Acta Physiol Scand 1981, 112: 495-496

Potential clamp analysis of the effect of anoxia on the nodal function of rat peripheral nerve fibres

TOM BRISMAR Department of Clinical Neurophysiology, Karolinska sjukhuset, 10401 Stockholm, Sweden

Potential clamp analysis of single myelinated nerve fibres from the rat showed that the acute effects of anoxia was ( I ) a decrease in the max. peak Na permeability (PNa) as measured at removed inactivation (h-= I ) , and (2) an increased inactiva- tion of P,, at the resting potential due to a shift of the h-/potential curve towards more negative po- tentials. This resulted in a reduction of the available PNa to about 20% in 15 min which blocks the nerv- ous impulse. Anoxia had no rapid effect on the membrane Na equilibrium potential, i.e. there was no indication of an internal Na accumulation due to anoxia within this time.

It is well known that uncoupling of the oxidative metabolism by e.g. dinitrophenol inhibits the active transport of Na out from the isolated squid axon (Hodgkin & Keynes 1955). In single rat nerve fibres anoxia decreases the action potential amplitude to only a graded response within about 20 min which was attributed to accumulation of Na inside the fibre (Maruhashi & Wright 1967).

The effect of anoxia was studied in the present experiments on single myelinated nerve fibres from the sciatic nerve of the rat. A single fibre was iso- lated and mounted in a recording chamber, and the membrane currents of a node of Ranvier were re- corded under potential clamp conditions (Brismar 1980). The nerve was kept in oxygenated Ringer solution until the effect of anoxia was tested on the single fibre. This was made by letting in 100% ni- trogen over the surface of the solution pools of the recording chamber, which rapidly depleted the oxygen content around the fibre about 0.1 mm be- low the fluid surface. The pH (7.4) of the tris-buf- fered Ringer solution (Brismar 1980) was unaffected

by the gas concentration changes. The experiments were performed at 24°C.

The Na equilibrium potential (UNa), the Na per- meability (PNa) curve, the steady state P,, inactiva- tion (h-), the max. K permeability (PK) and the leak conductance (gL) were repeatedly measured from the membrane current changes associated with var- ious potential steps. Anoxia decreased the PNa in two ways: ( I ) It decreased the max. peak Pxa meas- ured at large positive potential steps preceded by negative conditioning pulses (which removed in- activation, h,= 1). (2) I t shifted the curve relating h, to membrane potential in negative direction along the potential axis, so that the Pxa inactivation of the resting membrane was increased. The action upon the max. peak P,, level may have been due to a slow inactivation process (Brismar 1977), since its amplitude was increased by prolonged negative conditioning pulses. The potential dependence of the PNa8 activation\(U*,,, pNa) was not affected.

Anoxia had no significant effect (in 6 fibres) on UNa during the measuring period (20 rnin). A slow decrease in UNa (about 10 mV) was observed in some fibres during the experiment, but this was not notably accelerated by anoxia. This means that there was no indication of an internal Na accumula- tion due to anoxia within this time. The parameters g, and PK were also unaffected. Fig. 1 shows the time course and the magnitude of the changes in one fibre. The available PNa (=max. peak PN,Xh,) at resting potential was changed from about 0.70 be- fore to 0.22 after 13 min of anoxia. This change will render the fibre inexcitable. Oxygenation (for about 15 min) could not reverse these changes, but the decrease in PNa was halted.

Arm Physiol Scand 112

496 T. Brismar

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T i m e (mini

Fig. 1 . Short time effects of anoxia on max. peak PNa, h, (at resting potential, U=-80 mV), UlmaxpNs and UNa. Max. peak PN, of node obtained with the constant field equation from the peak Na current during positive potential steps preceded by a - 120 mV conditioning pulse (duration 8 ms), which removed the rapid inactivation (L= I ) . Utmax p N a i s the potential corresponding to the half value on the PNa-curve. h, (U=-80 mV) was obtained from the h,-curve in the usual way. UNB was estimated from Na-currentlpotential plots and from the reversal of the initial hump in the current record. Temp. 24°C.

It is concluded that anoxia blocks the impulse propagation in the isolated mammalian nerve fibre (in about 15 min) through a decrease in the max. peak PNa combined with an increased inactivation (negative shift of the ha-curve). Contrary to earlier assumptions, there was no indications of rapid changes in the axoplasmic “a].

This work was supported by the Foundations of the Karolinska institutet, the Swedish Diabetic Association and the Swedish Medical Research Council (Project No. 14X-4255).

REFERENCES BRISMAR, T. 1977. Slow mechanism for sodium permea-

bility inactivation in myelinated nerve fibre of Xenopus laevis. J Physiol (Lond) 270 283-297.

BRISMAR, T. 1980. Potential clamp analysis of mem- brane currents in ri t myelinated nerve fibres. J Physiol (Lond) 298: 171-18 1.

HODGKIN, A. L. & KEYNES, R. D. 1955. Active trans- port of cations in giant axons from Sepia and Loligo. J Physiol (Lond) 128: 28-60.

MARUHASHI, J. & WRIGHT, E. B. 1967. Effect of oxygen lack on the single isolated mammalian (rat) nerve fiber. J Neurophysiol30: 434-452.

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