Potassium Channels in Myelinated Nerve : Selective permeability to small cations

The permeability of K channels to various cations is studied in myelinated nerve. Ionic currents under voltage clamp are measured in Ringer solution containing tetrodotoxin and a high concentration of the test ion. Reversal potentials for current in K channels are determined and used with the Goldman-Hodgkin-Katz equation to calculate relative permeabilities. The ratios P (Tl):P (K):P (Rb):P (NH(NH4)) are 2.3:1.00:0.92:0.13. No other ions are found to be measurably permeant including Li(+), Na(+), Cs(+), methylamine, guanidine, hydrazine, or hydroxylamine. The ratio P (Na)/P (K) is less than 0.01. Potassium conductance is depressed at pH values below 5.0. Leakage conductance is higher in K, Rb, Cs, NH(4), and Tl Ringer than in Na Ringer, but the selectivity sequence probably is not the same as for K channels. The hypothesis is offered that the narrowest part of the K channel is a circle of oxygen atoms about 3 Å in diameter with low electrostatic field strength.