Effects of calcium and calcium‐chelating agents on the inward and outward current in the membrane of mollusc neurones

Abstract

Effects of internal and external Ca and Ca-chelating agents, EGTA [ethylene glycol bis (B-aminoethyl ether) tetraacetic acid] and EDTA on transmembrane ionic currents were studied in isolated, internally dialyzed neurons from the mollusks, Helix pomatia and Lymnaea stagnalis. The possible pharmacological effect of internally applied EGTA was investigated on the background of constant free Ca concentration (5.3 .times. 10-9 M). EGTA had no effect on Ca and Na inward currents but considerably depressed the delayed K outward current. No effective removal of this action could be achieved by the elevation of intracellular free Ca. In the absence of divalent cations in the external medium, EGTA (as well as EDTA) applied either intra- or extracellularly caused the appearance of a very large Na inward current with kinetics similar to those of Ca inward current and with the reversal potential around 10 mV. Effective concentrations of chelating agents were 0.1 mM (extracellular) and 1.0 mM (intracellular). Increase in intracellular Ca in the absence of EGTA (by dialysi of the cell with Ca-saturated solutions) did not produce any significant effect on the delayed K outward current. The small change observed in this current could be evaluated as a depression of maximum slope conductance and a shift to more negative membrane potential. Ca inward current was extremely sensitive to internal Ca; 5.8 .times. 10-8 M internal free Ca produced its complete depression. This effect was reversible. Na inward current was inhibited with 3.5 .times. 10-7 M intracellular Ca.