Mouse beta-TC6 insulinoma cells: high expression of functional alpha3beta4 nicotinic receptors mediating membrane potential, intracellular calcium, and insulin release.

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 alpha4beta2 antagonist dihydro-beta-erythroidine nor the selective alpha7 antagonist methyllycaconitine significantly blocked the nicotine-elicited depolarization or the calcium response. The elevation of intracellular calcium did not occur in calcium-free media, indicating that the increase in intracellular calcium was due to the influx of calcium. The rank order of potency for nicotinic agonists was as follows: epibatidine > nicotine = 3-(azetidinylmethoxy)pyridine (A-85380), cytisine, dimethylphenylpiperazinium (DMPP). Cytisine and DMPP seemed 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 the absence of beta2-containing receptors. Reverse transcription-polymerase chain reaction analyses indicated the presence of mRNA for alpha3 and alpha4 subunits and beta2 and beta4 subunits in beta-TC6 cells. The binding and functional data suggest that the major nicotinic receptor is composed of alpha3 and beta4 subunits. The beta-TC6 cells thus provide a model system for pharmacological study of such nicotinic receptors.