Sodium and potassium currents in squid axons perfused with fluoride solutions

Abstract

1. Axons perfused with a K-free solution containing 300 mM-NaF + sucrose to maintain isotonicity (referred to as 300 mM-NaF) and placed in K-free artificial sea-water usually depolarized spontaneously to around 0 mV. The membrane could be hyperpolarized to -70 to -100 mV with a small inwardly directed current; in one experiment the holding current was measured and was found to be less than 20 muA/cm(2).2. Membrane currents associated with a step depolarization from a potential which varied from -70 to -100 mV showed three phases: (a) an initial capacitative transient, (b) an early current which was inward for small depolarizations and outward for large ones, (c) a smaller maintained current. The currents in (b) and (c) are considered to be carried by Na ions since they both reversed direction at the same potential which was on the average within 0.3 mV of the equilibrium potential for Na ions, 10.4 mV at 0 degrees C and 11 mV at 16.5 degrees C, as estimated from measurements made with a cation-sensitive glass electrode.3. The instantaneous current-voltage relation was determined at the time of peak current and at the end of a long prepulse when the current had reached a steady level. In both cases the curve was approximately linear with a slight deviation at negative potentials.4. Prepulses, lasting 11-48 msec, to a potential of 33-64 mV (0-3.5 degrees C) produced a shift in the equilibrium potential of 0.6-3.3 mV. This small change can be accounted for by assuming that Na ions accumulate in the Frankenhaeuser-Hodgkin space.5. Both peak and steady-state components of Na current were blocked by tetrodotoxin (10(-7) g/ml.) in the external solution.6. The values of peak and steady-state Na conductance were strongly voltage-dependent for V less than -20 mV; for V more negative than -40 mV the peak and steady-state values increased e-fold for a change in potential of 4 and 6-8 mV respectively. At positive potentials the peak conductance was relatively independent of potential, whereas the steady-state curve showed an increase; at 50 mV the steady-state conductance was on the average 0.44 times the peak value for temperatures -0.3 to 4 degrees C and 0.24 times the peak value for a temperature of 16.5 degrees C.7. Following an 18-164 min perfusion period with 300 mM-NaF, the delayed K currents with 300 mM-KF were reduced in amplitude to less than one-tenth the initial level. This apparent removal of the delayed rectifier was not accompanied by any significant change in either the relation between peak early current and voltage or the associated equilibrium potential.8. In an experiment in which tetrodotoxin was used to block the early channel, K currents were determined before and after NaF perfusion. In both cases the kinetics on depolarization followed the Hodgkin-Huxley n(4) relationship and the rate constants were similar, although after NaF perfusion the amplitude was reduced to 0.07 times the control level.9. In axons perfused with 300 mM-KF, following removal of the delayed rectifier by 300 mM-NaF, the ratio of steady-state Na current: peak Na current was estimated to be about half the value obtained with NaF. A similar decrease was obtained in an axon which was perfused with 300 mM-CsF; on subsequent perfusion with 300 mM-KF, following 35 min with CsF, about half the original delayed current was present.10. The general conclusion is that in axons perfused with 300 mM-NaF the Na conductance is not fully inactivated by depolarizations which last for tens of milliseconds. The maintained component may underlie the plateau phase of long lasting action potentials which have been recorded under similar conditions.