Effect of glucose on the kinetics of bicarbonate reabsorption in the early and middle proximal tubule

ABSTRACT

The kinetics of bicarbonate reabsorption in the early (EPT) and middle proximal tubule (MPT) was studied in Munich Wistar rats by means of microperfusion techniques. The early proximal tubule (EPT) had a higher capacity of reabsorbing bicarbonate than the middle segment. The calculated bicarbonate flow (JHCO3) was 4.47 nmol s**-1 cm**-2 int the EPT and 2.30 nmol s**-1 cm**-2 in the MPT and the half time of injected bicarbonate (t/2) was less in the EPT (2.24 s) than in the MPT (5.20 s). The presence of 20 mM glucose in the lumen led to a reduction in the velocity of tubular acidification in the EPT (3.11 s) and caused a decrease in JHCO3 in both segments (2.46 nmol s**-1 cm**-2 in the EPT and 1.64 nmols s**-1 cm**-2 in the MPT). These alterations may be the result either of lumen-to-cell sodium gradient dissipation or electrical changes induced by sodium-glucose cotransport that may depolarize the basolateral membrane leading to intracellular alkalinization. This effect may then impair the Na +/H+ exchanger, thus decreasing bicarbonate reabsorption