The reverse mode of the Na+/Ca2+ exchanger provides a source of Ca2+ for store refilling following agonist-induced Ca2+ mobilization

Asthma Research Group, Firestone Institute for Respiratory Health, St. Joseph's Healthcare, and Department of Medicine, McMaster University, Hamilton, Ontario, Canada Submitted 15 June 2006 ; accepted in final form 11 October 2006 Agonist-induced contraction of airway smooth muscle (ASM) can be triggered by an elevation in the intracellular Ca 2+ concentration, primarily through the release of Ca 2+ from the sarcoplasmic reticulum (SR). The refilling of the SR is integral for subsequent contractions. It has been suggested that Ca 2+ entry via store-operated cation (SOC) and receptor-operated cation channels may facilitate refilling of the SR. Indeed, depletion of the SR activates substantial inward SOC currents in ASM that are composed of both Ca 2+ and Na + . Accumulation of Na + within the cell may regulate Ca 2+ handling in ASM by forcing the Na + /Ca 2+ exchanger (NCX) into the reverse mode, leading to the influx of Ca 2+ from the extracellular domain. Since depletion of the SR activates substantial inward Na + current, it is conceivable that the reverse mode of the NCX may contribute to the intracellular Ca 2+ pool from which the SR is refilled. Indeed, successive contractions of bovine ASM, evoked by various agonists (ACh, histamine, 5-HT, caffeine) were significantly reduced upon removal of extracellular Na + ; whereas contractions evoked by KCl were unchanged by Na + depletion. Ouabain, a selective inhibitor of the Na + /K + pump, had no effect on the reductions observed under normal and zero-Na + conditions. KB-R7943, a selective inhibitor of the reverse mode of the NCX, significantly reduced successive contractions induced by all agonists without altering KCl responses. Furthermore, KB-R7943 abolished successive caffeine-induced Ca 2+ transients in single ASM cells. Together, these data suggest a role for the reverse mode of the NCX in refilling the SR in ASM following Ca 2+ mobilization. calcium handling; airway smooth muscle; sodium-calcium exchanger Address for reprint requests and other correspondence: L. J. Janssen, L-314, St. Joseph's Healthcare, 50 Charlton Ave. East, Hamilton, Ontario, Canada L8N 4A6 (e-mail: janssenl{at}mcmaster.ca )