title: 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)
mesh_terms: D000818:Animals; D001693:Biological Transport, Active; D004032:Diet; D004285:Dogs; D007672:Kidney Cortex / Q000378:metabolism* / Q000648:ultrastructure; D007687:Kidney Tubules, Proximal / Q000378:metabolism*; D008297:Male; D008871:Microvilli / Q000378:metabolism; D009399:Nephrons / Q000378:metabolism; D010710:Phosphates / Q000378:metabolism*; D010758:Phosphorus / Q000008:administration & dosage; D012964:Sodium / Q000378:metabolism
chemical_list: D010710:Phosphates; D010758:Phosphorus; D012964:Sodium