Aldosterone (ALDO) increases transmembrane influx of Na+ in vascular smooth muscle (VSM) cells through increased synthesis of Na+ channels

Glomerular Mesangial Cell pH Homeostasis Mediates Mineralocorticoid Receptor-Induced Cell Proliferation

Mineralocorticoids (e.g., aldosterone) support chronic inflammatory tissue damage, including glomerular mesangial injury leading to glomerulosclerosis. Furthermore, aldosterone leads to activation of the extracellular signal-regulated kinases (ERK1/2) in rat glomerular mesangial cells (GMC). Because ERK1/2 can affect cellular pH homeostasis via activation of Na⁺/H⁺-exchange (NHE) and the resulting cellular alkalinization may support proliferation, we tested the hypothesis that aldosterone affects pH homeostasis and thereby cell proliferation as well as collagen secretion also in primary rat GMC. Cytoplasmic pH and calcium were assessed by single-cell fluorescence ratio imaging, using the dyes BCECF or FURA2, respectively. Proliferation was determined by cell counting, thymidine incorporation and collagen secretion by collagenase-sensitive proline incorporation and ERK1/2-phosphorylation by Western blot. Nanomolar aldosterone induces a rapid cytosolic alkalinization which is prevented by NHE inhibition (10 µmol/L EIPA) and by blockade of the mineralocorticoid receptor (100 nmol/L spironolactone). pH changes were not affected by inhibition of HCO₃^- transporters and were not dependent on HCO₃^-. Aldosterone enhanced ERK1/2 phosphorylation and inhibition of ERK1/2-phosphorylation (10 µmol/L U0126) prevented aldosterone-induced alkalinization. Furthermore, aldosterone induced proliferation of GMC and collagen secretion, both of which were prevented by U0126 and EIPA. Cytosolic calcium was not involved in this aldosterone action. In conclusion, our data show that aldosterone can induce GMC proliferation via a MR and ERK1/2-mediated activation of NHE with subsequent cytosolic alkalinization. GMC proliferation leads to glomerular hypercellularity and dysfunction. This effect presents a possible mechanism contributing to mineralocorticoid receptor-induced pathogenesis of glomerular mesangial injury during chronic kidney disease.

Eplerenone — A Novel Selective Aldosterone Blocker

OBJECTIVE:

To review the pharmacology, pharmacokinetics, clinical efficacy, and safety of eplerenone, a new selective aldosterone blocker.

DATA SOURCES:

Primary literature and review articles were obtained via MEDLINE search (1966–April 2002). Additional studies and abstracts were identified from the bibliographies of reviewed literature.

STUDY SELECTION AND DATA EXTRACTION:

Studies and review articles related to eplerenone, aldosterone, aldosterone antagonist, and spironolactone were reviewed. Data pertinent to this article were included.

DATA SYNTHESIS:

Eplerenone is a selective aldosterone blocker. Recent data have demonstrated the deleterious effects of aldosterone in several chronic disease states including hypertension and heart failure. Animal studies using eplerenone have shown a positive role for aldosterone antagonism in the treatment of hypertension, heart failure, myocardial infarction, renal disease, and atherosclerosis. In humans, eplerenone appears to be effective for the treatment of hypertension. An ongoing study will examine the effect of eplerenone for heart failure. To date, the incidence of adverse effects with eplerenone has been slightly lower than with spironolactone.

CONCLUSIONS:

Eplerenone appears to be a promising drug in a new class of agents called selective aldosterone blockers. The drug may be approved for treatment of hypertension in 2002. Additional studies are ongoing that may provide information on other clinical uses for this medication.

TAT fusion protein transduction into isolated mitochondria is accelerated by sodium channel inhibitors

Stringent control of ion and protein transport across the mitochondrial membranes is required to maintain mitochondrial function and biogenesis. In particular, the inner mitochondrial membrane is generally impermeable to proteins entering the matrix except via tightly regulated protein import mechanisms. Recently, cell penetrant peptides have been shown to move across the inner mitochondrial membrane in a manner suggesting an independent mechanism. HIV-1 transactivator of transcription (TAT) is an arginine-rich cell penetrant peptide, 47YGRKKRRQRRR57, which can transduce full-length proteins not only across the cell membrane but also into intracellular organelles. In this study, we investigated the ability of a TAT-containing protein to move into the mitochondrial matrix. Using a novel FACS assay for isolated, purified mitochondria, we show that TAT can deliver a modified fluorescent protein, mMDH-GFP, to the matrix of mitochondria and it is subsequently processed by the matrix peptidases. In addition, transduction of TAT-mMDH-GFP into mitochondria is independent of canonical protein import pathways as well as mitochondrial membrane potential. In direct contrast to published reports regarding the cell membrane where the sodium channel inhibitor, amiloride, blocks endocytosis and inhibits TAT transduction, TAT transduction into mitochondria is markedly increased by this same sodium channel inhibitor. These results confirm that the cell penetrant peptide, TAT, can readily transduce a protein cargo into the mitochondrial matrix. These results also demonstrate a novel role for mitochondrial sodium channels in mediating TAT transduction into mitochondria that is independent of endocytotic mechanisms. The mechanism of TAT transduction into mitochondria therefore is distinctly different from transduction across the cell membrane.

Renoprotection, renin inhibition, and blood pressure control: the impact of aliskiren on integrated blood pressure control

Hypertension (HTN) is an important factor in progressive loss of renal function. The kidney can be both a contributor to and a target of HTN. The functional integrity of the kidney is vital for the maintenance of cardiovascular homeostasis. Chronic activation of the renin system causes HTN and, ultimately, end-organ damage. Direct renin inhibitors (DRIs) inhibit plasma renin activity (PRA), thereby preventing the conversion of angiotensinogen to angiotensin I; consequently, the levels of both Ang I and Ang II are reduced. There is no compensatory increase in PRA activity with DRIs as seen with angiotensin-converting enzyme inhibitors (ACEIs) or angiotensin receptor blockers (ARBs). There are reasons to speculate that renin inhibition might prove to be a superior strategy for blocking the renin-angiotensin-aldosterone system compared with ACEIs or ARBs. Evidence for the efficacy of aliskiren (a DRI) is considered to be relatively strong, based on published, short-term, double-blind, randomized, controlled trials showing that aliskiren is as effective as other antihypertensive agents in reducing blood pressure (BP), with no rebound effects on BP after treatment withdrawal. When combined with diuretics, fully additive BP reduction is seen. When given with an ACEI or ARB, aliskiren produces significant additional BP reduction indicative of complimentary pharmacology and more complete renin-angiotensin system blockade.

Enhancement of receptor-operated cation current and TRPC6 expression in arterial smooth muscle cells of deoxycorticosterone acetate-salt hypertensive rats

OBJECTIVES

In deoxycorticosterone acetate (DOCA)-salt hypertensive rats, altered reactivity of blood vessels to vasoactive agonists is frequently associated with an elevation in blood pressure. Canonical transient receptor potential (TRPC) channels are believed to encode receptor-operated cation channels (ROC), the activation of which is involved in smooth muscle depolarization and vasoconstriction. The aims of the present study were to investigate whether the ROC current is increased in DOCA-hypertensive rats and determine whether aldosterone directly enhances the expression of TRPC.

METHODS

The nystatin-perforated patch-clamp technique was used for the recording of receptor-stimulated ion currents in mesenteric arterial smooth muscle cells, which were enzymatically dispersed from sham-operated and DOCA-salt hypertensive rats. Expressions of TRPCs were evaluated by reverse transcriptase-polymerase chain reaction (RT-PCR) and by Western blot analysis.

RESULTS

Receptor-stimulated currents activated by 5-hydroxytryptamine (serotonin) and norepinephrine were increased significantly in the mesenteric arterial smooth muscle cells of DOCA-salt hypertensive rats compared to sham-operated rats. Ion-substitution experiments revealed that the enhanced currents were cation currents (ROC currents). Enhanced expression of TRPC6 in mesenteric arteries from DOCA-salt hypertensive rats was demonstrated by real-time RT-PCR. Up-regulation of TRPC6 by aldosterone treatment in vitro was also observed in A7r5 cells by RT-PCR and in western blots.

CONCLUSION

These results suggest that aldosterone enhances TRPC6 expression and ROC currents in vascular smooth muscle cells, and that this may in turn contribute to altered vascular reactivity and to hypertension.

Serotonin-induced ion channel modulations in mesenteric artery myocytes from normotensive and DOCA-salt hypertensive rats

Although serotonin (5-hydroxytryptamine, 5-HT) has been found to be a potent vasoconstrictor, a pivotal role of 5-HT in the control of appetite and mood control by the modulation of neuronal synapse has also been proposed. Selective 5-HT reuptake inhibitors (SSRIs) are frequently used to suppress appetite and treat depressive disorder, and the target protein of SSRIs is the 5-HT transporter (5-HTT) in the neuronal synapse. However, SSRIs may increase the free 5-HT concentration in circulating blood because platelets and vascular smooth muscles express functional 5-HTT. In addition, enhanced vasoactive action of 5-HT and alterations in 5-HT receptor subtypes have been reported in some types of hypertension. Therefore, we can infer that the use of drugs such as SSRIs in some hypertensive patients is potentially risky. Altered functional expression of ion channels in vascular smooth muscle is suggested to be a mechanism for the enhanced vasoconstriction by vasoactive agonists, including 5-HT. In this brief review, we compared the electrophysiological properties of mesenteric artery myocytes and their modulation by 5-HT between sham-operated control and deoxycorticosterone acetate (DOCA)-salt hypertensive rats.

Aldosterone in the development and progression of renal injury

BACKGROUND

The renin-angiotensin-aldosterone system (RAAS) contributes to hypertension and nephropathy. Until recently, aldosterone either has not been considered or has been considered a relatively minor component of the process-a contribution that could be negated with angiotensin-converting enzyme (ACE) inhibition or angiotensin receptor blockade.

METHODS

A Medline search was performed to identify relevant literature describing the role of aldosterone in the pathogenesis of renal dysfunction.

RESULTS

Growing evidence from experimental and clinical studies indicates that increased aldosterone is an independent contributor to small- and medium-sized arterial injury and nephropathy. Excess mineralocorticoid receptor stimulation of local and systemic origin promotes target organ dysfunction, vascular injury, and fibrosis, independent of the effects of other elements of the RAAS. Blockade of the RAAS with ACE inhibition or angiotensin receptor blockade often does not confer optimal protection from the effects of mineralocorticoids on small- and medium-sized blood vessels. Recent preliminary data from clinical studies indicate that aldosterone blockade protects the kidneys, sharply decreases proteinuria, beyond the activities of ACE inhibition or angiotensin receptor blockade and independent of beneficial blood pressure effects, and can protect patients from vascular injury associated with diabetes mellitus and hypertension.

CONCLUSION

Aldosterone blockade with the selective aldosterone blocker eplerenone, in combination with other RAAS inhibitors, is probably renoprotective and should be considered as a component of the treatment regimens of diabetic and hypertensive patients at risk for renal or cardiovascular disease expression. A high priority should be placed on developing the randomized, controlled trials required to establish that role.

Sodium transport antagonism reduces thrombotic microangiopathy in stroke-prone spontaneously hypertensive rats

We examined whether amiloride, an agent that possesses epithelial sodium channel (ENaC)- and sodium/hydrogen exchange (NHE)-inhibitory activities, would exhibit renal vascular protection in saline-drinking, stroke-prone spontaneously hypertensive rats (SHRSP). SHRSP received amiloride (1.0 mg·kg−1·day−1, n = 6) or deionized water (3 mg·kg−1·day−1, n = 6) for 5 wk starting at 61 days of age. Systolic blood pressure (SBP) did not differ among the groups, and there was no difference in the average daily urine output, sodium excretion, or potassium excretion. Terminal urinary protein excretion, blood urea nitrogen, and renal thrombotic microangiopathic lesions were markedly reduced in the amiloride group with no difference in plasma renin activity (PRA). In a survival protocol, SHRSP infused subcutaneously with benzamil (0.7 mg·kg−1·day−1, n = 8), a selective ENaC inhibitor, dimethylamiloride (0.7 mg·kg−1·day−1, n = 8), a selective NHE inhibitor, or vehicle ( n = 7) had comparable SBP. Dimethylamiloride nonetheless prolonged survival of SHRSP ( P < 0.005 vs. vehicle), and benzamil-treated SHRSP lived even longer ( P < 0.0001 vs. vehicle; P < 0.05 vs. dimethylamiloride). In a separate series, plasma potassium concentration was elevated by dimethylamiloride (3.4 ± 0.1 meq/l, n = 8) and benzamil (3.3 ± 0.1 meq/l, n = 8) relative to vehicle (3.0 ± 0.1 meq/l, n = 8) at 4 but not at 24 h after dosing. These findings suggest the involvement of a sodium transport mechanism in the development of thrombotic microangiopathy in SHRSP, unrelated to marked changes in arterial pressure, PRA, plasma potassium, or urinary water and electrolyte excretion.

Aldosterone, mineralocorticoid receptors and vascular inflammation

For the past 50 years, the physiological action of aldosterone was considered to be on epithelial tissues to maintain fluid and electrolyte homeostasis. Recently, a nonepithelial, pathophysiologic, proinflammatory role for aldosterone has been inferred from studies on mineralocorticoid/salt administration, with or without mineralocorticoid receptor (MR) blockade, in experimental animals, and from clinical studies such as RALES and EPHESUS. More recently still, it has become clear that the pathophysiologic trigger for the vascular inflammatory response observed is not necessarily aldosterone per se, but inappropriate activation of vascular wall MR. MR can be inappropriately activated by aldosterone in the context of an inappropriate salt status, or by glucocorticoids in the context of tissue damage. The studies supporting this latter conclusion, and the novel mechanisms proposed to support this concept, are details in the text to follow.

Combined effects of enalapril and spironolactone in hamsters with dilated cardiomyopathy

Chronic angiotensin I-converting enzyme inhibition can be associated with aldosterone escape. We investigated the effects of enalapril, spironolactone, and their combination on hemodynamics and cardiac remodeling in cardiomyopathic hamsters to determine whether these drugs could exert additive effects. Cardiomyopathic hamsters, Bio TO-2 dilated strain, were orally treated with enalapril (20 mg. kg. day ) and/or spironolactone (20 mg. kg. day ) according to a 2 x 2 factorial design from 120 days of age. Animals were investigated at 180 (10 animals per group) and 240 (16 animals per group) days of age. Compared with corresponding untreated groups, enalapril significantly decreased mean blood pressure (-18%); enalapril and spironolactone significantly increased cardiac output (+28%, +11%) and femoral blood flow (+10%, +12%) and significantly decreased systemic (-38%, -17%) and femoral (-26%, -13%) vascular resistances. Enalapril and spironolactone significantly decreased left ventricle cavity area (-21%, -26%) and left (-34%, -47%) and right (-37%, -48%) ventricle collagen density. Spironolactone significantly increased left ventricle wall thickness (+4%). There were significant enalapril x spironolactone interactions for most variables (compared with control group, +52%, +36%, +45% for cardiac output; +26%, +28%, +26% for femoral blood flow; -50%, -30%, -45% for systemic vascular resistance; -33%, -20%, -35% for femoral vascular resistance; -27%, -31%, -40% for left ventricle cavity area; and -46%, -58%, -60% for left and -39%, -50%, -66% for right ventricle collagen density in enalapril, spironolactone, and enalapril + spironolactone groups, respectively). In cardiomyopathic hamsters, enalapril and spironolactone in combination did not improve hemodynamics more than enalapril alone but induced stronger effects than each drug alone on cardiac remodeling.

Combined effects of metoprolol and spironolactone in dilated cardiomyopathic hamsters

The use of beta-blockers reduces angiotensin II levels, but could not adequately suppress aldosterone production. Thus, the combination of a beta-blocker with an aldosterone receptor antagonist could exert additive effects. The effects of metoprolol and spironolactone and their combination on hemodynamics and cardiac remodeling in cardiomyopathic hamsters (CMH) were investigated. The Bio TO-2 dilated strain of CMH was treated orally with metoprolol (10 mg/kg/day), spironolactone (20 mg/kg/day), or both according to a 2 x 2 factorial design (24 animals per group) from 120 days of age and during 120 days. As compared to corresponding untreated groups, metoprolol significantly decreased mean blood pressure (-7%), and metoprolol and spironolactone significantly increased cardiac output (18% and 19%, respectively), mesenteric blood flow (11% and 14%), and femoral blood flow (13% and 17%), and significantly decreased systemic (-24% and -15%), mesenteric (-14% and -13%) and femoral (-19% and -10%) vascular resistances. Metoprolol significantly increased renal blood flow (22%) and significantly decreased renal vascular resistance (-23%). Metoprolol and spironolactone significantly decreased the cavity area of the left ventricle (-21% and -32%, respectively) and the collagen density of the left (-36% and -39%) and right (-38% and -43%) ventricles. Although the combination did not induce stronger effects than each drug alone on the systemic and most regional hemodynamic variables, it did have a stronger effect on the cardiac remodeling (compared to control group: -24%, -34%, and -46% for the left ventricle cavity area, -33%, -35%, and -62% for collagen density in the left ventricle, and -52%, -57%, and -59% for collagen density in the right ventricle, respectively, in the metoprolol, spironolactone, and metoprolol + spironolactone groups). In CMH, metoprolol and spironolactone combined did not improve hemodynamics more than each drug alone, but did exert additive effects on cardiac remodeling.

Aldosterone as a mediator in cardiovascular injury

The renin-angiotensin-aldosterone system plays a central role in the development of hypertension and the progression of end-organ damage. Although angiotensin-converting enzyme inhibitors and angiotensin II receptor antagonists can initially suppress plasma aldosterone, it is now well established that aldosterone escape may occur, whereby aldosterone levels return to or exceed baseline levels. The classic effects of aldosterone relate mainly to its action on epithelial cells to regulate water and electrolyte balance. However, blood pressure reduction or fluid loss could not account for the results of the Randomized Aldactone Evaluation Study, which showed that a low dose of spironolactone in addition to conventional therapy could decrease the overall risk of mortality by 30% among patients with severe congestive heart failure. The action of aldosterone at nonepithelial sites in the brain, heart, and vasculature is consistent with the presence of mineralocorticoid receptors in these tissues. Aldosterone has a number of deleterious effects on the cardiovascular system, including myocardial necrosis and fibrosis, vascular stiffening and injury, reduced fibrinolysis, endothelial dysfunction, catecholamine release, and production of cardiac arrhythmias. Several studies have now shown vascular and target-organ protective effects of aldosterone receptor antagonism in the absence of significant blood pressure lowering, consistent with a major role for endogenous mineralocorticoids as mediators of cardiovascular injury. The advent of selective aldosterone receptor antagonists such as eplerenone should prove of great therapeutic value in the prevention of cardiovascular disease and associated end-organ damage.

Role of central mineralocorticoid receptors in cardiovascular disease

Mineralocorticoids act directly through their receptors in specific centers in the central nervous system, kidneys, heart, and vascular smooth muscle to mediate hemodynamic homeostasis. These steroids also modulate renal and cardiovascular function indirectly through the autonomic nervous system. Complex homeostatic mechanisms under normal hormonal control become pathogenic when there is an excess of regulatory hormone. Experiments in which mineralocorticoid receptor antagonists or antisense oligodeoxynucleotides were administered centrally have clearly shown that centrally mediated effects on salt appetite, baroreceptor function, and autonomic drive to the renal and cardiovascular systems are crucial to the pathogenesis of hypertension and cardiovascular disease of hyperaldosteronism, and certain forms of genetic hypertension.

Aldosterone and the hypertensive kidney: its emerging role as a mediator of progressive renal dysfunction: a paradigm shift

End-stage renal disease (ESRD) comprises an enormous public health burden, with an increasing incidence and prevalence. Hypertension is a major risk factor for progressive renal disease. This escalating prevalence suggests that newer therapeutic interventions and strategies are needed to complement current antihypertensive approaches. Although much evidence demonstrates that angiotensin II mediates progressive renal disease, recent evidence also implicates aldosterone as an important pathogenetic factor in progressive renal disease. Several lines of experimental evidence demonstrate that selective blockade of aldosterone, independent of renin-angiotensin blockade, reduces proteinuria and nephrosclerosis in the spontaneously hypertensive stroke-prone rat model and reduces proteinuria and glomerulosclerosis in the subtotally nephrectomized rat model (i.e. remnant kidney). Whereas pharmacological blockade with angiotensin II receptor blockers and angiotensin-converting enzyme inhibitors reduces proteinuria and nephrosclerosis/ glomerulosclerosis, selective reinfusion of aldosterone restores these abnormalities despite continued renin-angiotensin blockade. Aldosterone may promote fibrosis by several mechanisms, including plasminogen activator inhibitor-1 expression and consequent alterations of vascular fibrinolysis, by stimulation of transforming growth factor-beta 1, and by stimulation of reactive oxygen species. Based on this theoretical construct, randomized clinical studies will be initiated to delineate the potential renal-protective effects of antihypertensive therapy utilizing aldosterone receptor blockade.

Aldosterone as a mediator of progressive renal disease: pathogenetic and clinical implications

End-stage renal disease is an enormous public health burden with an increasing incidence and prevalence. This escalating prevalence suggests that newer therapeutic interventions and strategies are needed to complement current antihypertensive approaches. Although much evidence shows that angiotensin II mediates progressive renal disease, recent evidence also implicates aldosterone as an important pathogenetic factor in progressive renal disease. Several lines of experimental evidence show that selective blockade of aldosterone, independent of renin-angiotensin blockade, reduces proteinuria and nephrosclerosis in the spontaneously hypertensive stroke-prone rat model and reduces proteinuria and glomerulosclerosis in the subtotally nephrectomized rat model (ie, remnant kidney). Although pharmacological blockade with angiotensin II-receptor blockers and angiotensin-converting enzyme inhibitors reduces proteinuria and nephrosclerosis and/or glomerulosclerosis, selective reinfusion of aldosterone restores these abnormalities despite continued renin-angiotensin blockade. Based on this theoretic construct, randomized clinical studies will be initiated to delineate the potential renal-protective effects of antihypertensive therapy using aldosterone-receptor blockade. This is a US government work. There are no restrictions on its use.

Insights Into Glucocorticoid-Associated Hypertension

The association between excess glucocorticoids and hypertension has been much discussed but poorly understood. From both clinical observations and laboratory studies, it is clear that glucocorticoids exert their effects at many different sites responsible for blood pressure regulation. Isoforms of the enzyme 11ss-hydroxysteroid dehydrogenase (11ss-HSD), located in steroid-responsive tissues, metabolize endogenously produced glucocorticoids. These enzymes limit steroid access to mineralocorticoid and/or glucocorticoid receptors. In the kidney, synthetic and endogenous glucocorticoids are capable of enhancing transepithelial sodium transport in the presence of 11ss-HSD inhibition. Proximal tubule reabsorption of sodium can be indirectly augmented after chronic exposure to glucocorticoids. In this segment, steroids have a permissive effect, increasing the expression of both Na(+), K(+) adenosine triphosphatase along the basolateral membrane and Na(+)-H(+) exchanger along the apical membrane of epithelial cells. Although glucocorticoids themselves produce no increase in sodium reabsorption in this segment, angiotensin II-stimulated sodium transport is significantly greater in proximal tubular cells pretreated with glucocorticoids. The increased transport in distal renal segments is more direct and stems in part from glucocorticoid cross-over binding to mineralocorticoid receptors. In vascular tissue, synthetic and endogenous glucocorticoids, after inhibition of the dehydrogenase reaction, magnify the response to circulating vasoconstrictors. The effects of glucocorticoids in vascular tissue is indirect, upregulating the expression of receptors to many vasoconstrictors and downregulating the effects of potential vasodilators. Thus, glucocorticoids have the potential to alter both circulating volume and vascular resistance.

Aldosterone and vascular damage

Although the aldosterone escape mechanism is well known, aldosterone has often been neglected in the pathophysiologic consequences of the activated renin-angiotensin-aldosterone system in arterial hypertension and chronic heart failure. There is now evidence for vascular synthesis of aldosterone aside from its secretion by the adrenal cortex. Moreover, aldosterone is involved in vascular smooth muscle cell hypertrophy and hyperplasia, as well as in vascular matrix impairment and endothelial dysfunction. The mechanisms of action of aldosterone may be either delayed (genomic) or rapid (nongenomic). Deleterious effects of aldosterone leading to vascular target-organ damage include (besides salt and water retention) decreased arterial and venous compliance, increased peripheral vascular resistance, and impaired autonomic vascular control due to baroreflex dysfunction.

General overview of mineralocorticoid hormone action

The adrenal cortex elaborates two major groups of steroids that have been arbitrarily classified as glucocorticoids and mineralocorticoids, despite the fact that carbohydrate metabolism is intimately linked to mineral balance in mammals. In fact, glucocorticoids assured both of these functions in all living cells, animal and photosynthetic, prior to the appearance of aldosterone in teleosts at the dawn of terrestrial colonization. The evolutionary drive for a hormone specifically designed for hydromineral regulation led to zonation for the conversion of 18-hydroxycorticosterone into aldosterone through the catalytic action of a synthase in the secluded compartment of the adrenal zona glomerulosa. Corticoid hormones exert their physiological action by binding to receptors that belong to a transcription factor superfamily, which also includes some of the proteins regulating steroid synthesis. Steroids stimulate sodium absorption by the activation and/or de novo synthesis of the ion-gated, amiloride-sensitive sodium channel in the apical membrane and that of the Na+/K+-ATPase in the basolateral membrane. Receptors, channels, and pumps apparently are linked to the cytoskeleton and are further regulated variously by methylation, phosphorylation, ubiquination, and glycosylation, suggesting a complex system of control at multiple checkpoints. Mutations in genes for many of these different proteins have been described and are known to cause clinical disease.