The interpretation of tracer fluxes in terms of membrane structure

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Abstract
Very early in the era of isotopic tracers it became apparent that tracer fluxes did not always come out as one had predicted from classical measurements of transport by osmosis or diffusion. Thus Hevesy, Hofer & Krogh (1935) determined the water permeability of frog skin from osmotic uptake of water as well as from measurements of its permeability to heavy water. They found the first method to give values some 4–5 times higher than the latter. Although this result at first seemed disappointing, it was soon realized that the two types of experiments provided more information about the nature of osmosis through real membranes than had been available before. Thus it was shown that the results might be explained in terms of interaction between water molecules moving in bulk, possibly through pores in the membrane (Ussing, 1952; Koefoed-Johnsen & Ussing, 1953; Andersen & Ussing, 1957).