Estrogens are widely used for contraception and osteoporosis prevention. The aim of the present study was to investigate the effect of 17 beta-estradiol on calcium (Ca(2+)) transport by the nephron luminal membranes, independently of any other Ca(2+)-regulating hormones. Proximal and distal tubules of rabbit kidneys were incubated with 17 beta-estradiol or the carrier for various periods of time, and the luminal membranes of these tubules were purified and vesiculated. Ca(2+) uptake by membrane vesicles was measured using the Millipore filtration technique. Incubation of proximal tubules with the hormone did not influence Ca(2+) uptake by the luminal membranes. In contrast, incubation of distal tubules with 10(-8) M 17 beta-estradiol for 30 min decreased the initial uptake of 0.5 mM Ca(2+) from 0.34+/-0.04 (s.e.m. ) to 0.17+/-0.04 pmol/microg per 5 s (P<0.05). In the presence of 100 mM Na(+), 0.5 mM Ca(2+) uptake was strongly diminished and the effect of 17 beta-estradiol disappeared (0.17+/-0.01 and 0.21+/-0.07 pmol/microg per 5 s in vesicles from the control and treated tubules). Direct incubation of the membranes with 17 beta-estradiol, however, failed to show any influence of the hormone on Ca(2+) transport. The action of 17 beta-estradiol was dose-dependent, with a half-maximal effect at approximately 10(-9) M. Ca(2+) uptake by the distal tubule membranes presents dual kinetics. 17 beta-Estradiol decreased the V(max) value of the high-affinity component from 0.42+/-0.02 to 0.31+/-0.03 pmol/microg per 10 s (P<0.02). In contrast with the effect of the hormone on Ca(2+) 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 Ca(2+) reabsorption by the high-affinity Ca(2+) channels of the distal luminal membranes, and enhances Na(+) transport by the membranes from proximal and distal nephrons.

Clinical and animal studies have shown that treatment with angiotensin-converting enzyme (ACE) inhibitors or angiotensin II (Ang II) receptor antagonists slows the progression of nephropathy in diabetes, indicating that Ang II plays an important role in its development. We have reported previously that insulin inhibits the stimulatory effect of high glucose levels on angiotensinogen (ANG) gene expression in rat immortalized renal proximal tubular cells (IRPTCs) via the mitogen-activated protein kinase (p44/42 MAPK) signal transduction pathway. We hypothesize that the suppressive action of insulin on ANG gene expression might be attenuated in renal proximal tubular cells (RPTCs) of rats with established diabetes. Two groups of male adult Wistar rats were studied: controls and streptozotocin (STZ)-induced diabetic rats at 2, 4, 8 and 12 weeks post-STZ administration. Kidney proximal tubules were isolated and cultured in either normal glucose (i.e. 5 mM) or high glucose (i.e. 25 mM) medium to determine the inhibitory effect of insulin on ANG gene expression. Immunoreactive rat ANG (IR-rANG) in culture media and cellular ANG mRNA were measured by a specific radioimmunoassay and reverse transcription-polymerase chain reaction assay respectively. Activation of the p44/42 MAPK signal transduction pathway in rat RPTCs was evaluated by p44/42 MAPK phosphorylation employing a PhosphoPlus p44/42 MAPK antibody kit. Insulin (10(-7) M) inhibited the stimulatory effect of high glucose levels on IR-rANG secretion and ANG gene expression and increased p44/42 MAPK phosphorylation in normal rat RPTCs. In contrast, it failed to affect these parameters in diabetic rat RPTCs. In conclusion, our studies demonstrate that hyperglycaemia induces insulin resistance on ANG gene expression in diabetic rat RPTCs by altering the MAPK signal transduction pathway.