EXTRACELLULAR ATP DIRECTLY GATES A CATION-SELECTIVE CHANNEL IN RABBITAIRWAY CILIATED EPITHELIAL-CELLS

1. A membrane conductance activated by extracellular ATP was identifie d and characterized in freshly dissociated rabbit airway ciliated cell s using the whole-cell and outside-out patch configurations of the pat ch-clamp technique. 2. In solutions designed to maximize currents thro ugh voltage-gated calcium channels, there were no indications of volta ge-gated Ba2+ currents. 3. Extracellular ATP (but not UTP or ADP) acti vated a membrane conductance which remained activated for several minu tes in the presence of ATP. The conductance was permeable to monovalen t and divalent cations with approximate relative permeabilities (P) fo r P-Ba: P-Cs : P-TEA of 4 : 1 : 0.1. Permeability to Cl- was negligibl e. 4. Including GDP-beta-S in the intracellular solution did not inhib it the effects of BTP, nor did GTP-gamma-S irreversibly activate the c onductance. 5. In outside-out membrane patches, with GDP-beta-S in the pipette solution, ATP activated ion channels which had a chord conduc tance of approximately 6 pS in symmetrical 150 mM CsCl solutions at -1 20 mV. 6. Suramin (100 mu M) inhibited the whole-cell currents activat ed by ATP (200 mu M) by 93 +/- 3 %. Similar effects of suramin were ob served on ATP-activated channels in outside-out membrane patches. 7. E xtracellular ATP had a priming action on the response to subsequent ex posure 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 concentra tion of ATP. The priming action of ATP was not inhibited by including GDP-beta-S in the intracellular solution. 8. The initial rate of activ ation increased with the concentration of ATP, and was voltage sensiti ve. 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 c onductance shifted from 210 +/- 30 to 14 +/- 4 mu M added ATP when CaC l2, in the extracellular solution was reduced from 1.58 to 0.01 mM. Th e Kill coefficient for ATP was 1.2 in both solutions. 10. These observ ations suggest that a form of ATP uncomplexed with divalent cations di rectly gates an ion channel (P2X receptor) in rabbit airway ciliated c ells, which serves as a pathway for Ca2+ influx. This purinoceptor may contribute to sustained ciliary activation during prolonged exposures to ATP.