DIFFERENTIATION OF ACTIVE AND PASSIVE COMPONENTS OF D-GLUCOSE TRANSPORT IN PROXIMAL TUBULE OF RAT KIDNEY

Both, the net and unidirectional transport of d-glucose across the proximal tubule of rat kidney were studied by the technique of continuous microperfusion under the condition of zero netflux of water and sodium chlorid. When the active transport component was completely abolished by 10-4 M of phlorizin a small passive component could be demonstrated. The passive component was also observed in the non-phlorizin-poisoned stat to be additive to the active transport. Vmax of the active d-glucose transport was 6×10-10 mol×cm-2×sec-1. The passive glucose flux is proportional to the concentration difference between perfusate and serum. There is no difference between data of influx and efflux measurements. From this it is concluded that the passive d-glucose flux is a simple diffusion process. The permeability coefficient for d-glucose (PG) calculated from 3 different sets of experiments is 1.7×10-5 cm×sec-1. Active and passive glucose transport component can cancel out each other at a transtubular concentration difference of 33 mmol/l. Changes of the specific activity in the perfusate were measured under three different conditions (d-glucose concentration lumen: > plasma, lumen = plasma, lumen < plasma). These changes were in agreement with our theoretical prediction using the above mentioned values of Vmax and PG for the calculation (see appendix). The permeability of d-glucose across the proximal tubules is small. Therefore, under normal free flow condition the passive transport component plays no role in the d-glucose absorption. However, with greatly increasing transtubular concentration difference of d-glucose, e.q. due to a low glomerular filtration rate and a high serum d-glucose concentration, the passive transport component may be the rate limiting factor for the net transport of d-glucose. © 1969 Springer-Verlag.