Proximal tubule glucose efflux in the rat as a function of delivered load

The effect of altering the delivered load of glucose by either increasing the rate of perfusion of the initial glucose concentration on the absorptive flux of glucose was examined in rat proximal tubule using the in vivo microperfusion technique. The passive flux coefficient was determined to be 1.27 pmol.min^-1.mm.^-1.mM^-1 glucose concentration gradient. With initial glucose concentrations of either 11.1 or 22.2 mM, the glucose flux increased as the rate of perfusion was increased. At similar rates of perfusion, glucose flux was higher from the 22.2 mM glucose solution than from the 11.1 mM solution. Estimation of the driving force for passive glucose efflux indicates that the increase in glucose efflux when flow rate is increased cannot be accounted for by passive changes in efflux out of the proximal tubule. When large transepithelial glucose gradients are imposed across the tubule, passive glucose flux plays a more significant role, but cannot totally account for higher rates of glucose efflux observed with the perfusion solution containing 22.2 mM glucose. These results are considered in the light of the recent model of the flow dependency of nonelectrolyte absorption of Barfuss and Schafer.