| Abstract: |
We have reported previously that in the presence of an osmotic gradient, facilitative glucose transporters (GLUTs) act as a transmembrane pathway for water flow. Here, we find evidence that they also allow water passage in the absence of an osmotic gradient. We applied the linear diffusion technique to measure the diffusional permeability (P(d)) of tritiated water (H-3-H2O) through plasma membranes of J774 murine macrophage-like cells. Untreated cells had a P(d) of 30.9 +/- 1.8 mum/s; the inhibitors of facilitative glucose transport cytochalasin B (10 muM) and phloretin (20 muM) reduced that value to 15.3 +/- 1.8 (50%) and 11.0 +/- 0.7 (62%) mum/s, respectively. In contrast, no significant effect on P(d) was observed in cells treated with dihydrocytochalasin B (P(d) = 28.4 +/- 1.5 mum/s). pCMBS (3 mM) inhibited glucose uptake by greater than 95%, and H-3-H2O diffusion by approximately 30% (P(d) = 22.9 +/- 1.5 mum/s). The combination of cytochalasin B plus pCMBS reduced P(d) by about 87% (P(d) = 3.9 +/- 0.3 mum/s). Moreover, 1 mM pCMBS did not affect the osmotic water permeability in Xenopus laevis oocytes expressing the brain/erythroid form of facilitative glucose transporters (GLUT1). These results indicate for the first time that about half of the total P(d) of J774 cells may be accounted for by water passage across GLUTs. Hence, they highlight the multifunctional properties of these transporters serving as conduits for both water and glucose. Our results also suggest for the first time that pCMBS blocks glucose transport without affecting water permeation through GLUTs. Lastly, because pCMBS decreases the P(d) of J774 cells, this suggests the presence in their plasma membranes of another protein(s) exhibiting water channel properties. |
