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

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Abstract
1 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 μm) inhibited the Ca2+ transient by 78 ± 10 %. The N-type Ca2+ channel blocker ω-conotoxin GVIA (0.1 μm) reduced the amplitude by −35 ± 4 %, whereas nifedipine (10 μm; L-type) and ω-conotoxin MVIIC (0.1 μm; P/Q type) were ineffective. 4 Caffeine (10 mm), ryanodine (10 μm), cyclopiazonic acid (30 μm) or CCCP (10 μm) had no detectable effects. 5 Blockade of large and small conductance Ca2+-dependent K+ channels with iberiotoxin (0.1 μm) and apamin (1 μm), respectively, or Ca2+-dependent Cl− channels by niflumic acid (100 μm) did not alter Ca2+ transients. 6 In contrast, the non-specific K+ channel blockers tetraethylammonium (10 mm) and 4-aminopyridine (10 mm) markedly increased the amplitude of the Ca2+ transient. Blockade of delayed rectifiers and A-like K+ channels, by tityustoxin-K (α) (0.1 μm) and pandinustoxin-K (α) (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 per cent 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 KV1.2) but not by Ca2+-activated ion channels.