The expression and functional role of nicotinic acetylcholine receptors in rat adipocytes

To clarify whether nicotine has a direct effect on the function of adipocytes, we evaluated nicotinic acetylcholine receptor (nAChR) expression in adipocytes by reverse transcriptase-polymerase chain reaction (RT-PCR) and immunocytochemistry and the direct effects of nicotine on the production of adipocytokines by enzyme-linked immunosorbent assay and Western blot analysis. Receptor binding assays were performed using [3H]nicotine. RT-PCR studies revealed that alpha1-7, 9, 10, beta1-4, delta, and epsilon subunit mRNAs are expressed in adipocytes. Immunocytochemical experiments also suggested the presence of alpha7 and beta2 subunits. The receptor binding assay revealed a binding site for nicotine (Kd = 39.2 x 10(-9) M) on adipocytes. Adipocytes incubated with nicotine for 12 and 36 h released tumor necrosis factor-alpha (TNF-alpha), adiponectin, and free fatty acid (FFA) into the medium in a dose-dependent manner with increasing nicotine concentration from 6 x 10(-8) to 6 x 10(-4) M. However, TNF-alpha protein levels in adipocytes incubated for 12 and 36 h decreased in a dose-dependent manner with increasing nicotine concentration from 6 x 10(-8) to 6 x 10(-4) M. These results show that adipocytes have functional nAChRs and suggest that nicotine reduces TNF-alpha protein production in adipocytes through the activation of nAChRs. Nicotine may temporarily lower insulin sensitivity by stimulating the secretion of TNF-alpha and FFA, whereas long-term direct stimulation of nAChRs by nicotine in addition to autonomic nervous system stimulation may contribute to better insulin sensitivity in vivo through a modulated secretion of adipocytokines.