Cromakalim and lemakalim activate Ca(2+)-dependent K+ channels in canine colon

The effects of cromakalim (BRL 34915) and its (-) optical isomer, lemakalim (BRL 38227) on the activity of 265-pS Ca(2+)-activated K+ channels (BK channels) were examined in cell-attached and inside-out patches from canine colonic myocytes. In cell-attached patches lemakalim increased the open probability (Po) of BK channels. Mean NPo, where N is the number of channels per patch, at +50 mV increased from 0.08 to 0.26 (20 microM lemakalim). In inside-out patches, cromakalim and lemakalim increased channel NPo rapidly and reversibly. This increase in NPo was due to a shift in half-maximal activation. Glyburide (20 microM) prevented the increase in NPo caused by lemakalim in cell-attached patches and reversed the increase in NPo in inside-out patches. Under conditions where Ca(2+)-activated K+ channels were maximally activated, lemakalim failed to increase current or induce a second type of K+ channel activity. When tetraethylammonium (200 microM) was added to the pipette solution to block the BK channel half maximally, lemakalim also failed to induce a second type of channel. Adenosine triphosphate (1 or 2 mM) applied to the inner surface of inside-out patches had no effect on Po of BK channels. Finally, the effects of lemakalim on ensemble average currents, constructed from multiple openings of BK channels in cell-attached patches was found to successfully mimic the effects of the drug on whole-cell membrane currents. We conclude that cromakalim and lemakalim activate BK channels in canine colonic cells. Whether this action participates in the membrane hyperpolarization and the decrease in frequency and duration of slow waves produced by these compounds in intact colonic muscles remains to be investigated.