Cell volume regulation in human neutrophils: 2-(aminomethyl)phenols as Cl- channel inhibitors

When subjected to hypotonic stress, human peripheral neutrophils initially swell due to rapid water entry and thereafter recover toward the normal cell size (approximately 330 microns 3). Neutrophils do not behave as perfect osmometers: when resuspended in half-isotonic medium (150 mosM), they swell by only approximately 40% rather than doubling in size as predicted for ideal behavior. As with lymphocytes, restoration to the normal cell size involves the net loss of K+ and Cl- from the cytosol through independent conductance pathways. Volume regulation is sensitive to 0.4-1 mM of quinine, UK-5099, 3,5-diiodosalicylate (DISA), MK-473 (an indanyloxyacetate derivative), and to MK-447 [a 2-(aminomethyl)phenol]. From correlation of drug effects on the time course of cell volume recovery and the associated volume-activated 86Rb+ and 36Cl- fluxes, it was evident that quinine blocked only K+ channels, whereas MK-447 acted as a selective inhibitor of Cl- channels. In contrast, UK-5099, DISA, and MK-473 were nonspecific in that the compounds displayed comparable suppressive effects on all three parameters. Structure-activity relationships in the MK-447 series revealed the critical elements of the molecule responsible for drug potency. In particular, the importance of the neighboring ionizable 1-hydroxyl and 2-aminomethyl groups and the formation of secondary ring structures for biological activity is emphasized. The most potent derivative thus far identified, termed analogue A [inhibitor constant (Ki) approximately 16 microM], had a potency approximately sixfold greater than that of the parent compound (Ki approximately 90 microM). These findings define the mechanism of action of a relatively new class of agents that behave as inhibitors of swelling-activated Cl- channels in these cells.