Calcium channel subtypes contributing to acetylcholine release from normal, 4‐aminopyridine‐treated and myasthenic syndrome auto‐antibodies‐affected neuromuscular junctions

Acetylcholine release at the neuromuscular junction relies on rapid, local and transient calcium increase at presynaptic active zones, triggered by the ion influx through voltage‐dependent calcium channels (VDCCs) clustered on the presynaptic membrane. Pharmacological investigation of the role of different VDCC subtypes (L‐, N‐, P/Q‐ and R‐type) in spontaneous and evoked acetylcholine (ACh) release was carried out in adult mouse neuromuscular junctions (NMJs) under normal and pathological conditions. ω‐Agatoxin IVA (500 nM), a specific P/Q‐type VDCC blocker, abolished end plate potentials (EPPs) in normal NMJs. However, when neurotransmitter release was potentiated by the presence of the K+ channel blocker 4‐aminopyridine (4‐AP), an ω‐agatoxin IVA‐ and ω‐conotoxin MVIIC‐resistant component was detected. This resistant component was only partially sensitive to 1 μM ω‐conotoxin GVIA (N‐type VDCC blocker), but insensitive to any other known VDCC blockers. Spontaneous release was dependent only on P/Q‐type VDCC in normal NMJs. However, in the presence of 4‐AP, it relied on L‐type VDCCs too. ACh release from normal NMJs was compared with that of NMJs of mice passively injected with IgGs obtained from patients with Lambert‐Eaton myasthenic syndrome (LEMS), a disorder characterized by a compromised neurotransmitter release. Differently from normal NMJs, in LEMS IgGs‐treated NMJs an ω‐agatoxin IVA‐resistant EPP component was detected, which was only partially blocked by calciseptine (1 μM), a specific L‐type VDCC blocker. Altogether, these data demonstrate that multiple VDCC subtypes are present at the mouse NMJ and that a resistant component can be identified under ‘pharmacological’ and/or ‘pathological’ conditions. British Journal of Pharmacology (2002) 136, 1135–1145. doi:10.1038/sj.bjp.0704818