Free serum cortisol during the postoperative acute phase response determined by equilibrium dialysis liquid chromatography-tandem mass spectrometry

Prevention of Adrenal Crisis: Cortisol Responses to Major Stress Compared to Stress Dose Hydrocortisone Delivery

CONTEXT

Patients with adrenal insufficiency require increased hydrocortisone cover during major stress to avoid a life-threatening adrenal crisis. However, current treatment recommendations are not evidence-based.

OBJECTIVE

To identify the most appropriate mode of hydrocortisone delivery in patients with adrenal insufficiency who are exposed to major stress.

DESIGN AND PARTICIPANTS

Cross-sectional study: 122 unstressed healthy subjects and 288 subjects exposed to different stressors (major trauma [N = 83], sepsis [N = 100], and combat stress [N = 105]). Longitudinal study: 22 patients with preserved adrenal function undergoing elective surgery. Pharmacokinetic study: 10 patients with primary adrenal insufficiency undergoing administration of 200 mg hydrocortisone over 24 hours in 4 different delivery modes (continuous intravenous infusion; 6-hourly oral, intramuscular or intravenous bolus administration).

MAIN OUTCOME MEASURE

We measured total serum cortisol and cortisone, free serum cortisol, and urinary glucocorticoid metabolite excretion by mass spectrometry. Linear pharmacokinetic modeling was used to determine the most appropriate mode and dose of hydrocortisone administration in patients with adrenal insufficiency exposed to major stress.

RESULTS

Serum cortisol was increased in all stress conditions, with the highest values observed in surgery and sepsis. Continuous intravenous hydrocortisone was the only administration mode persistently achieving median cortisol concentrations in the range observed during major stress. Linear pharmacokinetic modeling identified continuous intravenous infusion of 200 mg hydrocortisone over 24 hours, preceded by an initial bolus of 50-100 mg hydrocortisone, as best suited for maintaining cortisol concentrations in the required range.

CONCLUSIONS

Continuous intravenous hydrocortisone infusion should be favored over intermittent bolus administration in the prevention and treatment of adrenal crisis during major stress.

Relative adrenal insufficiency in cirrhotic patients with ascites (hepatoadrenal syndrome)

AIM

Relative adrenal insufficiency (RAI) has been reported in critically ill patients with cirrhosis. We evaluated the prevalence of RAI and its relationship to clinical course in non-septic cirrhosis patients with ascites.

METHODS

The study included 66 consecutive non-septic cirrhosis patients with ascites. RAI was defined by a delta cortisol lower than 9 μg/dL and/or a peak cortisol lower than 18 μg/dL.

RESULTS

Sixty-six patients with cirrhosis and ascites were studied. The mean Child-Turcotte-Pugh (CTP) and model for end stage liver disease (MELD) scores were 10.6 ± 1.9 and 21.5 ± 7.3, respectively. The prevalence of RAI in patients with cirrhosis and ascites was 47% (31/66). The prevalence of RAI in patients with and without spontaneous bacterial peritonitis, renal failure and type 1 hepatorenal syndrome (HRS) was comparable. Baseline hyponatremia was common in RAI (42% versus 17%, p = 0.026). There was a significant correlation of prevalence of RAI with prothrombin time, international normalized ratio, MELD scores and CTP class. During follow-up, there was no association between RAI and the risk to develop new infections, severe sepsis, type 1 HRS and death.

CONCLUSIONS

RAI is common in non-septic cirrhotic patients with ascites and its prevalence increases with severity of liver disease. However, it does not affect the short-term outcome in these patients.

The cortisol stress response induced by surgery: A systematic review and meta-analysis

OBJECTIVE

Surgery is a stressor that can be categorized by duration and severity and induces a systemic stress response that includes increased adrenal cortisol production. However, the precise impact of surgical stress on the cortisol response remains to be defined.

DESIGN

We performed a systematic review and meta-analysis to assess the cortisol stress response induced by surgery and to stratify this response according to different parameters.

METHODS

We conducted a comprehensive search in several databases from 1990 to 2016. Pairs of reviewers independently selected studies, extracted data and evaluated the risk of bias. Cortisol concentrations were standardized, pooled in meta-analysis and plotted over time.

RESULTS

We included 71 studies reporting peri-operative serum cortisol measurements in 2953 patients. The cortisol response differed substantially between moderately/highly invasive and minimally invasive surgical procedures. Minimally invasive procedures did not show a peri-operative cortisol peak, whereas more invasive surgeries caused a cortisol surge that was more pronounced in older subjects, women and patients undergoing open surgery and general anaesthesia. The duration of the procedure and the use of etomidate for induction of anaesthesia did not affect the cortisol response.

CONCLUSIONS

The peri-operative cortisol stress response is dynamic and influenced by patient-specific, surgical and anaesthetic features. However, the available evidence is derived from highly heterogeneous studies, with only two of 71 studies measuring cortisol by mass spectrometry, which currently prevents a precise and reproducible definition of this response.

Three calculations of free cortisol versus measured values in the critically ill

OBJECTIVES

To investigate the agreement between the calculated free cortisol levels according to widely applied Coolens and adjusted Södergård equations with measured levels in the critically ill.

DESIGN AND METHODS

A prospective study in a mixed intensive care unit. We consecutively included 103 patients with treatment-insensitive hypotension in whom an adrenocorticotropic hormone (ACTH) test (250μg) was performed. Serum total and free cortisol (equilibrium dialysis), corticosteroid-binding globulin and albumin were assessed. Free cortisol was estimated by the Coolens method (C) and two adjusted Södergård (S1 and S2) equations. Bland Altman plots were made.

RESULTS

The bias for absolute (t=0, 30 and 60min after ACTH injection) cortisol levels was 38, -24, 41nmol/L when the C, S1 and S2 equations were used, with 95% limits of agreement between -65-142, -182-135, and -57-139nmol/L and percentage errors of 66, 85, and 64%, respectively. Bias for delta (peak-baseline) cortisol was 14, -31 and 16nmol/L, with 95% limits of agreement between -80-108, -157-95, and -74-105nmol/L, and percentage errors of 107, 114, and 100% for C, S1 and S2 equations, respectively.

CONCLUSIONS

Calculated free cortisol levels have too high bias and imprecision to allow for acceptable use in the critically ill.

Update on adrenal insufficiency in patients with liver cirrhosis

Liver cirrhosis is a major cause of mortality worldwide, often with severe sepsis as the terminal event. Over the last two decades, several studies have reported that in septic patients the adrenal glands respond inappropriately to stimulation, and that the treatment with corticosteroids decreases mortality in such patients. Both cirrhosis and septic shock share many hemodynamic abnormalities such as hyperdynamic circulatory failure, decreased peripheral vascular resistance, increased cardiac output, hypo-responsiveness to vasopressors, increased levels of proinflammatory cytokines [interleukine(IL)-1, IL-6, tumor necrosis factor-alpha] and it has, consequently, been reported that adrenal insufficiency (AI) is common in critically ill cirrhotic patients. AI may also be present in patients with stable cirrhosis without sepsis and in those undergoing liver transplantation. The term hepato-adrenal syndrome defines AI in patients with advanced liver disease with sepsis and/or other complications, and it suggests that it could be a feature of liver disease per se, with a different pathogenesis from that of septic shock. Relative AI is the term given to inadequate cortisol response to stress. More recently, another term is used, namely "critical illness related corticosteroid insufficiency" to define "an inadequate cellular corticosteroid activity for the severity of the patient's illness". The mechanisms of AI in liver cirrhosis are not completely understood, although decreased levels of high-density lipoprotein cholesterol and high levels of proinflammatory cytokines and circulatory endotoxin have been suggested. The prevalence of AI in cirrhotic patients varies widely according to the stage of the liver disease (compensated or decompensated, with or without sepsis), the diagnostic criteria defining AI and the methodology used. The effects of corticosteroid therapy on cirrhotic patients with septic shock and AI are controversial. This review aims to summarize the existing published information regarding AI in patients with liver cirrhosis.

Does music influence stress in mechanically ventilated patients?

OBJECTIVES

Mechanically ventilated patients experience profound stress. Interventions are needed to ameliorate stress that does not cause adverse effects. The purpose of this study was to explore the influence of music on stress in a sample of patients over the duration of ventilatory support.

RESEARCH METHODOLOGY/DESIGN

Randomised controlled trial; randomised patients (56.8+16.9 years, 61% male, APACHE III 57.2+18.3) receiving ventilatory support to: (1) patient-directed music (PDM) where patients self-initiated music listening whenever desired from a preferred collection, (2) headphones only to block ICU noise, or (3) usual ICU care. Twenty-four hour urinary cortisol samples were collected from a sub-set of subjects with intact renal function and not receiving medications known to influence cortisol levels (n=65).

SETTING

12 ICUs in the Midwestern United States.

MAIN OUTCOME MEASURES

Urinary free cortisol (UFC), an integrative biomarker of stress.

RESULTS

Controlling for illness severity, gender, and baseline UFC (29-45 mg/day), mixed models analysis revealed no significant differences among groups in UFC over the course of ventilatory support.

CONCLUSION

While music did not significantly reduce cortisol, less profound spikes in UFC levels were observed but that, given the limitations of the research, this observation could have occurred merely by chance.

Regulation of cortisol bioavailability--effects on hormone measurement and action

Routine assessment of the hypothalamic-pituitary-adrenal axis relies on the measurement of total serum cortisol levels. However, most cortisol in serum is bound to corticosteroid-binding globulin (CBG) and albumin, and changes in the structure or circulating levels of binding proteins markedly affect measured total serum cortisol levels. Furthermore, high-affinity binding to CBG is predicted to affect the availability of cortisol for the glucocorticoid receptor. CBG is a substrate for activated neutrophil elastase, which cleaves the binding protein and results in the release of cortisol at sites of inflammation, enhancing its tissue-specific anti-inflammatory effects. Further tissue-specific modulation of cortisol availability is conferred by corticosteroid 11β-dehydrogenase. Direct assessment of tissue levels of bioavailable cortisol is not clinically practicable and measurement of total serum cortisol levels is of limited value in clinical conditions that alter prereceptor glucocorticoid bioavailability. Bioavailable cortisol can, however, be measured indirectly at systemic, extracellular tissue and cell levels, using novel techniques that have provided new insight into the transport, metabolism and biological action of glucocorticoids. A more physiologically informative approach is, therefore, now possible in the assessment of the hypothalamic-pituitary-adrenal axis, which could prove useful in clinical practice.

Challenges for the endocrine laboratory in critical illness

The endocrine laboratory must provide accurate and timely results for the critically ill patient. A number of pathophysiological factors affect assay systems for adrenal, thyroid and gonadal function tests. The effects are primarily on estimates of 'free hormone' concentration through abnormal binding protein concentrations and the effects of drugs and metabolites on hormone-protein binding. The limitations of the principal analytical techniques (immunoassay and chromatography-mass spectrometry) include drug effects, endogenous antibody interference and ion suppression. These effects are not always easily identified. Analytical specificity and standardisation result in differences in bias between assays and thus a requirement for assay specific decision limits and reference ranges. Good communication between clinician and laboratory is needed to minimise these effects. Developments in mass spectrometry should lead to greater sensitivity and wider applicability of the technique. International efforts to develop higher order reference materials and reference method procedures should lead to greater comparability of results.

Quantification of unbound prednisolone, prednisone, cortisol and cortisone in human plasma by ultrafiltration and direct injection into liquid chromatrography tandem mass spectrometry

BACKGROUND

The existing methods for quantitative analysis of free corticosteroids require high volume of plasma and laborious extraction processes. Development of ultrafiltration followed by the liquid chromatrography tandem mass spectrometry (LC-MS/MS) method that requires 300 microL of plasma, does not entail any offline extraction and achieves good sensitivity was described.

METHODS

Unbound corticosteroids were separated by the ultrafiltration of plasma using Microcon centrifugal filter devices (10,000 Dal nominal molecular weight limit). A 30 microL aliquot of the ultrafiltrate was directly injected into a two-dimensional high-performance liquid chromatography clean-up and separation system coupled with API-4000 mass spectrometer. The clean-up was performed on a Strata-X on-line extraction cartridge. A Zorbax-SB Phenyl, Rapid Resolution HT (2.1 x 100 mm) column was employed to chromatographically resolve cortisol and prednisolone from each other, from cortisone and prednisone as well as from interferences found in plasma from stable kidney transplant recipients.

RESULTS

Intra- and inter-run imprecision and inaccuracy within +/-15% were achieved during a three-batch validation for quality control samples at six concentrations in ultrafiltrate from charcoal-stripped plasma and three concentrations from normal plasma, over a 2000-fold dynamic range. The lower limit of quantification was 0.100 ng/mL for all four corticosteroids.

CONCLUSIONS

A highly selective, sensitive, simple and robust LC-MS/MS method was developed for the simultaneous quantification of free cortisol, cortisone, prednisolone and prednisone. The performance of the Strata-X on-line extraction cartridge was maintained for over 700 injections. The assay was successfully applied for the analysis of the analytes in over 500 plasma samples from stable kidney transplant recipients.

Assessing adrenal insufficiency of corticosteroid secretion using free versus total cortisol levels in critical illness

PURPOSE

To study the value of free versus total cortisol levels in assessing relative adrenal insufficiency during critical illness-related corticosteroid insufficiency.

METHODS

A prospective study in a mixed intensive care unit from 2004 to 2007. We consecutively included 49 septic and 63 non-septic patients with treatment-insensitive hypotension in whom an adrenocorticotropic hormone (ACTH) test (250 μg) was performed. Serum total and free cortisol (equilibrium dialysis), corticosteroid-binding globulin (CBG) and albumin were assessed.

RESULTS

Although a low CBG resulted in a high free cortisol level relative to total cortisol, free and total cortisol and their increases were well correlated (r = 0.77-0.79, P < 0.001). In sepsis, hypoalbuminemia did not affect total and free cortisol, and increases in total cortisol upon ACTH predicted increases in free cortisol regardless of low binding proteins. In non-sepsis, total cortisol was lower with than without hypoalbuminemia; free cortisol did not differ, since hypoalbuminemia concurred with a low CBG. Increases in total cortisol depended less on binding proteins than on raw levels. The areas under the receiver operating characteristic curve for predicting increases in free from total cortisol were 0.93-0.97 in sepsis and 0.79-0.85 in non-sepsis (P = 0.044 or lower for sepsis vs. non-sepsis).

CONCLUSIONS

Although the biologically active free cortisol fraction depends on binding proteins, total cortisol correlates to free cortisol in treatment-insensitive hypotension during critical illness. In sepsis, albumin is not an important binding molecule. Subnormal increments in total cortisol upon ACTH suffice in assessing relative adrenal insufficiency, particularly in sepsis.

Assessment of adrenocortical reserve in stable patients with cirrhosis

BACKGROUND & AIMS

Adrenal insufficiency (AI) is reported in critically ill patients with cirrhosis and is associated with increased mortality. It is unclear if AI is an underlying condition or triggered by critical events (e.g. sepsis). We investigated AI in cirrhosis without infection or hemodynamic instability.

METHODS

A total of 101 consecutive patients with cirrhosis were studied. AI was defined by a total serum cortisol (TC) <18 μg/dl at 20 or 30 min after injection of 1 μg of tetracosactrin. Transcortin, calculated free cortisol (cFC), and free cortisol index (FCI) were assessed in a subgroup of 41 patients, with FCI>12 representing normal adrenal function.

RESULTS

AI was present in 38 patients (38%). Child score (median, 10 vs 7, p<0.0001), MELD score (median, 17 vs 12, p<0.0001), ascites (68% vs 37%, p<0.01), basal TC (median,7.6 vs 14.9 μg/dl, p<0.001), albumin (28 ± 0.8 vs 33 ± 0.7 g/L, p<0.0001), INR (median, 1.6 vs 1.2, p<0.0001), total bilirubin (median, 51 vs 31 μmol/L, p<0.05), total cholesterol (median, 120 vs 142, p<0.05), and LDL (median, 76 vs 81, p<0.05) were significantly different between those with and without AI. ROC curves showed a basal TC ≤ 12.8 μg/dl to be a cut-off value closely associated with AI. The cFC was significantly related to TC for baseline values (R=0.94, p<0.0001), peak values (R=0.90, p<0.0001), and delta values (R=0.95, p<0.0001), in patients with and without AI. However, no patient had a FCI<12.

CONCLUSIONS

AI defined by an abnormal response to 1 μg tetracosactrin is frequent in stable patients with cirrhosis, in the absence of infections or hemodynamic instability and is related to the severity of liver disease. However, evaluation of the true incidence of AI should comprise direct assays of free cortisol. Clinical consequences of AI need to be explored.

HIF1alpha synergizes with glucocorticoids to promote BFU-E progenitor self-renewal

With the aim of finding small molecules that stimulate erythropoiesis earlier than erythropoietin and that enhance erythroid colony-forming unit (CFU-E) production, we studied the mechanism by which glucocorticoids increase CFU-E formation. Using erythroid burst-forming unit (BFU-E) and CFU-E progenitors purified by a new technique, we demonstrate that glucocorticoids stimulate the earliest (BFU-E) progenitors to undergo limited self-renewal, which increases formation of CFU-E cells > 20-fold. Interestingly, glucocorticoids induce expression of genes in BFU-E cells that contain promoter regions highly enriched for hypoxia-induced factor 1α (HIF1α) binding sites. This suggests activation of HIF1α may enhance or replace the effect of glucocorticoids on BFU-E self-renewal. Indeed, HIF1α activation by a prolyl hydroxylase inhibitor (PHI) synergizes with glucocorticoids and enhances production of CFU-Es 170-fold. Because PHIs are able to increase erythroblast production at very low concentrations of glucocorticoids, PHI-induced stimulation of BFU-E progenitors thus represents a conceptually new therapeutic window for treating erythropoietin-resistant anemia.

An evaluation of measured and calculated serum free cortisol in a group of patients with known adrenal suppression

Background

Since more than 90% of cortisol is bound to protein, serum free cortisol (SFC) may be a more appropriate marker of adrenal status than total cortisol. However, measurement of SFC is technically difficult and calculated SFC may offer a more practical alternative.

Methods

SFC, measured by equilibrium dialysis coupled with immunoassay, and calculated using Coolens' equation from total cortisol and corticosteroid binding globulin (CBG) concentrations, was compared in short Synacthen test (SST) serum from 42 patients, of whom 20 demonstrated a suppressed adrenal response.

Results

Considering the patient group as a whole, calculated SFC was found to be significantly lower than measured SFC, pre- and post-Synacthen ( P < 0.05 and <0.001, respectively). Upon classifying the patients as pass or fail based on total cortisol response to Synacthen, the difference in calculated and measured SFC only reached statistical significance for post-Synacthen concentrations in the pass group ( P < 0.01), suggesting a greater discrepancy at higher cortisol concentrations. There was no difference in CBG levels between the pass and fail groups and both measured and calculated SFC gave a diminished 30 min response in subjects deemed to have failed the SST.

Conclusion

Coolens' equation was found to underestimate measured SFC in this cohort of outpatients, as has been previously demonstrated, particularly in patients with a pronounced acute phase response. Although calculated SFC gave a diminished response in individuals deemed to have failed the SST, the concentration-dependent nature of the discrepancy may limit the usefulness of this method for assessing adrenal status.

"Relative" adrenal insufficiency in critical illness

OBJECTIVE

To present a recommended approach to the problem of "relative" adrenal insufficiency (RAI) in the intensive care unit (ICU).

METHODS

We examine historical data that support the traditional concepts of adrenal insufficiency and the idea that the increase in cortisol secretion during stress is needed to survive the stress. The controversial use of treatment with glucocorticoids (GCs) in patients with sepsis and septic shock in the ICU (and thus survival benefit) is also briefly discussed.

RESULTS

During the past decade, the concept of RAI as the failure of cortisol secretion to increase in response to stress to sustain the patient through that stress has gained strength. In some studies, it has been suggested that as many as 75% of patients in an ICU setting have RAI. Experimental support for the concept is not possible because there is no clinically useful laboratory measure of GC action. Therefore, diagnosis is generally based on interpretation of the cosyntropin stimulation test.

CONCLUSION

The best clinical judgment should always guide interpretation of any test results, and sharp categorization of patients on the basis of a single cutoff criterion should be avoided. Overall, the concept of RAI has no clinical utility. In these cases, administration of GCs adds cost without benefit and with increased risk.

Endocrine assessments during critical illness

The evaluation of hormonal status in critically ill patients is challenging and has many pitfalls. This article reviews proper assessment of glycemic status AND adrenal and thyroid function during critical care.

Free serum cortisol: quantification applying equilibrium dialysis or ultrafiltration and an automated immunoassay system

BACKGROUND

Quantification of bioactive, free serum cortisol concentrations can characterize adrenocortical function more appropriately compared to total serum cortisol measurement. Ultrafiltration or equilibrium dialysis of serum samples allow direct measurement of free serum cortisol concentrations but respective methods have poorly been validated so far. The aim of our study was to investigate the analytical performance of free serum cortisol measurement employing equilibrium dialysis and ultrafiltration.

METHODS

Two commercially available ultrafiltration devices and self-assembled dialysis cells, respectively, were studied. Cortisol was quantified in filtrate or dialysate using an automated immunoassay system. Using two serum pools, the inter-assay coefficient of variation was determined for the three methods and a method comparison was performed.

RESULTS

Inter-assay coefficients of variation (n=10) between 3.2% and 14.8% were observed in the imprecision study. Method comparison demonstrated close agreement between free serum cortisol results obtained by ultrafiltration and equilibrium dialysis, respectively (equilibrium dialysis=1.2xultrafiltration+3.9 nmol/L; r=0.99; n=35).

CONCLUSIONS

Direct quantification of free serum cortisol after equilibrium dialysis or ultrafiltration of the samples offers acceptable reproducibility and results in close agreement can be obtained. Both methods can potentially be introduced into a routine laboratory setting.

Effect of temperature on protein binding of cortisol

OBJECTIVES

Biological effects of cortisol are substantially determined by protein binding of the hormone. The aim of our study was to characterize temperature effects on cortisol protein binding by use of an equilibrium dialysis method.

DESIGN AND METHODS

Serum samples obtained from ten healthy volunteers were submitted to equilibrium dialysis. Each sample from the individuals was incubated for 16 h at 37 degrees C, 38 degrees C, 39 degrees C, 40 degrees C and 41 degrees C, respectively. In the dialysate samples obtained, cortisol concentrations were measured by immunoassay.

RESULTS

For samples incubated at 37 degrees C, a mean dialysate cortisol concentration of 0.41 microg/dL (SD=0.14) was found. Gradual increase of dialysate cortisol concentration was observed with increasing incubation temperatures. For samples incubated at 41 degrees C, a mean dialysate cortisol of 0.75 microg/dL (SD=0.24) was found. Thus, the mean percentage of free-to-total cortisol increased by about 80% from 3.7% (SD=1.1) at 37 degrees C to 6.7% (SD=1.8) at 41 degrees C.

CONCLUSIONS

The results of our in vitro experiments suggest that during fever the free-to-total ratio of cortisol is increased substantially compared to normal conditions, and that administration of antipyretic drugs might potentially be associated with substantial changes in the bioavailability of cortisol.

MineralocorticoidversusGlucocorticoid Receptor Occupancy Mediating Aldosterone-Stimulated Sodium Transport in a Novel Renal Cell Line

Aldosterone controls sodium balance by regulating an epithelial sodium channel (ENaC)-mediated sodium transport along the aldosterone-sensitive distal nephron, which expresses both mineralocorticoid (MR) and glucocorticoid receptors (GR). Mineralocorticoid specificity is ensured by 11beta-hydroxysteroid dehydrogenase type 2, which metabolizes cortisol or corticosterone into inactive metabolites that are unable to bind MR and/or GR. The fractional occupancy of MR and GR by aldosterone mediating the sodium transport response in the aldosterone-sensitive distal nephron cannot be studied in vivo. For answering this question, a novel mouse cortical collecting duct cell line (mCCD(cl1)), which expresses significant levels of MR and GR and a robust aldosterone sodium transport response, was used. Aldosterone elicited a biphasic response: Low doses (K(1/2) = approximately 0.5 nM) induced a transient and early increase of sodium transport (peaking at 3 h), whereas high doses (K(1/2) = approximately 90 nM) entailed an approximately threefold larger, long-lasting response. At 3 h, the corticosterone dose-response curve was shifted to the right compared with that of aldosterone by more than two log concentrations, an effect that was fully reverted in the presence of the 11beta-hydroxysteroid dehydrogenase type 2 inhibitor carbenoxolone. Low doses of dexamethasone (0.1 to 1 nM) failed to induce an early response, but high doses elicited a long-lasting response (K(1/2) = approximately 8 nM), similar to that observed for high aldosterone concentrations. Equilibrium binding assays showed that both aldosterone and corticosterone bind to a high-affinity, low-capacity site, whereas dexamethasone binds to one site. Within the physiologic range of aldosterone concentrations, sodium transport is predicted to be controlled by MR occupancy during circadian cycles and by MR and GR occupancy during salt restriction or acute stress.