Flux Ratios and Isotope Interaction in an Ion Exchange Membrane

Abstract

IN spite of the widespread use of radioactive tracers in the study of permeation of biological tissues, it is well known that the permeability coefficient derived from tracer exchange may differ appreciably from that derived from net flow^1–4, as, for example, for water flow in the frog skin and toad bladder^1–3 and sugar transport in red blood cells⁴. There is a similar ambiguity in the use of the “flux ratio” to evaluate the forces promoting transport. For simple passive flow across a membrane the original formulation^2,5,6 for the ratio of “unidirectional fluxes” was given as where X is the negative electrochemical potential difference of the test species, and R and T are the gas constant and absolute temperature, respectively. Deviations from this formula (that is an “abnormality” of the flux ratio) were considered to indicate additional forces such as solvent drag or active transport. Although the flux ratio equation has been useful in characterizing the forces which influence net transport, important exceptions have been noted^1,2,7,8. A clear example is the potassium flow in the poisoned squid axon, in which the flux ratio is markedly abnormal in spite of the apparent absence of either solvent drag or active transport⁷.