The Mechanism of Water Transport in Na +-Coupled Glucose Transporters Expressed in Xenopus Oocytes

Abstract

It is well established that cotransporters transport water, but how they do it is debated. Two mechanisms have been suggested: cotransport of water along with the nonaqueous substrates, and osmosis where the cotransporters simply act as water channels. In a recent article, Charron et al. investigated this question for the Na+-coupled glucose transporter expressed in Xenopus laevis oocytes. They focused upon the posttransport period, i.e., when external sugar was removed after a period of inwardly directed Na+ and sugar transport, and argued, on partly empirical grounds, that the data could be explained exclusively by osmosis. In this Comment to the Editor, we have reinterpreted these data by a numerical model of the oocyte which reflects the physical transport processes taking place: cotransport and/or osmosis across the membrane and diffusion and mass balance of the nonaqueous substrates in the cytoplasm. We find that the experiments and analysis as performed in Charron et al. are inadequate with respect to distinguishing between the cotransport hypothesis and the osmotic hypothesis for water transport in cotransporters.