Study of the calcium dynamics of the human α4β2, α3β4 and α1β1γδ nicotinic acetylcholine receptors

In this study three major subtypes of nicotinic acetylcholine receptors were characterized pharmacologically using the calcium influx through the ion channel as a robust functional assay system. Human alpha3beta4 receptors and alpha4beta2 receptors were cloned and stably expressed in HEK293 cells. [I-125]epibatidine saturation binding yielded a B-max of 4420 +/- 840 fmol/mg protein for the alpha4beta2 receptor (n=4) and 518 +/- 15 fmol/mg protein for the alpha3beta4 receptor (n=4). As a source for muscle type of nicotinic receptor, the TE671 cell line was used which expresses endogenously the human fetal alpha1betagammadelta subtype of nicotinic receptor. Stimulation of these nicotinic receptor subtypes in the different cell lines led to calcium transients that peaked 5-10 s after agonist application and declined thereafter. Eleven agonists were tested in this study and their efficacy and potency at the three nicotinic receptor subtypes were determined (epibatidine, ABT594, anatoxin, ABT418, nicotine, DMPP, cytisine, ABT089, choline, GTS21, AAR17779). This pharmacological characterization of agonist-induced elevation of intracellular free Ca2+ revealed a distinct rank order of agonist potency for each receptor subtype. Epibatidine showed at all three subtypes the highest potency and was a full agonist. The agonist-elicited response could be blocked by co-incubation of different antagonists from which mecamylamine did not display a strong subtype specificity. These data illustrate that the assessment of calcium transients upon receptor stimulation is a powerful tool for rapid examination of the functional properties of nicotinic receptors.