1 Acetylcholine release at the neuromuscular junction relies on rapid, local and transient calcium increase at presynaptic active zones, triggered by the ion in¯ux through voltage-dependent calcium channels (VDCCs) clustered on the presynaptic membrane. Pharmacological investigation of the role of di erent 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. 2 o-Agatoxin IVA (500 nM), a speci®c 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 o-agatoxin IVA-and o-conotoxin MVIICresistant component was detected. This resistant component was only partially sensitive to 1 mM oconotoxin 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. 3 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. Di erently from normal NMJs, in LEMS IgGs-treated NMJs an o-agatoxin IVA-resistant EPP component was detected, which was only partially blocked by calciseptine (1 mM), a speci®c L-type VDCC blocker. 4 Altogether, these data demonstrate that multiple VDCC subtypes are present at the mouse NMJ and that a resistant component can be identi®ed under`pharmacological' and/or`pathological' conditions.

Voltage-operated calcium channels play crucial roles in stimulus-secretion coupling in pancreatic beta cells. A growing body of evidence indicates that these channels in beta cells are heterogeneous. In particular, not all the high-threshold calcium channels expressed belong to the best known L-type. In rat insulinoma cells, for example, L, N, and P/Q-type channels are present, while in human beta cells L-type and P/Q-type dominate. Where present, N-type and P/Q-type channels participate, alongside with the dominant L-type, in the control of sugar- or depolarization-induced hormone release. Distinct biophysical properties and selective modulation of the channel subtypes are likely to play important physiological roles. T-type channels are involved in beta cell apoptosis, while calcium channel autoantibodies recognizing high-threshold channels in beta cells, have been described both in neurological and diabetic patients. Subtype-selective calcium channel drugs have the potential for being beneficial in beta cell pathological states.

Lambert-Eaton myasthenic syndrome (LEMS) is an autoimmune disorder of neuromuscular transmission in which antibodies are directed against voltage-gated calcium channels (VGCCs). We studied the action of LEMS immunoglobulin G (IgG) on cloned human VGCCs stably transfected into human embryonic kidney cells (HEK293). All LEMS IgGs tested bound to the surface of the P/Q-type VGCC cell line and caused a significant reduction in whole-cell calcium currents in these cells. By contrast, only 2 out of 6 IgGs bound extracellularly to the N-type VGCC cell line, and none of the LEMS IgGs tested was able to reduce whole-cell calcium currents in these cells. We used this apparent specificity of LEMS IgG for the P/Q-type VGCC to investigate the action of these IgGs on model systems where a number of different VGCC populations exist in equilibrium. LEMS IgG caused a significant downregulation in the omega-agatoxin IVA-sensitive P/Q-type VGCCs of cultured rat cerebellar neurons, but this was accompanied by a concomitant rise in the "resistant" R-type VGCCs. By using the passive transfer model of LEMS, similar results were observed at the mouse neuromuscular junction, where a significant reduction in P/Q-type VGCCs was paralleled by an increase in L- and R-type VGCCs. These results demonstrate an unexpected plasticity in the expression of VGCCs in mammalian neurons and may represent a mechanism by which the pathogenic effects of LEMS IgG are reduced.