Structural Requirements in the Steroid Lateral Chain for the Activation of β1 Subunit-Containing BK Channels by 5β-Cholanic Acid-3α-Ol Analogues

Abstract

BK channels regulate many physiological processes and thus constitute an attractive target for drug discovery. Numerous BK channel activators are available. However, these agents usually interact with the ubiquitously distributed channel-forming α subunit and cannot selectively target a particular tissue. Here, we performed a structure-activity relationship study of lithocholic acid (LCA), a cholane that was previously shown to activate BK channels via a steroid site in the accessory BK β1 subunit (Bukiya et al., 2011). This protein is highly abundant in smooth muscle but scarce in most other tissues (Brenner et al., 2000). Modifications to the LCA lateral chain length and functional group yielded a total of 14 compounds. Bath application of these LCA analogs (1-300 μM) to the cytosolic side of Xenopus oocyte membrane patches co-expressing BK channel-forming cbv1 and β1 subunits demonstrates that channel activation (EC50∼50 μM) requires the combination of a critical lateral chain length (optimal at C24), rather small volume of its functional group (65-92 A3), and net negative charge (−0.1 to −0.9 [e]). Thus, analogs having cyano or nitro groups substituting for the carboxyl at C24 in the LCA molecule constitute two novel BK channel activators that satisfy the aforementioned criteria. Data provide detailed structural information that helps us to better understand ligand recognition by the cholane steroid site present in the BK β1 protein, and will be useful to advance a pharmacophore in search of β1 subunit-selective BK channel activators. These compounds are expected to evoke smooth muscle relaxation, which would be beneficial in the pharmacotherapy of prevalent human disorders associated with increased smooth muscle contraction, such as systemic hypertension, cerebral or coronary vasospasm, bronchial asthma, bladder hyperactivity, and erectile dysfunction.Support: R01-HL104631; R37-AA011560 (AMD).