Methods: We screened a number of drugs using in-vitro isometric contraction studies of mouse trachea and bronchi. In addition, we conducted calcium imaging from dissociated mouse and human airway muscle cells.
Results: : We found two structurally unrelated membrane permeant SK antagonists relax ASM by ~90%. One is NS8593 ((R)-N-(Benzimidazol-2-yl)-1,2,3,4-tetrahydro-1-naphthylamine) that reduces channel opening by decreasing Ca2+-sensitivity. The other is UCL 1684 (6,10-diaza-3(1,3)8,(1,4)-dibenzena-1,5(1,4)-diquinolinacy clodecaphane), a SK channel-specific pore blocker. The airway muscle relaxant effect of these drugs occur at submicromolar concentrations and appear to be mediated through a novel mechanism: intracellular (sarcoplasmic reticulum) SK potassium channels. Non-permeant SK blocker (Apamin) had no affect on contractility. Consistent with effects on sarcoplasmic reticulum, calcium imaging revealed a reduced cholinergic-evoked calcium release, and reduced sarcoplasmic reticulum calcium load with UCL1684. The affect of these drugs are specific to airway smooth muscle since vascular (aorta) muscle contractility was unaffected.
Conclusions: Given the unique reliance of airway smooth muscle on calcium-routed through sarcoplasmic reticulum, antagonists of sarcoplasmic reticulum SK channels may provide a novel and specific drug target as bronchodilators for asthma and COPD.