Amiloride sensitive and insensitive sodium pathways and the cellular sodium transport pool of colonic epithelium in rats.

Methods for measurement of the epithelial Na transport pool (Nat) using epithelial scrapings and for analysing the transepithelial ionic fluxes of rat distal colon in vivo into transcellular and paracellular components have been used to study the amiloride sensitive (a.s.) and amiloride insensitive (a.i.) transcellular pathways in relation to variations of Nat. In the Na-replete normal rats, substitution of SO4 for Cl in the lumen approximately halved the Na transported by a.i. pathways and reduced Nat by about 60%, but in the Na-depleted rats, substitution of SO4 did not affect either the Na transported by a.s. pathways or Nat. The value of Nat for normal rats, with 150 mM-NaCl in the lumen, was 6-7 nmol Na mg-1 dry weight (corresponding to about 2-3 mmol kg-1 cell water) and fell by about 60% when lumen Na concentration was reduced to 50 mM. Its turnover half-time was 0.6 min. Nat was about threefold greater in the Na-depleted than in the normal rats but became undetectable when amiloride was in the lumen. Amiloride did not affect Nat in normal rats. We conclude that the increased Na absorption in Na depletion depended on substitution of a.s. for a.i. apical membrane pathways allowing increased Na entry into the epithelial cells so expanding Nat and stimulating the basolateral Na pumps.