Effects of extracellular volume expansion on the tubular reabsorption of glucose. A microinjection study

Clearance and intratubular injections of (14C) glucose were performed in glucose loaded rats, during control (C) and extracellular fluid volume expansion (VE) to 10% of body weight. VE resulted in a significant decrease in hematocrit from 47.50 +/- 1.06 to 38.80 +/- 1.14% and plasma protein from 6.23 +/- 0.25 to 4.13 +/- 0.21 gm/100 ml. Glomerular filtration rate (GFR) increased by 51% from 1.06 +/- 0.07 to 1.60 +/- 0.35 ml/min. Fractional excretion of sodium increased significantly from 0.42 +/- 0.07 to 12.58 +/- 1.25%. Maximal glucose reabsorption (TmG) was unchanged from 3.47 +/- 0.42 to 3.29 +/- 0.41 mg/min. However, TmG/GFR decreased significantly from 3.14 +/- 0.22 to 1.94 +/- 0.21 mg/ml GFR. As compared to C, VE resulted in a significant increase in (14C) glucose recovery after injection into the early and late proximal tubules, from 63 +/- 3 to 81 +/- 2% to 88 +/- 1% respectively. After distal tubular injections (14C) glucose recovery was complete in both C and VE; early distal injection 97 +/- 1 vs 98 +/- 1%, late distal injection 98 +/- 1 vs 99 +/- 1%. These results indicate an inhibitory effect of VE on fractional glucose reabsorption in the superficial nephron. There is no evidence for glucose reabsorption in the superficial distal nephron during C and VE.