Existence, properties, and functional expression of "Maxi-K"-type, Ca2+-activated K+ channels in short-term cultured hepatocytes.
A large-conductance, Ca2+-activated K+ channel was identified and characterized in embryonic chick hepatocytes using the patch-electrode voltage-clamp technique. The channel conductance was 213 pS in excised patches bathed in symmetrical 145 mM KCI and 1 mM Ca2+. Current-voltage relationships were linear with high K+ on both sides of the membrane but showed constant field rectification as the K+ gradient was increased. The reversal potential shifted 58 mV per 10-fold change in the ratio of external to internal K+. Channel openings occurred at potentials higher than +50 mV in cell-attached patches. The open probability X voltage relationship shifted to more negative potentials in excised, inside-out patches exposed to a solution containing high Ca2+. The voltage sensitivity of the channel was not significantly affected by changes in internal Ca2+ concentration. Conversely, channel gating, reflected in the half-activation potential, shifted 118 mV per 10-fold change in internal Ca2+ at concentrations less than approximately 2 microM, although at higher Ca2+, this parameter was Ca2+ insensitive. Channel open probability in cell-attached patches increased significantly following exposure of the cells to either the Ca2+ ionophore A-23187 or L-alanine, a cell-volume modulator. Channel density increased with time spent in culture from no observations in 10-hr cells, through 13 and 80% of patches in 24-and 48-hr cultured cells, respectively. The implications of delayed functional expression for ion channel studies in acutely dissociated cells is discussed.
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Department of Physiology, Queen's University, Kingston, Ontario, Canada.
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D000818:Animals; D000001:Calcimycin / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism*; D002478:Cells, Cultured; D002645:Chickens; D015640:Ion Channel Gating; D007476:Ionophores / Q000494:pharmacology; D008099:Liver / Q000378:metabolism*; D015221:Potassium Channels / Q000378:metabolism*
D007476:Ionophores; D015221:Potassium Channels; D000001:Calcimycin; D002118:Calcium