Transport properties of an in situ jejunal reservoir in dogs

To evaluate partial or total replacement of renal function using gut, we measured in vivo transport of nitrogen metabolites, electrolytes, and water into a jejunal segment configured as a continent reservoir in the dog. Reservoir contents were sampled and analyzed at serial time intervals during a 3-h period after instillation of solution containing (in mM) 40 NaCl, 10 NaHCO₃, 220 mannitol, pH 8.5, without or with added urease. At 10 min postinstillation, the amount of urea in the solution without added urease was 3-5 times greater than in the presence of added urease, but accumulation of NH₄/⁺ was 14-21 times greater in the solution containing added urease, giving a luminal NH₄/⁺ concentration up to 10,000 times that of plasma. In the absence of urease, HCO₃/^- concentration fell to 0, and pH declined to 6 at 3 h; in the presence of urease, HCO₃/^- concentration was 4.5 mM, and pH was 7.8 at 3 h. We conclude 1) urea is secreted by the reservoir; 2) H⁺ is secreted and/or formed in the reservoir; 3) in the presence of urease, urea hydrolyzed to NH₃ is converted to NH₄/⁺ by H⁺ and trapped in the lumen; and 4) in the urease solution, H⁺ binding by NH₃ preserves luminal HCO₃/^- , maintaining the initial pH. Thus the continent jejunal reservoir may supplement or replace impaired renal function.