Presynaptic mechanism of action of 4-aminopyridine: changes in intracellular free Ca2+ concentration and its relationship to B-50 (GAP-43) phosphorylation. 
Recently we have shown that 4-aminopyridine (4-AP), a drug known to enhance transmitter release, stimulates the phosphorylation of the protein kinase C substrate B-50 (GAP-43) in rat brain synaptosomes and that this effect is dependent on the presence of extracellular Ca2+.  Hence, we were interested in the relationship between changes induced by 4-AP in the intracellular free Ca2+ concentration ([Ca2+]i) and B-50 phosphorylation in synaptosomes.  4-AP (100 microM) elevates the [Ca2+]i (as determined with fura-2) to approximately the same extent as depolarization with 30 mM K+ (from an initial resting level of 240 nM to approximately 480 nM after treatment).  However, the underlying mechanisms appear to be different: In the presence of 4-AP, depolarization with K+ still evoked an increase in [Ca2+]i, which was additive to the elevation caused by 4-AP.  Several Ca2+ channel antagonists (CdCl2, LaCl3, and diphenylhydantoin) inhibited the increase in B-50 phosphorylation by 4-AP.  It is interesting that the increase in [Ca2+]i and the increase in B-50 phosphorylation by 4-AP were attenuated by tetrodotoxin, a finding pointing to a possible involvement of Na+ channels in this action.  These results suggest that 4-AP (indirectly) stimulates both Ca2+ influx and B-50 phosphorylation through voltage-dependent channels by a mechanism dependent on Na+ channel activity.