ω-Conotoxin-sensitive, voltage-operated Ca 2+ channels in insulin-secreting cells

The properties of voltage-operated Ca 2+ channel subtypes were investigated in insulin-secreting RINm5F cells. Two types of channels were identified: a dihydropyridine-sensitive (L-type) channel, and an ω-conotoxin-sensitive (ω-type) channel. 125 I-ω-Conotoxin bound with high affinity (K d 46.7 pM) to a saturable number of binding sites (10.3 fmol/mg of protein). Toxin binding was not antagonized by L-type channels ligands, but was sensitive to Ca 2+ and neomycin. 125 I-ω-Conotoxin-labeled Ca 2+ channels were recognized by autoantibodies of Lambert-Eaton myasthenic patients. These antibodies are known to be specific for the neuronal ω-type channel. High-voltage-activated Ca 2+ currents, investigated with the patch-clamp technique, consisted of a major dihydropyridine-sensitive (L-type) component, and a minor fraction irreversibly blocked by ω-conotoxin. Depolarizing secretagogues, such as D-glyceraldehyde and alanine, induced Ca 2+ -dependent insulin secretion, which was attenuated by ω-conotoxin. Taken together, these results show that voltage-operated Ca 2+ channels in insulin-secreting RINm5F cells are heterogeneous and, in particular, that an ω-type channel, pharmacologically, immunologically and electrophysiologically similar to the neuronal ω-type channel, is also expressed in endocrine cells where it might have a role in the control of hormone secretion.