Some further observations on the electrogenic sodium pump in non-myelinated nerve fibres

1. A study has been made of the hyperpolarization that follows a period of electrical activity (post-tetanic hyperpolarization) and of the hyperpolarization which develops when potassium is readmitted after bathing the desheathed vagus nerve of the rabbit in potassium-free Locke solution during 15 min (potassium-activated response).2. Reduction of the external chloride concentration increases the membrane resistance and the potassium-activated response without changing the time constant of the response. A linear relation between the amplitude of the potassium-activated response and the membrane resistance was found. When chloride was replaced completely by isethionate (sodium salt) and by sulphate (other salts) the potassium-activated response increased by a factor of 5.3. The membrane resistance is decreased during the post-tetanic hyper-polarization elicited in isethionate Locke solution: the decrease is more pronounced after a longer period of electrical stimulation of the nerve.4. A small increase of the membrane resistance was found during the potassium-activated response. The changed membrane potential during the response can account for the alteration of the membrane resistance observed.5. The amplitude of the potassium-activated response is increased during hyperpolarization and reduced during external depolarization of the nerve, whereas the time constant is not affected. The potassium-activated response appears to be independent of polarization of the membrane after correction for the changed membrane resistance.6. The maximum amplitude of the activated response and the external potassium concentration are related following Michaelis-Menten kinetics; the time constant of the response is inversely related to the external potassium concentration.7. The area of the electrogenic response activated by high potassium concentrations (5.6-20 mM) is almost constant, but is reduced at lower potassium concentrations. The amplitude and area of the thallium-activated response are increased (about 1.5 times) compared with the potassium-activated response.8. It was concluded that the electrogenic response, reflected by post-tetanic hyperpolarization, is not directly related to activity of the electrogenic pump, which is probably due to accumulation of potassium in the periaxonal space; that the potassium activated response is produced entirely by activity of the electrogenic sodium pump; and that the current produced by activity of the electrogenic sodium pump is independent of the electrochemical gradient and membrane resistance.