Single-channel properties of BK-type calcium-activated potassium channels at a cholinergic presynaptic nerve terminal

1. A high-conductance calcium-activated potassium channel (BK K-Ca) was cha racterized at a cholinergic presynaptic nerve terminal using the calyx syna pse isolated from the chick ciliary ganglion. 2. The channel had a conductance of 210 pS in a 150 mM:150 mM K+ gradient, was highly selective for K+ over Na+, and was sensitive to block by externa l charybdotoxin or tetraethylammonium (TEA) and by internal Ba2+. At +60 mV it was activated by cytoplasmic calcium [Ca2+](i) with a K-d of similar to 0.5 mu M and a Hill coefficient of similar to 2.0. At 10 mu M [Ca2+](i) th e channel was 50% activated (V-1/2) at -8.0 mV with a voltage dependence (B oltzmann slope-factor) of 32.7 mV. The V-1/2 values hyperpolarized with an increase in [Ca2+](i) while the slope factors decreased. There were no over t differences in conductance or [Ca2+](i) sensitivity between BK channels f rom the transmitter release face and the non-release face. 3. Open and closed times were fitted by two and three exponentials, respect ively. The slow time constants were strongly affected by both [Ca2+](i) and membrane potential changes. 4. In cell-attached patch recordings BK channel opening was enhanced by a p repulse permissive for calcium influx through the patch, suggesting that th e: channel can be activated by calcium ion influx through neighbouring calc ium channels. 5. The properties of the presynaptic BK channel are well suited for rapid a ctivation during the presynaptic depolarization and Ca2+ influx that are as sociated with transmitter release. This channel may play an important role in terminating release by rapid repolarization of the action potential.