Background Perioperative bronchospasm refractory to β-agonists continues to challenge anesthesiologists and intensivists. The TMEM16A calcium-activated chloride channel modulates airway smooth muscle (ASM) contraction. We hypothesized that TMEM16A antagonists would relax ASM contraction by modulating membrane potential and calcium flux. Methods Human ASM, guinea pig tracheal rings or mouse peripheral airways were contracted with acetylcholine (Ach) or leukotriene D4 (LTD4) and then treated with the TMEM16A antagonists: benzbromarone, T16Ainh-A01, MONNA or B25. In separate studies, guinea pig tracheal rings were contracted with Ach and then exposed to increasing concentrations of isoproterenol (0.01nM-10μM) ± benzbromarone. Plasma membrane potential and intracellular calcium concentrations were measured in human ASM cells. Results Benzbromarone was the most potent TMEM16A antagonist tested for relaxing an Ach-induced contraction in guinea pig tracheal rings (n=6). Further studies were done to investigate benzbromarone’s clinical utility. In human ASM, benzbromarone relaxed either an acetylcholine- or LTD4-induced contraction (n=8). Benzbromarone was also effective in relaxing peripheral airways (n=9) and potentiating relaxation by β-agonists (n=5–10). In cellular mechanistic studies, benzbromarone hyperpolarized human ASM cells (n=9–12) and attenuated intracellular calcium flux from both the plasma membrane and sarcoplasmic reticulum (n=6–12). Conclusions TMEM16A antagonists work synergistically with β-agonists and through a novel pathway of interrupting ion flux both at the plasma membrane and sarcoplasmic reticulum to acutely relax human airway smooth muscle.

Emerging evidence indicates that hypnotic anesthetics affect immune function. Many anesthetics potentiate γ-aminobutyric acid A receptor (GABAR) activation, and these receptors are expressed on multiple subtypes of immune cells, providing a potential mechanistic link. Like immune cells, airway smooth muscle (ASM) cells also express GABARs, particularly isoforms containing α-subunits, and activation of these receptors leads to ASM relaxation. We sought to determine if GABAR signaling modulates the ASM contractile and inflammatory phenotype of a murine allergic asthma model utilizing GABAR α-subunit global knockout (KO; ) mice. Wild-type (WT) and KO mice were sensitized with house dust mite (HDM) antigen or exposed to PBS intranasally 5 days/wk for 3 wk. Ex vivo tracheal rings from HDM-sensitized WT and KO mice exhibited similar magnitudes of acetylcholine-induced contractile force and isoproterenol-induced relaxation ( = not significant; = 4). In contrast, in vivo airway resistance (flexiVent) was significantly increased in KO mice ( < 0.05, = 8). Moreover, the KO mice demonstrated increased eosinophilic lung infiltration ( < 0.05; = 4) and increased markers of lung T-cell activation/memory (CD62L low, CD44 high; < 0.01, = 4). In vitro, KO CD4 cells produced increased cytokines and exhibited increased proliferation after stimulation of the T-cell receptor as compared with WT CD4 cells. These data suggest that the GABAR α-subunit plays a role in immune cell function during allergic lung sensitization. Thus GABAR α-subunit-specific agonists have the therapeutic potential to treat asthma via two mechanisms: direct ASM relaxation and inhibition of airway inflammation.