COMPARISON OF LARGE-CONDUCTANCE CA2-ACTIVATED K+ CHANNELS IN ARTIFICIAL BILAYER AND PATCH-CLAMP EXPERIMENTS()

We compared the gating, ion conduction, and pharmacology of large-cond uctance Ca2+-activated K+ channels (BK channels) from canine colon in artificial lipid bilayers and in excised patches. Both protocols ident ified 270-pS K+-selective channels activated by depolarization and Ca2 + (similar to 130-mV shift of half-activation voltage per 10-fold chan ge in Ca2+) that were inhibited by extracellular tetraethylammonium (T EA) and charybdotoxin. These similarities suggest that the same BK cha nnels are studied in the two techniques. However, we found three quant itative differences between channels in artificial bilayers and patche s. 1) Channels in artificial bilayers required fivefold higher free Ca 2+ or 80-mV stronger depolarization for activation. 2) The voltage dep endence of TEA block was smaller for channels in artificial bilayers. The apparent distance across the membrane field for the TEA binding si te was 0.031 for channels in artificial bilayers and 0.23 for channels in patches. 3) ATP (2 mM) decreased open probability (P-o) of channel s in artificial bilayers, whereas channels in patches were unaffected. Neither GTP nor UTP reduced P-o of channels in artificial bilayers. I t is possible that these differences may be due to a lack of molecular identity between the channels studied in the two protocols. Alternati vely, they may be attributed to alterations in channel properties duri ng reconstitution or to influences of the artificial lipid environment .