Chloride distribution in Aplysia neurones

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Abstract
1. The intracellular Cl− concentration (Cli) and the membrane potential (Em) were measured in the medial pleural neurones of Aplysia under various experimental conditions designed to determine the Cl− conductance of the neurones and investigate the possibility of an active Cl− transport.2. The magnitude of the Cl− conductance of the cell depends on the experimental conditions.3. In normal sea water, large changes of Em produced by passing current across the cell membrane caused no change of Cli, suggesting that the Cl− conductance was low. Similarly, moderate changes of ECl produced by decreasing Clo or increasing Cli had little or no effect on Em.4. A high Cl− conductance was observed in high Ko or very low Clo. It was greatly reduced if the external Ca2+ was replaced by Co2+, or in the presence of tubocurarine, or if the experiment was performed on an isolated cell soma. The high Cl− conductance is therefore attributed to the release of ACh and perhaps other transmitters from synaptic terminals.5. High concentrations of tetraethylammonium ions or procaine induced a depolarization of the cell, but a decrease of Cli. The rate of fall of Cli was increased by lowering external K+ or raising external Ca2+, and was decreased by replacing external Ca2+ by Co2+.6. NH4+ ions applied externally had effects similar to those of K+ ions. In situations in which intracellular NH4+ might be increased a fall in Cli was observed.7. The changes of Cli caused by TEA, procaine, or internal NH4+ occur against the driving force for passive Cl− movements. They are still observed in isolated cell bodies, and cannot be attributed to the activation of synaptic channels.8. Some interpretations of these anomalous Cl− movements are discussed which could also account for the difference between ECl and Em observed under normal conditions.