Received for publication May 23, 2005.
Revised December 6, 2005.
Accepted for publication December 6, 2005.

Mouse Beta-TC6 Insulinoma Cells: High Expression of Functional 34 Nicotinic Receptors Mediating Membrane Potential, Intracellular Calcium and Insulin Release

Abstract

Nicotine elicited membrane depolarization, elevation of intracellular calcium, rubidium efflux and release of insulin from mouse Beta-TC6 insulinoma cells. Such responses were blocked by the nicotinic antagonist mecamylamine, but not by the muscarinic antagonist atropine. Neither the selective ₄₂ antagonist dihydro--erythroidine, nor the selective ₇ antagonist methyllycaconitine significantly blocked the nicotine-elicted depolarization or calcium response. The elevation of intracellular calcium did not occur in calcium-free media, indicating that the rise in intracellular calcium was due to influx of calcium. The rank order of potency for nicotinic agonists was as follows: epibatidine > nicotine = A- 85380, cytosine, dimethylphenylpiperazinium (DMPP). Cytisine and DMPP appeared to be partial agonists. The density of nicotinic receptors measured by [³H] epibatidine binding was 7-fold higher in membranes from Beta-TC6 cells than in rat brain membranes. No binding of [¹²⁵I]A-85380 was detected, indicating absence of 2-containing receptors. RT-PCR analyses indicated the presence of mRNA for 3 and 4 subunits, and 2 and 4 subunits in Beta-TC6 cells. The binding and functional data suggest that the major nicotinic receptor is composed of 3 and 4 subunits. The Beta-TC6 cells thus provide a model system for pharmacological study of such nicotinic receptors.

Key words: Muscarinic cholinergic, Nicotinic cholinergic, Insulin, Ca imaging, Receptor binding studies