Apical and basolateral ATP stimulates tracheal epithelial chloride secretion via multiple purinergic receptors

Stimulation of Cl^- secretion across the airway epithelium by ATP or UTP as agonists has therapeutic implications for cystic fibrosis. Our results demonstrate that ATP stimulates Cl^- secretion in rat tracheal epithelial cell monolayers in primary culture from the apical or basolateral side of the monolayer. Multiple types of ATP-sensitive Cl^- conductances in intact monolayers were elucidated through inhibition by Cl^- channel-blocking drugs. Multiple Cl^- conductances stimulated by ATP and adenosine 3',5'-cyclic monophosphate (cAMP) (tested for comparison) were also deciphered more specifically by nystatin permeabilization of the basolateral membrane, subsequent imposition of symmetrical Cl^-, I^-, or Br^- solutions to test halide permselectivity, inhibition by Cl^- channel-blocking drugs, and construction of current-voltage plots to study time and voltage dependence of the currents. Apical ATP stimulates Cl^- secretion through P(2U) (or P(2Y2)) purinergic receptors via both intracellular Ca(i)/²⁺ (Ca(i)/²⁺)-dependent and Ca(i)/²⁺-independent signaling pathways by opening outwardly rectifying Cl^- channels (ORCCs), cystic fibrosis transmembrane conductance regulator (CFTR) Cl^- channels, and Ca(i)/²⁺-dependent Cl^- channels. Basolateral ATP stimulates Cl^- secretion via a combination of receptor subtypes (P(2T) and P(2U)) or a novel type of receptor (P(2Y3)), independent of Ca(i)/²⁺ or cAMP signaling by opening only CFTR channels. cAMP also stimulated multiple types of Cl^- conductances, consistent with simultaneous activation of CFTR and ORCCs. Together, these results suggest that ATP as an agonist stimulates Cl^- secretion via multiple purinergic receptors and multiple signal transduction pathways activated in different membrane domains of tracheal epithelia.