Phosphate transport by brushborder membranes from superficial and juxtamedullary cortex

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 (Na(o) > Na(i)) 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 [Δ + 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 < 0.05) higher apparent V(max) (mean ± SEM, nmoles Pi/0.5 min/mg/protein) for Pi in BBMV-SC (7.5 ± 1.57) compared with V(max) 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. In dogs fed LPD, the initial Na-dependent uptake of Pi was significantly (P < 0.05) higher in BBMV-SC (Δ + 170%) and in BBMV-JM (Δ + 71%) compared to corresponding BBMV-SC and BBMV-JM prepared from dogs fed HPD. The enhancement in rate of Na-dependent Pi uptake elicited by LPD was significantly (P < 0.05) greater in BBMV-SC (net increase 1878 ± 440 pmoles/0.5 min/mg protein) than in BBMV-JM (714 ± 128), showing that BBMV-SC undergo an adaptive increase in Pi transport capacity which is greater in BBMV-SC than in BBMV-JM. These results demonstrate that luminal BBMV from SC and JM cortex differ in their capacity for Na-dependent Pi transport in vitro and also in the adaptive changes elicited by phosphate deprivation. Such differences in luminal BBM may in part account for heterogeneity of proximal tubular phosphate reabsorption observed in vivo.