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

Abstract

Isolated and intracellularly perfused neurons from the snail H. pomatia were investigated under voltage-clamp conditions. Ca inward current and asymmetric displacement current were examined. Two components of the asymmetric displacement current were distinguished. One was irreversibly blocked by intracellular Fl- together with the Ca inward current. Another component (about 20%) was not affected. The relation of the Fl--sensitive asymmetric displacement current to the activity of Ca channels was investigated. The amounts of charge displaced by the one- and off-responses of the asymmetric current were similar. The maximum charge displacement was between 1500-2000 electron charges/.mu.m2. The normalized steady-state voltage distribution curve of the displaced charge coincided with the square root of the normalized Ca conductance. The slope was 4 mV per e-fold change for the Ca conductance and 8 mV for the charge distribution. The effective valencies were close to 6 (for the Ca conductance) and 3 (for the charge distribution), suggesting 2 gating particles per Ca channel. The on-process of the Ca inward current fitted m2-kinetics. The time constant .tau.m corresponded to the time constant .tau.onas of the asymmetric current in a wide range of tested voltages. This correspondence failed only at small depolarizations where .tau.m exceeded .tau.onas. The off time constants of the Ca inward current (.tau.offCa) and the asymmetric current (.tau.offas) examined at -40 mV did not depend significantly on the test pulse height. The ratio .tau.offas/.tau.offCa varied between 1.7 and 2.5. Ca 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. The Fl--sensitive asymmetric displacement current is related to the activation of Ca channels.