Ca(2+) permeability of human heteromeric nAChRs expressed by transfection in human cells

The Ca(2+) permeability of the human heteromeric alpha 3 beta 4, alpha 4 beta 2 and alpha 4 beta 4 neuronal nicotinic acetylcholine receptors (nAChRs) was estimated by measuring the fractional Ca(2+) current (P(f)) flowing through the ligand-activated receptor-channels. Simultaneous recordings of transmembrane currents and fluorescence transients, using the whole-cell patch-clamp technique combined with fura-2 fluorescence microscopy, were performed in transiently transfected human cells. The human alpha 4 beta 2 nAChR showed a P(f) value of 2.6%, while the human alpha 3 beta 4 nAChR showed a similar P(f) value of 2.7%. Conversely, alpha 4 beta 4 nAChR exhibited a P(f) value (1.5%) significantly smaller than those of both alpha 4 beta 2 and alpha 3 beta 4 nAChRs. In test experiments performed in HEK 293 cells stably expressing rat GluR1 AMPA receptor subunit, we repeated the determination of P(f), whose value (3.2%) has previously been reported by others using the same fluorescent dye; and we found a very similar P(f) value (3.5%). In further test experiments, we found that P(f) values of chick alpha 3 beta 4 (4.4%) and alpha 4 beta 4 (2.1%) matched those previously reported by us using confocal fluorescence microscopy. Thus, our findings are consistent with those elsewhere reported even using different experimental procedures, giving a strong support to the following sequence of Ca(2+) permeability: h-alpha 3 beta 4>h-alpha 4 beta 2>h-alpha 4 beta 4.