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

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 P-Ba: P-Cs : P-TEA of 4 : 1 : 0.1. Permeability to Cl- was negligible. 4. Including GDP-beta-S in the intracellular solution did not inhibit the effects of BTP, nor did GTP-gamma-S irreversibly activate the conductance. 5. In outside-out membrane patches, with GDP-beta-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 mu M) inhibited the whole-cell currents activated by ATP (200 mu M) 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 (t(1/2)) was 46 +/- 9 a during the first exposure to 200 mu M: 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-beta-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 mu M ATP, t(1/2) at +40 mV was 4-fold longer than t(1/2) at -40 mV. 9. Half-maximal activation of the conductance shifted from 210 +/- 30 to 14 +/- 4 mu M added ATP when CaCl2, in the extracellular solution was reduced from 1.58 to 0.01 mM. The Kill 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.