K+ channel cAMP activated in guinea pig gallbladder epithelial cells

In guinea pig gallbladder epithelial cells, an increase in intracellular cAMP levels elicits the rise of anion channel activity. We investigated by patchclamp techniques whether K1 channels were also activated. In a cell-attached configuration and in the presence of theophylline and forskoline or 8-Br-cAMP in the cellular incubation bath, an increase of the open probability (Po) values for Ca21-activated K1 channels with a single-channel conductance of about 160 pS, for inward current, was observed. The increase in Po of these channels was also seen in an inside-out configuration and in the presence of PKA, ATP, and cAMP, but not with cAMP alone; phosphorylation did not influence single-channel conductance. In the inside-out configuration, the opioid loperamide (1025 M) was able to reduce Po when it was present either in the microelectrode filling solution or on the cytoplasmic side. Detection in the epithelial cells by RT-PCR of the mRNA corresponding to the a subunit of largeconductance Ca21-activated K1 channels (BKCa) indicates that this gallbladder channel could belong to the BK family. Immunohistochemistry experiments con- firm that these cells express the BK asubunit, which is located on the apical membrane. Other K1 channels with lower conductance (40 pS) were not activated either by 8-Br-cAMP (cell-attached) or by PKA 1 ATP 1 cAMP (inside-out). These channels were insensitive to TEA1 and loperamide. The data demonstrate that under conditions that induce secretion, phosphorylation activates anion channels as well as Ca21- dependent, loperamide-sensitive K1 channels present on the apical membrane.