Purinergic regulation of the epithelial Na+ channel

1. The epithelial Na⁺ channel (ENaC) is a major conductive pathway that transports Na⁺ across the apical membrane of the distal nephron, the respiratory tract, the distal colon and the ducts of exocrine glands. The ENaC is regulated by hormonal and humoral factors, including extracellular nucleotides that are available from the epithelial cells themselves.
2. Extracellular nucleotides, via the P2Y2 receptors (P2Y2Rs) at the basolateral and apical membrane of the epithelia, trigger signalling systems that inhibit the activity of the ENaC and activate Ca²⁺ -dependent Cl⁻ secretion.
3. Recent data from our laboratory suggest that stimulation of the P2Y2Rs at the basolateral membrane inhibits ENaC activity by a signalling mechanism that involves Gβγ subunits freed from a pertussis toxin (PTX)-sensitive G-protein and phospholipase C (PLC) β4. A similar signalling mechanism is also partially responsible for inhibition of the ENaC during activation of apical P2Y2Rs.
4. Stimulation of apical P2Y2Rs also activates an additional signalling mechanism that inhibits the ENaC and involves the activated Gα subunit of a PTX-insensitive G-protein and activation of an unidentified PLC. The effect of this PTX-insensitive system requires the activity of the basolateral Na⁺/K⁺/2Cl⁻ cotransporter.