SUMMARY1. The blocking by external Cs of inward tail currents through the K channel of the squid giant axon, as seen by the effect of Cs on the 'instantaneous' I-V curve is described.2. Block onset is complete within 50-100 /tsec of a step in voltage. The block produces a negative slope region in the inward current quadrants of the I-V plots.3. The experiments were performed in the presence of external concentrations of 240 mM-K and 0-200 mM-Cs, with the external ionic strength maintained by substituting Tris or Na for the Cs. Essentially the same results were obtained with either Tris or Na as the substitute ion.4. The concentration of Cs required to block 50 % of the K channels at zero transmembrane voltage, in intact axons, was estimated to be approximately 1 M.5. The slope of the dose-response curve is steeper than expected for a blocking reaction with 1:1 stoichiometry when membrane voltage is in the neighbourhood of -100 mV.6. With Cs concentrations > 50 mM, the voltage dependence of the block is too steep to be accounted for by the binding of a single Cs ion per channel within the membrane electric field.7. In perfused axons, the block occurred at less negative internal voltages when internal K concentration was reduced.8. These observations are qualitatively consistent with a multisite channel showing single file properties.

Summary. Potassium currents of various durations were obtained from squid giant axons voltage-clamped in artificial seawater solutions containing sufficient tetrodotoxin to block the sodium conductance completely. From instantaneous potassium currentvoltage relations, the reversal potentials immediately at the end of these currents were determined. On the basis of these reversal potential measurements, the potassium ion concentration gradient across the membrane was shown to decrease as the potassium current duration increased. The kinetics of this change was shown to vary monotonically with the potassium ion efflux across the membrane estimated from the integral over time of the potassium current divided by the Faraday, and to be independent of both the external sodium ion concentration and the presence or absence of membrane series resistance compensation. It was assumed that during outward potassium current flow, potassium ions accumulated in a periaxonal space bounded by the membrane and an external diffusion barrier. A model system was used to describe this accumulation as a continuous function of the membrane currents. On this basis, the mean peria'~onal space thickness and the permeability of the external barrier to K + were found to be 357 A and 3.21 • 10 -4 cm/sec, respectively. In hyperosmotic seawater, the value of the space thickness increased significantly even though the potassium currents were not changed significantly. Values of the resistance in series with the membrane were calculated from the values of the permeability of the external barrier and these values were shown to be roughly equivalent to series resistance values determined by current clamp measurements. Membrane potassium ion conductances were determined as a function of time and voltage. When these were determined from data corrected for the potassium current reversal potential changes, larger maximal potassium conductances were obtained than were obtained using a constant reversal potential. In addition, the potassium conductance turn-on with time at a variety of membrane potentials was shown to be slower when potassium conductance values were obtained using a variable reversal potential than when using a constant reversal potential.

AB S T R A £I T Giant axons were voltage-clamped in solutions of constant sodium concentration (230 m_~) and variable potassium concentrations (from 0 to 210 rnM). The values of the peak initial transient current, Ip, were measured as a function of conditioning prepulse duration over the range from less than I rnsec to over 3 rain. Prepulse amplitudes were varied from F_~ ---20 mv to E,, = --160 mv. The attenuation of the Ip values in high [K¢] was found to vary as a function of time when long duration conditioning potentials were applied. In both high and low [Ko], Ip values which had reached a quasisteady-state level within a few milliseconds following a few milliseconds of hyperpolarization were found to increase following longer hyperpolarization. A second plateau was reached with a time constant of about 100-500 msec and a third with a time constant in the range of 30 to 200 sec. The intermediate quasi-steady-state level was absent in K-free ASW solutions. Sodium inactivation curves, normalized to ll, mlx values obtained at either the first or second plateaus, were significantly different in different [Ko]. The inactivation curves, however, tended to superpose after about I rain of hyperpolarizing conditioning. The time courses and magnitudes of the intermediate and very slow sodium conductance restorations induced by long hyperpolarizing pulses are in agreement with those predicted from the calculated rates and magnitudes of [K -~] depletion in the space between the axolemma and the Schwann layer. I N T R O D U C T I O NIn 1952, H o d g k i n and H u x l e y (1952 a) d e m o n s t r a t e d t h a t the inward transient c u r r e n t in the voltage-clamped squid giant axon is n o r m a l l y carried b y sodium ions. Recently, it has been shown t h a t the peak a m p l i t u d e of this inw a r d transient current, [~, is inversely related to the external potassium con-589