Calcium current inactivation in identified neurones of Helix aspersa.

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Abstract
A 2-electrode voltage clamp method was used to study Ca inward currents in identified H. aspersa neurons bathed in 25 mM Ca, Na-free saline with tetraethylammonium and 4-AP. Inward currents were blocked by CdCl2. In Cd delayed outward currents appeared at +30 mV. When 2 identical depolarizations were separated by a gap, the inward current turned off to the same level during the 2 pulses up to +20 mV; above this potential the records cross over. The turn-off of inward current at potentials up to +20 mV was unaffected by 0.2 mM quinine, which reduced outward currents at more depolarized potentials. Inward currents declined exponentially over the first 100 ms with a time constant around 60 ms at 0 mV. Double-pulse experiments gave the same time course of turn-off. When Ca inward current was reduced by lowering [Ca]o or by partial block by Cd the rate and extent of turn-off was reduced. The inactivation curve (obtained using a double pulse with gap method) was U-shaped. The curve was not significantly changed by addition of quinine (0.2 mM) or by changing test pulse size. Recovery of inward currents after a depolarizing prepulse was a double-exponential process, with time constants of 120 ms and 9.4 s at 10-11.degree. C. The results are discussed in terms of possible Ca-dependent Ca inactivation and in terms of the possibility of development of an outward Ca-dependent K current.