Blocking of the squid axon potassium channel by external caesium ions.

Abstract

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.

Properties of the current if in the sino‐atrial node of the rabbit compared with those of the current iK, in Purkinje fibres.

“…3 and 4. Caesium is known to block the inward-rectifying K channel in skeletal muscle (Gay & Stanfield, 1977) and to reduce K permeability in the starfish egg (Hagiwara et al 1976), eel electroplaque (Ruiz-Manresa et al 1970) and squid axon (Adelman & French, 1978). It has also been recently shown to block the K channels 'K, and 'K, in sheep Purkinje fibres (Isenberg, 1977).…”

“…Caesium blocks the inward rectifying K channel in skeletal muscle (Beauge, Medici & Sjodin, 1973) and reduces K permeability in squid axon (Adelman & French, 1978), eel electroplaque (Ruiz-Manresa, Ruarte, Schwartz & Grundfest, 1970) and egg cells (Hagiwara & Takahashi, 1974). Also, Cs has been shown to greatly reduce the currents iK, and in, in Purkinje fibres in concentrations up to 20 mm (Isenberg, 1977).…”

Fast inward‐rectifying current accounts for anomalous rectification in olfactory cortex neurones.

“…The presence of a 'leak' K+ conductance complicated our interpretation of the blocking effects of Cs+ and Ba2+ on Ifi r, since these cations also induced a negative slope conductance region in the I-V relation of 'linear' cells that lacked If.i r., External Cs+ blocks only inward K+ currents (Hagiwara et al 1976;Gay & Stanfield, 1977;Adelman & French, 1978) whereas Ba2+ can block both inward and outward K+ currents Standen & Stanfield, 1978;Schwindt Crill, 1980 …”

Amphiphilic Blockers Punch through a Mutant CLC-0 Pore

“…In these studies, the voltage dependence of the blocker affi nity is multiphasic, as if the blocker, which carries a charge, fi rst binds to a specifi c site in the pore, and then traverses the entire pore. These examples include the following: the block of K + channels by methylammonium, Na + and Cs + ions ( French and Wells, 1977 ;Adelman and French, 1978 ;French and Shoukimas, 1985 ;Nimigean and Miller, 2002 ); the acetylcholine block of the ACh Correspondence to Tsung-Yu Chen: t y c c h e n @ u c d a v i s . e d u Abbreviations used in this paper: CPA, p -chlorophenoxy acetate; HEK, human embryonic kidney.…”