Pfliigers Arch (1991) 419:249--255 003167689100172Y Journal of Physiology 9 Springer-Verlag1991 Glucose transporters do not serve as water channels in renal and intestinal epithelia Jacqueline A. Dempster, Alfred N. van Hock, Maarten D. de Jong, and Card H. van Os Department of Physiology, University of Nijmegen, P.O. Box 9101, NL-6500 HB Nijmegen, The Netherlands Received February 12, 1991/Received after revision April 29, 1991/Accepted July 8, 1991 Abstract. Glucose carriers have been shown to serve as water channels in macrophages and in oocytes injected with messenger ribonucleic acid (mRNA) encoding the glucose carrier protein (Fischbarg et al. [4, 5]). The con- tribution, therefore, of glucose carriers to osmotic water permeability (Pf) in renal and intestinal epithelial cells was investigated. Pf of brush border membrane vesicles (BBMVs) and of basolateral membrane vesicles (BLMVs) was studied using stopped-flow spectrophotometry. Os- motic shrinkage of renal vesicles exhibited fast and slow components at 4 ~ and 37 ~ The fast component could be inhibited by HgC12 or dimethylsulphoxide (DMSO) at these temperatures, whereas the slow component was in- hibited only at 4~ Osmotic shrinkage of intestinal BBMVs and BLMVs was homogeneous at 4 ~ and 37 ~ and was slightly inhibitable by HgCI2 or DMSO at 4 ~ but not 37 ~ In both tissues, vesicle uptake of glucose was sensitive to HgC12, but not to DMSO. Phlorizin and phloretin inhibited D-glucose uptake in BBMVs and BLMVs respectively, but had no significant effect on Pf. In membrane vesicles of kidney origin, Pf was tenfold higher than in membranes from intestine. This difference was not reflected by the phlorizin- and phloretin-sensitive D-glucose uptakes. Our study concludes that glucose transporters do not serve as water channels in kidney or intestine. Although membrane proteins contribute slight- ly to Pf at 4~ this contribution is insignificant at 37 ~ A membrane protein serving specifically as a water channel could only be demonstrated in renal cortical membranes. Key words: Water channels - Glucose transporters - Membrane vesicles - Rat kidney cortex - Rat intestine Introduction Water transport is believed to occur by permeation through the lipid bilayer and is sufficient for water ex- Offprint requests to: J.A. Dempster change in most cells [3]. In some membranes, however, osmotic water permeability (Pf) is faster than explained by lipid-mediated transport and this has led to investiga- tions of a specific water channel. It is now fairly estab- lished that, in brush border and basolateral membranes of the kidney proximal tubule cells, such a channel is pro- teic in nature. This is illustrated in renal membranes by high Pf valves [10, 12, 21, 22], low activation energies [10, 12, 16], and inhibition by mercuric reagents by one order of magnitude [10, 12, 19, 21, 22], and is compatible with properties found in red blood ceils of various species [15], vasopressin-treated toad bladder [2] and cortical collecting tubules [9]. With this knowledge, several attempts have been made to identify the membrane protein responsible for water transport through channels. Water transport via urea car- riers, ion channels or other membrane proteins in red blood cells has been dismissed, although Band 3 remains a controversial contender, as reviewed by Macey [15]. An- other contender has been the glucose transporter follow- ing studies in macrophages and in oocytes injected with messenger ribonucleic acid (mRNA) encoding the brain/Hep G2, adult skeletal muscle/adipocyte, or liver forms of the glucose carrier protein [4, 5]. Brush border membrane Na+-dependent and basolateral membrane Na+-independent glucose transport have been demon- strated in both kidney [17, 20] and intestine [13]. Howev- er, Pf values have been shown to be severalfold higher in brush border and basolateral membrane vesicles (BBMVs and BLMVs respectively) from kidney than from intestine [10, 121. This study investigates the proposed correlation be- tween Pf and glucose transporters in BBMV and BLMV from rat renal cortical, jejunal and ileal epithelia. The ef- fects of inhibitors of water transport, namely HgC12 and dimethylsulphoxide (DMSO) [12], and inhibitors of glu- cose transport on water transport and solute uptake were studied to evaluate the contribution of typical membrane carrier proteins to Pf in these epithelial membranes.