Effect of D-glucose and L-glutamate on the kinetics of bicarbonate reabsorption in rat proximal tubules

RESUMO

The effect of D-glucose or L-glutamate on the kinetics of bicarbonate reabsorption in the early (EPT) and middle proximal tubule (MPT) was studied in vivo in Munich-Wistar rats by microperfusion techniques. The presence of 20 mM D-glucose in the lumen increased acidification half-time (t/2) (from 2.54 + or - 0.09s to 3.11 + or - 0.17 s in EPT and from 4.75 + or - 0.20 s to 6.04 + or - 0.49 s in MPT). Bicarbonate reabsorption 9JHCO-3) decreased as a consequence of this change (from 3.80 + or - 0.17 to 2.46 + or - 0.20 nmol cm-2s-1 in EPT and from 2.30 + or - 0.10 to 1.64 + or - 0.10 nmol cm-2s-1 in MPT). In this situation the basolateral membrane potential difference (BLMPD) in the MPT decereased from -41.6 + or - 2.47 to -29.7 + or - 2.45 mV and returned to control values after perfusion with D-glucose. The addition of 20 mM L-glutamate to the luminal perfusion caused an opposite effect, i.e., a decrease in t/2 (1.54 + or - 0.21 s in EPT and 3.25 + or - 0.26 s in MPT) and a consequent increase in JHCO-3 in both segments (5.09 + or - 0.58 nmol cm-2s-1 in EPT and 3.92 + or - 0.30 nmol cm-2s-1 in MPT). The BLMPD of MPT increased during L-glutamate perfusion (-39 + or - 2.48 mV in control and -52.0 + or - 2.72 mV with L-glutamate) and returned to control values after perfusion. The results observed may be the consequence of coupled Na+ - substrate transport altering the BLMPD with modifies the electrical driving force for coupled Na+ -HCO-3 efflux across the basolateral membrane. The alteration of this process may in turn affect intracellular pH, which is an important modulator of luminal Na+./H+ enchange. This possibility is supported by the observed depolarization of BLMPD by D-glucose (electroneutral molecule), and hyperpolarization by glutamate anion