The properties of calcium-activated potassium ion channels in guinea-pig isolated hepatocytes.
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.
Capiod
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Department of Pharmacology, University College London.
Ogden
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D000200:Action Potentials / Q000187:drug effects; D000818:Animals; D001030:Apamin / Q000494:pharmacology; D002118:Calcium / Q000494:pharmacology*; D004533:Egtazic Acid / Q000494:pharmacology / Q000031:analogs & derivatives; D006168:Guinea Pigs; D066298:In Vitro Techniques; D007700:Kinetics; D008099:Liver / Q000502:physiology*; D011188:Potassium / Q000494:pharmacology; D015221:Potassium Channels / Q000187:drug effects*; D014403:Tubocurarine / Q000494:pharmacology
D015221:Potassium Channels; D001030:Apamin; D004533:Egtazic Acid; C025603:1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid; D011188:Potassium; D002118:Calcium; D014403:Tubocurarine