Maxi K+ channels and their relationship to the apical membrane conductance in Necturus gallbladder epithelium.
Using the patch-clamp technique, we have identified large-conductance (maxi) K+ channels in the apical membrane of Necturus gallbladder epithelium, and in dissociated gallbladder epithelial cells. These channels are more than tenfold selective for K+ over Na+, and exhibit unitary conductance of approximately 200 pS in symmetric 100 mM KCl. They are activated by elevation of internal Ca2+ levels and membrane depolarization. The properties of these channels could account for the previously observed voltage and Ca2+ sensitivities of the macroscopic apical membrane conductance (Ga). Ga was determined as a function of apical membrane voltage, using intracellular microelectrode techniques. Its value was 180 microS/cm2 at the control membrane voltage of -68 mV, and increased steeply with membrane depolarization, reaching 650 microS/cm2 at -25 mV. We have related maxi K+ channel properties and Ga quantitatively, relying on the premise that at any apical membrane voltage Ga comprises a leakage conductance and a conductance due to maxi K+ channels. Comparison between Ga and maxi K+ channels reveals that the latter are present at a surface density of 0.09/microns 2, are open approximately 15% of the time under control conditions, and account for 17% of control Ga. Depolarizing the apical membrane voltage leads to a steep increase in channel steady-state open probability. When correlated with patch-clamp studies examining the Ca2+ and voltage dependencies of single maxi K+ channels, results from intracellular microelectrode experiments indicate that maxi K+ channel activity in situ is higher than predicted from the measured apical membrane voltage and estimated bulk cytosolic Ca2+ activity. Mechanisms that could account for this finding are proposed.
Segal
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Department of Physiology and Biophysics, University of Texas Medical Branch, Galveston 77550-2781.
Reuss
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D000818:Animals; D002118:Calcium; D004553:Electric Conductivity; D004848:Epithelium / Q000378:metabolism; D005704:Gallbladder / Q000378:metabolism*; D066298:In Vitro Techniques; D008433:Mathematics; D008564:Membrane Potentials; D008839:Microelectrodes; D015233:Models, Statistical; D009338:Necturus maculosus; D011188:Potassium; D015221:Potassium Channels / Q000378:metabolism / Q000502:physiology*; D012964:Sodium
D015221:Potassium Channels; D012964:Sodium; D011188:Potassium; D002118:Calcium