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,αβγδ) value of 2.1%. The substitution of the γ subunit with the ε 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,α3β4) was 4.6%, while P(f,α4β4) and P(f,α4β2) were 3.0% and 2.9%, respectively. 4. The amplitude of the current elicited by the activation of α3β4 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 (αβεδ) is more permeable to Ca2+ than the fetal-type (αβγδ), while ganglionic-like α3β4 nAChR is more permeable to Ca2+ than the examined α4-containing nAChRs. The functional significance is discussed.
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