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omega-Conotoxin-sensitive, voltage-operated Ca2+ channels in insulin-secreting cells
1Department of Medical Pharmacology, University of Milan, Italy.
Abstract:
The properties of voltage-operated Ca2+ channel subtypes were investigated in insulin-secreting RINm5F cells. Two types of channels were identified: a dihydropyridine-sensitive (L-type) channel, and an omega-conotoxin-sensitive (omega-type) channel. 125I-omega-Conotoxin bound with high affinity (Kd 46.7 pM) to a saturable number of binding sites (10.3 fmol/mg of protein). Toxin binding was not antagonized by L-type channel ligands, but was sensitive to Ca2+ and neomycin. 125I-omega-Conotoxin-labeled Ca2+ channels were recognized by autoantibodies of Lambert-Eaton myasthenic patients. These antibodies are known to be specific for the neuronal omega-type channel. High-voltage-activated Ca2+ currents, investigated with the patch-clamp technique, consisted of a major dihydropyridine-sensitive (L-type) component, and a minor fraction irreversibly blocked by omega-conotoxin. Depolarizing secretagogues, such as D-glyceraldehyde and alanine, induced Ca(2+)-dependent insulin secretion, which was attenuated by omega-conotoxin. Taken together, these results show that voltage-operated Ca2+ channels in insulin-secreting RINm5F cells are heterogeneous and, in particular, that an omega-type channel, pharmacologically, immunologically and electrophysiologically similar to the neuronal omega-type channel, is also expressed in endocrine cells where it might have a role in the control of hormone secretion.
Insights
Voltage-operated calcium channels in insulin-secreting cells include L-type and omega-type channels. The omega-type channel, similar to neuronal channels, plays a role in hormone secretion control.
Area of Science:
- Endocrinology
- Neuroscience
- Cell Physiology
Background:
- Voltage-operated calcium channels (VOCCs) are crucial for cellular functions, including hormone secretion.
- RINm5F cells are a model system for studying insulin secretion and calcium channel activity.
- The heterogeneity of VOCCs in endocrine cells is not fully understood.
Purpose of the Study:
- To characterize the subtypes of voltage-operated calcium channels in insulin-secreting RINm5F cells.
- To investigate the pharmacological, immunological, and electrophysiological properties of these channels.
- To determine the role of specific calcium channel subtypes in regulating insulin secretion.
Main Methods:
- Radioligand binding assays using 125I-omega-conotoxin.
- Patch-clamp electrophysiology to record high-voltage-activated calcium currents.
- Immunological characterization using autoantibodies from Lambert-Eaton myasthenic patients.
- Assessment of insulin secretion in response to secretagogues and channel blockers.
Main Results:
- Two distinct VOCC subtypes were identified: dihydropyridine-sensitive (L-type) and omega-conotoxin-sensitive (omega-type).
- 125I-omega-conotoxin exhibited high-affinity binding to saturable sites, distinct from L-type channel ligands.
- Omega-type channels were recognized by Lambert-Eaton myasthenic syndrome antibodies, indicating similarity to neuronal omega-type channels.
- Patch-clamp studies revealed a major L-type and a minor omega-type calcium current component.
- Depolarizing secretagogues induced insulin secretion, which was partially inhibited by omega-conotoxin.
Conclusions:
- RINm5F cells express heterogeneous voltage-operated calcium channels, including an omega-type channel.
- This omega-type channel shares characteristics with neuronal omega-type channels and is present in endocrine cells.
- The omega-type channel likely contributes to the regulation of calcium-dependent insulin secretion.