Sodium-Calcium Exchange in Intracellular Calcium Handling of Human Airway Smooth Muscle

Enhanced airway contractility following inflammation by cytokines such as tumor necrosis factor alpha (TNF alpha) or interleukin-13 (IL-13) involves increased intracellular Ca2+ ([Ca2+](i)) levels in airway smooth muscle (ASM). In ASM, plasma membrane Ca2+ fluxes form a key component of [Ca2+](i) regulation. There is now growing evidence that the bidirectional plasma membrane Na+/Ca2+ exchanger (NCX) contributes to ASM [Ca2+](i) regulation. In the present study, we examined NCX expression and function in human ASM cells under normal conditions, and following exposure to TNFa or IL-13. Western blot analysis showed significant expression of the NCX1 isoform, with increased NCX1 levels by both cytokines, effects blunted by inhibitors of nuclear factor NF-kappa B or mitogen-activated protein kinase. Cytokine-mediated increase in NCX1 involved enhanced transcription followed by protein synthesis. NCX2 and NCX3 remained undetectable even in cytokine-stimulated ASM. In fura-2 loaded human ASM cells, NCX-mediated inward Ca2+ exchange as well as outward exchange (measured as rates of change in [Ca2+](i)) was elicited by altering extracellular Na+ and Ca2+ levels. Contribution of NCX was verified by measuring [Na+](i) using the fluorescent Na+ indicator SBFI. NCX-mediated inward exchange was verified by demonstrating prevention of rising [Ca2+](i) or falling [Na+](i) in the presence of the NCX inhibitor KBR7943. Inward exchange-mode NCX was increased by both TNF beta and IL-13 to a greater extent than outward exchange. NCX siRNA transfection substantially blunted outward exchange and inward exchange modes. Finally, inhibition of NCX expression or function blunted peak [Ca2+](i) and rate of fall of [Ca2+](i) following histamine stimulation. These data suggest that NCX-mediated Ca2+ fluxes normally exist in human ASM (potentially contributing to rapid Ca2+ fluxes), and contribute to enhanced [Ca2+](i) regulation in airway inflammation.