intracellular potentials during extrusion of sodium into solutions containing potassium show:(a) an internal potential more negative than the potassium equilibrium potential (E(K)); at 20 degrees C the difference is nearly 20 mV.(b) that a difference between the membrane potential and E(K) is dependent on temperature and is abolished by 10(-5)M ouabain.(c) an internal potential which becomes more negative in the presence of 0.1% cocaine, a concentration of cocaine which substantially increases the membrane resistance to potassium movement. In the absence of potassium or rubidium no such hyperpolarization occurs.3. When muscles extrude into solutions which contain rubidium they have internal potentials which are 10-20 mV more negative than when extruding sodium into corresponding solutions containing potassium.4. Measurements of electrical conductance in the potassium solution suggest that the electrochemical potential difference for potassium ions may be large enough to account for the measured inward potassium movements during sodium extrusion. The reliability of the measurements does not, however, exclude the possibility that some part of the inward potassium movement is chemically linked to outward movement.5. Measurements of membrane conductance in solutions containing rubidium, and of net movements of rubidium in the presence and absence of ouabain, lead to the conclusion that at least 90% of the inward rubidium movement during sodium extrusion must be chemically linked to the sodium movement.6. The hyperpolarization during extrusion of sodium could be explained on the basis of a fall of the potassium or rubidium concentration in a region of the extracellular space immediately external to the membrane. It is argued that certain characteristics of the hyperpolarization make it difficult to explain the hyperpolarization on this basis alone, though some part of it may be due to extracellular depletion of either potassium or rubidium.The main conclusion is that the sodium pump is capable of transferring electric charge across the membrane in which it is operating, but that, in a given time, the net charge transferred is less than the charge on the sodium ions that the pump has transported, by an amount that corresponds to the charge on the potassium or rubidium ions chemically transported by the pump.