Voltage Clamp Studies on the Effect of Internal Cesium Ion on Sodium and Potassium Currents in the Squid Giant Axon

Abstract

Isolated and cleaned giant axons of Loligo pealii were internally perfused with solutions containing cesium sulfate and potassium fluoride. Membrane currents obtained as a function of clamped membrane potentials indicated a severe depression of the delayed outward current component normally attributed to potassium ion movement. Steady-state currents showed a negative slope in the potential range from -45 to -5 mv which corresponded to the negative slope for the peak sodium current relation vs. membrane potential which suggested long duration sodium currents. Using sodium-free sea water externally, sodium currents were separated from total currents and these persisted for longer times than normal. This result suggested that internal cesium ion delays the sodium conductance turnoff. The separated nonsodium currents showed an abnormal rectification as compared with those predicted by the independence principle, such that while potassium permeability appeared normal at the resting potential, its value decreased progressively with increasing depolarization. Pickard et al. (1964) have recently shown that when sea water in which the sodium was replaced with cesium was applied externally to the isolated squid giant axon the delayed or potassium component of outward membrane current in the voltage clamp was not decreased as might be expected if Cs+ were acting chemically in a way similar to K+ ion. Previous studies on the internally perfused squid axon (Baker et al., 1962) showed that replacing an isotonic K 2 SO 4 perfusion solution with isotonic Cs 2 SO 4 solution caused a rapid decrease in resting potential and an increase in the duration of the action potential elicited during the brief period before inexcitability occurred as a result of the depolarization. These authors were uncertain as to whether the increase in action potential duration was a direct result of the internal Cs+ ion or was an indirect effect resulting from internal K+ ion reduction. More recently, Chandler and Meves (1965), upon voltage-clamping squid axons internally perfused with half Cs and half K isotonic solution, have

Enhancement of inward current by serotonin in neurons of Aplysia

“…Since iontophoretic application of serotonin requires holding at depolarized potentials for prolonged periods of time while perfusion of serotonin only requires stepping to these potentials for 2 s or so, one possible explanation of this differential effect is a voltage-dependent blockade by the potassium channel blocking agents. Intracellular cesium has been reported to be more effective at depolarized potentials (Adelman and Senft, 1966). Therefore, the blockade by cesium when holding at -10 mV t o elicit the iontophoretic response might be greater than that when holding at -60 mV and stepping to -10 mV for 2 s with serotonin in the bath.…”

Ammonium Ion Currents in the Squid Giant Axon

“…The peak currents were consistently lower than the values predicted by the independence principle. Adelman and Senft (1966) have proposed that the permeable ions often have a twofold effect--they carry current and they can block current-carrying pathways. For example Cs is slightly permeable in the fast channel, but internal Cs blocks the outward slow K current.…”

Ionic currents through the membrane of the mammalian oocyte and their comparison with those in the tunicate and sea urchin.

“…The I-V curve indicates that there is a marked outward-going rectification above -15 mV. In squid giant axon, the internal perfusion of Cs ions abolishes the delayed rectification (Adelman & Senft, 1966;Bezanilla & Armstrong, 1972). In the sea urchin oocyts, the outward surge at +50 mV level was also effectively suppressed up to an eighth by intracellular injection of Cs ions (not illustrated).…”

“…The analysis of the electrical excitability was carried out by both current and voltage-clamp methods, as described above. In a few experiments the current electrode filled with 7 M-CsCl was used to inject Cs ions intracellularly to suppress the outward current (Adelman & Senft, 1966).…”