Regulation of ion transport via apical purinergic receptors in intact rabbit airway epithelium

We investigated purinergic receptors involved in ion transport regulation in the intact rabbit nasal airway epithelium. Stimulation of apical membrane P2Y receptors with ATP or UTP (200 M) induced transient increases in short-circuit current (I_sc) of 13 and 6% followed by sustained inhibitions to 8 and 17% below control level, respectively. Serosal application of nucleotides had no effect. The ATP-induced response appeared to involve additional activation of apical adenosine (P1) and P2X receptors. The inhibitory effect of ATP and UTP on I_sc was eliminated by pretreatment with amiloride (100 M), while the stimulatory effect was potentiated, indicating that ATP and UTP inhibit Na⁺ and stimulate Cl^– current. Ionomycin (1 M) induced responses similar to UTP and ATP and desensitized the epithelium to the nucleotides, indicating involvement of intracellular Ca²⁺ (Ca²⁺_i). Furthermore, ATP, UTP and ionomycin induced 21, 24, and 21% decreases, respectively, in transepithelial conductance. Measurements of unidirectional isotope fluxes showed a 39% decrease in the dominant net Na⁺ absorption in response to ATP, while the smaller net Cl^– secretion increased only insignificantly and unidirectional Cl^– fluxes decreased significantly. The results suggest that nucleotides released to the airway surface liquid exert an autocrine regulation of epithelial NaCl absorption mainly by inhibiting the amiloride-sensitive epithelial Na⁺ channel (ENaC) and paracellular anion conductance via a P2Y receptor-dependent increase in Ca²⁺_i, while stimulation of Cl^– secretion is of minor importance.