Figure 4.

Proximal tubule phosphate transport. (A) Concepts of renal inorganic phosphate (P_i) homeostasis by the proximal tubule. The flux of filtered and reabsorbed P_i is plotted against plasma phosphate concentration; the difference between the two yields the rate of excretion of P_i. There are a number of terms used to quantify the proximal tubule’s P_i reabsorption at the whole organism level. Fractional excretion of P_i (FEP) and tubular reabsorption of P_i (TRP) sum to unity (FEP=1−TRP). The maximal tubular reabsorptive capacity of P_i (TmP in units of mass/time) refers to the saturating transepithelial flux of P_i that the tubule can mount and is equal to the difference between filtered and absorbed phosphate when the filtered load is higher than TmP. The plasma concentration threshold at which P_i starts to appear in the urine is TmP/GFR (in units of mass/volume). (B) Cell model of proximal tubule P_i transport. Three apical transporters mediate P_i entry with different preferred valence of P_i, stoichiometry of Na⁺, electrogenicity, and pH gating. The affinities for Na⁺ are all approximately 30–50 mM but are much higher for phosphate (0.1, 0.07, and 0.025 mM for NaP_i-Ila, NaP_i-Ilc, and PiT-2, respectively). Distribution in the proximal tubule segments (S1, S2, S3). Basolateral P_i exit occurs via unknown mechanisms. Apical Na-coupled P_i transport is inhibited in acidosis by alteration in luminal substrate, directly gating of the transporter by pH, and decreased apical NaP_i transporters as described in Figure 3B.