Abstract

Nicotine elicited membrane depolarization, elevation of intracellular calcium, rubidium efflux, and release of insulin from mouse β-TC6 insulinoma cells. Such responses were blocked by the nicotinic antagonist mecamylamine but not by the muscarinic antagonist atropine. Neither the selective α₄β₂ antagonist dihydro-β-erythroidine nor the selective α₇ 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 β-TC6 cells than in rat brain membranes. No binding of ¹²⁵I-A-85380 was detected, indicating the absence of β2-containing receptors. Reverse transcription-polymerase chain reaction analyses indicated the presence of mRNA for α3 and α4 subunits and β2 and β4 subunits in β-TC6 cells. The binding and functional data suggest that the major nicotinic receptor is composed of α3 and β4 subunits. The β-TC6 cells thus provide a model system for pharmacological study of such nicotinic receptors.