Abstract
An exchange of Na+ for H+ has been proposed to explain why jejunal Na+ absorption is influenced by luminal concentrations of H+ and HCO3-. We studied the influence of luminal Na+ concentration on net HCO3- absorption by perfusing rat jejunum in vivo. When Na+ was omitted from the perfusion fluid, HCO3- absorption diminished by a fixed amount over a range of initial HCO3- concentrations of 15 to 80 mM. This change was not caused by alterations in transmural PD or direction of water movement. Because the rate of HCO3- absorption decreased as the luminal HCO3- concentration lessened, Na+-dependent HCO3- absorption accounted for an increasing percent of total absorption as the luminal concentration of HCO3- diminished.
The effect of Na+ on HCO3- absorption is mediated, at least in part, by H+ secretion, because luminal CO2 production (manifested by luminal PCO2) dimished as HCO3- absorption decreased. The changes in PCO2 are caused by reaction of H+ with HCO3- in the luminal fluid because luminal PCO2 is augmented by the presence of HCO3- and is diminished by addition of phosphate or Tris buffer.
Whether all H+ secretion requires luminal Na+ cannot be determined with these experimental techniques because mucosal permeability to Na+ and the unstirred layer make it impossible to eliminate Na+ ions from the luminal cell surface. The nature of the mechanism for HCO3- transport that is not sodium dependent remains to be determined.
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Selected References
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