Close correlation between estrogen treatment and renal phosphate reabsorption capacity

To determine the influence of estrogen on the activity of renal proximal tu bular reabsorption of inorganic phosphate (Pi) in women, we examined the ch anges of the renal threshold phosphate concentration (also denoted as TmP/G FR), as well as the changes in the concentrations of mineral components in the circulation in two groups of women-one receiving hormone replacement th erapy (HRT) and one receiving gonadotropin-releasing hormone agonists (GnRH -a) therapy. We also examined the changes in the concentrations of serum PT H in the GnRH-a group. The patients in the HRT group were continuously trea ted with 0.625 mg conjugated equine estrogens plus 2.5 mg medroxyprogestero ne acetate per day. The patients in the GnRH-a group were treated with a mo nthly injection of 3.75 mg leuprolide acetate depot for 6 months. The value s of TmP/GFR decreased in all of the patients who received HRT. The mean pe rcentage change in TmP/GFR was -14.5% (range, -24.3% to -9.6%). In contrast , in all of the patients treated with GnRH-a, the values of TmP/GFR increas ed after 6 months of treatment (the mean percentage change was 28.5%; range , 18.2-78.3%) and returned to the preadministration level at 12 weeks after stopping therapy. In these patients, both the values of TmP/GFR and the co ncentrations of serum Pi correlated significantly with circulating estradio l levels (r = -0.767, P < 0.01 and r = -0.797, P < 0.01, respectively), but the concentrations of serum corrected calcium did not correlate. Moreover, in the same patients, the levels of serum intact PTH decreased significant ly (P < 0.05) after 6 months of treatment, but at 12 weeks after stopping t herapy the trends of these levels varied among individual patients. These r esults suggest that estrogen could act directly to suppress sodium-dependen t Pi reabsorption in the renal proximal tubules.