Potential clamp analysis of membrane currents in rat myelinated nerve fibres.

Abstract

1. Large myelinated nerve fibres were isolated from rats and the membrane action potential was recorded in single nodes of Ranvier. Potential clamp experiments were performed at 24 degrees C and in one fibre also at 33 degrees C. 2. Positive potential steps were associated with an initial, mainly Na carried, current with an equilibrium potential (Ue) of 40 +/‐ 13 mV. The INa vs. U curve showed rectification at large positive potentials. 3. The Na permeability (PNa) curve was characteristically S‐shaped with a half value at ‐48 +/‐ 6 mV and a ceiling value of 3.7 +/‐ 0.7 cm.sec‐1.10(‐3). PNa as defined by the constant field equation accounted for the rectification of the INa/U curve. 4. The potential and time dependence of the rapid inactivation (h) process was described quantitatively in terms of its steady state (h infinity) vs. U curve and its rate constants (alpha h and beta h) vs. U curves. h infinity (U = ‐80 mV) was 0.68 +/‐ 0.09. 5. The delayed currents were small and the ionic specificity of the delayed permeability changes was not identified. Tentatively calculated PK at large potentials was 0.21 +/‐ 0.06 cm.sec‐1.10(‐3). The leak conductance (gL) was 130 +/‐ 33 mS.cm‐2. 6. A temperature rise in one experiment from 24 to 33 degrees C increased the rate constants (alpha h and beta h), but did not significantly change the size of the delayed currents. 7. INa was completely blocked by 25 nM‐TTX in the outside solution. External application of 5 mM‐TEA only slowly and incompletely blocked the small delayed currents. This effect was not fully reversible. 8. A comparison with frog fibres showed that the node in rat fibres had (a) a lower Ue of the initial current (probably because of a lower Na selectivity of the channel for the initial current), (b) approximately equal max. peak PNa, (c) about 10 mV negatively shifted PNa vs. U and h infinity vs. U curves, (d) quantitatively similar relations between rate constants of inactivation (alpha h and beta h) and U at 20‐‐24 degrees C, (e) PK that was 1/5 of PK in frog fibres, (f) 4‐‐5 times larger leak conductance, (g) membrane action potentials of smiliar amplitude and duration.