Phosphate transport by brushborder membranes from superficial and juxtamedullary cortex

Abstract

In vivo studies indicate that the extent of phosphate (Pi) reabsorption differs in proximal tubules of superficial (SC) and juxtamedullary (JM) nephrons. Since Na-gradient (Nao greater than Nai) dependent uptake of Pi by the luminal brushborder membrane (BBM) may be the rate-determining step in proximal tubular reabsorption, we studied this transport system in brushborder membrane vesicles (BBMV) prepared from SC and JM renal cortex of dogs fed either a low phosphorus diet (LPD, 0.07% Pi) or high phosphorus diet (HPD, 1.2% Pi). In the initial uphill phase (that is, "overshoot"), the rate of Na-gradient dependent uptake of Pi was significantly greater [delta + 35%] in BBMV from the SC cortex (BBMV-SC) than in BBMV from the JM cortex (BBMV-JM) of the dogs fed LPD. Higher Na-dependent Pi uptake was due to significantly (P less than 0.05) higher apparent Vmax (mean +/- SEM, nmoles Pi/0.5 min/mg protein) for Pi in BBMV-SC (7.5 +/- 1.57) compared with Vmax in BBMV-JM (6.05 +/- 1.74). Higher transport of Pi in BBMV-SC compared with BBMV-JM of dogs fed LPD was a difference relatively specific for the Na-dependent Pi uptake system; Na+ independent uptake of Pi and Na-dependent uptake of D-glucose were lower in BBMV-SC than in BBMV-JM. The size of BBMV or rate of Na+ uptake did not differ between BBMV-SC and BBMV-JM. The Na-gradient dependent uptake of Pi was no different between BBMV-SC and BBMV-JM from dogs stabilized on HPD.(ABSTRACT TRUNCATED AT 250 WORDS)