The electrogenic potential in rat C nerve fibres: some effects of lithium and thallium

1. Subsequent to bathing a desheathed vagus nerve in a K‐free medium Li (5‐50 m M ) generates a hyperpolarizing response in the same manner as does K except that Li is 23 times less potent than K. Preincubation with small concentrations of either Li or K inhibits a subsequent K response. 2. The Li response is less phasic than the K response. In the presence of a cardiac glycoside Li has no effect whereas K depolarizes the preparation. Thus Li is presumed to generate a relatively pure ouabain‐sensitive electrogenic response. 3. If a Li response is always terminated with 50 m M ‐Li, but various concentrations are used during the first half, the total response has a constant area despite the wide range of activation during the first half. This implies that the electrogenic pump ratio is constant over this range of activation. 4. Thallium (1 m M ) also generates a ouabain‐sensitive response, but even short exposures (5 min) produces a strong and lasting inhibition of subsequent potassium responses. On a slower time scale Tl also inhibits the electrogenic post‐tetanic hyperpolarization. After exposure to Tl a test action potential shows that the afterpotential is also decreased. 5. If action potentials are produced during an electrogenic response the hyperpolarizing afterpotential reverses polarity. It is supposed that this is due to a short‐circuiting of the electrogenic potential during this time.