Flow-sensitive K+-coupled ATP Secretion Modulates Activity of the Epithelial Na+ Channel in the Distal Nephron

The epithelial Na+ channel (ENaC) in the aldosterone-sensitive distal nephron (ASDN) is under tonic inhibition by a local purinergic signaling system responding to changes in dietary sodium intake. Normal BKCa channel function is required for flow-sensitive ATP secretion in the ASDN. We tested here whether ATP secreted through connexin channels in a coupled manner with K+ efflux through BKCa channels is required for inhibitory purinergic regulation of ENaC in response to increases in sodium intake. Inhibition of connexin channels relieves purinergic inhibition of ENaC. Deletion of the BK-beta 4 regulatory subunit, which is required for normal BKCa channel function and flow-sensitive ATP secretion in the ASDN, suppresses increases in urinary ATP in response to increases in sodium intake. As a consequence, ENaC activity, particularly in the presence of high sodium intake, is inappropriately elevated in BK-beta 4 nul lmice. ENaC in BK-beta 4 null mice, however, responds normally to exogenous ATP, indicating that increases in activity do not result from end-organ resistance but rather from lowered urinary ATP. Consistent with this, disruption of purinergic regulation increases ENaC activity in wild type but not BK-beta 4 null mice. Consequently, sodium excretion is impaired in BK-beta 4 null mice. These results demonstrate that the ATP secreted in the ASDN in a BKCa channel-dependent manner is physiologically available for purinergic inhibition of ENaC in response to changes in sodium homeostasis. Impaired sodium excretion resulting form loss of normal purinergic regulation of ENaC in BK-beta 4 null mice likely contributes to their elevated blood pressure.