Intracellular chloride activity and the effects of acetylcholine in snail neurones

1. Cl − ‐sensitive micro‐electrodes were used to measure intracellular Cl − in snail neurones. The electrodes consisted of a sharpened and chlorided silver wire mounted inside a glass micropipette. 2. The electrodes appeared to record changes in internal Cl − accurately but in H cells the chloride equilibrium potential ( E Cl ) as measured by the Cl − ‐sensitive electrode was always less negative than E ACh . 3. In some H cells ACh caused a measurable increase in internal Cl − when the cell was at its resting potential. In voltage‐clamped cells there was a close correlation between the change in internal Cl − and the extra clamp current caused by a brief application of ACh. This confirmed that ACh increases the cell's membrane permeability only to Cl − ions, and that E ACh was equal to E Cl . 4. There was good agreement between the measured change in internal Cl − and that calculated from the cell size and clamp charge only when it was assumed that a constant voltage offset was added to the potential of the Cl − ‐sensitive electrode while it was inside the nerve cell. 5. Cl − ‐sensitive electrodes with AgCl as the sensitive material appear to be unsuitable for intracellular measurement of Cl − , although they might be suitable for following changes in E Cl . 6. In certain D cells ACh also caused an increase in internal Cl − although it decreased the membrane potential. In the presence of hexamethonium, ACh caused a hyperpolarization and a smaller increase in internal chloride. 7. It is concluded that the intracellular Cl − in both H and D cells is about 8·3 m M , giving an E Cl of about −58 mV.