Superoxide Anions Inhibit Intracellular Calcium Response in Porcine Airway Smooth Muscle Cells

Background Superoxide anions (O-2(-)) have multiple effects on pulmonary parenchyma altering cell proliferation, cellular metabolism, and airway smooth muscle (ASM) contraction. Intracellular calcium ([Ca2+](i)) concentration plays a significant role in the regulation of ASM contraction, relaxation, proliferation, and gene expression. Objective We investigated the effects of O-2(-) on agonist-stimulated changes in [Ca2+](i) in ASM cells. Design/Methods Fura-2 AM-loaded, freshly isolated porcine ASM (PASM) cells were used to examine [Ca2+](i) release in response to acetylcholine (ACh), histamine, endothelin, caffeine, and thapsigargin (TPG) in the presence or absence of extracellular Ca2+. Results Exposure of PASM cells to xanthine and xanthine oxidase (X + XO) resulted in a time-dependent generation of O-2(-), inhibited by superoxide dismutase (SOD). Preincubating PASM cells with X + XO for 15- or 45-minute inhibited net [Ca2+](i) responses to ACh, histamine, caffeine, and TPG compared with control cells. Pretreating PASM cells with SOD for 30 minutes mitigated the inhibitory effect of X + XO treatment on ACh-induced Ca2+ elevation suggesting role of O-2(-). X + XO treatment also inhibited caffeine- and TPG-induced Ca2+ elevation suggesting effect of O-2(-) on [Ca2+](i) release and reuptake mechanisms. Conclusion Superoxide attenuates [Ca2+](i) release, reuptake, and may interfere with physiological functions of ASM cells.