Ca2+ permeability through rat cloned α9-containing nicotinic acetylcholine receptors

We investigated the functional properties of rat α9 and α9α10 nicotinic acetylcholine receptors (nAChRs) expressed by transient transfection in the rat GH4C1 cell line, using both Ca2+ imaging and whole-cell recording. Acute applications of ACh generated short-delay fast-rising and quick-decaying Ca2+ transients, suppressed in Ca2+-free medium and invariably accompanied by the activation of whole-cell inward currents. The mean amplitude of ACh-induced currents was as small as −16 pA in α9 subunit cDNA-transfected GH4C1 cells (α9-GH4C1), while they were much larger (range: −150 to −300 pA) in α9α10 subunit cDNAs-transfected GH4C1 cells (α9α10-GH4C1). Currents were not activated by nicotine, were blocked by methyllycaconitine and were ACh concentration-dependent. Because the Ca2+ permeability of α9-containing nAChRs has been estimated in immortalized cochlear UB/OC-2 mouse cells, we also characterized the ACh-induced responses in these cells. Unlike α9- and α9α10-GH4C1 cells, UB/OC-2 cells responded to ACh with both long-delay methyllycaconitine-insensitive whole-cell currents and long-lasting Ca2+ transients, the latter being detected in the absence of Ca2+ in the extracellular medium and being suppressed by the Ca2+-ATPase inhibitor thapsigargin, known to deplete IP3-sensitive stores. These results indicated the involvement of muscarinic nAChRs and the lack of functional ACh-gated receptor channels in UB/OC-2 cells. Thus, we measured the fractional Ca2+ current (Pf, i.e. the percentage of total current carried by Ca2+ ions) in α9α10-GH4C1, obtaining a Pf value of 22 ± 4%; this is the largest value estimated to date for a ligand-gated receptor channel. The physiological role played by Ca2+ entry through α9-containing nAChRs gated by ACh is discussed.