Nucleotide regulation of paracellular Cl− permeability in natural rabbit airway epithelium

In this study, we demonstrate a novel regulatory mechanism by which mucosal nucleotides via P2Y receptors decrease paracellular Cl− ion permeability in natural rabbit airway epithelium (in addition to a decrease in active Na+ absorption). In contrast to primary cultures, the natural airway epithelium is a low-resistance epithelium, and an equivalent circuit model predicts that changes of more than ∼12% in transepithelial conductance (G t) must include an effect on paracellular conductance (G s). Mucosal P2Y receptor stimulation with uridine triphosphate (UTP; 200 μM) decreased G t by up to 50% (average, 24%) and simultaneously decreased the paracellular Cl− permeability (mucosa-to-serosa Cl− flux) by 16%, but had no effect on mannitol permeability. The G t response to UTP was mimicked and attenuated by ionomycin (1 μM), suggesting a dependence on Ca2+ i. Amiloride (100 μM) and hyperosmolarity (+75 mM mannitol) also decreased G t, indicating a role of cell shrinkage. Elevation of cAMP with forskolin (8 μM) or isoproterenol (10 μM) increased G t by 55 and 32%, and forskolin increased paracellular Cl− permeability by 37% without affecting mannitol permeability. The opposite effects of Ca2+ i and cAMP on G t suggest an autocrine nucleotide signaling sequence where P2Y-dependent decrease in passive, paracellular Cl− transport is succeeded by a reversion of this effect due to P1-receptor-stimulated cAMP formation by adenosine originating from a time-dependent breakdown of mucosal ATP.