Activation of the recombinant human α₇ nicotinic acetylcholine receptor significantly raises intracellular free calcium

The α₇ nicotinic acetylcholine receptor (nAChR) subtype, unlike other neuronal nicotinic receptors, exhibits a relatively high permeability to Ca⁺⁺ ions. Although Ca⁺⁺ entry through this receptor subtype has been implicated in various Ca⁺⁺-dependent processes in the central nervous system, little is known about how this receptor modulates mammalian intracellular Ca⁺⁺ dynamics. Intracellular Ca⁺⁺ responses evoked by activation of the human α₇ nAChRs stably expressed in HEK-293 (human embryonic kidney) cells were studied. Inward current and intracellular Ca⁺⁺ transients were recorded simultaneously in response to a fast drug application system. Current recordings under whole-cell voltage-clamp and fast ratiometric intracellular Ca⁺⁺ imaging acquisition were synchronized to drug pulses. The mean peak [Ca⁺⁺](i) observed with 100 μM (-)-nicotine was 356 ± 48 nM (n = 8). The magnitude of the intracellular Ca⁺⁺ elevation corresponds to a 20% fractional current carried by Ca⁺⁺ ions. The EC₅₀ of the intracellular Ca⁺⁺ responses for (-)-nicotine, (±)-epibatidine, 1,1 dimethyl-4-phenyl-piperazinium and acetylcholine were 51, 3.5, 75 and 108 μM, respectively. These EC₅₀ values strongly correlate with those recorded for the cationic inward current through α₇ nAChR. α-Bungarotoxin, methyllcaconitine or extracellular Ca⁺⁺ chelation ablated (-)-nicotine- evoked increase in intracellular Ca⁺⁺ concentration. This study provides evidence that cation influx through the human α₇ nAChR is sufficient to mediate a significant, transient, rise in intracellular Ca⁺⁺ concentration.