Free-flow reabsorption of glucose, sodium, osmoles and water in rat proximal convoluted tubule

Abstract

1. Reabsorption of glucose, sodium, total solute (osmoles) and water in the rat proximal tubule (pars convoluta) were studied by free-flow micropuncture at normal (saline-infused), suppressed (saline with phlorizin) and elevated (glucose infusion) glucose reabsorption rates. 2. Phlorizin completely inhibited net glucose reabsorption, approximately halved reabsorption of sodium, total solutes and water, and reduced single nephron glomerular filtration rate (SNGFR). 3. In saline and glucose-infused groups, there were no significant differences between SNGFR nor between reabsorptions (fractional and absolute) of either sodium, total solute or water, which were uniformly distributed along segments assessible to micropuncture. 4. Glucose reabsorptive capacity existed along the entire pars convoluta, with highest reabsorptive rates in convolutions closest to the glomerulus (in saline-infused rats, 90% fractional reabsorption at 2 mm, over 95% at end pars convoluta; in glucose-infused rats, 55 and 90%, respectively). 5. In saline and glucose infused rats, a significant correlation existed between net glucose and sodium reabsorption, but the regression slopes differed and correlations became non-significant when the reabsorptive fluxes were factored by SNGFR. 6. For all groups, the majority of tubular fluid (TF) concentrations of osmoles and sodium were lower than those in plasma (over-all mean TFosm)Posm = 0.973 +/- 0.004, P less than 0.001; TFNa /PNa = 0.964 +/- 0.005, P less than 0.001). 7. Correspondingly, calculated osmolal and sodium concentrations in the reabsorbate were greater than those in plasma, and were significantly correlated with distance to puncture site with maximal values in the most proximal convolutions (for osmolality, approximately +79 m-osmole kg-1 water at 1 mm).