Ionic basis of the resting potential of submucosal arterioles in the ileum of the guinea-pig.

1. The changes in the resting membrane potential of arterioles produced by rapid and brief changes in external ionic concentrations were measured. 2. The resting membrane potential was insensitive to changes in the external concentrations of both sodium and chloride ions but sensitive to changes in the external concentration of potassium ions. 3. Increasing the external concentrations of potassium ions produced depolarizations that were well described by the Nernst equation. 4. Decreased external concentrations of potassium ions produced membrane depolarizations which appeared to result not from inhibition of an electrogenic sodium pump but rather from a change in the resting conductance of the arteriolar membrane to potassium ions. 5. Ouabain caused both membrane depolarization and an increase in membrane resistance. 6. It is suggested that at rest, arteriolar smooth muscle is permeant predominantly to potassium ions, with only small contributions from chloride and sodium ions. No evidence was obtained which would support the idea that an appreciable proportion of the resting membrane potential depended upon current flow from an electrogenic sodium pump.