Contributions of various ions to the resting and action potentials of crayfish medial giant axons

An axonal spike in an identified fast crab motor neuron has sodium and calcium components

1. High intensity intracellular stimulation of the FBE axon produced a spike discharge. The spikes divided into two components, each with a different frequency. 2. Ion replacement and blocking specific ion channels revealed that the FBE spike has calcium and sodium components. 3. The pronounced depolarizing wave that follows the FBE spike is not produced by changes in calcium conductance.

A comparative study of the effects of tetrodotoxin and the removal of external Na+ on the resting potential: evidence of separate pathways for the resting and excitable Na currents in squid axon

To investigate whether the Na permeability of the resting membrane is determined predominantly by the excitable Na channel, we examined the effects of tetrodotoxin (TTX) and the complete removal of external Na+ on the resting potential. In the intact squid axon bathed in K-free artificial seawater, both TTX and the removal of Na+ produced small hyperpolarizations. The effect of Na removal, however, was larger than that of TTX. In the perfused squid axon, the hyperpolarization produced by the removal of external Na+ was greatly enhanced when the internal K concentration ([K+]i) was reduced. The effect of TTX, on the other hand, was not sensitive to the [K+]i or to the membrane potential. For [K+]i = 50 mM and [K+]o = 0, the average hyperpolarization produced by TTX was 1.2 mV, while the hyperpolarization produced by Na removal was approximately 21 mV. The difference between these two effects suggests that the majority of the resting Na current passes through pathways other than the excitable Na channel.

Ammonium sulfate induced uncouplings of crayfish septate axons with and without increased junctional resistance

Cytoplasmic pH (pH1) of crayfish lateral giant axons was monitored using antimony microelectrodes placed near septate junctions and variations of internal pH was induced by short applications of ammonium sulfate in the perfusing bath of the preparation. This treatment produced a rapid cell alkalinization followed, after wash, by acidification rebound. Simultaneously, two successive phases of uncoupling of the septate junction occurred; they had the same time course as those of their associated pH1 movements. Calculation of the electronic coupling parameters indicated that, during alkalinization, coupling was accompanied by an increased axonal membrane conductance (the intimate origin of which was beyond the scope of this study) and resulted from a shunt of the gap junctions; the resistance proper of the latter was unaffected; thus involvement of Ca2+ was ruled out and uncoupling was only an indirect consequence of the electrotonic junction's network configuration. In contrast, and as expected from previous investigations, the junctional membrane resistance was increased during the second phase of cytoplasmic acidification. Evidence that uncoupling can be brought about by a non-junctional membrane increased permeability raises questions about some of the criteria commonly used during investigations of electrotonic transmission.

Changes in excitatory and inhibitory synaptic potentials leading to epileptogenic activity

The effects of the epileptogenic agent, penicillin, on excitatory and inhibitory postsynaptic potentials (EPSPs and IPSPs) were studied in the hippocampal in vitro slice preparation. The actions of penicillin were compared to those of bicuculline, an antagonist of gamma-aminobutyric acid. Neither substance enhanced monosynaptic EPSP amplitude in CA1 pyramidal cells, but both penicillin and bicuculline depressed IPSPs. Large depolarizations that gave rise to cellular bursting activity did develop after addition of penicillin or bicuculline to the bathing medium. There was also an increase in the incidence of fast prepotentials of 'd-spikes.' These observations are interpreted according to the hypothesis that penicillin and bicuculline block cellular IPSPs, and consequently allow remote intrinsic excitatory events to invade the cell soma and trigger action potentials.

Effect of external potassium on the coupled sodium: potassium transport ratio of axons

1. Resting membrane potential and the current-voltage relation were measured in crayfish giant axons bathed in various potassium solutions with and without ouabain. 2. Ouabain caused a depolarization of the membrane at each [K]o used but did not affect membrane resistance. 3. The ouabain-sensitive transport current was least (3 microamperemeter/cm2) in 0 mM [K]o and greatest (7 microamperemeter/cm2) in 16.2 and 21.6 mM [K]o. 4. The assumption was made an some indirect evidence presented that axons equilibrated in various potassium solutions maintain constant internal sodium and potassium concentrations for up to 3 h. 5. On the basis of this assumption, the apparent ratio of coupled Na : K transport was calculated. It was found to be least (-1.3/1) in 0 mM [K]o and to approach infinity in 16.2 and 21.6 mM [K]o. 6. The data indicate that the apparent variability of the Na : K exchange ratio likely represents an intrinsic property of the exchange mechanism and is less likely to be explained by a fixed-ratio coupled Na : K transport operating in parallel with electro-neutral Na : Na or K : K exchange.

The influence of chloride on the ouabain-sensitive membrane potential and conductance of crayfish giant axons

1. Resting potential and current-voltage relation were measured in crayfish giant axons bathed in chloride-free and sodium-free solutions with and without ouabain. 2. Chloride-free solution caused a transient depolarization but did not alter the steady-state membrane potential. Utilizing isethionate as an anion substitute, the membrane resistance increased 12.5%. 3. In the absence of extracellular chloride, ouabain (0.5-1 mM) depolarized the axon 6-7 mV. The shape of the current-voltage relation did not change but the curve was shifted along the current axis. 4. These results indicate that ouabain inhibits a steady-state hyperpolarizing electrogenic pump current of approximately 3 muA/cm2. 5. Extracellular sodium removal from axons equilibrated in chloride-free solutions transiently hyperpolarized the membrane 6-7 mV without a change in membrane resistance. The transient hyperpolarization was ouabain and temperature sensitive. The steady-state potential reached in sodium-free and chloride-free solution was not ouabain sensitive. Temperature sensitivity of the steady-state membrane potential was greatly reduced. 6. The transient hyperpolarization produced by extracellular sodium removal was metabolically driven and may present the expression of a sodium efflux transport current of 7.0-7.5 muA/cm2. 7. Using electrophysiologically measured parameters, sodium and potassium conductance, influx and efflux currents and the coupling ratio for sodium/potassium transport are calculated from a modification of the conductance equation. 8. The sodium/potassium transport coupling ratio for steady-state conditions was estimated at 5:3 (1.67:1).

Evidence for Ca-2+ control of the transducer mechanism in crayfish stretch receptor

Recording from the dendrite membrane indicated a resting potential of minus-51.6 mV, which was reduced by inhibition of the Na+/K+ pump. Voltage clamp at rest revealed a small inward current between minus-50 and minus-80 mV and a larger outward current at clamp potentials of minus-40 to +30 mV. Using ramp-changes of muscle tension as stimuli a time-variant tension-induced inward current (TIC) became apparent, the amplitude of which decreased towards larger depolarizing voltages until at +18 mV the current reversed the direction. The time course of the conductance changes corresponds to similar phases in the generator potential. The outward current only responded to fast reductions in tension, decreasing transiently. A contribution of the active Na+/K+ pump to the hyperpolarizing potential response is suggested by the effects of K-removal or Na-substitution by Li+. In Na-free choline chloride media the generator potential and the TIC was depressed by 70-85%. Additional removal of Ca-2+ abolished the TIC. In contrast, lowering the Ca-2+ level in presence of Na+ decreased the membrane resistance and markedly enhanced the TIC (maximally eightfold at 10-minus 5M Ca-2+) while 75-150 mM Ca-2+ or intracellular application of a Ca-ionophore had the reverse effect.