Free calcium ions in neurones of Helix aspersa measured with ion‐selective micro‐electrodes.

1. Intracellular free calcium concentration, [Ca2+]i, was measured in giant neurones of the sub‐oesophageal ganglia of Helix aspersa, using Ca‐selective micro‐electrodes containing a PVC‐gelled, neutral‐ligand sensor. 2. In calibration solutions the electrodes had a virtually ideal, Nernstian, response down to 1 microM‐Ca2+ in the presence of 0.125 M‐K+, 18‐24 mV from 1 to 0.1 microM‐Ca2+ and 8‐14 mV from 0.1 to 0.01 microM‐Ca2+. Interference from H+ and Mg2+ was negligible. The small response to Na+ at sub‐micromolar Ca2+ was taken into account, when necessary, in measurement of [Ca2+]i. 3. Measurements of basal [Ca2+]i were made in ganglia from animals kept only a few weeks in captivity, in a bathing solution equilibrated with air and containing 2 mM‐Ca2+. In thirteen measurements from impalements which met stringent criteria for electrode performance and cell viability, the mean basal pCa (‐‐log10[Ca2+]) was 6.77 +/‐ 0.07 (S.E.), corresponding to a mean free Ca2+ concentration of 0.17 microM. 4. The basal [Ca2+]i in neurones from a group of snails kept hibernating for several months was higher, mean pCa 6.15, for ganglia handled in 2 mM‐Ca2+ solution. 5. Intracellular injections of Ca2+ or EGTA raised and lowered, respectively, the indicated basal [Ca2+]i, showing that the electrodes responded appropriately inside the cells and that unknown or untested components of cytoplasm were not significantly interfering with the Ca‐sensor. 6. Altering the external Ca2+ concentration between 0.1 and 10 mM usually produced only small, +/‐ 0.1 pCa units, changes in basal [Ca2+]i of satisfactorily impaled, quiescent cells. 7. In cell 1F, which has repetitive spikes with a substantial Ca current, changes in Ca gradient or blockade of voltage‐dependent Ca channels sometimes markedly altered [Ca2+]i, showing that Ca entry with the spikes was elevating [Ca2+]i. 8. Replacing external Na+ with Li+ or bis(2‐hydroxyethyl)dimethylammonium had little effect on [Ca2+]i. 9. Elevating CO2 to 5% or 79% lowered [Ca2+]i by an average of 0.16 and 0.26 pCa units respectively.