Phosphate uptake by superficial and deep nephron brush border membranes

Abstract

Dietary phosphate restriction and acute parathyroidectomy in rat are known to be associated with a selective increase in phosphate uptake by renal cortical brush border membranes (BBM). Conversely, phosphate loading and parathyroid hormone (PTH) administration result in a decrease of this uptake. In the present study, we investigated whether the response of the membrane to these various stimuli implies similar or different modifications of the kinetic properties of this membrane, whether these modifications affect one or both of the two systems of phosphate transport previously described, whether both superficial and deep nephron populations are involved, and whether the two stimuli: dietary phosphate, and parathyroid activity, are additive or not. Kinetic studies of phosphate (PO₄) uptake by BBM vesicles were performed in seven groups of rats: control (N), acutely thyroparathyroidectomized (TPTX), PTH loaded (PTH), phosphate loaded (P⁺), phosphate depleted (P⁻) phosphate depleted with acute thyroparathyroidectomy (P⁻TPTX), and phosphate depleted-PTH loaded (P⁻PTH). In each of these experimental conditions, superficial and deep nephrons were investigated. Results indicate that 1. BBM from deep nephrons present a greater capacity for PO₄ transport than those from superficial nephrons; 2. Whereas a dual system of PO₄ uptake is observed in superficial BBM, deep BBM present only one single system; 3. Phosphate in the diet influences PO₄ uptake by BBM to a greater extent in the deep than in the superficial nephrons; 4. PTH status on the contrary, equally influences both populations; 5. TPTX does not significantly enhance PO₄ uptake in phosphate depleted rats; however, PTH loading curtails this uptake; 6. In the deep BBM neither the PTH status nor the phosphate content of the diet modify the apparentK_m. In the superficial BBM, the apparentK_m of the high affinity system (low substrate concentrations) varies with the PTH activity.