The effect of the antimineralocorticoid RU 28318 on aldosterone biosynthesis in vitro

Contrasting effects of eplerenone and spironolactone on adrenal cell steroidogenesis

Spironolactone and eplerenone are widely used as mineralocorticoid antagonists. Spironolactone has several nonspecific actions including inhibition of androgen receptor and steroid hormone biosynthesis. While studies have shown that eplerenone does not exhibit nonspecific actions on androgen receptor, its effects on steroid hormone production have not been reported. Herein, the effects of eplerenone (0.1-30 microM) and spironolactone (0.1-30 microM) on steroid production were examined in human adrenocortical H295R cells. Spironolactone inhibited basal production of cortisol (91%) and aldosterone (53%). Treatment of H295R cells with angiotensin II (Ang II) for 24 h increased aldosterone production by 11-fold. Spironolactone inhibited Ang II stimulation of aldosterone production by 80%. Addition of pregnenolone increased aldosterone (9-fold) and cortisol (3-fold) production. Spironolactone inhibited pregnenolone metabolism to aldosterone (67%) and cortisol (74%). The inhibitory effects of spironolactone occurred at concentrations far higher than those needed to block mineralocorticoid receptor, suggesting an action directly on the enzymes involved in steroid production. In contrast, eplerenone did not inhibit basal, Ang II, forskolin, pregnenolone-stimulated cortisol, or aldosterone production. Together, these data demonstrate that opposed to spironolactone, pharmacologic concentrations of eplerenone do not inhibit adrenal cell aldosterone or cortisol production.

Rapid glucocorticoid effects on excitatory amino acid levels in the hippocampus: a microdialysis study in freely moving rats

Glucocorticoids can rapidly affect neuronal function and behaviour in mammals. Several studies have suggested the possible existence of rapid, non-genomic effects of glucocorticoids in the hippocampus. To investigate whether glucocorticoids could affect neurotransmission in the hippocampus through rapid, non-genomic mechanisms, we studied the effects of acute glucocorticoid administration on extracellular amino acid levels in the CA1 area of the hippocampus. By means of microdialysis on freely moving rats, we observed that an intraperitoneal injection of corticosterone (2.5 mg/kg) induced a rapid (within 15 min) and transient (returning to basal levels by 35-45 min) increase in extracellular aspartate and glutamate levels ( approximately 155-160%), both in sham-operated and adrenalectomized rats. These effects occurred in parallel with a rise in corticosterone concentration, also detected by microdialysis, in this hippocampal area. Intrahippocampal perfusion of corticosterone by retrodialysis also produced the same fast and reversible effects on excitatory amino acid (EAA) levels. Extracellular concentrations of taurine and gamma-aminobutyric acid (GABA) were unchanged after intrahippocampal glucocorticoid administration. This corticosterone-mediated rise in EAA levels was not inhibited by the presence of specific antagonists for the two types of intracellular corticosteroid receptors, nor by a protein synthesis inhibitor, anisomycin. Perfusion of dexamethasone, a synthetic glucocorticoid, elicited a similar effect to that observed with corticosterone treatment in all studied cases. However, non-glucocorticoid steroids did not affect amino acid transmission in this hippocampal area. These results indicate that glucocorticoids induce a rapid and transient increase in hippocampal EAA levels in vivo that might be exerted through a novel non-genomic mechanism of action.

Corticosterone modifies muscarinic receptor immunoreactivity in rat hippocampus

In the present study we report the effect of corticosterone in the regulation of hippocampal muscarinic acetylcholine receptor immunoreactivity (mAChR-ir) expression in rats. Adrenalectomy (ADX) or a single injection of a mineralocorticoid antagonist RU-28318 (1.0 mg/100 g body weight (b.w.)) in adrenally intact rats 24 h prior to sacrifice revealed an increased mAChR-ir in hippocampal CA1 and CA3 areas. Corticosterone replacement (100 microg/100 g b.w.) prevented the increase in mAChR-ir of ADX animals. However, glucocorticoid receptor antagonist (RU38486) treatment in adrenally intact rats failed to affect the mAChR immunolabeling. These results point to a modulation of muscarinic receptors by corticosterone that is predominantly mediated by the mineralocorticoid receptor.

Corticosteroid-dependent sodium transport in a novel immortalized mouse collecting duct principal cell line

The final control of sodium balance takes place in the cortical collecting duct (CCD) of the nephron, where corticosteroid hormones regulate sodium reabsorption by acting through mineralocorticoid (MR) and/or glucocorticoid (GR) receptors. A clone of principal CCD cells (mpkCCDc14) has been established that is derived from a transgenic mouse (SV40 large T antigen under the control of the SV40 enhancer/L-type pyruvate kinase promoter). Cells grown on filters form polarized monolayers with high electrical transepithelial resistance (R(T) approximately 4700 ohm x cm2) and potential difference (P(D) approximately -50 mV) and have an amiloride-sensitive electrogenic sodium transport, as assessed by the short-circuit current method (Isc approximately 11 microA/cm2). Reverse transcription-PCR experiments using rat MR primers, [3H]aldosterone, and [3H]dexamethasone binding and competition studies indicated that the mpkCCDc14 cells exhibit specific MR and GR. Aldosterone increased Isc in a dose- (10(-10) to 10(-6) M) and time-dependent (2 to 72 h) manner, whereas corticosterone only transiently increased Isc (2 to 6 h). Consistent with the expression of 11beta-hydroxysteroid dehydrogenase type 2, which metabolizes glucocorticoids to inactive 11-dehydroderivates, carbenoxolone potentiated the corticosterone-stimulated Isc. Aldosterone (5x10(-7) M)-induced Isc (fourfold) was associated with a three- to fivefold increase in alpha-ENaC mRNA (but not in those for beta- or gamma-ENaC) and three- to 10-fold increases in alpha-ENaC protein synthesis. In conclusion, this new immortalized mammalian CCD clonal cell line has retained a high level of epithelial differentiation and sodium transport stimulated by aldosterone and therefore represents a useful mammalian cell system for identifying the genes controlled by aldosterone.

Evaluation of RU28318 and RU40555 as selective mineralocorticoid receptor and glucocorticoid receptor antagonists, respectively: receptor measures and functional studies

Corticosterone regulates a wide range of physiological parameters. Two receptors for corticosterone have been identified, the mineralocorticoid (type I) receptor (MR) and the glucocorticoid (type II) receptor (GR). To determine the relative role of these two receptors in mediating the effects of endogenous corticosterone, many studies have relied on the use of putative selective corticosteroid receptor antagonists. This study further examined the in vivo receptor selectivity of two compounds, RU28318 and RU40555 that are believed to be selective antagonists for MR and GR, respectively. Acute treatment of adrenalectomized rats with RU28318 (10-50 mg/kg) selectively decreased ex-vivo available MR binding in the hippocampus, whereas acute treatment with RU40555 (10-30 mg/kg) selectively decreased available GR binding in the hippocampus and pituitary. These receptor binding measures suggest that RU28318 in vivo selectively occupied MR, and that RU40555 in vivo selectively occupied GR. In functional studies, RU28318 (50 mg/kg) blocked the normalizing effect of aldosterone (120 microg/kg) on saline intake of adrenalectomized rats. RU40555 (30 mg/kg) blocked the suppressive effect of dexamethasone (50 microg/kg) on acute stress-induced corticosterone secretion. These studies further support the in vivo corticosteroid receptor selectivity of these two compounds and confirms their effective corticosteroid antagonistic properties.

Repeated blockade of mineralocorticoid receptors, but not of glucocorticoid receptors impairs food rewarded spatial learning

Corticosteroids from the adrenal cortex influence a variety of behaviours including cognition, learning and memory. These hormones act via two intracellular receptors, the mineralo-corticoid receptor (MR) and the glucocorticoid receptor (GR). These two receptor types display a high concentration and distinct distribution in the hippocampus, a brain region which is directly involved in the regulation of spatial orientation and learning. In this study, repeated subcutaneous administration of the mineralocorticoid receptor antagonist RU28318 (1.0 mg/100 g body weight), the glucocorticoid receptor blocker RU38486 (2.5 mg/100 g body weight), or a combination of both antagonists were investigated for their effects on working--and reference memory in morning and afternoon trials during 8 subsequent days in food rewarded spatial learning in a hole board task. Each rat received one dose of either vehicle (2% ethanol in PEG 400), RU28318, RU38486 or the combination of both antagonists directly after the first trial on training days 1, 3, 5, and 7. The experiments demonstrated that repeated blockade of mineralocorticoid receptors impairs reference memory reflected in the morning--as well as in the afternoon trial, whereas blockade of glucocorticoid receptors has little effect on this type of cognitive behaviour. Furthermore, combined blockade of MRs and GRs resulted in a decrease, in both daily trials, in reference memory as well as working memory performance. These findings suggest that in this spatial learning paradigm, the impairment of working memory required blockade of both receptor types, while reference memory performance involves predominantly the mineralocorticoid receptors.

Mineralocorticoid and glucocorticoid receptor antagonists in animal models of anxiety

The behavioral effects of intracerebroventricular (i.c.v.) administration of a specific mineralocorticoid receptor (MR) antagonist [RU28318 (10-50 ng/2 microliters)], a glucocorticoid receptor (GR) antagonist [RU38486 (1-50 ng/2 microliters)], or both antagonists (50 ng/2 microliters), were studied in two different animal models of fear and anxiety in rats. In the defensive burying paradigm simultaneous blockade of MR and GR increased immobility behavior, whereas a small decrease in defensive burying was seen. In the fear-potentiated startle test concurrent MR and GR blockade led to an increase in fear-potentiated startle at the highest loudness level (105 dB). In both tests the antagonists were not effective when given separately. The findings are discussed in terms of the involvement of GR and MR in neural mechanisms of fear and anxiety.

Anxiolytic-like effects of selective mineralocorticoid and glucocorticoid antagonists on fear-enhanced behavior in the elevated plus-maze

The effects of intracerebroventricular (ICV) administration of the mineralocorticoid receptor (MR) antagonist, RU28318, and the glucocorticoid receptor (GR) antagonist, RU38486, were studied on behavior of rats exposed to a compartment previously associated with a stressor, and placed subsequently in an elevated plus-maze test. Fear-motivated immobility behavior was attenuated by the MR antagonist in a dose of 50 or 100 ng ICV, whereas the GR antagonist alone or simultaneous administration of both antagonists had no significant effect. In the elevated plus-maze, immediately after the exposure to the conditioned stressor, both the GR antagonist (50 ng) and MR antagonist (50 ng) increased the percentage of time the rats spent on open arms, and increased the amount of entries into these open arms. These data are interpretated in terms of the involvement of the GR and MR in fear and anxiety.

Steroid hormone stimulation of Na+ transport in A6 cells is mediated via glucocorticoid receptors

The A6 cell line derived from the toad kidney forms polarized, highly differentiated epithelial monolayers in culture and has been utilized as an experimental model for studying regulation of transepithelial Na+ transport by aldosterone. In the present study we evaluated the specific role(s) of glucocorticoid and mineralocorticoid receptors in mediating this enhanced electrogenic Na+ transport, which was measured experimentally as an increase in short-circuit current (Isc). Our data demonstrate that specific glucocorticoid agonists (100 nM), including RU 28362 and RU 26988, elicit "mineralocorticoid-like" increases in Isc that are blocked by the glucocorticoid antagonist RU 38486 but are unaffected by mineralocorticoid antagonists including RU 28318 and RU 26752. The stimulatory effects of aldosterone (100 nM) were also blocked by RU 38486 and not by mineralocorticoid antagonists. These data extend earlier studies suggesting that in this cell line aldosterone mediates its physiological effects via binding with relatively low affinity (dissociation constant Kd congruent to 25-50 nM) to glucocorticoid receptors, despite the presence of apparently normal mineralocorticoid receptors. Our in vitro biochemical studies also demonstrate that A6 glucocorticoid receptor complexes can be thermally activated or transformed to DNA binding forms which exhibit altered elution profiles from anion-exchange resins. Thus, based on several criteria, these amphibian glucocorticoid receptors appear very similar to classical mammalian receptors and are capable of mediating all of the stimulatory effects of aldosterone on net Na+ transport.

Cellular responses to steroids in the enhancement of Na+ transport by rat collecting duct cells in culture. Differences between glucocorticoid and mineralocorticoid hormones

It has recently been discovered that both mineralocorticoid (MC) and glucocorticoid (GC) hormones can stimulate electrogenic Na+ absorption by mammalian collecting duct cells in culture. In primary cultures of rat inner medullary collecting duct (IMCD) cells, 24-h incubation with either MC or GC agonist stimulates Na+ transport approximately threefold. We have now determined that the effects were not additive, but the time courses were different. As aldosterone is known to stimulate citrate synthase, Na+/K+ ATPase activity, and ouabain binding in cortical collecting duct principal cells, we determined the effects of steroids on these parameters in IMCD cells. MC and GC agonists both produced a small increase in citrate synthase activity. There was no increase in Na+/K+ ATPase activity but specific ouabain binding was increased more than two-fold by either agonist. To determine the role of apical Na+ entry in the steroid-induced effects, the Na+ channel inhibitor, benzamil, was used. Benzamil did not alter the stimulation of citrate synthase activity by either steroid. In contrast, GC stimulation of ouabain binding was prevented by benzamil, whereas MC stimulation was not. We conclude that there are differences in the way that MC and GC hormones produce an increased Na+ transport. Both appear to produce translocation (or activation) of pumps into the basolateral membrane. GC stimulation of pump translocation requires increased Na+ entry whereas MC stimulation does not.

Immunophotochemical analysis of mineralocortin by polyclonal antibodies against the native receptor from rat kidney

We have obtained a polyclonal antiserum by immunizing fawn Burgundy rabbits with the mineralocorticoid receptor (MCR) purified biochemically from rat kidneys. High titers of anti-MCR activity were obtained in radioimmunoassays within 3 weeks and increased with a booster shot. In Western blot analysis, the antibody revealed a major band of 94-98 kDa in renal cytosol from rat and beef kidneys. We also developed a fluorographic procedure where the MCR linked covalently to tritiated R-5020, following ultraviolet irradiation, gave imprints superimposable on the Western blot profile. The fluorographic pattern was specific since it was largely abolished in the presence of cold RU 26752 that is specific to MCR, or mineralocortin. The immune IgG precipitated rat renal MCR(-)[3H]RU 26752 complexes in a dose-dependent manner and also recognized MCR bound to the natural hormone aldosterone. During gel permeation chromatography on Sephacryl, the elution profile of [3H]RU 26752 shifted to high-molecular-weight regions in the presence of immune IgG. The receptor protein could be immunolocalized primarily to the principal cells of the collecting duct in rat kidney but the intercalated cells and glomeruli were not labeled, contrary to beef kidney where a uniform pattern of immunostaining was evident. These should permit large-scale purification of the MCR for detailed physicochemical studies and for screening of the MCR-positive tissues during various pathophysiological syndromes.

Generation of polyclonal antibodies against the mineralocorticoid receptor and analysis of mineralocortin in rat myocardium by immunophotochemistry

Fawn, Burgundy rabbits were immunized with the mineralocorticoid receptor (MCR) purified biochemically from rat kidney by a simple, two step procedure. High anti-MCR titers were observed in radioimmunoassays just 3 weeks after the initial injection and increased further with time. Western blot analysis revealed a single band of 94-98 kDa in renal and cardiac cytosol from the rat, like the antigen prepared biochemically. The two atria from beef heart exhibited far greater MCR-positivity compared to the two ventricles, suggesting physiological relevance. The receptor was also photolabelled for the first time with promegestone in this very 94-98 kDa region which could be displaced by the antagonist RU 26752 specific to MCR. The immune IgG precipitated 3H-aldosterone or 3H-RU 26752-MCR complexes from rat heart, and displaced the MCR-antagonist complex to high molecular weight regions during gel permeation chromatography on Sephacryl columns. Immunofluorescent labelling showed that MCR was widely distributed in the cytoplasm in rat myocardium with limited staining in what appeared to be the nuclear compartment. These open up the possibility of large scale purification of the endogenous mineralocorticoid binding protein, mineralocortin, for detailed physicochemical characterization. The technique of photoaffinity labelling presented here should also help delineate the nature of the steroid binding domain in the MCR.

Assessment of the antimineralocorticoid effect of RU 28318 in healthy men with induced exogenous and endogenous hypermineralocorticism

The antimineralocorticoid effect of a single dose of RU 28318, has been assessed in healthy men with exogenous or endogenous hypermineralocorticism. For exogenous hypermineralocorticism induced by ingestion of 9 alpha-fluorohydrocortisone (9 alpha-FHC) and aldosterone infusion, RU 28318 100 mg (9 alpha-FHC ingestion) or 200 mg (aldosterone infusion) was administered, and its effect compared with identical doses of spironolactone or a placebo. For endogenous hypermineralocorticism induced by ingestion of furosemide, RU 28318 100 and 300 mg was tested in comparison with 100 mg spironolactone or placebo. In all 3 studies, both RU 28318 and spironolactone significantly raised the urinary Na/K ratio when compared to placebo administration. No significant difference was apparent between RU 28318 and spironolactone. Thus, a single dose of RU 28318 in man has an antimineralocorticoid effect identical to those produced by the identical molar dose of spironolactone. In addition, the results show that furosemide-induced hyperaldosteronism constitutes a simple and reproducible test for assessing the antimineralocorticoid effect of a drug.