Characterization of action potential-evoked calcium transients in mouse postganglionic sympathetic axon bundles

1. Action potential-evoked Ca2+ transients in postganglionic sympathetic axon bundles in mouse vas deferens have been characterized using confocal microscopy and Ca2+ imaging. 2. Axonal Ca2+ transients were tetrodotoxin sensitive. The amplitude depended on both the frequency of stimulation and the number of stimuli in a train. 3. Removal of extracellular Ca2+ abolished the Ca2+ transient. Cd2+ (100 muM) inhibited the Ca2+ transient by 78 +/- 10%. The N-type Ca2+ channel blocker omega -conotoxin GVIA (0.1 muM) reduced the amplitude by -35 +/- 4%, whereas nifedipine (10 muM; L-type) and omega -conotoxin MVIIC (0.1 muM; P/Q type) were ineffective. 4. Caffeine (10 mM), ryanodine (10 muM), cyclopiazonic acid (30 muM) or CCCP (10 muM) had no detectable effects. 5. Blockade of large and small conductance Ca2+-dependent K+ channels with iberiotoxin (0.1 muM) and apamin (1 muM), respectively, or Ca2+-dependent Cl- channels by niflumic acid (100 muM) did not alter Ca2+ transients, 6. In contrast, the non-specific K+ channel blockers tetraethylammonium (10 mm) and 4-aminopyridine (10 nim) markedly increased the amplitude of the Ca2+ transient. Blockade of delayed rectifiers and A-like K+ channels, by tityustoxin-K (alpha) (0.1 muM) and pandinustoxin-K (alpha) (10 nm), respectively, also increased the Ca2+ transient amplitude. 7. Thus, Ca2+ transients are evoked by Na+-dependent action potentials in axons. These transients originate mainly from Ca2+ entry through voltage-dependent Ca2+ channels (80% Cd2+ sensitive of which 40 % was attributable to N-type). Twenty percent of the Ca2+ transient was not due to Ca2+ entry through voltage-gated Ca2+ channels. Intracellular stores and mitochondria were not involved in the generation of the transient. Ca2+ transients are modulated by A-like K+ channels and delayed rectifiers (possibly K(V)1.2) but not by Ca2+-activated ion channels.