Ion migration and inactivation in the calcium channel.

1. Calcium conduction has been studied under voltage clamp in isolated nerve cell bodies of Helix aspersa. A suction pipette method was used to produce internal perfusion and low access resistance to the cell's interior. 2. Ca current does not obey the independence principle and saturates with increasing extracellular Ca concentration. The effect is voltage-dependent. 3. The unit conductance for the Ca channel is between 10(-12) and 10(-13) S, values consistent with impeded conduction. 4. Inactivation is partly dependent upon an increase of intracellular Ca activity due to Ca current. However, a component remains after the increase in [Ca]i has been prevented with EGTA. 5. The Ca channel may have two open states or there may be two sets of Ca channels.