Voltage-clamp studies of the calcium inward current in an identified snail neurone: comparison with the sodium inward current.

1. Membrane currents were recorded under voltage clamp from cell A of the snail Helix aspersa. 2. In sodium-free saline the inward current was reduced to 75% of that in normal saline (containing both sodium and calcium). 3. The inward current in sodium-free saline was dependent on the external calcium concentration. 4. Calcium-free saline reduced the inward current to 30% of that in normal saline. (Na, Ca)-free saline abolished the inward current. 5. Changes in calcium concentration shifted the curve relating calcium conductance to membrane potential along the voltage axis. 6. Inactivation of inward current in both normal saline and sodium-free saline developed exponentially with time. 7. Steady-state inactivation of calcium inward current was similar to that for sodium current and it is concluded that the conductance mechanisms for these two ions bear a close resemblance.