Effect of Luminal Sodium Concentration on Bicarbonate Absorption in Rat Jejunum

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 P_{CO2) 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.}