ENERGETICS OF COUPLED NA+ AND CL- ENTRY INTO EPITHELIAL-CELLS OF BULLFROG SMALL-INTESTINE

Na+, K+ and Cl- concentrations (cji) and activities (aji), and mucosal membrane potentials (Em) were measured in epithelial cells of isolated bullfrog (Rana catesbeiana) small intestine. Segments of intestine were stripped of their external muscle layers, and bathed (at 25°C and pH 7.2) in oxygenated Ringer solutions containing 105 mM Na+ and Cl- and 5.4 mM K+. Na+ and K+ concentrations were determined by atomic absorption spectrometry and Cl- concentrations by conductometric titration following extraction of the dried tissue with 0.1 M HNO3. 14C-labelled inulin was used to determine extracellular volume. Em was measured with conventional open tip microelectrodes, aCli with solid-state Cl--selective silver microelectrodes and aNai and aKi with Na+- and K+-selective liquid ion-exchanger microelectrodes. The average Em recorded was -34 mV. cNai, cKi and cCli were 51, 105 and 52 mM. The corresponding values for aNai, aKi and aCli were 18, 80 and 33 mM. These results suggest that a large fraction of the cytoplasmic Na+ is 'bound' or sequestered in an osmotically inactive form, that all, or virtually all the cytoplasmic K+ behaves as if in free solution, and that there is probably some binding of cytoplasmic Cl-. aCli significantly exceeds the level corresponding to electrochemical equilibrium across the mucosal and baso-lateral cell membranes. Earlier studies showed that coupled mucosal entry of Na+ and Cl- is implicated in intracellular Cl- accumulation in this tissue. This study permitted estimation of the steady-state transapical Na+ and Cl- electrochemical potential differences (Δμ̄Na and Δμ̄Cl). Δμ̄Na (-7000 J · mol-1; cell minus mucosal medium) was energetically more than sufficient to account for Δμ̄Cl (1000-2000 J · mol-1). © 1979.