Action of ANP on the nongenomic dose-dependent biphasic effect of aldosterone on NHE1 in proximal S3 segment

Signaling pathways involved in the rapid biphasic effect of aldosterone on Na+/H+ exchanger in rat proximal tubule cells

The receptors and signaling pathways for nongenomic effects of aldosterone (Aldo) on the proximal Na⁺/H⁺ exchanger are still unknown; therefore, the aim of this study was to investigate the mineralocorticoid receptor (MR) and/or glucocorticoid receptor (GR) participation in rapid Aldo effects on NHE1 (basolateral Na⁺/H⁺ exchanger isoform) and cytosolic calcium concentration ([Ca²⁺]_i). In addition, phospholipase C (PLC), protein kinase C (PKC), and mitogen-activated protein kinase kinase (MEK) involvement in signaling pathways of such effects was evaluated, using immortalized proximal tubule cells of rat (IRPTC) as an experimental model. MR and GR expression was investigated using reverse transcription polymerase chain reaction and immunoblotting. The intracellular pH recovery rate (after acid loading) and [Ca²⁺]_i were determined by the probes BCECF-AM and FURA 2-AM, respectively. Aldo (10^-12 M) promoted a moderate increase in [Ca²⁺]_i and stimulation of NHE1, whereas Aldo (10^-6 M) greatly increased the [Ca²⁺]_i, but inhibited the NHE1. BAPTA-AM (a calcium chelator), GR antagonism and inhibition of PLC, PKC and MEK pathway abolished the biphasic and dose-dependent effect of Aldo on NHE1 and decreased the [Ca²⁺]_i; whereas MR do not appear to participate in this rapid signaling in IRPTC cells. The reduction of GR content, by gene silencing, abolished the Aldo effect on NHE1, in low concentration, confirming the importance of this receptor in the rapid modulation of proximal sodium and hydrogen transports.

Effect of neprilysin inhibition on renal function in patients with type 2 diabetes and chronic heart failure who are receiving target doses of inhibitors of the renin-angiotensin system: a secondary analysis of the PARADIGM-HF trial

BACKGROUND

Neprilysin inhibition has favourable effects on experimental diabetic nephropathy. We sought to assess the effects of neprilysin inhibition on the course of renal function in patients with type 2 diabetes.

METHODS

In the randomised, double-blind PARADIGM-HF trial, the effects of sacubitril/valsartan (97 mg/103 mg twice daily) were compared with enalapril (10 mg twice daily) in 8399 patients with mild-to-moderate chronic heart failure and systolic dysfunction. In this secondary intention-to-treat analysis, we assessed the change in estimated glomerular filtration rate (eGFR) over a 44-month follow-up period in patients with (n=3784) and those without (n=4615) diabetes. PARADIGM-HF is registered with ClinicalTrials.gov, number NCT01035255.

FINDINGS

eGFR decreased by 1·1 mL/min per 1·73 m² per year (95% CI 1·0-1·2) in patients without diabetes, but by 2·0 mL/min per 1·73 m² per year (1·9-2·1) in those with diabetes (p<0·0001). Compared with patients treated with enalapril, those treated with sacubitril/valsartan had a slower rate of decline in eGFR (-1·3 vs -1·8 mL/min per 1·73 m² per year; p<0·0001), and the magnitude of the benefit was larger in patients with versus those without diabetes (difference 0·6 mL/min per 1·73 m² per year [95% CI 0·4-0·8] in patients with vs 0·3 mL/min per 1·73 m² per year [0·2-0·5] in those without diabetes; p_interaction=0·038). The greater effect of neprilysin inhibition in patients with diabetes could not be explained by the effects of treatment on the course of heart failure or on HbA_1c. The incremental benefit of sacubitril/valsartan in patients with diabetes was no longer apparent when changes in eGFR were adjusted for urinary cyclic guanosine monophosphate (p=0·41).

INTERPRETATION

In patients in whom the renin-angiotensin system is already maximally blocked, the addition of neprilysin inhibition attenuates the effect of diabetes to accelerate the deterioration of renal function that occurs in patients with chronic heart failure.

FUNDING

Novartis.

Activation and Inhibition of Sodium-Hydrogen Exchanger Is a Mechanism That Links the Pathophysiology and Treatment of Diabetes Mellitus With That of Heart Failure

The mechanisms underlying the progression of diabetes mellitus and heart failure are closely intertwined, such that worsening of one condition is frequently accompanied by worsening of the other; the degree of clinical acceleration is marked when the 2 coexist. Activation of the sodium-hydrogen exchanger in the heart and vasculature (NHE1 isoform) and the kidneys (NHE3 isoform) may serve as a common mechanism that links both disorders and may underlie their interplay. Insulin insensitivity and adipokine abnormalities (the hallmarks of type 2 diabetes mellitus) are characteristic features of heart failure; conversely, neurohormonal systems activated in heart failure (norepinephrine, angiotensin II, aldosterone, and neprilysin) impair insulin sensitivity and contribute to microvascular disease in diabetes mellitus. Each of these neurohormonal derangements may act through increased activity of both NHE1 and NHE3. Drugs used to treat diabetes mellitus may favorably affect the pathophysiological mechanisms of heart failure by inhibiting either or both NHE isoforms, and drugs used to treat heart failure may have beneficial effects on glucose tolerance and the complications of diabetes mellitus by interfering with the actions of NHE1 and NHE3. The efficacy of NHE inhibitors on the risk of cardiovascular events may be enhanced when heart failure and glucose intolerance coexist and may be attenuated when drugs with NHE inhibitory actions are given concomitantly. Therefore, the sodium-hydrogen exchanger may play a central role in the interplay of diabetes mellitus and heart failure, contribute to the physiological and clinical progression of both diseases, and explain certain drug-drug and drug-disease interactions that have been reported in large-scale randomized clinical trials.

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.

Aldosterone induces rapid sodium intake by a nongenomic mechanism in the nucleus tractus solitarius

The purpose of this study was to determine whether aldosterone has a rapid action in the nucleus tractus solitarius (NTS) that increases sodium intake, and to examine whether this effect of aldosterone, if present, is mediated by G protein-coupled estrogen receptor (GPER). Adult male Sprague-Dawley rats with a stainless-steel cannula in the NTS were used. Aldosterone was injected into the NTS at the doses of 1, 5, 10 and 20 ng 0.1 μl⁻¹. A rapid dose-related increase of 0.3 M NaCl intake was induced within 30 min and this increase was not suppressed by the mineralocorticoid receptor (MR) antagonist spironolactone (10 ng 0.1 μl⁻¹). Water intake was not affected by aldosterone. The GPER agonist G-1 produced a parallel and significant increase in sodium intake, while pre-treatment with GPER antagonist G15 (10 ng 0.1 μl⁻¹) blocked the G-1 or aldosterone-induced rapid sodium intake. In addition, sodium intake induced by sodium depletion or low-sodium diet fell within 30 min after injection into the NTS of the MR antagonist spironolactone, while G15 had no effect. Our results confirm previous reports, and support the hypothesis that aldosterone evokes rapid sodium intake through a non-genomic mechanism involving GPER in NTS.

Glucocorticoids Reduce Renal NHE8 Expression

The proximal tubule reabsorbs most of the filtered bicarbonate which is mediated in large part by Na⁺/H⁺ exchange (NHE). We have previously demonstrated that there is an isoform switch during postnatal maturation from NHE8 to NHE3 that is concordant with the postnatal increase in serum glucocorticoid levels. To examine if glucocorticoids may be responsible for this isoform switch, we administered dexamethasone daily to mice at 7–10 days of age, a time prior to the normal isoform switch. We show that compared to vehicle-treated controls, dexamethasone caused a premature increase in renal NHE3 and decrease in NHE8 mRNA, total protein, and brush border membrane protein abundance. To examine if there was a direct epithelial action of dexamethasone on NHE8, we studied normal rat kidney (NRK) cells in vitro which express NHE8 on their apical membrane. Dexamethasone decreased NHE8 mRNA, total protein, and apical protein abundance. Dexamethasone also decreased Na⁺/H⁺ exchanger activity. These studies provide evidence that glucocorticoids may play a role in the developmental isoform switch from NHE8 to NHE3 and cause a decrease in NHE8 expression and activity.

Aldosterone regulates Na(+), K(+) ATPase activity in human renal proximal tubule cells through mineralocorticoid receptor

The mechanisms by which aldosterone increases Na(+), K(+) ATPase and sodium channel activity in cortical collecting duct and distal nephron have been extensively studied. Recent investigations demonstrate that aldosterone increases Na-H exchanger-3 (NHE-3) activity, bicarbonate transport, and H(+) ATPase in proximal tubules. However, the role of aldosterone in regulation of Na(+), K(+) ATPase in proximal tubules is unknown. We hypothesize that aldosterone increases Na(+), K(+) ATPase activity in proximal tubules through activation of the mineralocorticoid receptor (MR). Immunohistochemistry of kidney sections from human, rat, and mouse kidneys revealed that the MR is expressed in the cytosol of tubules staining positively for Lotus tetragonolobus agglutinin and type IIa sodium-phosphate cotransporter (NpT2a), confirming proximal tubule localization. Adrenalectomy in Sprague-Dawley rats decreased expression of MR, ENaC α, Na(+), K(+) ATPase α1, and NHE-1 in all tubules, while supplementation with aldosterone restored expression of above proteins. In human kidney proximal tubule (HKC11) cells, treatment with aldosterone resulted in translocation of MR to the nucleus and phosphorylation of SGK-1. Treatment with aldosterone also increased Na(+), K(+) ATPase-mediated (86)Rb uptake and expression of Na(+), K(+) ATPase α1 subunits in HKC11 cells. The effects of aldosterone on Na(+), K(+) ATPase-mediated (86)Rb uptake were prevented by spironolactone, a competitive inhibitor of aldosterone for the MR, and partially by Mifepristone, a glucocorticoid receptor (GR) inhibitor. These results suggest that aldosterone regulates Na(+), K(+) ATPase in renal proximal tubule cells through an MR-dependent mechanism.