Rat proximal tubule D-glucose transport as a function of concentration,flow, and radius

Summary From earlier microperfusion studies ofD-glucose and water reabsorption in the proximal surface nephron of the rat,D-glucose was found to be removed by a saturable carrier and by an apparent coupling with net fluid reabsorption. Equations appropriate to describe this system were developed. They incorporated carrier-mediatedD-glucose transport, net water transport, and water-coupled solute transport. Water reabsorption was assumed to be constant either per unit surface area, or per unit volume of the nephron, and the rate of carrier-transportedD-glucose was assumed constant per unit length, per unit surface area, or per unit volume of the tubule. The possibility thatD-glucose could be reabsorbed via two carrier systems was also explored analytically. It was observed from this treatment that the fraction ofD-glucose reabsorbed would change if the perfusion rate was changed. With an increase in perfusion rate, a decrease in reabsorbed fraction was seen which indicates that if net fluid reabsorption is proportional to volume, carrier-mediated sugar transport is proportional to surface area or length of the tubule. From these relationshipsJ _max, the maximal rate of carrier-transported sugar, was calculated to be 3.3×10⁻¹⁰M/cm² sec, a value comparable to that reported from other laboratories. The results of this analysis are compatible with the data obtained both by micropuncture experiments during free flow and by glucose clearance studies until theT _mG is reached. The possibility that theT _mG obtained in clearance studies is due to a decrease in the fraction of fluid reabsorbed in the proximal tubule or to a second saturable carrier is discussed. It is observed that, in either case, if load is increased by increasing the glomerular filtration rate, noT _mG would be reached, or stated another way, one would predict from the analysis thatT _mG would be proportional to glomerular filtration rate. Partially supported by NIH-AM-10779-02 and “Deutsche Forschungsgemeinschaft”. Partially presented at the XXIV International Congress of Physiological Societies, Washington, D.C., 1968. PHS post doctoral fellow 1-F2-AM-37056-01 and Dr. Henry C. and Bertha H. Buswell Fellow Dr. Henry C. and Bertha H. Buswell Fellow