Pore properties and pharmacological features of the P2X receptor channel in airway ciliated cells

Airway ciliated cells express an ATP-gated P2X receptor channel of unknown subunit composition (P2X_cilia) which is modulated by Na⁺ and by long exposures to ATP. P2X_cilia was investigated by recording currents from freshly dissociated rabbit airway ciliated cells with the patch-clamp technique in the whole-cell configuration. During the initial continuous exposure to extracellular ATP, P2X_cilia currents gradually increase in magnitude (priming), yet the permeability to N-methyl-D-glucamine (NMDG) does not change, indicating that priming does not arise from a progressive change in pore diameter. Na⁺, which readily permeates P2X_cilia receptor channels, was found to inhibit the channel extracellular to the electric field. The rank order of permeability to various monovalent cations is: Li⁺, Na⁺, K⁺, Rb⁺, Cs⁺, NMDG⁺ and TEA⁺, with a relative permeability of 1.35, 1.0, 0.99, 0.91, 0.79, 0.19 and 0.10, respectively. The rank order for the alkali cations follows an Eisenman series XI for a high-strength field site. Ca²⁺has been estimated to be 7-fold more permeant than Na⁺. The rise in [Ca²⁺]_i in ciliated cells, induced by the activation of P2X_cilia, is largely inhibited by either Brilliant Blue G or KN-62, indicating that P2X₇ may be a part of P2X_cilia. P2X_cilia is augmented by Zn²⁺ and by ivermectin, and P2X₄ receptor protein is detected by immunolabelling at the basal half of the cilia, strongly suggesting that P2X₄ is a component of P2X_cilia receptor channels. Taken together, these results suggest that P2X_cilia is either assembled from P2X₄ and P2X₇ subunits, or formed from modified P2X₄ subunits.