Calcium inward current and related charge movements in the membrane of snail neurones.

1. Isolated and intracellularly perfused neurones from the snail Helix pomatia have been investigated under voltage-clamp conditions. Calcium inward current and asymmetric displacement current were examined. 2. Two components of the asymmetric displacement current have been distinguished. One of them was irreversibly blocked by intracellular fluoride together with the calcium inward current. Another component (about 20%) was not affected. The relation of the fluoride-sensitive asymmetric displacement current to the activity of calcium channels was investigated. 3. The amounts of charge displaced by the on- and off-responses of the asymmetric current were similar. The maximum charge displacement was between 1500 and 2000 electron charges/micrometers 2. 4. The normalized steady-state voltage distribution curve of the displaced charge coincided with the square root of the normalized calcium conductance. The slope was 4 mV per e-fold change for the calcium conductance and 8 mV for te charge distribution. Thus, the effective valencies were close to 6 (for the calcium conductance) and 3 (for the charge distribution), suggesting two gating particles per calcium channel. 5. The on-process of the calcium inward current fitted m2 kinetics. The time constant tau m corresponded to the time constant tau ason of the asymmetric current in a wide range of tested voltages. This correspondence failed only at small depolarizations where tau m exceeded tau ason. 6. The 'off' time constants of the calcium inward current (tau Caoff) and the asymmetric current (tau asoff) examined at -40 mV did not depend significantly on the test pulse height. The ratio tau asoff/tau Caoff varied between 1.7 and 2.5. 7. Calcium inward-current relaxation kinetics were measured for small shifts of membrane potential (Vtest = -2 mV) applied at the peaks of the current. The time constants were smaller than tau m and depended only weakly on voltage. 8. It is concluded that the fluoride-sensitive asymmetric displacement current is related to the activation of calcium channels.