Inhibition of acetylcholine release from presynaptic terminals of skate electric organ by calcium channel antagonists: a detailed pharmacological study

Abstract

Release of acetylcholine (ACh) from the presynaptic terminals in skate electric organ was tested for its sensitivity to calcium channel antagonists. A pharmacological profile was established by measuring inhibition of K+-stimulated release of [3H]ACh from prelabelled tissue slices. Peptide antagonists of N-type (ω-conotoxins GVIA and MVIIA) and P-type (ω-agatoxin-IVA) channels had no effect, whereas both ω-conotoxins MVIIC and SVIB produced concentration-dependent inhibition and could completely block ACh release. ω-Conotoxin GVIA and ω-agatoxin IVA did not attenuate the block by ω-conotoxin MVIIC. The inorganic ions, Cd2+ and Ni2+, also produced a full inhibition of release (Cd2+≫Ni2+) and Gd3+ a partial one. Drugs targeting L-type channels (diltiazem, nifedipine and verapamil) at low μM concentrations and a synthetic analogue of the polyamine toxin from funnel web spider venom (sFTX) at 1mM were all non-inhibitory. Inhibition by ω-conotoxins MVIIC (IC50 25nM) and SVIB (IC50 500nM) was reversible and modulated by external concentrations of Ca2+. Inhibitory potency was increased by lowering and decreased by elevating external Ca2+. This “antagonistic” effect of Ca2+ was also seen with Cd2+ inhibition. The inhibitory potency of ω-conotoxin MVIIC was unaffected by predepolarisation. End plate potentials generated by release of endogenous ACh in electrically-stimulated slices were also reversibly blocked by Cd2+ and ω-conotoxins MVIIC and SVIB but were unaffected by ω-conotoxin GVIA and ω-agatoxin IVA. It is concluded that ACh release in skate electric organ depends on presynaptic calcium channels which have different pharmacological properties from established sub-types.