Ca²⁺-mediated agonists, including UTP, are being developed for therapeutic use in cystic fibrosis (CF) based on their ability to modulate alternative Cl conductances. As CF is also characterized by hyperabsorption of Na⁺, we determined the effect of mucosal UTP on transepithelial Na⁺ transport in primary cultures of human bronchial epithelia (HBE). In symmetrical NaCl, UTP induced an initial increase in short-circuit current (I_sc) followed by a sustained inhibition. To differentiate between effects on Na⁺ absorption and Cl secretion, I_sc was measured in the absence of mucosal and serosal Cl (I_Na). Again, mucosal UTP induced an initial increase and then a sustained decrease that reduced amiloride-sensitive I_Na by 73%. The Ca²⁺-dependent agonists histamine, bradykinin, serosal UTP, and thapsigargin similarly induced sustained inhibition (62-84%) of I_Na. Mucosal UTP induced similar sustained inhibition (half-maximal inhibitory concentration 296 nM) of I_Na in primary cultures of human CF airway homozygous for the F508 mutation. BAPTA-AM blunted UTP-dependent inhibition of I_Na, but inhibitors of protein kinase C (PKC) and phospholipase A₂ had no effect. Indeed, direct activation of PKC by phorbol 12-myristate 13-acetate failed to inhibit Na⁺ absorption. Apyrase, a tri- and diphosphatase, did not reverse inhibitory effects of UTP on I_Na, suggesting a long-term inhibitory effect of UTP that is independent of receptor occupancy. After establishment of a mucosa-to-serosa K⁺ concentration gradient and permeabilization of the mucosal membrane with nystatin, mucosal UTP induced an initial increase in K⁺ current followed by a sustained inhibition. We conclude that increasing cellular Ca²⁺ induces a long-term inhibition of transepithelial Na⁺ transport across normal and CF HBE at least partly due to downregulation of a basolateral membrane K⁺ conductance. Thus UTP may have a dual therapeutic effect in CF airway: 1) stimulation of a Cl secretory response and 2) inhibition of Na⁺ transport.

cystic fibrosis; cystic fibrosis transmembrane conductance regulator; epithelial sodium channel; uridine 5'-triphosphate; human airway

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