To assess the mechanism by which estrogen replacement therapy (EKT) enhances renal calcium conservation in perimenopausal women, we studied 18 normal women in early postmenopause before and after 6 months of ERT (cyclic treatment with transdermal estradiol at 100 μg/day and medroxyprogesterone acetate at 10 mg/day for the first 12 days of each cycle). ITie changes after ERT were: serum ionized calcium and ultrafiltrable calcium, no change; serum intact PTH, 38.2% increase (P < 0.0001); serum 1,25-dihydroxyvitamin D, 23.8% increase (P < 0.0001); urinary calcium excretion, 33.3% decrease (P < 0.001); and deoxypyridinoline (a marker for bone resorption), 19.5% decrease (P < 0.0001). Also, ERT increased tubular reabsorption of calcium (TRCa; 97.6% ± 0.2% to 98.7% ± 0.1%; P < 0.0001), and this increase correlated with that in serum PTH (r = 0.49; P < 0.05). After the infusion of human Kl'H-(1-34), the TRCa maximum was greater after ERT than at baseline (99.4% ± 0.1% vs. 99.0% ± 0.1%; P < 0.0001), resulting in decreased calcium excretion (0.9 ± 0.20 vs. 1.43 ± 0.20 ¿imol/dL glomerular filtrate; P < 0.001). Thus, in early postmenopause, the major mechanism of increased renal calcium conservation after ERT is an increase in TRCa due to an increase in serum PTH because of estrogen-induced inhibition of bone resorption. However, ERT also may directly increase the TRCa maximum in response to PTH.
ASJC Scopus subject areas
- Endocrinology, Diabetes and Metabolism
- Clinical Biochemistry
- Biochemistry, medical