Calcium-dependent depression of a late outward current in snail neurons

Neuron cell bodies of Helix pomatia were voltage-clamped with a 300-millisecond depolarizing test pulse (pulse II) delivered I second after a depolarizing conditioning pulse (pulse I). The outward current, measured 200 milliseconds after the onset of pulse. II, exhibited a strong depression that was dependent on the presence of pulse. I. The maximum depression of the pulse II outward current occurred when pulse I voltages lay in the range over which calcium influx is inferred to be greatest; depression of the pulse II current subsided as pulse I potentials approached the putative calcium equilibrium potential. In the presence of extracellular [ethylenebis(oxyethylenenitrilo)]tetraacetic acid (EGTA) or D600, the intensity of the pulse II current became largely independent of pulse I, approaching the values of maximal depression seen in normal Ringer solution. On the other hand, lowering the intracellular pH with extracellular carbon dioxide-carbonate buffer had no measurable effect on the outward currents. Other experiments showed that it is primarily the calcium-dependent, outward-current hump of the N-shaped late current-voltage curve that is depressed by presentation of the conditioning pulse. It was concluded that distinct from an early potassium-activating role, calcium entering during a depolarization leads, during a subsequent depolarization, to a depression of the calcium-activated potassium system that persists for many seconds.