d(-)3-Hydroxybutyrate cotransport with Na in rat renal brush border membrane vesicles

Abstract

Which are the driving forces ford(-)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 (andK _i=K _o) 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 (pH_i=pH_o=6.4 vs. 7.4) or inwardly directed H⁺ gradients (pH_o/pH_i=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 mM¹⁴C-HB was significantly increased by 5–10 mM intravesicular HB under Na-equilibrated conditions. Thus, the rate of translocation of the free carrier appears to limit HB influx through the cotransport system.