D(-)3-hydroxybutyrate cotransport with Na in rat renal brush border membrane vesicles

Which are the driving forces for D(-)3-hydroxybutyrate (HB) transport in rat renal brush border membranes (RBB)? Sodium, even in the absence of gradients, accelerates the unidirectional (1-5 s) flux of HB into rat RBB vesicles. Valinomycin (and Ki = Ko) does not significantly alter the NaCl gradient driven HB influx. Thus, the Na-dependent HB influx is driven by the chemical Na+ gradient but it is not driven by changes in the transmembrane electrical potential. Indeed, in valinomycin-treated membranes, vesicle-inside more negative potentials (K-gluconate in-Na-gluconate out) sufficient to accelerate Na-glucose cotransport, did not stimulate HB influx, in the presence of inwardly directed Na+ gradients, and did not significantly inhibit when in the absence of Na+. Thus, cotransport of HB with Na in rat RBB membranes does not involve the net transfer of positive charge and the passive conductance of this membrane for HB- is not large. However, vesicle inside more negative potentials (induced by inwardly directed NaNO3 gradients or by outwardly directed K+ gradients and valinomycin in the presence of inwardly directed Na+ gradients) inhibited HB influx, suggesting that another potential sensitive mechanism, perhaps redistribution of intramembrane charges, may influence HB influx. Acidification (pHi = pHo = 6.4 vs. 7.4) or inwardly directed H+ gradients (pHo/pHi = 6.4/7.4) did not alter HB influx, in the absence of Na+. Thus there is no evidence for a H+ driven HB influx. HB influx is significantly inhibited by high (100 mEq/l) trans concentration of Na+. Also, influx of 2.25 mM 14C-HB was significantly increased by 5-10 mM intravesicular HB under Na-equilibrated conditions.(ABSTRACT TRUNCATED AT 250 WORDS)