Mechanism of bicarbonate absorption and its relationship to sodium transport in the human jejunum
Authors: Leslie A. Turnberg, John S. Fordtran, N. W. Carter, Floyd C. Rector
Journal: Journal of Clinical Investigation (1970)

Using a constant perfusion technique, sodium and bicarbonate absorption was studied in human subjects. The following observations were made on sodium absorption from saline solution: (a) the rate of sodium absorption is markedly influenced by bulk water flow, (b) when net water flow is zero, sodium absorption is zero if there are no concentration gradients between plasma and lumen that favor net NaCl diffusion; and (c) the PD between abraded skin and jejunal lumen is near zero when saline is perfused and does not change with partial substitution of sulfate or bicarbonate for chloride. Based on these observations, we conclude that sodium absorption from saline is entirely passive in the human jejunum. On the other hand, in the presence of bicarbonate sodium is absorbed actively against electrochemical gradients. The mechanism of the link between bicarbonate and sodium absorption was studied in normal subjects and in 11 patients with pernicious anemia; the latter were chosen because they do not secrete gastric acid which can react with bicarbonate in the jejunal lumen. We observed that bicarbonate absorption (a) occurs against steep electrochemical gradients, (b) does not generate a potential difference between abraded skin and jejunal lumen, (c) is inhibited by acetazolamide, and (d) generates a high CO2 tension in jejunal fluid. These observations suggest that bicarbonate absorption is mediated by active hydrogen secretion, rather than by bicarbonate ion transport per se, and that the link between sodium and bicarbonate transport is best explained by a sodium-hydrogen exchange process.In this study of bicarbonate and sodium absorption in the intestine, absorption in a 30-cm segment of intestine was studied by the Ingelfinger triple-lumen perfusion system, which involves perfusion of test solutions into the intestine and sampling of gut contents 10 and 40 cm beyond the infusion marker. Human subjects were used. Observations made from these experiments on the mechanism of bicarbonate absorption and its relationship to sodium transport in the jejunum from saline solutions include: 1) the rate of sodium absorption is influenced greatly by bulk water flow; 2) when net water flow is zero, sodium absorption is zero in the absence of concentration gradients betwee plasma and lumen; and 3) the potential difference between abraded skin and jejunal lumen is near zero when saline is perfused and does not change when sulfate or bicarbonate is partially substituted for the chloride. It is concluded that sodium absorption from saline is entirely passive in the human jejunum; in the presence of bicarbonate, sodium is actively absorbed against electrochemical gradients. This study also looked at the mechanism of the link between bicarbonate and sodium absorption. Normal subjects and 11 patients with pernicious anemia were studied. Bicarbonate absorption was found to 1) occur against steep electrochemical gradients; 2) not generate a potential difference between abraded skin and jejunal lumen; 3) be inhibited by acetazolamide; and 4) generate a high carbon dioxide tension in jejunal. These observations led to the conclusion that bicarbonate absorption is mediated by active hydrogen secretion rather than by bicarbonate ion transport per se, making the best explanation for the link between sodium and bicarbonate transport a sodium-hydrogen exchange process.