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Department of Pharmacology, University College London.

1. Unitary currents due to calcium-activated potassium ion channels were studied in inside-out or outside-out excised membrane patches from guinea-pig hepatocytes. 2. Potassium ion channels were identified which were activated by internal calcium ions and blocked by external apamin (50 nM) or (+)-tubocurarine (10 microM). These properties are characteristic of the whole-cell potassium conductance increase evoked in guinea-pig hepatocytes by hormonal stimulation. 3. The single-channel conductance was 20 pS in inside-out or outside-out patches with external and internal K+ ion concentrations of 150 and 135 mM respectively and gluconate anion. Reducing external K+ concentration to 5 mM reduced the unitary conductance for outward current to 6 pS. 4. The calcium sensitivity was investigated with buffered internal Ca2+ ion concentrations in the range 0.3-2.2 microM. Tubocurarine-sensitive channels had an open probability of less than 0.05 at 0.3 microM-internal Ca2+. This increased steeply to a maximum of 0.85 at concentrations of 1.1 microM-Ca2+ or higher. 5. In patches with a single channel active, analysis of open and closed intervals showed that openings occurred in bursts. The increase of open probability at high internal Ca2+ concentration was associated with prolonged bursts of channel opening. 6. Comparison of these results with data from whole-cell conductance changes and with published levels of intracellular Ca2+ ion concentration (Woods, Cuthbertson & Cobbold, 1987) suggests that a large proportion, more than 40%, of potassium ion channels in guinea-pig hepatocytes are activated by hormonal stimulation.

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