Electronic measurement of the intracellular concentration and net flux of sodium in the squid axon.

John W Moore,William J Adelman

doi:10.1085/jgp.45.1.77

Abstract

A unique, rapid, and non-destructive determination of the intracellular sodium concentration of a squid axon may be provided by the "voltage clamp" technique, in which the potential across the axon membrane is under electronic control. The potential at which the early component of ionic current reverses following a membrane potential step was used as an index of the intracellular sodium concentration. Several types of experiments were used to test the applicability of this method for measurement of intracellular sodium and its net flux. The concentration was found to increase from 38 mM for a fresh axon to 50 mM in about an hour. From this change, the net flux for a fresh resting axon was estimated to be 40 pmoles/cm(2) sec. Rapid stimulation of an unclamped axon produced a marked increase in the rate of sodium accumulation. Rapid pulsing of the membrane in a voltage clamp to potentials more positive than the sodium potential moved sodium out fast enough to produce a definite decrease in internal concentration. The agreement between the results with this method and those with more direct methods is quite satisfactory. An attractive feature of this method of intracellular sodium determination is that the physiological function of the axon is maintained and other measurements may be made concurrently.

Highlights

Hodgkin and Huxley (1952 a) found that, when depolarizing potential steps were applied to the squid axon membrane in a voltage clamp, the direction of the early component of ionic current reversed when the membrane potential was pulsed beyond the assumed value for the sodium equilibrium potential
O n the assumption that the Nernst equation is as valid for changes in the internat concentration as for external changes, we may calculate the internal sodium concentration from sodium potential measurements when the external concentration is known
Until the early component of the ionic current can be demonstrated to be a flow of sodium ions by less equivocal techniques, probably the best procedure is to compare the various determinations made using this method to other measurements made previously by different techniques

Summary

INTRODUCTION

Hodgkin and Huxley (1952 a) found that, when depolarizing potential steps were applied to the squid axon membrane in a voltage clamp, the direction of the early component of ionic current reversed when the membrane potential was pulsed beyond the assumed value for the sodium equilibrium potential. The potential at which this reversal occurred followed rather closely the predicted change in the equilibrium potential upon variation of the external sodium concentration. The sodium potential has been followed as a function of time in the resting axons in order to obtain a measure of the resting sodium flux which can be compared with tracer results. Until the early component of the ionic current can be demonstrated to be a flow of sodium ions by less equivocal techniques, probably the best procedure is to compare the various determinations made using this method to other measurements made previously by different techniques

METHOD

RESULTS

DISCUSSION

Findings

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