Targeting the restricted α-subunit repertoire of airway smooth muscle GABA_A receptors augments airway smooth muscle relaxation

Abstract

The prevalence of asthma has taken on pandemic proportions. Since this disease predisposes patients to severe acute airway constriction, novel mechanisms capable of promoting airway smooth muscle relaxation would be clinically valuable. We have recently demonstrated that activation of endogenous airway smooth muscle GABAA receptors potentiates ␤-adrenoceptor-mediated relaxation, and molecular analysis of airway smooth muscle reveals that the ␣-subunit component of these GABAA receptors is limited to the ␣4-and ␣5-subunits. We questioned whether ligands with selective affinity for these GABAA receptors could promote relaxation of airway smooth muscle. RT-PCR analysis of GABAA receptor subunits was performed on RNA isolated by laser capture microdissection from human and guinea pig airway smooth muscle. Membrane potential and chloride-mediated current were measured in response to GABAA subunit-selective agonists in cultured human airway smooth muscle cells. Functional relaxation of precontracted guinea pig tracheal rings was assessed in the absence and presence of the ␣4-subunit-selective GABAA receptor agonists: gaboxadol, taurine, and a novel 8-methoxy imidazobenzodiazepine (CM-D-45). Only messenger RNA encoding the ␣4-and ␣5-GABAA receptor subunits was identified in RNA isolated by laser capture dissection from guinea pig and human airway smooth muscle tissues. Activation of airway smooth muscle GABAA receptors with agonists selective for these subunits resulted in appropriate membrane potential changes and chloride currents and promoted relaxation of airway smooth muscle. In conclusion, selective subunit targeting of endogenous airway smooth muscle-specific GABAA receptors may represent a novel therapeutic option for patients in severe bronchospasm.

Airway smooth muscle in airway reactivity and remodeling: what have we learned?

“…Furthermore, the role of chloride regulation in ASM is not well understood and may be important in ASM contractility vs. relaxation (130). For example, a recent study showed that GABA, acting via the ionotropic GABA-A receptors can potentiate ␤ 2 ARmediated relaxation but in fact causes membrane depolarization involving chloride-mediated currents (78 ] i to, not only remain elevated, but also contribute to the remodeling process. Surprisingly, there are little to no data on the calmodulin-dependent plasma membrane Ca 2ϩ ATPase (PMCA; a major efflux mechanism in other cell types) beyond its localization to caveolae (52).…”

Active Components of Ginger Potentiate β-Agonist–Induced Relaxation of Airway Smooth Muscle by Modulating Cytoskeletal Regulatory Proteins

“…Immortalized and primary human ASM cell lines were prepared as described previously (23,24) and grown in phenol red-free Dulbecco's modified Eagle's medium/F12 media (GIBCO, Grand Island, NY) with 10% fetal bovine serum and antibiotics. , solid squares), 6-gingerol (100 mM, solid triangles), 8-gingerol (100 mM, open circles), or 6-shogaol (100 mM, open triangles) was added concurrently with the 300 pM concentration of isoproterenol.…”

Effects of Ginger and Its Constituents on Airway Smooth Muscle Relaxation and Calcium Regulation

“…Immortalized and primary human ASM cell lines were prepared as described previously (17,18) and grown in phenol red-free Dulbecco's Modified Eagle's Medium/F12 media (GIBCO, Grand Island, NY), with 10% FBS and antibiotics.…”