AJP - Cell Physiology, Vol 266, Issue 3 C601-C610, Copyright © 1994 by American Physiological Society
Comparison of large-conductance Ca(2+)-activated K+ channels in artificial bilayer and patch-clamp experiments
C. L. Kapicka, A. Carl, M. L. Hall, A. L. Percival, B. W. Frey and J. L. Kenyon
Department of Physiology, University of Nevada School of Medicine, Reno 89557.
We compared the gating, ion conduction, and pharmacology of
large-conductance Ca(2+)-activated K+ channels (BK channels) from canine
colon in artificial lipid bilayers and in excised patches. Both protocols
identified 270-pS K(+)-selective channels activated by depolarization and
Ca2+ (approximately 130-mV shift of half-activation voltage per 10-fold
change in Ca2+) that were inhibited by extracellular tetraethylammonium
(TEA) and charybdotoxin. These similarities suggest that the same BK
channels are studied in the two techniques. However, we found three
quantitative differences between channels in artificial bilayers and
patches. 1) Channels in artificial bilayers required fivefold higher free
Ca2+ or 80-mV stronger depolarization for activation. 2) The voltage
dependence of TEA block was smaller for channels in artificial bilayers.
The apparent distance across the membrane field for the TEA binding site
was 0.031 for channels in artificial bilayers and 0.23 for channels in
patches. 3) ATP (2 mM) decreased open probability (Po) of channels in
artificial bilayers, whereas channels in patches were unaffected. Neither
GTP nor UTP reduced Po of channels in artificial bilayers. It is possible
that these differences may be due to a lack of molecular identity between
the channels studied in the two protocols. Alternatively, they may be
attributed to alterations in channel properties during reconstitution or to
influences of the artificial lipid environment.
