The classical giant axon preparation from the common North Atlantic squid (Loligo pealei) fires one and only one action potential in response to a sustained depolarizing current pulse regardless of pulse amplitude or duration, or temperature of the bathing medium (0-20 °C; Clay, 1998). By contrast, the Hodgkin & Huxley (1952) model predicts repetitive firing for a broad range of stimuli. A revised model based on current views of the underlying sodium and potassium ion currents, I_Na and I_K, respectively, is sufficient to mimic the electrical behaviour of the axon (Clay, 1998). Repetitive firing was observed in this study during intracellular perfusion when the pH of the buffer was raised to 7.7, or higher. (The axoplasmic pH is 7.3.) Moreover, the activity occurred without current stimulation, i.e. the axon is an autonomous pacemaker oscillator under these conditions.

Experiments were performed using axial wire current- and voltage-clamp techniques with intracellular perfusion as described elsewhere (Clay & Shlesinger, 1983). The external solution was filtered seawater. The internal perfusate consisted of 400 mM sucrose and 300 mM potassium glutamate. The pH (pH_i) of this buffer could be stably titrated in the 7.0-9.0 range with free glutamic acid. The temperature in these experiments (range of 0-20 °C) was maintained constant to within 0.1 °C with a Peltier device.

When the intracellular buffer was switched from one having a pH of 7.2 to a pH of 8.5, the membrane potential changed over a period of a few minutes from a stable resting level of -60 mV to spontaneously occurring subthreshold oscillations which gradually increased in amplitude until action potential threshold was crossed (Fig. 1). The preparation then fired repetitively for as long as the experiment lasted (up to 4 h). Similar results were observed in a total of 20 out of 24 axons. This result is not attributable to an effect of pH_i on I_Na. An effect of pH_i on I_K has been reported (Clay, 1990). Specifically, the steady-state inactivation curve was shifted rightward on the voltage axis as pH_i was elevated in the 6-10 range. That is, the I_K component at -60 mV is effectively increased by an increase in pH_i from 7.2 to 8.5, which cannot account for the results in this study. The non-specific cationic conductance of the axon (g _L in the Hodgkin & Huxley (1952) model) is also pH_i sensitive. This conductance is reduced by increases in pH_i above 7.2, i.e. g _L is activated by protons. The axon also has a small, persistent, tetrodotoxin-sensitive sodium ion current, I_NaP, at relatively negative potentials (theshold at -90 mV; Rakowski et al. 1985) which together with a pH_i-sensitive g _L is sufficient to explain the autonomous activity illustrated in Fig. 1. Specifically, the reduction of g _L allows the negative slope character of I_NaP at -60 mV to destabilize the equilibrium potential, thereby resulting in repetitive action potentials.

Figure 1. The pHi was initially 7.2. The resting potential was -60 mV. A few minutes after changing pHi to 8.5, spontaneous activity was observed.

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Clay, J.R. (1998). J. Neurophysiol. 80, 903-913.

Clay, J.R. & Shlesinger, M.F. (1983). Biophys. J. 42, 43-53.

Hodgkin, A.L. & Huxley, A.F. (1952). J. Physiol. 117, 500-544. abstract

Rakowski, R., Deweer, P. & Gadsby, D. (1985). Biophys. J. 47, 31a.