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

Abstract

The epithelial sodium channel (ENaC) plays a major role in the regulation of sodium balance and BP by controlling Na ϩ reabsorption along the renal distal tubule and collecting duct (CD). ENaC activity is affected by extracellular nucleotides acting on P2 receptors (P2R); however, there remain uncertainties over the P2R subtype(s) involved, the molecular mechanism(s) responsible, and their physiologic role. This study investigated the relationship between apical P2R and ENaC activity by assessing the effects of P2R agonists on amiloride-sensitive current in the rat CD. Using whole-cell patch clamp of principal cells of split-open CD from Na ϩ -restricted rats, in combination with immunohistochemistry and real-time PCR, we found that activation of metabotropic P2R (most likely the P2Y 2 and/or 4 subtype), via phospholipase C, inhibited ENaC activity. In addition, activation of ionotropic P2R (most likely the P2X 4 and/or 4/6 subtype), via phosphatidylinositol-3 kinase, either inhibited or potentiated ENaC activity, depending on the extracellular Na ϩ concentration; therefore, it is proposed that P2X 4 and/or 4/6 receptors might function as apical Na ϩ sensors responsible for local regulation of ENaC activity in the CD and could thereby help to regulate Na ϩ balance and systemic BP.

Purinergic signalling in the kidney in health and disease

“…Activation of P2Y 2 and P2Y 4 receptors resulted in PLCdependent inhibition of ENaC activity. Interestingly, activation of P2X4 and P2X4/6 receptors caused an inhibition of ENaC activity when luminal concentrations of sodium were high (145 mM); however, when sodium concentrations were reduced to more physiological levels (50 mM), activation of the receptors potentiated ENaC activity [412,413]. In accordance with a P2X4 component to purinergic regulation of sodium transport is the demonstration that in the 'distal-like' cell line, Xenopus A6 cells, activation of basolateral P2X4-like receptors resulted in increased apical membrane insertion of ENaC.…”

ENaC, renal sodium excretion and extracellular ATP

“…Using a split-tubule preparation of rat distal CDs, our own experiments reveal that PCs possess both apical P2Y and P2X receptors, and that each receptor subclass regulates ENaC activity [7]. We have located at least two types of P2Y receptor (P2Y 2 and P2Y 4 subtypes) on the apical border, identified immunohistochemically in the cell membrane and by real time-PCR within the CD.…”

“…It is now widely accepted that released ATP can act as a local hormone on both sides of the kidney tubule, where this molecule has ample choices for molecular targets. The presence of almost all of the molecularly identified subtypes of ionotropic (P2X) and metabotropic (P2Y) receptors for ATP has been demonstrated in the apical and/or basolateral membranes of native epithelial cells and immortalised epithelial cell lines [7,17,[27][28][29]. Ionotropic P2X receptors are represented by homomeric assemblies (P2X 1,2,3,4,5,6,7 ) and heteromeric assemblies (P2X 1/2 , P2X 1/4 , P2X 1/5 , P2X 2/3 , P2X 2/6 , P2X 4/6 , P2X 4/7 ).…”

“…The localisation of P2X receptor subtypes has been reported as highly regional, occasionally overlapping and sometimes discrete, but never ubiquitous. Metabotropic P2Y receptors are also represented by many of the known subtypes (chiefly P2Y 1 , P2Y 2 , P2Y 4 , P2Y 6 , P2Y 11 , but not yet P2Y 12 , P2Y 13 and P2Y 14 ), which again may be found in overlapping regional distributions, but never ubiquitously distributed [7,17,28,29].…”

“…Extracellular ATP-based regulation of sodium excretion is not only found in the kidney but also occurs in other hollow organs where sodium ions can be reclaimed (lungs, gastrointestinal tract, pancreatic duct, hepatic and biliary ducts and eccrine sweat ducts). The pharmacological manipulation of P2R populations linked to ENaC may have therapeutic potential in helping to correct a number of disease states, such as cystic fibrosis [1,2], polycystic kidney disease [3] (and see article by Turner et al [4] in this Special Issue) and vascular hypertension associated with salt-retention [5][6][7]. Here, we review this new body of work on P2R/ENaC interactions in renal epithelia and address the role of ATP regulatory mechanisms in controlling the sodium balance in the extracellular fluid.…”