Natural Bile Acids and Synthetic Analogues Modulate Large Conductance Ca²⁺-activated K⁺ (BK_Ca) Channel Activity in Smooth Muscle Cells

Bile acids have been reported to produce relaxation of smooth muscle both in vitro and in vivo. The cellular mechanisms underlying bile acid–induced relaxation are largely unknown. Here we demonstrate, using patch-clamp techniques, that natural bile acids and synthetic analogues reversibly increase BK_Ca channel activity in rabbit mesenteric artery smooth muscle cells. In excised inside-out patches bile acid–induced increases in channel activity are characterized by a parallel leftward shift in the activity-voltage relationship. This increase in BK_Ca channel activity is not due to Ca²⁺-dependent mechanism(s) or changes in freely diffusible messengers, but to a direct action of the bile acid on the channel protein itself or some closely associated component in the cell membrane. For naturally occurring bile acids, the magnitude of bile acid–induced increase in BK_Ca channel activity is inversely related to the number of hydroxyl groups in the bile acid molecule. By using synthetic analogues, we demonstrate that such increase in activity is not affected by several chemical modifications in the lateral chain of the molecule, but is markedly favored by polar groups in the side of the steroid rings opposite to the side where the methyl groups are located, which stresses the importance of the planar polarity of the molecule. Bile acid–induced increases in BK_Ca channel activity are also observed in smooth muscle cells freshly dissociated from rabbit main pulmonary artery and gallbladder, raising the possibility that a direct activation of BK_Ca channels by these planar steroids is a widespread phenomenon in many smooth muscle cell types. Bile acid concentrations that increase BK_Ca channel activity in mesenteric artery smooth muscle cells are found in the systemic circulation under a variety of human pathophysiological conditions, and their ability to enhance BK_Ca channel activity may explain their relaxing effect on smooth muscle.