Regulation of nephron acidification by corticosteroids

The present paper reviews work from our laboratories evaluating the importance of adrenal cortical hormones in acidification by proximal and cortical distal tubules. Proximal acidification was determined by stationary microperfusion, and measurement of bicarbonate reabsorption using luminal pH determination was performed with H+-ion-sensitive microelectrodes. Rats were adrenalectomized (ADX) 48 h before the experiments, and corticosteroids (aldosterone (A), corticosterone (B), and 18-OH corticosterone (18-OH-B)) were injected intramuscularly 100 and 40 min before the experiments. In ADX rats stationary pH increased significantly to 7.03 as compared to sham-operated rats (6.78). Bicarbonate reabsorption decreased from 2.65 ± 0.18 in sham-operated rats to 0.50 ± 0.07 nmol cm-2 s-1 after ADX. The administration of the three hormones stimulated proximal tubule acidification, reaching, however, only 47.2% of the sham values in aldosterone-treated rats. Distal nephron acidification was studied by measuring urine minus blood pCO2 differences (U-B pCO2) in bicarbonate-loaded rats treated as above. This pCO2 difference is used as a measure of the distal nephron ability to secrete H+ ions into an alkaline urine. U-B pCO2 decreased significantly from 39.9 ± 1.26 to 11.9 ± 1.99 mmHg in ADX rats. When corticosteroids were given to ADX rats before the experiment, U-B pCO2 increased significantly, but reached control levels only when aldosterone (two 3-µg doses per rat) plus corticosterone (220 µg) were given together. In order to control for the effect of aldosterone on distal transepithelial potential difference one group of rats was treated with amiloride, which blocks distal sodium channels. Amiloride-treated rats still showed a significant reduction in U-B pCO2 after ADX. Only corticosterone and 18-OH-B but not aldosterone increased U-B pCO2 back to the levels of sham-operated rats. These results show that corticosteroids stimulate renal tubule acidification both in proximal and distal nephrons and provide some clues about the mechanism of action of these steroid

MDCK cells express serotonin-regulable 11β-hydroxysteroid dehydrogenase type 2

Prior to this work, we found that adrenal as well as extra-adrenal factors activate the response of renal l 11β-hydroxysteroid dehydrogenase 2 to stressful situations. These results -showing ways through which the organism hinders the pathological occupation of mineralocorticoid receptors by glucocorticoids leading to sodium retention and hypertension- prompted the present study on the nature of the above-mentioned extra-adrenal factors. Serotonin was chosen because of its properties as a widely distributed neurohormone, known to interact with glucocorticoids at many sites, also exhibiting increased levels and effects under stressful situations. We studied serotonin effects on 11β-hydroxysteroid dehydrogenase 2 activity in a cell line derived from distal nephron polarized-epithelium, employing 3H-corticosterone as substrate. The end-product, 3H-11- dehydrocorticosterone was separated from the substrate by HPLC and quantified. Serotonin stimulated 11β-hydroxysteroid dehydrogenase 2 activity only at 2nM and 25pM, the magnitude of the response depending also on substrate concentration. The stimulation was blocked by the specific inhibitors methiothepin and ketanserin. We postulate that the organism partially prevents renal mineralocorticoid receptor occupancy by glucocorticoids, circulating at enhanced levels under stressful situations, through serotonin-mediated catabolic regulation of the 11β-hydroxysteroid dehydrogenase 2 activity. Given many, mostly positive, interactions between both hormones, this might eventually pave the way to studies on a new regulatory axis.Fil:Zallocchi, M.L. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina.Fil:Calvo, J.C. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina.Fil:Matkovic', L.B. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina

Modification of an essential amino group in the mineralocorticoid receptor evidences a differential conformational change of the receptor protein upon binding of antagonists, natural agonists and the synthetic agonist 11,19-oxidoprogesterone

The alkylation of amino groups of the mineralocorticoid receptor (MR) with pyridoxal 5′-phosphate or 2,4,6-trinitrobenzenesulphonate (TNBS) under controlled conditions modifies only one lysyl residue, which accounts for a 70% inhibition of steroid binding capacity. The Kd of aldosterone for MR is not affected by the treatment, but the total number of binding sites is greatly decreased. The modified receptor is capable of dynamically conserving its association with the hsp90-based heterocomplex. Importantly, the binding of natural agonists protects the hormone binding capacity of the MR from the inactivating action of alkylating agents. In contrast, antagonistic steroids are totally incapable of providing such protection. Like the antagonistic ligands, and despite its potent mineralocorticoid biological effect, the sole MR specific synthetic agonist known to date, 11,19-oxidoprogesterone (11-OP), shows no protective effect upon treatment of the MR with pyridoxal 5′-phosphate or TNBS. Limited digestion of the MR with α-chymotrypsin generates a 34 kDa fragment, which becomes totally resistant to digestion upon binding of natural agonists, but not upon binding of antagonists. Interestingly, the synthetic 21-deoxypregnanesteroid 11-OP exhibits an intermediate pattern of proteolytic degradation, suggesting that the conformational change generated in the MR is not equivalent to that induced by antagonists or natural agonists. We conclude that in the first steps of activation, the MR changes its conformation upon binding of the ligand. However, the nature of this conformational change depends on the nature of the ligand. The experimental evidence shown in this work suggests that a single lysyl group can determine the hormone specificity of the MR. © 2002 Elsevier Science B.V. All rights reserved.Fil:Ghini, A.A. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina.Fil:Burton, G. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina