Structure-activity relations of amiloride and its analogues in blocking the mechanosensitive channel in Xenopus oocytes.

1. Patch clamp recording techniques have been used to compare the block caused by amiloride and some of its structural analogues of the mechanosensitive (MS) cation selective channel in frog (Xenopus laevis) oocytes. 2. Like amiloride, the amiloride analogues dimethylamiloride (DMA), benzamil and bromohexamethyleneamiloride (BrHMA) block the MS channel in a highly voltage-dependent manner. 3. All analogues tested were more potent blockers than amiloride with IC50's of 500 microM (amiloride), 370 microM (DMA), 95 microM (benzamil) and 34 microM (BrHMA). 4. Hill plots gave Hill coefficients of 2 (amiloride), 1.8 (DMA), 1 (benzamil) and 1.2 (BrHMA) indicating that the binding of two ligand molecules may be necessary for the block caused by amiloride, DMA and possibly BrHMA whereas only a single ligand molecule may be required for the block by benzamil. 5. The potential use of BrHMA as a light-activated, covalent label of the MS channel protein is discussed. 6. The amiloride analogue 'fingerprinting' of the blocking site on the MS channel indicates it is structurally different from previously described amiloride-sensitive ion transport pathways but may be related to the amiloride binding site on outer hair cells of the ear.