Properties of a facilitating calcium current in pace‐maker neurones of the snail, Helix pomatia.

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Abstract
Simultaneous measurements of local voltage clamp currents from patches of soma membrane and K activity at the soma surface were used to analyze the time and voltage dependence of the slow inward current in bursting pace-maker neurons of the snail H. pomatia. At low levels of depolarization (.ltoreq. 20 mV) a net inward current was recorded simultaneously with an efflux of K+ from the cell. With larger depolarizations (20-170 mV from holding potential of -50 mV) the deficit in net outward charge transfer compared with K+ efflux, and the appearance of inward-going tail currents following repolarization, revealed a persistent inward-going current under these conditions. This inward current was carried primarily by Ca2+ as demonstrated by its voltage dependence (a minimum at about +115 mV) and its disappearance in Co-Ringer. It is identified with the slow inward Ca current Iin slow (Eckert and Lux, 1976). The inward current predicted from comparisons of current trajectories reached a maximum at 15-20 ms (for depolarizations from -50 to 0 mV) and gradually declined with sustained depolarization. Partial inactivation was removed by repolarization to -50 mV and the Ca dependent deficit was greater in the sum of repeated voltage clamp pulses than during sustained depolarization. It was largest for pulses of 25-100 ms duration, decreasing as pulse duration increased. Responses to repeated activation with 100 ms pulses with different repolarization intervals revealed a minimum Iin slow at short intervals (e.g., 20 ms) due to failure to remove partial inactivation. At intermediate intervals (e.g., 200-400 ms) Iin slow showed facilitation. This was revealed in calculations of the net charge transfer and current deficits and in the tail currents following repolarization. The deficit increased progressively with repetitive stimulation. With longer intervals (e.g., 800-1000 ms) defacilitation during repeated stimulation after the first 2 pulses was revealed in calculations of deficits, current trajectories and in the tail currents. Although facilitation depended on duration of repolarization between pulses, increasing intermediate hyperpolarizations from the holding potential of -50 mV were usually ineffective in increasing Iin slow. Strong preceding hyperpolarization could even decrease the magnitude of Iin slow and prevent its facilitation with repetitive stimulation, whereas preceding depolarizing pulses could increase Iin slow without preventing its facilitation with repetitive stimulation. The properties of Iin slow were contrasted with previously described membrane conductances and compared with properties attributed to Ca fluxes in other systems.