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Medical Research Council Neuropharmacology Research Group, Department of Pharmacology, School of Pharmacy, London.

1. The properties of single Ca2+-activated K+ channels in cultured rat superior cervical ganglionic neurones were studied in cell-attached and excised patches using the patch-clamp technique. 2. In cell-attached patches using an external K+ concentration ([K+]o) of 150 mM, approximately equal to the internal [K+], the channel slope conductance was approximately 200 pS and independent of membrane voltage between -50 and +50 mV. Using [K+]o of 4.7 mM (providing a near physiological K+ gradient), the I-V relationship was non-linear with a slope conductance of approximately 120 pS at 0 mV. 3. The channel was selective for K+ over Cs+ and Na+ which were impermeant from either side of the membrane. Both Na+ and Cs+ also blocked the movement of K+ through the channel. Cs+ was active on either side of the membrane, whereas Na+ apparently blocked the channel only when applied to the cytoplasmic side. 4. The channel was activated by increasing the Ca2+ concentration on the inside of the membrane ([Ca2+]i). The channel was virtually inactive when [Ca2+]i = 0.01 microM. Depolarizing the patch at a constant [Ca2+]i usually further increased the opening probability. 5. The gating properties of the channel were studied using cell-attached patches. At potentials more negative than the resting membrane potential, the open-time distribution was described by a single exponential. On depolarization, two exponentials were required. The closed-time distribution was fitted by three exponentials. 6. Depolarization of the patch caused the long mean open lifetime to increase whilst the short mean open and closed lifetimes were unaffected. Both the intermediate and long mean closed lifetimes decreased with depolarization from -60 to +60 mV. 7. In cell-attached patches, the long mean open lifetimes were usually smaller than those observed in excised patches at depolarized potentials (greater than 0 mV). 8. A fourth closed state, possibly representing an inactivated form of the channel, was infrequently observed. A 50% substate of the full single-channel current was also observed occasionally. This substate was always associated with openings to the full current state. 9. The channel was blockable by external tetraethylammonium (25 microM-1 mM), Ba2+ (1-10 mM), and quinine (10-200 microM). External d-tubocurarine (25-100 microM) also blocked this IC channel. However it was insensitive to apamin (100-300 nM), muscarine (10 microM) and 4-aminopyridine (1-3 mM). The channel was also blocked by internal tetraethylammonium (5-10 mM) or Ba2+ (0.3-1 mM).(ABSTRACT TRUNCATED AT 400 WORDS)

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