Measurement of axonal membrane conductances and capacity by means of a varying potential control voltage clamp

Abstract

A new mode of voltage clamping in the squid giant axon is introduced and its advantages are analyzed, tested, and utilized to investigate membrane conductances and capacity. This method replaces the constant command potentials of the standard voltage clamp with potentials which vary with time. Some of the advantages in using the varying potential clamp are: (1) slowly varying potentials generate practically pureI _K ; (2) rapidly varying potentials generate practically pureI _Na; (3) triangular waves generate, under proper conditions, pure capacity currents and easy-to-analyze leakage currents; (4) the method gives direct, on-line display of sodium or potassium I–V characteristics within milliseconds; (5) it enables rapid and accurateE _Na andE _K determinations; and (6) it enables simple and accurate determination ofC _m. The method was utilized to study the effects of various ions on membrane conductances and the effects of ionic composition, ionic strength, and temperature on membrane capacity. Membrane capacity was found to be practically independent of frequency in the 200 to 2,000 Hz range. Replacement of external sodium by Ca⁺⁺, by impermeable Tris⁺, or even by dextrose or sucrose (low ionic-strength solutions) had negligible effects onC _m.C _m showed a small, positive temperature coefficient of 1.39% per °C in the 3 to 21°C range, and little change with temperature in the 20 to 40°C range. Above 40°C, bothC _m andg _L increased considerably with temperature.