The effects of ouabain on the transmembrane potentials and intracellular potassium activity of canine cardiac Purkinje fibers.

Open-tip microelectrodes containing a potassium-sensitive liquid ion exchanger (Corning 477317) were used to study the effects of ouabain on the intracellular potassium activity and the transmembrane potentials of beating canine cardiac Purkinje fibers. The preparations were superfused with Tyrode's solution containing ouabain, 2 X 10(-7) M, and potassium, 4 mM, for 30 minutes. At the end of this period, intracellular potassium activity had decreased from the control value of 130.0 mM to 112.2 mM. The resting membrane potential determined through conventional 3 M KC1-filed microelectrodes decreased from -83.6 to -78.8 mV. Comparison of the decrease in the potassium equilibrium potential with the decrease in the resting membrane potential suggests that there was an accumulation of potassium at the exterior surface of the cell membrane. The effect of ouabain on the resting membrane potential, therefore, was due to a change in the transmembrane potassium ion gradient. This, in turn, resulted from a decrease in intracellular potassium activity and, apparently, from an increased potassium activity at the cell surface.