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

Abstract

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. Cation-selective ion channels activated by extracellular ATP (P2X receptor channels) are widely distributed in electrically excitable cells such as neurones, and in non-excitable cells such as immune cells and epithelial cells (North, 2002). Seven subunits of the mammalian P2X receptor family (P2X 1 to P2X 7 ) have been cloned, and all of them can form functional homomeric receptor channels in heterologous expression systems (North, 2002;Jones et al. 2004). Six functional heteromeric receptor channels (P2X 1 /P2X 2 , P2X 1 /P2X 4 , P2X 1 /P2X 5 , P2X 2 /P2X 3 , P2X 2 /P2X 6 and P2X 4 /P2X 6 ) have also been characterized (Torres et al. 1998;Le et al. 1998Le et al. , 1999King et al. 2000;Brown et al. 2002;Nicke et al. 2005). Co-immunoprecipitation assays performed on HEK293 cells co-expressing different P2X subunits suggest that additional hetero-oligomeric assemblies are possible among the different P2X receptor subunits (Torres et al. 1999). The pharmacological and biophysical properties of native P2X receptor channels often deviate from those of heterologously expressed P2X receptor channels. This suggests that native P2X receptor channels might be processed differently or that they are heteromeric channels of yet uncharacterized combinations of P2X subunits.Airway ciliated cells express a P2X receptor channel (P2X cilia ) of unknown composition. This channel is interesting both from a physiological and from a biophysical standpoint because of its modulation by both ATP and Na + . P2X cilia receptor currents gradually increase over tens of seconds during the first exposure to ATP whereas subsequent applications of ATP activate the current significantly faster (Korngreen et al. 1998). This priming phenomenon can be observed for several minutes after the initial application of ATP, and thus ATP can be considered both an agonist and a long-term modulator of the P2X cilia receptor channel. P2X cilia receptor channels are also modulated by Na + (Ma et al. 1999). The effect

Morphine enhances IL-1β release through toll-like receptor 4-mediated endocytic pathway in microglia

“…P2X7-dependent IL-1b release plays a critical role in LPS-activated microglia, but in inactivated microglial cells, P2X7 receptor shows little functional activity [15,16]. Microglia are co-expression of P2X4R with P2X7 receptor, and recent evidence has indicated a structural interaction between P2X4 and P2X7 receptors, and the expression of P2X4R leads to facilitation of P2X7-dependent release of IL-1β [17,18]. Chronic morphine enhances microglial P2X4R signaling which makes a crucial contribution to pathologically enhanced pain processing [19,20].…”

Sodium-Dependent Regulation of Renal Amiloride-Sensitive Currents by Apical P2 Receptors

“…23,24 Given that we found P2X 4 subunits to be the predominant apically expressed P2XR in CD PC and our [Na ϩ ] ext levels were relatively high compared with in vivo micropuncture studies of the rat distal nephron, 17 When extracellular Na ϩ concentrations were changed from 145 to 50 mM, maximal amplitudes of DIDS-insensitive currents evoked by ATP, 2meSATP, and ATP␥S (100 M, V h ϭ Ϫ60 mV) were significantly increased, by 11 Ϯ 4, 32 Ϯ 9, and 36 Ϯ 5%, respectively (P Ͻ 0.05; n ϭ 5). In contrast, the amplitude of UTP-evoked currents did not change significantly.…”

ENaC, renal sodium excretion and extracellular ATP

“…We had failed to observe any potentiation of recombinant ENaC activity, but we did not reduce extracellular sodium to low levels (50 mM). However, it is notable that the probability of channel opening and the magnitude of inward currents carried by P2X 4 -like receptors in airway epithelia can be enhanced when extracellular sodium concentrations are very low [41,42]. How such enhancements of P2X 4 -like receptor function might lead to a potentiation of renal ENaC activity is still to be resolved, but a hint comes from a series of interconnected findings.…”