Ca2+ permeability of human heteromeric nAChRs expressed by transfection in human cells

The Ca2+ permeability of the human heteromeric 34, 42 and 44 neuronal nicotinic acetylcholine receptors (nAChRs) was estimated by measuring the fractional Ca2+ current (Pf) 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 42 nAChR showed a Pf value of 2.6%, while the human 34 nAChR showed a similar Pf value of 2.7%. Conversely, 44 nAChR exhibited a Pf value (1.5%) significantly smaller than those of both 42 and 34 nAChRs. In test experiments performed in HEK 293 cells stably expressing rat GluR1AMPA receptor subunit,we repeated the determination of Pf, whose value (3.2%) has previously been reported by others using the same fluorescent dye; and we found a very similar Pf value (3.5%). In further test experiments, we found that Pf values of chick 34 (4.4%) and 44 (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 Ca2+ permeability: h-34 > h-42 > h-44.