Abstract
In the present work we studied the effects of phloretin on Na +-dependent d-glucose uptake by brush border membrane vesicles isolated from rabbit small intestine. (1) Phloretin had no inhibitory effect on Na +-dependent d-glucose uptake in the presence of equilibrated Na +, although it inhibited the process in the presence of a Na + gradient. Phlorizin inhibited Na +-dependent d-glucose uptake both in the presence and in the absence of a Na + gradient. Phloretin and phlorizin competed with each other for the inhibition of Na +energized d-glucose uptake. These results indicate that phloretin has no direct interaction with the Na +-dependent d-glucose transporter per se, though phloretin and phlorizin may have a common step(s) in their inhibition mechanisms. (2) Phloretin, but not phlorizin, was found to increase the equilibrium level of d-glucose uptake whether Na + was present or not. The increase in the equilibrium level was due to phloretin-induced binding of d-glucose to the brush border membrane, but not due to an increase in the intravesicular space. It seems to occur by a mechanism different from that by which phloretin inhibits Na +energized d-glucose uptake, because the equilibrium level of d-glucose uptake is increased by the ionized form of phloretin but Na +-energized d-glucose uptake is inhibited by the unionized form.
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