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

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 (--log10Ca2+) 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.