Calcium permeability of neuronal nicotinic acetylcholine receptor channels in PC12 cells.

The conductance properties of neuronal nicotinic acetylcholine receptors (neuronal nAChR channels) in PC12 cells were studied using single channel and whole cell gigaohm seal voltage clamp techniques. Acetylcholine receptor agonists were applied using external pipettes. Neuronal nAChR channels were selective for monovalent cations, and were impermeable to anions. Based on measurements of shifts in reversal potential with changes in external Na and K ion concentrations and evaluation of the GHK constant field relation, PK/PNa was 1.45 (ignoring Ca permeability). PCa/PK was 1.75 based on the shift in reversal potential seen when raising the external Ca concentration from 2 to 50 mM (holding the external Na ion concentration constant), and determining the best fit to the extended GHK constant field relation using ionic activities. This value is larger than that determined for the muscle-type nAChR channel. Evaluation of the extended GHK current equations using these permeability coefficients indicated that at subthreshold voltages approximately 5% of total current through neuronal nAChR channels will be carried by Ca ions. Thus, these channels may be important mediators of activity-dependent Ca influx in the nervous system.