Salt- and angiotensin II-dependent variations in amiloride-sensitive rectal potential difference in mice

In the rectum and distal nephron, sodium reabsorption is mediated by the am iloride-sensitive epithelial sodium channel (ENaC). The ENaC-mediated sodiu m transport is electrogenic and creates an amiloride-sensitive transepithel ial potential difference (PD). 2. We have evaluated the salt- and angiotensin (Ang)II-dependent variations in amiloride-sensitive rectal PD in mice and assessed their relationship w ith renal sodium handling. 3. Rectal PD was measured in vivo in mice maintained on a medium-, low- or high-sodium diet. On a medium-salt diet, the mean (+/-SEM) amiloride-sensit ive PD was larger in the afternoon than in the morning (-26.1+/-0.9 and -11 .2+/-0.7mV, respectively; P = 0.001), indicating a circadian cyclicity. Rec tal PD increased on a low-sodium diet and decreased on a high-sodium diet. 4. Amiloride-sensitive rectal PD correlated significantly with the urinary Na+/K+ ratio (P < 0.001) and with sodium reabsorption in the distal nephron as measured by the lithium clearance technique (P < 0.001). 5. In mice treated with an AngII AT(1) receptor antagonist, amiloride-sensi tive rectal PD was increased in the afternoon compared with controls (32.8/-2.0 vs -24.4+/-0.9, respectively; P < 0.001). 6. At high doses, AngII decreased the amiloride-sensitive rectal PD and thi s effect was blunted by an AT(1) receptor antagonist. 7. These results show the presence of a salt-dependent daily cyclicity of s odium transport in the mouse rectum that follows circadian changes in sodiu m handling in the distal nephron, Angiotensin II appears to modulate this d iurnal pattern of rectal amiloride-sensitive sodium transport.