Modulation of alpha7 nicotinic receptor-mediated calcium influx by nicotinic agonists

Our previous work had demonstrated stable expression of rat alpha7 alpha-bungarotoxin (alpha-BGT) binding sites by GH4C1 rat pituitary cells, a clonal line that does not endogenously express nicotinic receptors. The stably expressed alpha7 sites had similar binding affinities, pharmacological profiles, kinetic properties, and molecular size as rat brain alpha-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 alpha7/GH4C1 cells in a dose-dependent manner with EC50 values that correlate well with the affinity of these ligands for alpha7/GH4C, alpha-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 alpha7/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 alpha7/GH4C1 cells to K+ resulted in about a 2-fold increase in alpha-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 alpha7 receptor-mediated calcium influx by nicotinic agonists, suggest that these receptors may play an important role in neuronal function under control and depolarizing conditions.