Distribution of calcium currents in sprouting PC12 cells.

Whole-cell calcium and barium currents were recorded from PC12 cell bodies and growth cones during nerve growth factor (NGF)-induced neurite outgrowth. Depolarizing voltage steps were applied to activate the currents, and pharmacological agents were used to separate them from other ionic currents. In recordings from growth cones still attached to neurites, current flow from central parts of the cell could be distinguished from the growth cone currents. On the other hand, currents from neuritic shafts and growth cones contributed to whole-cell currents recorded in the soma. Such currents were isolated by alternatively comparing recordings of whole-cell calcium currents with recordings in which calcium currents of defined parts of the cell were suppressed by exposing these regions to laminar streams of solutions with low calcium. The boundary between such streams and the bath solution was shown to be sharp using a calcium-selective microelectrode. The current deficits recorded when the growth cones were exposed to solutions with low calcium (growth cone currents) were within 10-50% of the total cell currents and similar to the currents recorded when the whole cell except the growth cone was exposed to low calcium. The current densities in the growth cones during initial sprouting were 5.4 times higher than those in the somata. Growth cone currents showed more inactivation than currents originating from the soma during pulses of 200 msec. In most experiments no calcium currents could be resolved in the neuritic shaft during initial neurite growth (1-10 d of NGF application), indicating low current density. In proximal segments of the neurites, however, a somatofugal decrease of the current density was observed. It is concluded from these results, that in outgrowing neurites of PC12 cells high densities of calcium channels are maintained in the growth cones, whereas in the neuritic shaft calcium channel density is initially low and later increases during consolidation of the neurites.