Non-classical receptors for aldosterone in plasma membranes from pig kidneys

G protein-coupled estrogen receptor: a promising therapeutic target for aldosterone-induced hypertension

Aldosterone is one of the most essential hormones synthesized by the adrenal gland because it regulates water and electrolyte balance. G protein-coupled estrogen receptor (GPER) is a newly discovered aldosterone receptor, which is proposed to mediate the non-genomic pathways of aldosterone while the hormone simultaneously interacts with mineralocorticoid receptor. In contrast to its cardio-protective role in postmenopausal women via its interaction with estrogen, GPER seems to trigger vasoconstriction effects and can further induce water and sodium retention in the presence of aldosterone, indicating two entirely different binding sites and effects for estrogen and aldosterone. Accumulating evidence also points to a role of aldosterone in mediating hypertension and its risk factors via the interaction with GPER. Therefore, with this review, we aimed to summarize the research on these interactions to help (1) elucidate the role of GPER activated by aldosterone in the blood vessels, heart, and kidney; (2) compare the non-genomic actions between aldosterone and estrogen mediated by GPER; and (3) address the potential of GPER as a new promising therapeutic target for aldosterone-induced hypertension.

Non-genomic steroid signaling through the mineralocorticoid receptor: Involvement of a membrane-associated receptor?

Corticosteroid receptors in the mammalian brain mediate genomic as well as non-genomic actions. Although receptors mediating genomic actions were already cloned 35 years ago, it remains unclear whether the same molecules are responsible for the non-genomic actions or that the latter involve a separate class of receptors. Here we focus on one type of corticosteroid receptors, i.e. the mineralocorticoid receptor (MR). We summarize some of the known properties and the current insight in the localization of the MR in peripheral cells and neurons, especially in relation to non-genomic signaling. Previous studies from our own and other labs provided evidence that MRs mediating non-genomic actions are identical to the ones involved in genomic signaling, but may be translocated to the plasma cell membrane instead of the nucleus. With fixed cell imaging and live cell imaging techniques we tried to visualize these presumed membrane-associated MRs, using antibodies or overexpression of MR-GFP in COS7 and hippocampal cultured neurons. Despite the physiological evidence for MR location in or close to the cell membrane, we could not convincingly visualize membrane localization of endogenous MRs or GFP-MR molecules. However, we did find punctae of labeled antibodies intracellularly, which might indicate transactivating spots of MR near the membrane. We also found some evidence for trafficking of MR via beta-arrestins. In beta-arrestin knockout mice, we didn't observe metaplasticity in the basolateral amygdala anymore, indicating that internalization of MRs could play a role during corticosterone activation. Furthermore, we speculate that membrane-associated MRs could act indirectly via activating other membrane located structures like e.g. GPER and/or receptor tyrosine kinases.

The Interface of Nuclear and Membrane Steroid Signaling

Steroid hormones bind receptors in the cell nucleus and in the cell membrane. The most widely studied class of steroid hormone receptors are the nuclear receptors, named for their function as ligand-dependent transcription factors in the cell nucleus. Nuclear receptors, such as estrogen receptor alpha, can also be anchored to the plasma membrane, where they respond to steroids by activating signaling pathways independent of their function as transcription factors. Steroids can also bind integral membrane proteins, such as the G protein-coupled estrogen receptor. Membrane estrogen and progestin receptors have been cloned and characterized in vitro and influence the development and function of many organ systems. Membrane androgen receptors were cloned and characterized in vitro, but their function as androgen receptors in vivo is unresolved. We review the identity and function of membrane proteins that bind estrogens, progestins, and androgens. We discuss evidence that membrane glucocorticoid and mineralocorticoid receptors exist, and whether glucocorticoid and mineralocorticoid nuclear receptors act at the cell membrane. In many cases, integral membrane steroid receptors act independently of nuclear steroid receptors, even though they may share a ligand.

G Protein-Coupled Estrogen Receptor 1 (GPER1) Mediates Aldosterone-Induced Endothelial Inflammation in a Mineralocorticoid Receptor-Independent Manner

OBJECTIVE

It has been increasingly appreciated that G protein-coupled estrogen receptor 1 (GPER1) mediates both proinflammatory and anti-inflammatory response of estrogen. It is also involved in some rapid vascular effects of aldosterone in a mineralocorticoid receptor (MR) independent manner. However, whether GPER1 mediates aldosterone-induced inflammation response in endothelial cells and its relationship with MR are yet undetermined and therefore require further explanation.

METHOD

Based on the hypothesis that GPER1 plays a role in the aldosterone-related vascular inflammation, the present study utilized a model of human umbilical vein endothelial cells transfected with MR siRNA and induced for inflammatory response with increasing concentration of aldosterone.

RESULTS

It was discovered that induction of aldosterone had no effect on the expression of GPER1 but promoted the expression of MR. Suppression of MR did not influence GPER1 expression, and GPER1 was capable of mediating part of aldosterone-induced endothelial inflammatory response. This effect may involve phosphoinositide 3-kinases (PI3K) pathway signaling.

CONCLUSION

These findings not only demonstrated the role of GPER1 in aldosterone-induced vascular inflammation but also suggested an alternative for pharmaceutical treatment of hyperaldosteronism considering the unsatisfying effect on cardiovascular risks with MR antagonists.

Genomic and rapid effects of aldosterone: what we know and do not know thus far

Aldosterone is the most known mineralocorticoid hormone synthesized by the adrenal cortex. The genomic pathway displayed by aldosterone is attributed to the mineralocorticoid receptor (MR) signaling. Even though the rapid effects displayed by aldosterone are long known, our knowledge regarding the receptor responsible for such event is still poor. It is intense that the debate whether the MR or another receptor-the "unknown receptor"-is the receptor responsible for the rapid effects of aldosterone. Recently, G protein-coupled estrogen receptor-1 (GPER-1) was elegantly shown to mediate some aldosterone-induced rapid effects in several tissues, a fact that strongly places GPER-1 as the unknown receptor. It has also been suggested that angiotensin receptor type 1 (AT1) also participates in the aldosterone-induced rapid effects. Despite this open question, the relevance of the beneficial effects of aldosterone is clear in the kidneys, colon, and CNS as aldosterone controls the important water reabsorption process; on the other hand, detrimental effects displayed by aldosterone have been reported in the cardiovascular system and in the kidneys. In this line, the MR antagonists are well-known drugs that display beneficial effects in patients with heart failure and hypertension; it has been proposed that MR antagonists could also play an important role in vascular disease, obesity, obesity-related hypertension, and metabolic syndrome. Taken altogether, our goal here was to (1) bring a historical perspective of both genomic and rapid effects of aldosterone in several tissues, and the receptors and signaling pathways involved in such processes; and (2) critically address the controversial points within the literature as regarding which receptor participates in the rapid pathway display by aldosterone.

Rapid actions of aldosterone revisited: Receptors in the limelight

Steroid hormones like aldosterone have been conclusively shown to elicit both late genomic and rapid, nongenomically initiated responses. Aldosterone was among the first for which rapid, clinically relevant effects were even shown in humans. Yet, after over 30 years of research, the nature of receptors involved in rapid actions of aldosterone is still unclear. Such effects may be assigned to the classical, intracellular steroid receptors, in this case mineralocorticoid receptors (MR, class IIa action Mannheim classification). They typically disappear in knockout models and are blocked by MR-antagonists such as spironolactone, as shown for several cellular and physiological, e.g. renal or cardiovascular effects. In contrast, there is also consistent evidence suggesting type IIb effects involving structurally different receptors ("membrane receptors") being insensitive to classic antagonists and persistent in knockout models; IIb effects have lately even been confirmed by atomic force detection of surface receptors which bind aldosterone but not spironolactone. Type IIa and b may coexist in the same cell with IIa often augmenting early IIb effects. So far cloning of IIb receptors was unsuccessful; therefore results on G-protein coupled estrogen receptor 1 (GPER1) being potentially involved in rapid aldosterone action raised considerable interest. Surprisingly, GPER1 does not bind aldosterone. Though under these circumstances GPER1 should not yet be considered as IIb-receptor, it might be an intermediary signaling enhancer of mineralocorticoid action as shown for epithelial growth factor receptors reconciling those results. We still seem to be left without IIb-receptors whose identification would however be highly desirable and essential for clinical translation.

30 YEARS OF THE MINERALOCORTICOID RECEPTOR: Nongenomic effects via the mineralocorticoid receptor

The mineralocorticoid receptor (MR) belongs to the steroid hormone receptor family and classically functions as a ligand-dependent transcription factor. It is involved in water-electrolyte homeostasis and blood pressure regulation but independent from these effects also furthers inflammation, fibrosis, hypertrophy and remodeling in cardiovascular tissues. Next to genomic effects, aldosterone elicits very rapid actions within minutes that do not require transcription or translation and that occur not only in classical MR epithelial target organs like kidney and colon but also in nonepithelial tissues like heart, vasculature and adipose tissue. Most of these effects can be mediated by classical MR and its crosstalk with different signaling cascades. Near the plasma membrane, the MR seems to be associated with caveolin and striatin as well as with receptor tyrosine kinases like EGFR, PDGFR and IGF1R and G protein-coupled receptors like AT1 and GPER1, which then mediate nongenomic aldosterone effects. GPER1 has also been named a putative novel MR. There is a close interaction and functional synergism between the genomic and the nongenomic signaling so that nongenomic signaling can lead to long-term effects and support genomic actions. Therefore, understanding nongenomic aldosterone/MR effects is of potential relevance for modulating genomic aldosterone effects and may provide additional targets for intervention.

Mineralocorticoid regulation of cell function: the role of rapid signalling and gene transcription pathways

The mineralocorticoid receptor (MR) and mineralocorticoids regulate epithelial handling of electrolytes, and induces diverse effects on other tissues. Traditionally, the effects of MR were ascribed to ligand-receptor binding and activation of gene transcription. However, the MR also utilises a number of intracellular signalling cascades, often by transactivating unrelated receptors, to change cell function more rapidly. Although aldosterone is the physiological mineralocorticoid, it is not the sole ligand for MR. Tissue-selective and mineralocorticoid-specific effects are conferred through the enzyme 11β-hydroxysteroid dehydrogenase 2, cellular redox status and properties of the MR itself. Furthermore, not all aldosterone effects are mediated via MR, with implication of the involvement of other membrane-bound receptors such as GPER. This review will describe the ligands, receptors and intracellular mechanisms available for mineralocorticoid hormone and receptor signalling and illustrate their complex interactions in physiology and disease.

On the pleotropic actions of mineralocorticoids

Classical effects of mineralocorticoids include stimulation of Na(+) reabsorption and K(+) secretion in the kidney and other epithelia including colon and several glands. Moreover, mineralocorticoids enhance the excretion of Mg(2+) and renal tubular H(+) secretion. The renal salt retention following mineralocorticoid excess leads to extracellular volume expansion and hypertension. The increase of blood pressure following mineralocorticoid excess is, however, not only the result of volume expansion but may result from stiff endothelial cell syndrome impairing the release of vasodilating nitric oxide. Beyond that, mineralocorticoids are involved in the regulation of a wide variety of further functions, including cardiac fibrosis, platelet activation, neuronal function and survival, inflammation as well as vascular and tissue fibrosis and calcification. Those functions are briefly discussed in this short introduction to the special issue. Beyond that, further contributions of this special issue amplify on mineralocorticoid-induced sodium appetite and renal salt retention, the role of mineralocorticoids in the regulation of acid-base balance, the involvement of aldosterone and its receptors in major depression, the mineralocorticoid stimulation of inflammation and tissue fibrosis and the effect of aldosterone on osteoinductive signaling and vascular calcification. Clearly, still much is to be learned about the various ramifications of mineralocorticoid-sensitive physiology and pathophysiology.

Upregulation of store operated Ca channel Orai1, stimulation of Ca(2+) entry and triggering of cell membrane scrambling in platelets by mineralocorticoid DOCA

BACKGROUND/AIMS

Mineralocorticoid excess leads to vascular injury, which is partially due to hypertension but in addition involves increased concentration of cytosolic Ca(2+) concentration in platelets, key players in the pathophysiology of occlusive vascular disease. Mineralocorticoids are in part effective by rapid nongenomic mechanisms including phosphatidylinositide-3-kinase (PI3K) signaling, which involves activation of the serum & glucocorticoid inducible kinase (SGK) isoforms. SGK1 has in turn been shown to participate in the regulation of the pore forming Ca(2+) channel protein Orai1 in platelets. Orai1 accomplishes entry of Ca(2+), which is in turn known to trigger cell membrane scrambling. Platelets lack nuclei but are able to express protein by translation, which is stimulated by PI3K signaling. The present study explored whether the mineralocorticoid desoxycorticosterone acetate (DOCA) influences platelet Orai1 protein abundance, cytosolic Ca(2+)-activity ([Ca(2+)]i), phosphatidylserine abundance at the cell surface and/or cell volume.

METHODS

Orai1 protein abundance was estimated utilizing CF™488A conjugated antibodies, [Ca(2+)]i utilizing Fluo3-fluorescence, phosphatidylserine abundance utilizing FITC-labelled annexin V, and cell volume utilizing forward scatter in flow cytometry.

RESULTS

DOCA (10 µg/ml) treatment of murine platelets was followed by a significant increase of Orai1 protein abundance, [Ca(2+)]i, percentage of phosphatidylserine exposing platelets and platelet swelling. The effect on [Ca(2+)]i, phosphatidylserine abundance and cell volume were completely abrogated by addition of the specific SGK inhibitor EMD638683 (50 µM) CONCLUSIONS: The mineralocorticoid DOCA upregulates Orai1 protein abundance in the cell membrane, thus increasing [Ca(2+)]i and triggering phosphatidylserine abundance, effects paralleled by platelet swelling.

Nongenomic actions of aldosterone and progesterone revisited

After almost 30 years of research, the existence of nongenomic steroid actions is no longer disputed. Yet, there is still a debate on the nature of receptors involved, and answers to the inherent questions are important for translational activities. In the case of aldosterone, the existence of receptors different from the classic mineralocorticoid receptors (MR) had been postulated 25 years ago as the pharmacology of about 50% of rapid actions of aldosterone reported so far is incompatible with MR involvement (insensitivity to classic MR antagonists). Candidates proposed as alternatives to MR were protein kinase C, sodium-potassium ATPase or aberrant forms of MR, none of which supported convincing evidence to represent 'the aldosterone membrane receptor'. Early in 2011, data on GPR30 showed its involvement in rapid aldosterone action, and major pharmacological aspects of this action are compatible with the landmark deviations from MR pharmacology mentioned above. GPR30, therefore, may be a receptor candidate for nongenomic aldosterone action. Similarly, a variety of promising candidates mediating rapid progesterone action has been described, including progesterone receptor membrane component 1 (PGRMC1) which seems to be associated with tumor proliferation, and membrane progesterone receptor (mPR) originally identified in fish with potential linkage to reproductive processes. So far, no candidate was unanimously convincing. In 2010, two independent groups reported that CatSper, a calcium channel, is a strong receptor candidate for the rapid action of progesterone on sperm fertilization. With these novel receptors cloned, translational activities ultimately leading to new drugs for cardiovascular protection (in the case of aldosterone) or fertilization benefits (for progesterone) are much more promising.

Mechanisms underlying rapid aldosterone effects in the kidney

The steroid hormone aldosterone is a key regulator of electrolyte transport in the kidney and contributes to both homeostatic whole-body electrolyte balance and the development of renal and cardiovascular pathologies. Aldosterone exerts its action principally through the mineralocorticoid receptor (MR), which acts as a ligand-dependent transcription factor in target tissues. Aldosterone also stimulates the activation of protein kinases and secondary messenger signaling cascades that act independently on specific molecular targets in the cell membrane and also modulate the transcriptional action of aldosterone through MR. This review describes current knowledge regarding the mechanisms and targets of rapid aldosterone action in the nephron and how aldosterone integrates these responses into the regulation of renal physiology.

Role of aldosterone and angiotensin II in insulin resistance: an update

The role of the Renin-Angiotensin-Aldosterone system (RAAS) on the development of insulin resistance and cardiovascular disease is an area of growing interest. Most of the deleterious actions of the RAAS on insulin sensitivity appear to be mediated through activation of the Angiotensin II (Ang II) Receptor type 1 (AT(1)R) and increased production of mineralocorticoids. The underlying mechanisms leading to impaired insulin sensitivity remain to be fully elucidated, but involve increased production of reactive oxygen species and oxidative stress. Both experimental and clinical studies also implicate aldosterone in the development of insulin resistance, hypertension, endothelial dysfunction, cardiovascular tissue fibrosis, remodelling, inflammation and oxidative stress. There is abundant evidence linking aldosterone, through non-genomic actions, to defective intracellular insulin signalling, impaired glucose homeostasis and systemic insulin resistance not only in skeletal muscle and liver but also in cardiovascular tissue. Blockade of the different components of the RAAS, in particular Ang II and AT(1)R, results in attenuation of insulin resistance, glucose homeostasis, as well as decreased cardiovascular disease morbidity and mortality. These beneficial effects go beyond to those expected with isolated control of hypertension. This review focuses on the role of Ang II and aldosterone in the pathogenesis of insulin resistance, as well as in clinical relevance of RAAS blockade in the prevention and treatment of the metabolic syndrome and cardiovascular disease.

Aldosterone stimulates elastogenesis in cardiac fibroblasts via mineralocorticoid receptor-independent action involving the consecutive activation of Galpha13, c-Src, the insulin-like growth factor-I receptor, and phosphatidylinositol 3-kinase/Akt

We previously demonstrated that aldosterone, which stimulates collagen production through the mineralocorticoid receptor (MR)-dependent pathway, also induces elastogenesis via a parallel MR-independent mechanism involving insulin-like growth factor-I receptor (IGF-IR) signaling. The present study provides a more detailed explanation of this signaling pathway. Our data demonstrate that small interfering RNA-driven elimination of MR in cardiac fibroblasts does not inhibit aldosterone-induced IGF-IR phosphorylation and subsequent increase in elastin production. These results exclude the involvement of the MR in aldosterone-induced increases in elastin production. Results of further experiments aimed at identifying the upstream signaling component(s) that might be activated by aldosterone also eliminate the putative involvement of pertussis toxin-sensitive Galphai proteins, which have previously been shown to be responsible for some MR-independent effects of aldosterone. Instead, we found that small interfering RNA-dependent elimination of another heterotrimeric G protein, Galpha13, eliminates aldosterone-induced elastogenesis. We further demonstrate that aldosterone first engages Galpha13 and then promotes its transient interaction with c-Src, which constitutes a prerequisite step for aldosterone-dependent activation of the IGF-IR and propagation of consecutive downstream elastogenic signaling involving phosphatidylinositol 3-kinase/Akt. In summary, the data we present reveal new details of an MR-independent cellular signaling pathway through which aldosterone stimulates elastogenesis in human cardiac fibroblasts.

Novel hormone “receptors”

Our concepts of hormone receptors have, until recently, been narrowly defined. In the last few years, an increasing number of reports identify novel proteins, such as enzymes, acting as receptors. In this review we cover the novel receptors for the hormones atrial naturetic hormone, enterostatin, hepcidin, thyroid hormones, estradiol, progesterone, and the vitamin D metabolites 1,25(OH)(2)D(3) and 24,25(OH)(2)D(3).

Aldosterone induces elastin production in cardiac fibroblasts through activation of insulin-like growth factor-I receptors in a mineralocorticoid receptor-independent manner

Aldosterone is known to regulate electrolyte homeostasis, but it may also contribute to other processes, including the maladaptive remodeling of postinfarct hearts. Because aldosterone has been implicated in the stimulation of collagen production in the heart, we investigated whether it would also affect elastin deposition in cultures of human cardiac fibroblasts. We first demonstrated that treatment with 1 to 50 nmol/L aldosterone leads to a significant increase in collagen type I mRNA levels and in subsequent collagen fiber deposition. Pretreatment of cells with the mineralocorticoid receptor antagonist spironolactone, but not with the glucocorticoid receptor antagonist RU 486, inhibited collagen synthesis in aldosterone-treated cultures. Most importantly, we demonstrated that aldosterone also increases elastin mRNA levels, tropoelastin synthesis, and elastic fiber deposition in a dose-dependent manner. Strikingly, neither spironolactone nor RU 486 eliminated aldosterone-induced increases in elastin production. We further discovered that the proelastogenic effect of aldosterone involves a rapid increase in tyrosine phosphorylation of the insulin-like growth factor-I receptor and that the insulin-like growth factor-I receptor kinase inhibitor AG1024 or an anti-insulin-like growth factor-I receptor-neutralizing antibody inhibits both insulin-like growth factor-I and aldosterone-induced elastogenesis. Thus, we have demonstrated for the first time that aldosterone, which stimulates collagen production through the mineralocorticoid receptor-dependent pathway, also increases elastogenesis via a parallel mineralocorticoid receptor-independent pathway involving I insulin-like growth factor-I receptor signaling.

Analysis of aldosterone-induced differential receptor-independent protein patterns using 2D-electrophoresis and mass spectrometry

In the human body the mineralocorticoid aldosterone is responsible for maintaining water and electrolyte homeostasis and therefore controlling blood pressure. In addition, aldosterone has recently been associated with severe heart failure. Besides receptor-dependent action, the damaging effects of aldosterone may also be partly mediated through non-genomic mechanisms. The present study focuses on the mineralocorticoid receptor-independent action of aldosterone at the protein level. We chose the fission yeast Schizosaccharomyces pombe as a model organism, since this yeast does not contain nuclear steroid receptors, but many genes and regulatory mechanisms that are close to those of mammals. Using 2D-electrophoresis we identified for the first time protein spots affected by aldosterone in a nuclear receptor-free system. Mass spectrometry analysis using MALDI-TOF MS and nanoLC-MS/MS approaches allowed the unambiguous identification of 11 proteins that showed increased or decreased levels, which may represent newly identified players and pathways of aldosterone-induced action. Two proteins with a connection to osmotic regulation (NAD-dependent malic enzyme and glycerol-3-phosphate-dehydrogenase), as well as two proteins involved in the overall organization of the cytoskeleton, vip1 and glyceraldehyde-3-phosphate dehydrogenase, which was also found to be specifically affected by aldosterone in human HCT116 cells, are discussed.

Aldosterone receptor antagonists: Biology and novel therapeutic applications

A dysregulation of the aldosterone system has been involved in the pathophysiology of cardiovascular diseases, including myocardial failure and, partially, essential hypertension. In humans and in rat models, aldosterone action induces heart remodeling and interstitial and perivascular myocardial fibrosis. Therefore, a rationale for using aldosterone antagonists (ARAs) of the spironolactone family, which have been available for decades for the treatment of aldosterone excess syndromes, has now emerged. The development of compounds such as eplerenone, with a greater selectivity for mineralocorticoid receptors, is promising also in terms of reduction of endocrine side effects. The use of ARAs for the treatment of myocardial failure and selected cases of hypertension, in combination with the current therapy, has been strongly supported by trials such as the Randomized Aldactone Evaluation Study (RALES) and the Eplerenone Neurohormonal Efficacy and Survival Study (EPHESUS). Thus, the addition of ARAs to the conventional therapy appears beneficial, leading to an improved survival rate and a reduced incidence of cardiac complications.

The nongenomic actions of aldosterone

Aldosterone has physiological effects to regulate fluid and electrolyte homeostasis across epithelia and proinflammatory effects on a variety of nonepithelial cells in the context of inappropriate salt status. These effects are mediated by mineralocorticoid receptors, members of a large family of nuclear transcription factors, by DNA-directed, RNA-mediated protein synthesis. Rapid effects of aldosterone, insensitive to actinomycin D or cycloheximide and thus clearly nongenomic, have been convincingly documented in a variety of epithelial and nonepithelial tissues. Despite strenuous attempts, isolation of a nonclassical membrane receptor for aldosterone has proven unsuccessful, and rapid nongenomic effects mediated by classical mineralocorticoid receptors are increasingly recognized in the kidney, heart, and vascular wall. The mechanism of rapid nongenomic actions of aldosterone may vary between tissues in terms of pathways; in addition, what remains to be established is the physiological role of aldosterone action via such rapid nongenomic mechanisms and how they might synergize with the longer time course genomic actions of mineralocorticoids.

Rapid natriuretic action of aldosterone in the rat

Rapid, nongenomic actions of aldosterone have been demonstrated in a number of cell types in vitro, including renal cell lines, but there remains little direct evidence that it is able to exert rapid effects on the kidney in the whole animal. Accordingly, the aim of this study was to determine whether aldosterone induces rapid changes in the renal handling of electrolytes or acid-base balance in the anesthetized rat. With the use of a servo-controlled fluid replacement system, spontaneous urine output by anesthetized male Sprague-Dawley rats was replaced with 2.5% dextrose. After a 3-h equilibration and a 1-h control period, rats were infused with aldosterone (42 pmol/min) or vehicle for 1 h. Aldosterone infusion induced a rapid (within 15 min) increase in sodium excretion that peaked at 0.24 ± 0.08 compared with 0.04 ± 0.01 μmol·min−1 100·body weight−1 ( P = 0.041) in the vehicle-infused rats. This natriuresis was not associated with changes in glomerular filtration rate; urine flow rate; potassium, chloride, or bicarbonate excretion; or urine pH. The mechanisms involved are unclear, but because we have previously shown that aldosterone stimulates a rapid (4 min) increase in cAMP generation in the rat inner medullary collecting duct (IMCD) (Sheader EA, Wargent ET, Ashton N, and Balment RJ. J Endocrinol 175: 343–347, 2002), they could involve cAMP-mediated activation of the cystic fibrosis transmembrane conductance regulator chloride channel, which drives sodium secretion in the IMCD.

Mineralocorticoid receptor antagonists do not block rapid ERK activation by aldosterone

Aldosterone can elicit rapid nongenomic effects both in vivo and in vitro, often mediated by signal transduction cascades. However, it is not understood how these rapid effects are initiated. In this study we show that aldosterone leads to rapid activation of mitogen activated protein kinases ERK1/2 in the cortical collecting duct cell line M-1. Inhibitors of transcription and translation could not block this activation, which suggests an extranuclear (nongenomic) mechanism. Although it is known that M-1 cells do not contain a transcriptionally functional MR, it is not known whether a closely related protein still could mediate the effects, or an unrelated nonclassic receptor. To test this hypothesis, the effects of four classical mineralocorticoid receptor antagonists were studied. None of the compounds could block the response to aldosterone. Altogether, the data suggest that rapid aldosterone effects in M-1 cells are initiated by a receptor different from the classical mineralocorticoid receptor.

A quick glance at rapid aldosterone action

Not all of the actions of aldosterone are mediated by the classic genomic pathway involving transcription and translation. Non-genomic or non-classical rapid responses that do not require these steps have been known for some time, but have only attracted significant interest in the last decade. At the cellular level, second messengers and kinase cascades are commonly involved. Most of these non-classical effects are insensitive to inhibitors of the classical cytosolic mineralocorticoid receptor. Non-genomic aldosterone action has been observed in clinical studies particularly in the cardiovascular system, and further research may improve the understanding of their participation in the pathogenesis of aldosterone related diseases and eventually enhance the options for therapy.

Nongenomic vascular action of aldosterone in the glomerular microcirculation

Aldosterone (Aldo) accelerates hypertension, proteinuria, and glomerulosclerosis in animal models of malignant hypertension or chronic renal failure. Aldo may exert these deleterious renal effects by elevating renal vascular resistance and glomerular capillary pressure. To test this possibility, directly examined were the action of Aldo on the afferent (Af) and efferent (Ef) arterioles (Arts). Examined were the effect of Aldo added to both the bath and lumen on the intraluminal diameter (measured at the most responsive point) of rabbits. Aldo caused dose-dependent constriction in both arterioles with a higher sensitivity in Ef-Arts. Vasoconstrictor action of Aldo was not affected by a mineralocorticoid receptor antagonist spironolactone and was reproduced by membrane-impermeable albumin-conjugated Aldo, suggesting that the vasoconstrictor actions are nongenomic. Pretreatment with neomycin (a specific inhibitor of phospholipase C) abolished the vasoconstrictor action of Aldo in both arterioles. In addition, the vasoconstrictor action of Aldo on Af-Arts was inhibited by both nifedipine and efonidipine, whereas that on Ef-Arts was inhibited by efonidipine but not nifedipine. The results demonstrate that Aldo causes nongenomic vasoconstriction by activating phospholipase C with a subsequent calcium mobilization thorough L- or T-type voltage-dependent calcium channels in Af- or Ef-Arts, respectively. These vasoconstrictor actions on the glomerular microcirculation may play an important role in the pathophysiology and progression of renal diseases by elevating renal vascular resistance and glomerular capillary pressure.

Corticosteroid receptors, 11 beta-hydroxysteroid dehydrogenase, and the heart

Mineralocorticoid and glucocorticoid hormones are known as corticosteroid hormones and are synthesized mainly in the adrenal cortex; however, more recently the enzymes involved in their synthesis have been found in a variety of cells and tissues, including the heart. The effects of these hormones are mediated via both cytoplasmic mineralocorticoid receptors (MRs) and glucocorticoid receptors (GRs), which act as ligand-inducible transcription factors. In addition, rapid, nongenomically mediated effects of these steroids can occur that may be via novel corticosteroid receptors. The lipophilic nature of these hormones allows them to pass freely through the cell membrane, although the intracellular concentration of mineralocorticoids and glucocorticoids is dependent on several cellular factors. The main regulators of intracellular glucocorticoid levels are 11 beta-hydroxysteroid dehydrogenase (11 beta HSD) isoforms. 11 beta HSD1 acts predominantly as a reductase in vivo, facilitating glucocorticoid action by converting circulating receptor-inactive 11-ketoglucocorticoids to active glucocorticoids. In contrast, 11 beta HSD 2 acts exclusively as an 11 beta-dehydrogenase and decreases intracellular glucocorticoids by converting them to their receptor-inactive 11-ketometabolites. Furthermore, P-glycoproteins, by actively pumping steroids out of cells, can selectively decrease steroids and local steroid synthesis can increase steroid concentrations. Receptor concentration, receptor modification, and receptor-protein interactions can also significantly impact on the corticosteroid response. This review details the receptors and possible mechanisms involved in both mediating and modulating corticosteroid responses. In addition, direct effects of corticosteroids on the heart are described including a discussion of the corticosteroid receptors and the mechanisms involved in mediating their effects.

Aldosterone receptor antagonists: biology and novel therapeutical applications

Recent studies suggest that a dysregulation of the aldosterone system is involved in the pathophysiology of different cardiovascular diseases, including myocardial failure and several cases of essential hypertension. In both rat models and in humans, aldosterone action has been shown to induce heart remodeling and interstitial and perivascular fibrosis of the myocardium. For these reasons, a rationale for the use of aldosterone antagonists (ARAs) of the spirolactone family, which have been available for decades in the treatment of aldosterone excess syndromes, has now emerged. Moreover, the recent validation of their use, in combination with the current therapy, for the treatment of these cardiovascular diseases by trials like the RALES Study has further strenghtened this approach. The development of compounds, like eplerenone, with a greater selectivity for mineralocorticoid receptors, seems promising also in terms of reduction of endocrine side effects. The addition of aldosterone antagonists to the conventional therapy of myocardial failure and of selected cases of hypertension thus appears beneficial, resulting in an improved survival rate and a reduced incidence of cardiac complications. This review article, after a brief recall of the physiology of the aldosterone system, addresses the emerging role of aldosterone in cardiovascular diseases, considers the pharmacology of ARAs and the novel therapeutical applications of these compounds in hypertension and heart failure.

Nongenomic steroid action: controversies, questions, and answers

Steroids may exert their action in living cells by several ways: 1). the well-known genomic pathway, involving hormone binding to cytosolic (classic) receptors and subsequent modulation of gene expression followed by protein synthesis. 2). Alternatively, pathways are operating that do not act on the genome, therefore indicating nongenomic action. Although it is comparatively easy to confirm the nongenomic nature of a particular phenomenon observed, e.g., by using inhibitors of transcription or translation, considerable controversy exists about the identity of receptors that mediate these responses. Many different approaches have been employed to answer this question, including pharmacology, knock-out animals, and numerous biochemical studies. Evidence is presented for and against both the participation of classic receptors, or proteins closely related to them, as well as for the involvement of yet poorly understood, novel membrane steroid receptors. In addition, clinical implications for a wide array of nongenomic steroid actions are outlined.

Rapid aldosterone actions: from the membrane to signaling cascades to gene transcription and physiological effects

Nongenomic actions of aldosterone have been described in a number of cell culture and in vivo systems. They occur, in contrast to the classical genomic effects on gene transcription, rapidly within seconds to minutes after aldosterone administration. The primary effector is still unknown. Whether it is a so far unidentified membrane bound aldosterone receptor or the classical mineralocorticoid receptor or both is under debate. The downstream signaling cascade involved in such rapid actions begins to be elucidated. In this work, we discuss the nature of the putative membrane receptor for aldosterone and summarize observed rapid aldosterone effects in different in vitro and in vivo systems.

Nongenomic effects of aldosterone on human renal cells

The development of chronic renal insufficiency may be partially mediated by the nongenomic action of aldosterone. Here we investigate whether aldosterone could evoke a nongenomic action in primary cultures of human renal cells. Intracellular Ca(2+) ([Ca(2+)](i)) and cAMP were measured in human mesangial cells (MC), glomerular visceral epithelial cells (GVEC), and proximal and distal tubular epithelial cells (Ptec and Dtec) in the presence of aldosterone (10-100 nmol/liter) by fura-2 fluorescence and RIA, respectively. In MC, Ptec, and Dtec, aldosterone increased [Ca(2+)](i) within 1 min, whereas in GVEC, only a minor effect was found. Preincubation of cells with spironolactone did not blunt this effect. Hydrocortisone, used at a concentration 100-fold higher than that of aldosterone, did not affect [Ca(2+)](i.) In MC, Ptec, and Dtec, a dose-dependent increase ( approximately 1.3- to 1.5-fold) in intracellular cAMP levels was found. These data demonstrate a nongenomic action of aldosterone in human MC, Ptec, and Dtec. As these effects occur at concentrations close to free plasma aldosterone levels in man, they may be of physiological relevance and may contribute to renal injury.

Aldosterone inhibits HCO absorption via a nongenomic pathway in medullary thick ascending limb

Rapid actions of aldosterone that are independent of transcription and translation have been described in a variety of cells; however, whether nongenomic pathways mediate aldosterone-induced regulation of renal tubule transport has not been determined. We report here that aldosterone induces rapid (<3.5 min) inhibition of HCO absorption in the medullary thick ascending limb (MTAL) of the rat. This inhibition is observed over the physiological range of hormone concentrations (IC(50) approximately 0.6 nM) and is not affected by pretreatment with actinomycin D (12.5 microg/ml), cycloheximide (40 microg/ml), or spironolactone (10 microM). The glucocorticoids dexamethasone, cortisol, and corticosterone (1 or 500 nM) did not affect HCO absorption in the absence or presence of carbenoxolone. Thus the specificity of rapid aldosterone action is not dependent on 11beta-hydroxysteroid dehydrogenase activity. The inhibition by aldosterone is additive to inhibition by angiotensin II and vasopressin, indicating that these factors regulate MTAL transport through distinct pathways. These results demonstrate that aldosterone inhibits HCO absorption in the MTAL via a pathway that is rapid, highly selective, independent of transcription and protein synthesis, and not mediated through the classic mineralocorticoid receptor. The results establish a role for nongenomic pathways in mediating aldosterone-induced regulation of transepithelial transport in the mammalian kidney. The novel action of aldosterone to inhibit luminal acidification in the MTAL may play a role in enabling the kidney to regulate acid-base balance independently of Na(+) balance and extracellular fluid volume.

Nongenomically initiated steroid actions

The classical theory of steroid hormone action comprises binding to an intracellular receptor followed by modulation of transcriptional and translational events. These cumbersome model explains the characteristic latency of these genomic steroid effects. Over the past two decades, increasing evidence for rapid nongenomic effects of steroids, incompatible with the traditional model, has accumulated. These alternative steroid effects have been described for all classes of steroids and a multitude of species and tissues with different mechanisms of action.

Binding of 1alpha,25-dihydroxyvitamin D(3) to annexin II: effect of vitamin D metabolites and calcium

We have recently reported that annexin II serves as a membrane receptor for 1alpha,25-(OH)(2)D(3) and mediates the rapid effect of the hormone on intracellular calcium. The purpose of these studies was to characterize the binding of the hormone to annexin II, determine the specificity of binding, and assess the effect of calcium on binding. The binding of [(14)C]-1alpha,25-(OH)(2)D(3) bromoacetate to purified annexin II was inhibited by 1alpha, 25-(OH)(2)D(3) in a concentration-dependent manner. Binding of the radiolabeled ligand to annexin II was markedly diminished by 1alpha, 25-(OH)(2)D(3) at 24 microM, 18 microM, and 12 microM and blunted by 6 microM and 3 microM. At a concentration of 12 microM, 1beta, 25-(OH)(2)D(3) also diminished the binding of [(14)C]-1alpha, 25-(OH)(2)D(3) bromoacetate to annexin II, but cholecalciferol, 25-(OH)D(3), and 24,25-(OH)(2)D(3) did not. Saturation analyses of the binding of [(3)H]-1alpha,25-(OH)(2)D(3) to purified annexin II showed a K(D) of 5.5 x 10(-9) M, whereas [(3)H]-1beta,25-(OH)(2)D(3) exhibited a K(D) of 6.0 x 10(-9) M. Calcium, which binds to the carboxy terminal domain of annexin II, had a concentration-dependent effect on [(14)C]-1alpha,25-(OH)(2)D(3) bromoacetate binding to annexin II, with 600 nM calcium being able to inhibit binding of the radiolabeled analog. The inhibitory effect of calcium was prevented by EDTA. Homocysteine, which binds to the amino terminal domain of annexin II, had no effect on the binding of the bromoacetate analog to the protein. The data indicate that 1alpha,25-(OH)(2)D(3) binding to annexin II is specific and suggest that the binding site may be located on the carboxy terminal domain of the protein. The ability of 1beta,25-(OH)(2)D(3) to inhibit the binding of [(14)C]-1alpha, 25(OH)(2)D(3) bromoacetate to annexin II provides a biochemical explanation for the ability of the 1beta-epimer to inhibit the rapid actions of the hormone in vitro.

Steroid hormones use non-genomic mechanisms to control brain functions and behaviors: a review of evidence

Progestins, estrogens, androgens, and corticosteroids are capable of modifying brain functions and behaviors by mechanisms that involve the classic genomic model for steroid action. However, experimental evidence indicates that some responses to steroid hormones use non-classical, non-genomic mechanisms. This paper reviews the evidence that steroids can bind to receptors in the plasma membrane, activate cell signaling pathways, and regulate responses on a time scale of seconds or a few minutes. The existence of these alternative regulatory pathways for steroid hormones should make endocrinologists and neurobiologists change how they think about steroid hormones. It is no longer valid to assume that minute-to-minute changes in steroid concentrations are not regulating biologically important, short-term responses, or that the only steroids with biological functions are the ones that bind with high affinity to intracellular steroid receptors.

Mannheim classification of nongenomically initiated (rapid) steroid action(s)

There is increasing evidence for rapid effects of steroids that are incompatible with the classical model of genomic steroid action. To address the diversity of mechanisms for rapid steroid signaling described over the past years, a classification of rapid steroid effects has been proposed to promote the discussion and understanding of nongenomic steroid action.

Specific nongenomic actions of aldosterone

BACKGROUND

According to the traditional model, steroid hormones modulate gene transcription and protein synthesis. The considerable latency of these genomic steroid effects is the consequence of these time-consuming steps of action. Over the years, it has become increasingly clear that rapid actions of steroids exist that are incompatible with this "classic" genomic model of action. These rapid, nongenomic effects, which recently have been shown for virtually all groups of steroids, are likely to be transmitted by specific membrane receptors.

METHODS

A review of data mainly focusing on the nongenomic in vitro and in vivo effects of aldosterone is presented.

RESULTS

For rapid aldosterone effects, a prominent example of a receptor/effector cascade for nongenomic steroid effects has been described in various cell types. Nonclassic membrane receptors with a high affinity for aldosterone, but not for cortisol, seem to be involved. As an important second messenger, [Ca2+]i is consistently increased within minutes after the addition of aldosterone. The effects are half maximal at physiological concentrations of free aldosterone (approximately 0.1 nmol/L), while the classic mineralocorticoid antagonist canrenone is ineffective in blocking the action of aldosterone. In addition, cortisol is active only at supramicromolar concentrations. Aldosterone rapidly acts on further cell signaling systems, for example, phosphoinositide hydrolysis and cAMP generation.

CONCLUSIONS

For a better understanding of nongenomic aldosterone action even in a clinical context, future research will have to target the cloning of the first membrane receptor for aldosterone and the evaluation of the clinical relevance of rapid steroid effects in general.

Rapid, nongenomic steroid actions: A new age?

In the traditional theory of steroid action, steroids bind to intracellular receptors and modulate nuclear transcription after translocation of steroid-receptor complexes into the nucleus. Due to similarities of molecular structure, specific receptors for steroids, vitamin D(3) derivatives, and thyroid hormone are considered to represent a superfamily of steroid receptors. While genomic steroid effects characterized by their delayed onset of action and their sensitivity to blockers of transcription and protein synthesis have been known for several decades, rapid actions of steroids have been more widely recognized and characterized in detail only recently. Rapid effects of steroids, thyroid hormones, and the steroid hormone metabolite of vitamin D(3), 1alpha, 25-dihydroxyvitamin D(3), on cellular signaling and function may be transmitted by specific membrane receptors. Binding sites in membranes have been characterized, exposing binding features compatible with an involvement in rapid steroid signaling. Characteristics of putative membrane receptors are completely distinct from intracellular steroid receptors, a fact which is further supported by the inability of classic steroid receptor antagonists to block nongenomic steroid actions. A putative progesterone membrane receptor has been cloned and functionally expressed with regard to progesterone binding. Development of drugs that specifically affect nongenomic action alone or even both modes of action may find applications in various, areas such as in the cardiovascular and central nervous systems and treatment of preterm labor, infertility, and electrolyte abnormalities.

Rapid nongenomic effects of aldosterone in mineralocorticoid-receptor-knockout mice

In addition to genomic effects of aldosterone, rapid nongenomic effects of steroids have been reported in various tissues that were clearly incompatible with a genomic action of aldosterone. Rapid effects of aldosterone involve second messengers such as calcium and cAMP. Specific high affinity binding sites for aldosterone have been characterized in membranes for different cells, which probably transmit those rapid steroid effects. To date, it is unclear if these binding sites are modified classical mineralocorticoid receptors (MR) or if they represent an unrelated receptor protein. The aim of the present study was to investigate whether rapid aldosterone action still occurs in the absence of the classical MR. For this purpose we used the model of MR knockout mice. Rapid effects were analyzed in skin cells, measuring intracellular calcium and cAMP levels after stimulation with aldosterone. We found that rapid effects are not only present in MR knockout mice, but that the effects are even larger than in wild-type mice cells. The results of the present study demonstrate that the classic MR is dispensable for rapid aldosterone action. The study, thus, proves that a receptor different from the classic intracellular receptor is involved in rapid aldosterone signaling.

Segregation of retinoic acid effects on fetal ovarian germ cell mitosis versus apoptosis by requirement for new macromolecular synthesis

Retinoic acid (RA), a naturally occurring metabolite of vitamin A, plays an essential role in regulating cellular growth, differentiation, and death in a variety of tissues, particularly during fetal development. However, essentially nothing is known of the effects of RA on fetal gametogenesis. Using a recently validated system of culturing murine fetal ovaries, herein we sought to characterize the actions of RA on female germ cell proliferation and apoptosis during oogenesis. In the absence of trophic hormone support, approximately 90% of the oogonia and oocytes present in fetal ovaries at the start of culture underwent apoptosis over a 72 h culture period (P < 0.05), whereas provision of 0.01-1 microM RA dose dependently maintained germ cell numbers. In fact, ovaries cultured with 0.1 microM RA for 72 h possessed approximately 30% more oogonia and oocytes as compared with the preculture mean number (P < 0.05). Additional experiments, using in situ DNA 3'-end-labeling and cellular morphology to assess apoptosis coupled with 5-bromo-2'-deoxyuridine incorporation to assess proliferation, revealed that RA acts as both a mitogen and a survival factor for female germ cells. Furthermore, the ability of RA to stimulate germ cell proliferation in cultured fetal ovaries was completely suppressed (P < 0.05) by cotreatment with inhibitors of transcription (alpha-amanitin, 0.1 microg/ml) or protein synthesis (cycloheximide, 1.0 microg/ml), whereas RA-mediated suppression of germ cell apoptosis was not affected by cotreatment with either macromolecular synthesis inhibitor (P > 0.05). Moreover, cotreatment of fetal ovaries with 5 microM LY294002, an inhibitor of phosphatidylinositol 3'-kinase, had no effect on RA-promoted germ cell maintenance (P > 0.05). By comparison, the antiapoptotic effects of insulin-like growth factor I on germ cells in cultured fetal ovaries were significantly attenuated by cotreating ovaries with LY294002 (P < 0.05) but not with alpha-amanitin or cycloheximide (P > 0.05). Importantly, the effect of RA on the female germ line was also observed in vivo because a single oral administration of 100 mg/kg RA to timed-pregnant female mice resulted in a significantly (P < 0.05) larger endowment of primordial oocytes in female offspring. That these actions were mediated, at least in part, by specific retinoid receptors was demonstrated by the finding of retinoic acid receptor protein in fetal female gonocytes, as assessed by immunohistochemical localization experiments. Collectively, these data indicate that RA can function, in vitro and in vivo, as a potent germ cell survival factor and mitogen during fetal oogenesis in the mouse.

Nongenomic steroid actions: fact or fantasy?

In the common theory of steroid action, steroids bind to intracellular receptors and modulate nuclear transcription after translocation of steroid--receptor complexes into the nucleus. Due to homologies of molecular structure, specific receptors for steroids, vitamin D3, and thyroid hormone are considered to represent a superfamily of steroid receptors. While genomic steroid effects being characterized by their delayed onset of action and their sensitivity to blockers of transcription and protein synthesis have been known for several decades, very rapid actions of steroids have been more widely recognized and characterized in detail only recently. Rapid effects of steroids, vitamin D3, and thyroid hormones on cellular signaling and function may be transmitted by specific membrane receptors. Although no receptor of this kind has been cloned up to now, binding sites in membranes have been characterized exposing binding features compatible with an involvement in rapid steroid signaling. Characteristics of putative membrane receptors were completely different from those of intracellular steroid receptors, which was further supported by the inability of classic steroid receptor antagonists to inhibit nongenomic steroid actions. Development of drugs that specifically affect nongenomic action alone or even both modes of actions may find applications in various areas such as the cardiovascular and central nervous systems and treatment of preterm labor, infertility, and electrolyte homeostasis. To acquaint the reader with major aspects of nongenomic steroid actions, these effects on cellular function will be summarized, potentially related binding sites in membranes discussed, and the physiological or pathophysiological relevance of nonclassic actions exemplified.

Rapid cardiovascular action of aldosterone in man

Rapid nongenomic in vitro effects of aldosterone have been demonstrated recently in cultured vascular smooth muscle and endothelial cells. But there is, as yet, little evidence for corresponding in vivo effects. The present study thus investigates the rapid nongenomic effects of aldosterone on human cardiovascular function. In a double-blind placebo-controlled randomized parallel trial on 17 patients with suspected coronary heart disease, the effect of 1 mg aldosterone iv on cardiovascular function was assessed during cardiac catheterization. Hemodynamic parameters (such as heart rate, left ventricular and atrial pressures, arterial pressures, vascular resistances, and cardiac output) were measured before and 3 and 10 min after administration of aldosterone or placebo. Significant changes were found for systemic vascular resistance, cardiac output, and cardiac index, compared with the placebo group (Wilcoxon test, P < 0.02-0.05). The effect of aldosterone dissipated within 10 min. The results are in line with the in vitro data cited above and consistent with earlier findings on acute cardiovascular effects of aldosterone, which have now been confirmed and extended by contemporary techniques. The hypotheses of rapid nongenomic in vivo effects of aldosterone are further substantiated by this study.

Early aldosterone effects

Aldosterone stimulates sodium reabsorption across tight epithelia via corticosteroid receptors. These receptors are ligand-activated transcription factors which regulate a set of genes, leading to changes in the expression of proteins which are thought to mediate the stimulatory action on sodium reabsorption. This functional response starts after a lag period and can be schematically divided into an early phase of activation and a late phase of accumulation of effectors such as Na+ channels and pumps. Recently, the first gene products regulated sufficiently rapidly by aldosterone to possibly account for the early activation have been identified. However, the actual mediators of the physiological response remain to be determined. Besides these transcriptionally mediated effects, aldosterone has been shown to produce rapid, nongenomic actions on intracellular signalling cascades leading to an activation of Na+/H+ exchange. Such effects are mediated by a different type of receptor and have been described both in classical aldosterone target cells and other cells. Their physiological implications are as yet not clear. The aim of this minireview is to describe briefly both the classical early and rapid nongenomic effects of aldosterone and to address the question whether nongenomic effects might play a role for the Na+ transport response.

Characterization of high affinity progesterone-binding membrane proteins by anti-peptide antiserum

A chemically synthesized 15-mer oligopeptide derived from the N terminus of high affinity progesterone-binding membrane site(s) from porcine liver was used to generate site-specific antibodies. Western blotting experiments confirmed the specificity of the anti-peptide serum obtained. In further investigations this antiserum was used for the identification of the native progesterone-binding membrane protein complex that represents an oligomer with an apparent molecular mass of about 200 kDa. In temperature-induced Triton X-114 phase separation experiments combined with Western-blotting, the progesterone-binding site was identified as an hydrophobic (integral) membrane protein. In addition, in Western blotting analyses the antiserum reacted with the progesterone-binding or related proteins in membrane fractions from a wide array of different tissues in various species.

Nongenomic effects of aldosterone on intracellular calcium in porcine endothelial cells

Rapid in vitro effects of aldosterone on intracellular electrolytes, cell volume, and the sodium-proton antiport have been described in human mononuclear leukocytes and vascular smooth muscle cells. In the present study, we demonstrate rapid aldosterone effects on free intracellular calcium as determined by fura 2 fluorometry in single porcine endothelial cells. After addition of 100 nmol/l aldosterone, cells respond with a sustained rise in free intracellular calcium by approximately 50% of initial levels within 1-5 min. Elevations are predominantly seen in the subplasmalemmal space. Effective half-maximal concentration values for aldosterone are approximately 1 pmol/l and for cortisol approximately 1 nmol/l. These effects are blunted in calcium-free medium and absent after pretreatment by thapsigargine. They remain unchanged by a >1,000-fold excess of spironolactone. These findings indicate the existence of a nongenomic pathway for aldosterone action in porcine endothelial cells and may be related to known rapid cardiovascular effects of aldosterone in vivo mediated through the baroreceptor reflex.

Specific, nongenomic actions of steroid hormones

Traditionally, steroid hormone action has been described as the modulation of nuclear transcription, thus triggering genomic events that are responsible for physiological effects. Despite early observations of rapid steroid effects that were incompatible with this theory, nongenomic steroid action has been widely recognized only recently. Evidence for these rapid effects is available for steroids of all clones and for a multitude of species and tissues. Examples of nongenomic steroid action include rapid aldosterone effects in lymphocytes and vascular smooth muscle cells, vitamin D3 effects in epithelial cells, progesterone action in human sperm, neurosteroid effects on neuronal function, and vascular effects of estrogens. Mechanisms of action are being studied with regard to signal perception and transduction, and researchers have developed a patchy sketch of a membrane receptor-second messenger cascade similar to those involved in catecholamine and peptide hormone action. Many of these effects appear to involve phospholipase C, phosphoinositide turnover, intracellular pH and calcium, protein kinase C, and tyrosine kinases. The physiological and pathophysiological relevance of these effects is unclear, but rapid steroid effects on cardiovascular, central nervous, and reproductive functions may occur in vivo. The cloning of the cDNA for the first membrane receptor for steroids should be achieved in the near future, and the physiological and clinical relevance of these rapid steroid effects can then be established.

Non-genomic effects of estrogen and the vessel wall

Estrogen, like other steroids, is now believed to possess rapid membrane effects independent of the classical gene activation pathway of steroid action. The presence of membrane estrogen receptors has been demonstrated in different cell types, but not yet in vascular tissue. In vivo, estrogen administration rapidly promotes acetylcholine-induced vasodilation of the coronary and peripheral vascular beds of postmenopausal women. Estrogen also causes relaxation of precontracted isolated arterial segments and perfused organ preparations, within minutes of administration of the hormone. These rapid vasomotor effects of estrogen may be related to blockade of the cell membrane voltage-dependent calcium channels, resulting in inhibition of extracellular Ca2+ mobilization and flux. Recently, estradiol has been shown to rapidly affect cyclic nucleotide turnover in vascular segments, smooth muscle, and epithelial cell cultures, suggesting the possibility of a "cross-talk" between membrane-mediated events and nuclear receptor activation.

Characterization and solubilization of novel aldosterone-binding proteins in porcine liver microsomes

Using the radioligand [1,2,6,7-3H]aldosterone ([3H]aldosterone), specific binding sites for aldosterone were identified and characterized in microsomal preparations from porcine liver. The maximum binding capacity is approximately 700 fmol x mg-1 microsomal protein. The reversible binding of [3H]aldosterone was saturable and Scatchard analysis revealed two apparent dissociation constants (Kd), Kd1 < or = 11 nM and Kd2 = 118 nM. Binding was optimal at pH 7.2, thermolabile, and was reduced by more than 70% when membrane vesicles were pretreated with trypsin. Binding was selective for aldosterone with cortisol being a weak agonist at 1000-fold higher concentrations only. Among those detergents tested to optimize conditions for solubilization, n-octylglucoside (75 mM) was most favorable and solubilized 25% of the radioligand-binding protein complex in the undissociated form. These binding sites have unique pharmacological properties, which are similar to those found for aldosterone membrane binding in human lymphocytes and pig kidney, and for rapid aldosterone effects on sodium-proton exchange.