Estrogens are widely used for contraception and osteoporosis prevention. The aim of the present study was to investigate the effect of 17β-estradiol on calcium (Ca2+) transport by the nephron luminal membranes, independently of any other Ca2+-regulating hormones. Proximal and distal tubules of rabbit kidneys were incubated with 17β-estradiol or the carrier for various periods of time, and the luminal membranes of these tubules were purified and vesiculated. Ca2+ uptake by membrane vesicles was measured using the Millipore filtration technique. Incubation of proximal tubules with the hormone did not influence Ca2+ uptake by the luminal membranes. In contrast, incubation of distal tubules with 10 8 M 17βestradiol for 30 min decreased the initial uptake of 0· 5 mM Ca2+ from 0· 340· 04 (...) to 0· 170· 04 pmol/µg per 5s (P< 0· 05). In the presence of 100mM Na+, 0· 5 mM Ca2+ uptake was strongly diminished and the effect of 17β-estradiol disappeared (0· 170· 01 and

0· 210· 07 pmol/µg per 5 s in vesicles from the control and treated tubules). Direct incubation of the membranes with 17β-estradiol, however, failed to show any influence of the hormone on Ca2+ transport. The action of 17βestradiol was dose-dependent, with a half-maximal effect at approximately 10 9 M. Ca2+ uptake by the distal tubule membranes presents dual kinetics. 17β-Estradiol decreased the Vmax value of the high-affinity component from 0· 420· 02 to 0· 310· 03 pmol/µg per 10 s (P< 0· 02). In contrast with the effect of the hormone on Ca2+ transport, estradiol increased Na+ uptake by both the proximal and distal tubule luminal membranes. In conclusion, incubation of proximal and distal tubules with estrogen decreases Ca2+ reabsorption by the high-affinity Ca2+ channels of the distal luminal membranes, and enhances Na+ transport by the membranes from proximal and distal nephrons.