Extracellular ATP directly gates a cation‐selective channel in rabbit airway ciliated epithelial cells

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Abstract
1 A membrane conductance activated by extracellular ATP was identified and characterized in freshly dissociated rabbit airway ciliated cells using the whole-cell and outside-out patch configurations of the patch-clamp technique. 2 In solutions designed to maximize currents through voltage-gated calcium channels, there were no indications of voltage-gated Ba2+ currents. 3 Extracellular ATP (but not UTP or ADP) activated a membrane conductance which remained activated for several minutes in the presence of ATP. The conductance was permeable to monovalent and divalent cations with approximate relative permeabilities (P) for PBa:PCs:PTEA of 4:1:0.1. Permeability to Cl− was negligible. 4 Including GDP-β-S in the intracellular solution did not inhibit the effects of ATP, nor did GTP-γ-S irreversibly activate the conductance. 5 In outside-out membrane patches, with GDP-β-S in the pipette solution, ATP activated ion channels which had a chord conductance of approximately 6 pS in symmetrical 150 mM CsCl solutions at -120 mV. 6 Suramin (100 μM) inhibited the whole-cell currents activated by ATP (200 μM) by 93 ± 3 %. Similar effects of suramin were observed on ATP-activated channels in outside-out membrane patches. 7 Extracellular ATP had a priming action on the response to subsequent exposure to ATP. At -40 mV, the time to half-maximal current activation (t½) was 46 ± 9 s during the first exposure to 200 μM ATP and decreased to 5 ± 3 s during a second exposure to the same concentration of ATP. The priming action of ATP was not inhibited by including GDP-β-S in the intracellular solution. 8 The initial rate of activation increased with the concentration of ATP, and was voltage sensitive. During the first exposure to 200 μM ATP, t½ at +40 mV was 4-fold longer than t½ at -40 mV. 9 Half-maximal activation of the conductance shifted from 210 ± 30 to 14 ± 4 μM added ATP when CaCl2 in the extracellular solution was reduced from 1.58 to 0.01 mM. The Hill coefficient for ATP was 1.2 in both solutions. 10 These observations suggest that a form of ATP uncomplexed with divalent cations directly gates an ion channel (P2X receptor) in rabbit airway ciliated cells, which serves as a pathway for Ca2+ influx. This purinoceptor may contribute to sustained ciliary activation during prolonged exposures to ATP.