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Measurements were made of the kinetics and steady-state properties of the sodium conductance changes in the giant axon of the crab Carcinus maenas. The conductance measurements were made in the presence of small concentrations of tetrodotoxin and as much electrical compensation as possible in order to minimize errors caused by the series resistance. After an initial delay of 10-150 microsec, the conductance increase during depolarizing voltage clamp pulses followed the Hodgkin-Huxley kinetics. Values of the time constant for the activation of the sodium conductance lay on a bell-shaped curve with a maximum under 180 microsec at -40 mV (at 18 degrees C). Values of the time constant for the inactivation of the sodium conductance were also fitted using a bell-shaped curve with a maximum under 7 msec at -70 mV. The effects of membrane potential on the fraction of Na channels available for activation studied using double pulse protocols suggest that hyperpolarizing potentials more negative than -100 mV lock a fraction of the Na channels in a closed conformation.

Measurements were made of the kinetic and steady-state characteristics of the potassium conductance in the giant axon of the crabs Carcinus maenas and Cancer pagirus. The conductance increase during depolarizing voltage-clamp pulses was analyzed assuming that two separate types of potassium channels exist in these axons (M.E. Quinta-Ferreira, E. Rojas and N. Arispe, J. Membrane Biol. 66:171-181, 1982). It is shown here that, with small concentrations of conventional K+-channel blockers, it is possible to differentially inhibit these channels. The potassium channels with activation and fast inactivation gating (m3h, Hodgkin-Huxley kinetics) were blocked by external application of 4 amino-pyridine (4-AP). The potassium channels with standard gating (n4, Hodgkin-Huxley kinetics) were preferentially inhibited by externally applied tetraethylammonium (TEA). The differential blockage of the two types of potassium conductance changes suggests that they represent two different populations of potassium channels. It is further shown here that blocking the early transient conductance increase leads to the inhibition of the repetitive electrical activity induced by constant depolarizing current injection in fibers from Cardisoma guanhumi.

Permeability ratios for pairs of monovalent cations permeating the two potassium systems proposed for the giant axon of the crab Carcinus maenas (M.E. Quinta-Ferreira, E. Rojas & N. Arispe, J. Membrane Biol. 66:171-181, 1982b) were estimated from measurements of the reversal potential of the currents under voltage-clamp conditions. With K+ inside the axon, permeability ratios from the reversal potential of the currents through the late channel are: PRb/PK = 0.9, PNH4/PK less than 0.2 and PCS/PK = 0.18. With CS+ inside the ratios are: PK/PCS = 8.7, PRb/PCS = 7.1 and PNH4/PCS = 2.4. The analysis of the inward currents carried by Rb+ or NH+4 showed similar reversal potentials for the early transient component and the late sustained component. Whence, the sequence of permeabilities for the two types of potassium channels is: PK greater than PRb greater than PNH4 greater than PNa = PCS. The time constants for the activation of the two components recorded either in K-, Rb-, or NH4-artificial seawater are twice as large as the corresponding time constants measured in Na-artificial seawater.

Measurements were made of the kinetic and steady-state characteristics of the potassium conductance in the giant axon of the crab Carcinus maenas. These measurements were made in the presence of tetrodotoxin, using the feedback amplifier concept introduced by Dodge and Frankenhaeuser (J. Physiol, (London) 143:76-90). The conductance increase during depolarizing voltage-clamp pulses was analyzed assuming that two separate potassium channels exist in these axons. The first potassium channel exhibited activation and fast inactivation gating which could be fitted using the m3h, Hodgkin-Huxley formalism. The second potassium channel exhibited the standard n4 Hodgkin-Huxley kinetics. These two postulated channels are blocked by internal application of caesium, tetraethylammonium and sodium ions. External application of 4 amino-pyridine also blocks these channels.