1. Department of Chemistry, Ben-Gurion University of the Negev, Beer-Sheva 84105, Israel
2. Department of Life Sciences and The Zlotowski Center for Neuroscience, Ben-Gurion University of the Negev, Beer-Sheva 84105, Israel
3. These authors contributed equally to this work
4. Present address: Max-Planck Institut für medizinische Forschung Abteilung Zellphysiologie, Jahnstrasse 29, D-69120, Heidelberg, Germany

Correspondence to: Shai D. Silberberg⁴ Correspondence and requests for materials should be addressed to S.D.S. (e-mail: Email: silber@bgumail.bgu.ac.il).

The mucociliary system is responsible for clearing inhaled particles and pathogens from the airways. This important task is performed by the beating of cilia and the consequent movement of mucus from the lungs to the upper airways¹,². Because ciliary motility is enhanced by elevated intracellular calcium concentrations, inhibition of calcium influx could lead to disease by jeopardizing mucociliary clearance. Several hormones and neurotransmitters stimulate ciliary motility, one of the most potent of which is extracellular ATP (ATP_o)¹, which acts by releasing calcium ions from internal stores and by activating calcium influx^{3, 4, 5}. Here we show that, in airway ciliated cells, extracellular sodium ions (Na_o ⁺) specifically and competitively inhibit an ATP_o-gated channel that is permeable to calcium ions, and thereby attenuate ATP_o-induced ciliary motility. Our finding points to a physiological role for Na_o ⁺ in ciliary function, and indicates that mucociliary clearance might be improved in respiratory disorders such as chronic bronchitis and cystic fibrosis by decreasing the sodium concentration of the airway surface fluid in which the cilia are bathed.