A new dissection procedure for preparing Myxicola giant axons for observation under voltage clamp is described. Preparation time is generally 40-45 min. 65-70% of the preparations attempted may be brought through the entire procedure, including insertion of the long internal electrode, and support an initial action potential amplitude of 100 mv or greater. Mean values for axon diameter, resting membrane potential, action potential amplitude, maximum peak inward transient current, and resting membrane resistance are 560 micro, -66.5 mv, 112 mv, 0.87 ma/cm(2) and 1.22 K ohm cm (2) respectively. Cut branches do not seem to be a problem in this preparation. Behavior under voltage clamp is reasonably stable over several hours. Reductions in maximum inward transient current of 10% and in steady-state current of 5-10% are expected in the absence of any particular treatment. Tetrodotoxin blocks the action potential and both the inward and outward transient current, but has no effect on either the resting membrane potential or the steady-state current. This selective action of tetrodotoxin on the transient current is taken as an indication that this current component is probably carried by Na.

Action Potentials↗

PubMed1969

Current separations in Myxicola giant axons.

The effect of reducing the external sodium concentration, [Na](o), on resting potential, action potential, membrane current, and transient current reversal potential in Myxicola giant axons was studied. Tris chloride was used as a substitute for NaCl. Preliminary experiments were carried out to insure that the effect of Tris substitution could be attributed entirely to the reduction in [Na](o). Both choline and tetramethylammonium chloride were found to have additional effects on the membrane. The transient current is carried largely by Na, while the delayed current seems to be independent of [Na](o). Transient current reversal potential behaves much like a pure Nernst equilibrium potential for sodium. Small deviations from this behavior are consistent with the possibility of some small nonsodium component in the transient current. An exact P(Na)/P(K) for the transient current channels could not be computed from these data, but is certainly well greater than unity and possibly quite large. The peak of the action potential varied with [Na](o) as expected for a sodium action potential with some substantial potassium permeability at the time of peak. Resting membrane potential is independent of [Na](o). This finding is inconsistent with the view that the resting membrane potential is determined only by the distribution of K and Na, and P(Na)/P(K). It is suggested that P(Na)/P(K)'s obtained from resting membrane potential-potassium concentration data do not always have the physical meaning generally attributed to them.

Action Potentials↗

PubMed1969

Leak current rectification in Myxicola giant axons. Constant field and constant conductance components.

Early leak current, i.e. for times similar to the time to peak of the transient current was measured in Myxicola giant axons in the presence of tetrodotoxin. The leak current-voltage relation rectifies, showing more current for strong depolarizing pulses than expected from symmetry around the holding potential. A satisfactory practical approximation for most leak corrections is constant resting conductance. The leak current-voltage curve rectifies less than expected from the constant field equation. These curves cannot be reconstructed by summing the constant field currents for sodium and potassium using a P(Na)/P(K) ratio obtained in the usual way, from zero current constant field fits to resting membrane potential data. Nor can they be reconstructed by summing the constant field current for potassium with that for any other single ion. They can be reconstructed, however, by summing the constant field current for potassium with a constant conductance component. It is concluded that the leak current and the resting membrane potential, therefore, are determined by multiple ionic components, at least three and possibly many. Arguments are presented suggesting that ion permeability ratios obtained in the usual way, by fitting the constant field equation to resting membrane potential data should be viewed with skepticism.

Animals↗

PubMed1969