Na and K activity coefficients in bile and bile salts determined by glass electrodes

Mathematical formulations for transmembrane potential differences are expressed in terms of ionic activities rather than ionic concentrations, and require knowledge of the activity coefficients of a given ionic species in mixed solutions. Cation-selective glass electrodes have been used to determine sodium and potassium activity coefficients in pure bile salt solutions and in native bile, relative to standard NaCl or KCl solutions. Comparison was made with osmotic coefficients determined by freezing-point depression. Both sodium and potassium activity coefficients in bile salt solutions and in bile were lower than those for NaCl or KCl solutions at corresponding concentrations, with potassium coefficients being lower than those for sodium. These derived activity coefficients have been used experimentally in in vivo and in vitro gall bladder preparations with close agreement between observed potentials and those predicted by the Hodgkin-Katz equation.