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 PTl:PK:PRb:PNHNH4 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 PNa/PK is less than 0.01. Potassium conductance is depressed at pH values below 5.0. Leakage conductance is higher in K, Rb, Cs, NH4, 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 A in diameter with low electrostatic field strength.

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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. Re- versal potentials for current in K channels are determined and used with the Goldman-Hodgkin-Katz equation to calculate relative permeabilities. The ratios PTI:PK:PRb:PNH 4 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 PN,/PK 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 TI Ringer than in Na Ringer, but the selectivity se- quence 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 A in diameter with low electrostatic field strength.

Potassium Channels in Myelinated Nerve

1. Two types of after-potentials in the stretch receptor neurone of crayfish are described.

2. A short-duration after-hyperpolarization associated with a single spike or a few spikes is diminished and reversed on applying hyperpolarizing currents. However, a much longer-lasting post-tetanic hyperpolarization (PTH) is enhanced by conditioning hyperpolarization; thus, no reversal potential can be obtained.

3. No changes in membrane conductance occur during PTH.

4. Reducing K concentration in the bathing fluid diminishes PTH, while it shifts the reversal potential of the short after-potential toward greater negativity.

5. Replacement of Na with Li, or addition of 2,4-dinitrophenol in the bathing fluid suppresses PTH in a reversible manner.

6. Electrophoretic injection of Na into the cell induces a long-lasting hyperpolarization.

7. No change in K-equilibrium potential, as indicated by the reversal point of the short after-potential, is detected during PTH.

8. It is concluded that the short after-potential is caused by a permeability increase for potassium ion, whereas PTH is produced by an electrogenic Na-pump.

Post‐tetanic hyperpolarization and electrogenic Na pump in stretch receptor neurone of crayfish

The authors have agreed that this Chapter should not be an exhaustive compilation of all papers. Instead they intend to submit the essential modern knowledge necessary to neurobiologists working with peripheral nerve of frogs which, in their opinion, is an essential test object to develop the theoretical and experimental basis of the function of excitable membranes of vertebrates. They want, in particular, to present a chapter which in the future can be used to familiarize newcomers with the methods and theories used in this field. Since there is almost no work on unmyelinated amphibian fibers, the discussion is almost exclusively on myelinated nerve.

Electrophysiology of the Peripheral Myelinated Nerve

Recovery Processes and Metabolism of Nerve

Stimulation of intact or desheathed frog sciatic nerves produced an increase in the sodium content and a decrease in the potassium content of this tissue. In desheathed preparations the magnitudes of the changes in ionic contents decreased as the concentration of the potassium in the bathing solution was increased, while changing the external sodium concentration produced small effects on the ionic shifts. During tetanization, the rate of decline of the compound action potential also decreased as the external potassium concentration increased. Eliminating the activity respiration with 0.2 mM azide did not greatly modify the changes in sodium and potassium distribution that accompanied activity in either intact or desheathed nerves.

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Tomoaki Asano

Papers

Effects of potassium, sodium, and azide on the ionic movements that accompany activity in frog nerves