RT Journal Article SR Electronic T1 Modulation of α7 Nicotinic Receptor-Mediated Calcium Influx by Nicotinic Agonists JF Molecular Pharmacology JO Mol Pharmacol FD American Society for Pharmacology and Experimental Therapeutics SP 499 OP 506 VO 51 IS 3 A1 M. Quik A1 J. Philie A1 J. Choremis YR 1997 UL http://molpharm.aspetjournals.org/content/51/3/499.abstract AB Our previous work had demonstrated stable expression of rat α7 α-bungarotoxin (α-BGT) binding sites by GH4C1 rat pituitary cells, a clonal line that does not endogenously express nicotinic receptors. The stably expressed α7 sites had similar binding affinities, pharmacological profiles, kinetic properties, and molecular size as rat brain α-BGT receptors, suggesting that they represent a good model system for studying receptor function. The present data show that nicotinic receptor agonists increase intracellular calcium levels ([Ca2+]i), as assessed using Fura-2, in α7/GH4C1 cells in a dose-dependent manner with EC50 values that correlate well with the affinity of these ligands for α7/GH4C1 α-BGT receptors. Nicotinic receptor antagonists inhibited agonist-induced increases in [Ca2+]i, with IC50 values in the nanomolar to micromolar range. The nicotinic agonist-induced increase in [Ca2+]i required extracellular calcium and did not occur in the presence of CdCl2, suggesting that agonist-induced increases in [Ca2+]i are due to an influx of extracellular calcium through voltage-gated calcium channels. Preexposure of the α7/GH4C1 cells to 8-bromo cAMP resulted in an enhanced [Ca2+]i in response to agonist, suggesting that phosphorylation by adenylate cyclase may regulate receptor responsiveness. Interestingly, short-term preexposure (40–60 sec) of the cells to subthreshold concentrations of nicotinic agonist-enhanced receptor-stimulated calcium influx (up to 55%) while activating agonist concentrations completely blocked receptor-mediated responses. Long-term exposure of α7/GH4C1cells to K+ resulted in about a 2-fold increase in α-BGT receptors and in agonist-evoked calcium influx. The sensitivity of these up-regulated receptors were modulated by subthreshold and activating concentrations of agonist in a manner similar to control receptors. The present results, demonstrating a biphasic regulation of α7 receptor-mediated calcium influx by nicotinic agonists, suggest that these receptors may play an important role in neuronal function under control and depolarizing conditions. The American Society for Pharmacology and Experimental Therapeutics