Slow mechanism for sodium permeability inactivation in myelinated nerve fibre of Xenopus laevis.

SUMMARY1. We used a patch clamp to measure Na currents across 10-15 #tm diameter circular patches of frog and rat skeletal muscle membrane. We tested for electrophoretic mobility of Na channels, by applying steady lateral fields (of the order of 10 mV #sm-) across the wall of the patch pipette.2. Application of steady negative potentials to the inside of the pipette resulted in a fall in the number of functional Na channels in the patch. This fall took several minutes to complete and was reversible. It was assayed by applying suitable depolarizations at approximately 11 sec intervals.3. When a steady lateral field was applied in the absence of changes in membrane potential of the patch, the loss of Na current was virtually abolished. Thus it was not due to electrophoretic movement of channels, but instead to depolarization of the sarcolemma. Evidently, a very slow inactivation of Na conductance operates in skeletal muscle.4. In frog muscle, the rate constants for loss and recovery of Na current were about 0-1 min-(17 'C) at resting potential. Rate constants were higher at more positive and at more negative membrane potentials. Current amplitude was reduced to 0 5 at about -76 mV.5. Roughly similar results were found in rat omohyoid muscle. A further inactivation mechanism, whose rate was intermediate between conventional fast inactivation and the very slow process described here, was present also in both rat and frog muscle.6. In frog muscle, lateral fields do not alter the potential dependence of fast inactivation. Either the surface charge due to membrane lipids does not influence inactivation or the lipids immediately surrounding the Na channel are restricted in their mobility.

Section: Resultsmentioning

confidence: 99%

Slow changes in currents through sodium channels in frog muscle membrane.

Almers¹,

Stanfield²,

Stühmer³

1983

“…3, which gave p = 0-26. It is concluded that the initial current was mainly a Na current (denoted INa below), but that the Na selectivity was lower than in the frog node (p 0-05, Frankenhaeuser & Moore, 1963b;p=008, Hille, 1975 (Drouin & Neumcke, 1974;Brismar, 1977), which causes a potential drop outside the membrane depending on the inward current amplitude. It may be concluded from Fig.…”

Section: Membiane Currentsmentioning

confidence: 99%

“…2 that there is a mechanism for PNo inactivation in the rat node that resembles the h-system in squid and frog nerve (Hodgkin &;Huxley, 1952b;Frankenhaeuser, 1959Frankenhaeuser, , 1960 (Brismar, 1977) the time course consisted of a rapid initial phase that was fitted by an exponential time constant (as) and a slow variation ia peakINc that was observed as a deviation from the steady state at long lasting conditioning pulses. The rapid inactivation system (h) could generally be distinguished from the slow inactivation changes, because the time courses were sufficiently different.…”

Section: Pna Inactivation Systemmentioning

confidence: 99%

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

Brismar¹

1980

Self Cite

197

SUMMARY1. 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 0C and in one fibre also at 33 0C.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-'. 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 (ho*,) vs. U curve and its rate constants (ac and fik) VS. U curves. h. (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-'. 10-. The leak conductance (gL) was 130+ 33 mS. cm-2.6. A temperature rise in one experiment from 24 to 330C increased the rate constants (ah and fik), 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.: vs. U curves, (d) quantitatively similar relations between rate constants of inactivation (ash and fl) and U at 20-24 0C, (e) PK that was 1/5 of PK in frog fibres, (f) 4-5 times larger leak conductance, (g) membrane action potentials of similar amplitude and duration.

“…Studies in nerve (Adelman & Palti, 1969;Chandler & Meves, 1970;Peganov, Khodorov & Shishkova, 1973;Fox, 1976;Schauf, Pencek & Davis, 1976;Brismar, 1977;Chiu, 1977;Rudy, 1981;Sigworth, 1981;Collins, Rojas & Suarez-Isla, 1982) and in muscle (Reuter, 1968;Brown, Lee & Powell, 1981;Almers, Stanfield & StUhmer, 1983;Patlak & Oritz, 1985; Ruff, Simoncini & Stiihmer, 1987;Simoncini & Stiihmer, 1987) have shown that sodium channels in these excitable tissues display multiple components of inactivation, components that can be distinguished by their different kinetics. In this paper we show that sodium channels in mammalian Schwann cells, cells which are not generally regarded as excitable, also show kinetically distinct components of inactivation.…”

Section: Introductionmentioning

confidence: 99%

Multiple kinetic components of sodium channel inactivation in rabbit Schwann cells.

Howe

Ritchie

1992

SUMMARY1. We have studied the kinetics of inactivation of sodium currents evoked in Schwann cells from neonatal and adult rabbits with patch-clamp recording techniques. The decay both of whole-cell currents and of ensemble currents obtained from outside-out patches was reasonably well-described by single exponential fits which gave values for the time constant, Th, similar to those found for such currents in nerve (about 0-5 ms at 0 mV). Although inclusion of an additional exponential component usually improved the fits to the decay of these currents, the relative amplitude of the slower component (time constant 3-6 ms) was always small.2. The time course of recovery from steady-state inactivation clearly consisted of two exponential components. At -120 mV, recovery from steady-state inactivation at -50 mV consisted of a fast component with a time constant of 2-2 + 02 ms and a much slower component with a time constant of 12 + 02 s (n = 9). The relative amplitude of the slow component (expressed as a fraction of the sodium current when inactivation was removed completely) was 0-56 + 0 03. The corresponding amplitudes of the slow component when similar experiments were done from holding potentials of 0 and -70 mV were 0-80 + 0-04 and 0-36 + 0-03, respectively (n = 5 and 8).3. The onset of steady-state inactivation also followed a bi-exponential time course. The time constant of the slower component was similar at each potential examined (0, -50 and -70 mV), being 6-7 s. The relative amplitude of the slow component of onset depended on membrane potential, and it was similar (at each potential examined) to the corresponding amplitude of the slow component of recovery.4. The inclusion of 40 mM-iodate ions in the pipette solution slowed the decay of whole-cell sodium currents, as did the extracellular application of venom (10 jug ml-') from the scorpion Leiurus quinquestriatus. Prolonged exposure of the cells to Leiurus venom appeared to increase the steady-state amount of slow inactivation. 5. Records of single-channel sodium currents tended to cluster into records which either did, or did not, contain openings. This apparently non-random behaviour depended on membrane potential, and on the frequency at which the test steps were repeated, in the way expected if it resulted from slow inactivation.6. The results indicate that sodium channels in Schwann cells can exist in at least three different inactivated states and provide further evidence of the similarity between these sodium channels and sodium channels in excitable cells.MS 9300