Ca2+ permeability of mouse and chick nicotinic acetylcholine receptors expressed in transiently transfected human cells

1. Combinations of cDNAs encoding mouse and chick nicotinic acetylcholine receptor (nAChR) subunits were transiently transfected into human BOSC 23 cells, and the expressed receptors were studied by simultaneously recording transmembrane currents and fluorescence transients using the whole-cell patch-clamp technique, and confocal microscopy with the Ca2+ indicator dye fluo-3. 2. The fractional Ca2+ current, P-f, of nAChRs was evaluated as the normalized ratio of nicotine-evoked fluorescence transient over total charge entering the cell (F/Q ratio). Mouse fetal muscle nAChR channels had a P-f,P-alpha beta gamma delta value Of 2.1%. The substitution of the gamma subunit with the epsilon subunit resulted in a 2-fold increase in P-f (4.2%). The difference in Ca2+ permeability was confirmed by determination of Ca2+/Cs+ permeability ratios. 3. Among the chick neuronal nAChRs tested, P-f,P-alpha 3 beta 4 was 4.6%, while P-f,P-alpha 4 beta 4 and P-f,P-alpha 4 beta 2 were 3.0% and 2.9%, respectively. 4. The amplitude of the current elicited by the activation of alpha 3 beta 4 nAChRs increased as the external Ca2+ concentration was raised from 2 to 110 mM, whereas currents flowing through all other nAChRs tested were reduced to various extents. 5. Our findings indicate that the adult-type muscle nAChR (alpha beta epsilon delta) is more permeable to Ca2+ than the fetal-type (alpha beta gamma delta), while ganglionic-like alpha 3 beta 4 nAChR is more permeable to Ca2+ than the examined alpha 4-containing nAChRs. The functional significance is discussed.