Co-existence of muscarinic and nicotinic receptors and their functional interaction in mouse Beta-TC6 cells

Mouse Beta-TC6 insulinoma cells possessing nicotinic receptor [Ohtani, M., Oka, T., Badyuk, M., Xiao, Y., Kellar, KJ., Daly, JW., 2006. Mouse β-TC6 insulinoma cells: high expression of functional α3β4 nicotinic receptors mediating membrane potential, intracellular calcium, and insulin release. Mol. Pharmacol. 69, 899–907.] also expressed M3 and M4 muscarinic receptors. Carbamylcholine, a mixed muscarinic/nicotinic receptor agonist, or oxotremorine M, a selective muscarinic agonist, elicited an elevation of cytoplasmic Ca2+ concentration ([Ca2+]i) and release of insulin. The maximal [Ca2+]i response induced by carbamylcholine was larger than that of oxotremorine M or that of nicotine, suggesting that carbamylcholine enhanced the [Ca2+]i response by stimulating two types of receptor. M3 and M4 muscarinic receptor antagonists inhibited the [Ca2+]i responses to carbamylcholine and oxotremorine M, suggesting the involvement of these muscarinic receptors in the regulation of [Ca2+]i. In addition, pretreatment with carbamylcholine inhibited the [Ca2+]i responses to oxotremorine M or nicotine, indicating that the effect of carbamylcholine on [Ca2+]i was mediated by both muscarinic and nicotinic receptors. A phospholipase C (PLC) inhibitor U73122, a protein kinase C (PKC) inhibitor chelerythrine and a phospholipase A2 (PLA2) inhibitor AACOCF3 inhibited the [Ca2+]i response to carbamylcholine or oxotremorine M, while these inhibitors did not block the effect of nicotine. Carbamylcholine induced a smaller extent of insulin secretion than oxotremorine M, suggesting that concomitant stimulation of muscarinic and nicotinic receptors by carbamylcholine resulted in the negative type of the receptor interaction.