Increased Na(+)-H+ exchange in red blood cells of patients with primary aldosteronism

DOCA sensitive pendrin expression in kidney, heart, lung and thyroid tissues

BACKGROUND/AIMS

Pendrin (SLC26A4), a transporter accomplishing anion exchange, is expressed in inner ear, thyroid gland, kidneys, lung, liver and heart. Loss or reduction of function mutations of SLC26A4 underlie Pendred syndrome, a disorder invariably leading to hearing loss with enlarged vestibular aqueducts and in some patients to hypothyroidism and goiter. Renal pendrin expression is up-regulated by mineralocorticoids such as aldosterone or deoxycorticosterone (DOCA). Little is known about the impact of mineralocorticoids on pendrin expression in extrarenal tissues.

METHODS

The present study utilized RT-qPCR and Western blotting to quantify the transcript levels and protein abundance of Slc26a4 in murine kidney, thyroid, heart and lung prior to and following subcutaneous administration of 100 mg/kg DOCA.

RESULTS

Slc26a4 transcript levels as compared to Gapdh transcript levels were significantly increased by DOCA treatment in kidney, heart, lung and thyroid. Accordingly pendrin protein expression was again significantly increased by DOCA treatment in kidney, heart, lung and thyroid.

CONCLUSION

The observations reveal mineralocorticoid sensitivity of pendrin expression in kidney, heart, thyroid and lung.

Effect of protein kinase C activation on Na+-H+ exchange in erythrocytes of frog Rana temporaria

The treatment of frog erythrocytes incubated in standard nitrate medium with 100 nM phorbol ester (PMA) induced a sharp increase in the 22Na uptake by the cells and intracellular Na(+) concentration. The PMA-induced enhancement in 22Na uptake was stimulated by the addition of 0.1 mM ouabain to the incubation medium and completely blocked by 1 mM amiloride. The time course of 22Na uptake by frog red cells in the presence of PMA showed a lag phase ( approximately 5 min), after which was linear within 5-15 min. The calculated Na(+) influx in erythrocytes treated with PMA was 49.4+/-3.7 mmol l(-1) cells h(-1) as compared with 1.2+/-0.25 mmol l(-1) h(-1) for control cells. 5-(N-ethyl-N-isopropyl)-amiloride, selective blocker of NHE1, caused a dose-dependent inhibition of the PMA-induced Na(+) influx with IC(50) of 0.27 microM. The PMA-induced Na(+) influx was almost completely inhibited by 0.1 microM staurosporine, protein kinase C blocker. Pretreatment of frog red blood cells for 5, 10 or 15 min with 10 mM NaF, non-selective inhibitor of protein phosphatase, led to a progressive stimulation of the PMA effect on Na(+) influx. Both amiloride and NaF did not affect the basal Na(+) influx in frog erythrocytes. The data indicate that the Na(+)-H(+) exchanger in the frog erythrocytes is quiescent under basal conditions and can be markedly stimulated by PMA.

Rapid and non-genomic reduction of intracellular [Ca(2+)] induced by aldosterone in human bronchial epithelium

1. Using a Ca(2+) imaging system and fura-2 AM (5 microM) we showed that exposure of polarised monolayers of human bronchial epithelial cells (16HBE14o- cell line) to aldosterone produced a fast intracellular [Ca(2+)] ([Ca(2+)](i)) decrease, in 70 % of cells. Exposure to aldosterone (1 nM) reduced the [Ca(2+)](i) by 39 +/- 9 nM (n = 282, P < 0.0001) within 10 min, from a basal [Ca(2+)](i) of 131 +/- 19 nM (n = 282). 2. The effect of aldosterone on [Ca(2+)](i) was not affected by inhibitors of the classical genomic pathway, cycloheximide (1 microM) or spironolactone (10 microM). The aldosterone-induced [Ca(2+)](i) decrease was inhibited by thapsigargin (1 microM), pertussis toxin (24 h at 200 ng ml(-1)), the adenylate cyclase inhibitors 2',3'-dideoxyadenosine (200 microM) and MDL-12,330A hydrochloride (500 microM), and the protein kinase A inhibitor R(P)-adenosine 3',5'-cyclic monophosphorothioate (200 microM). In addition, treatment of 16HBE14o- monolayers with aldosterone (1 nM) inhibited by approximately 30 % the large and transient [Ca(2+)](i) increase induced by apical exposure to uridine triphosphate (UTP, 0.1 mM), a known secretagogue in airway epithelia. 3. Our results demonstrate for the first time that in human bronchial epithelial cells, aldosterone decreases [Ca(2+)](i) levels via a non-genomic mechanism. The hormone-induced changes to [Ca(2+)](i) involve stimulation of thapsigargin-sensitive Ca(2+)-ATPase, via G-protein-, adenylate cyclase- and protein kinase A-coupled signalling pathways.

Aldosterone activates Na+/H+ exchange in vascular smooth muscle cells by nongenomic and genomic mechanisms

BACKGROUND

In vascular smooth muscle cells (VSMCs), Na+/H+ exchange (NHE) plays an important role in intracellular pH (pHi) regulation. Recently, nongenomic effect of aldosterone (ALDO) on NHE activity has been suggested in VSMCs. However, the nongenomic and genomic effects of ALDO on NHE and the intracellular signaling mechanisms for these effects have not fully been determined in VSMCs.

METHODS

The effects of short- (3 hr) and long- (24 hr) term exposure to ALDO on NHE activity were examined in cultured VSMCs from rat thoracic aortae by using single-cell pHi measurement with the pH-sensitive dye 2'7'-bis(carboxyethyl)-5(6)-carboxyfluorescein. The NHE activity was calculated from the initial rate of Na+-dependent pHi recovery after acid load.

RESULTS

The NHE activity significantly increased after short- and long-term exposure of VSMCs to ALDO (10(-6) M). The inhibitors of gene transcription (actinomycin D) and of protein synthesis (cycloheximide) had no effect on the short-term ALDO effect, but inhibited the long-term ALDO effect. The antagonists of the mineralocorticoid receptor (MR) (spironolactone) and of the glucocorticoid receptor (GR) (RU38486) caused no effect on the short-term ALDO effect, but inhibited the long-term ALDO effect. Two protein kinase C (PKC) inhibitors (staurosporine A and calphostin C) and PKC down-regulation (24 hr pre-exposure to phobol 12-myristate 13-acetate, PMA) inhibited both the short- and long-term ALDO effects. Exposure of VSMCs to PMA for 3 hours mimicked the short-term effect of ALDO on NHE activity. ALDO significantly increased PKC activity in VSMCs. The short-term ALDO effect was inhibited by disruptors of microtubule (colchicine) and of filamentous-actin (cytochalasin B). Long-term exposure of ALDO caused a threefold increase in NHE (NHE-1) mRNA levels.

CONCLUSIONS

The short-term effect of ALDO on NHE activity is not mediated through either MR or GR, occurs independent of gene transcription and protein synthesis, and occurs through a mechanism involving the structural elements of cytoskeleton. The long-term effect of ALDO on NHE activity occurs through both MR and GR and requires gene transcription and protein synthesis. Both short- and long-term effects of ALDO are mediated through PKC activation. Therefore, ALDO activates NHE by nongenomic and genomic mechanisms in VSMCs.

Genetic and biochemical determinants of abnormal monovalent ion transport in primary hypertension

Data obtained during the last two decades show that spontaneously hypertensive rats, an acceptable experimental model of primary human hypertension, possess increased activity of both ubiquitous and renal cell-specific isoforms of the Na+/H+ exchanger (NHE) and Na+-K+-2Cl- cotransporter. Abnormalities of these ion transporters have been found in patients suffering from essential hypertension. Recent genetic studies demonstrate that genes encoding the beta- and gamma-subunits of ENaC, a renal cell-specific isoform of the Na+-K+-2Cl- cotransporter, and alpha3-, alpha1-, and beta2-subunits of the Na+-K+ pump are localized within quantitative trait loci (QTL) for elevated blood pressure as well as for enhanced heart-to-body weight ratio, proteinuria, phosphate excretion, and stroke latency. On the basis of the homology of genome maps, several other genes encoding these transporters, as well as the Na+/H+ exchanger and Na+-K+-2Cl- cotransporter, can be predicted in QTL related to the pathogenesis of hypertension. However, despite their location within QTL, analysis of cDNA structure did not reveal any mutation in the coding region of the above-listed transporters in primary hypertension, with the exception of G276L substitution in the alpha1-Na+-K+ pump from Dahl salt-sensitive rats and a higher occurrence of T594M mutation of beta-ENaC in the black population with essential hypertension. These results suggest that, in contrast to Mendelian forms of hypertension, the altered activity of monovalent ion transporters in primary hypertension is caused by abnormalities of systems involved in the regulation of their expression and/or function. Further analysis of QTL in F2 hybrids of normotensive and hypertensive rats and in affected sibling pairs will allow mapping of genes causing abnormalities of these regulatory pathways.

Enhanced Na+/H+ exchange in Cushing's syndrome reflects functional hypermineralocorticoidism

BACKGROUND

Patients with Cushing's syndrome exhibit a bimodal distribution of maximal rates of the erythrocyte amiloride-sensitive Na+/H+ exchange (NHE). Enhanced erythrocyte NHE has recently been found in patients with primary aldosteronism.

OBJECTIVE

To test the hypothesis that occult hypermineralocorticoidism in a subset of patients with Cushing's syndrome is responsible for the greater than normal NHE.

METHODS

NHE was measured as maximal initial rate (Vmax) of amiloride-inhibited efflux of H+ into an alkaline Na+-containing medium, for 47 patients with hypercortisolism (20 with pituitary adenomas, 18 with adrenal adenomas, and nine with ectopic production of adrenocorticotropin). Clinical appearance, blood pressure levels, plasma aldosterone and deoxycorticosterone levels, serum electrolytes, and urine (tetrahydrocortisol plus 5-alpha-tetrahydrocortisol) : tetrahydrocortisone ratios were assessed for all patients. Twenty patients (10 with greater than normal NHE and 10 with low-to-normal NHE) were randomly selected from 47 patients with hypercortisolism, and treated with 200 mg/day spironolactone for 7 days. NHE in these patients was assessed before starting the treatment and 2 days after its cessation.

RESULTS

Greater than normal NHE (Vmax) was associated with peripheral edema, high diastolic blood pressure, hypokalemia, and high urine (tetrahydrocortisol plus 5-alpha-tetrahydrocortisol) : tetrahydrocortisone ratios. The enhanced NHE was rapidly normalized by treatment with spironolactone.

CONCLUSION

Erythrocyte NHE in patients with hypercortisolism and functional hypermineralocorticoidism is greater than normal due to incomplete peripheral conversion of cortisol (which binds to mineralocorticoid receptors) into metabolically inactive cortisone.

Can we use erythrocytes for the study of the activity of the ubiquitous Na+/H+ exchanger (NHE-1) in essential hypertension?

Both Na+/Li+ countertransport and electrochemical proton gradient (delta mu(H+))-induced Na+ and H+ fluxes are increased in erythrocytes from patients with essential hypertension. It was assumed that these abnormalities are related to ubiquitous (housekeeping) forms of the Na+/H+ exchanger (NHE-1). To examine this hypothesis, we compared kinetic and regulatory properties of erythrocyte Na+/Li+ countertransport and delta mu(H+)-induced Na+ and H+ fluxes with data obtained for cloned isoforms of the Na+/H+ exchanger. In human erythrocytes, Na+/Li+ countertransport exhibited a hyperbolic dependence on [Na+]0 with a K0.5 of approximately 30 to 40 mmol/L. The activity of this carrier was increased by two-fold in the fraction of erythrocytes enriched with the old cells, was inhibited by 0.1 mmol/L phloretin, and was insensitive to both 1 mmol/L amiloride and ATP depletion. In contrast, delta mu(H+)-induced 22Na influx was exponentially increased at [Na+]0 > 60 mmol/L, was insensitive to phloretin, was partly decreased by both 1 mmol/L amiloride and ATP depletion, and was the same in total erythrocytes and in the old cells. The values of Na+/Li+ countertransport and delta mu(H+)-induced Na+ influx in erythrocytes from different species were not correlating and their ratio in human, rat, and rabbit erythrocytes was 10:1:170 and 1:5:1 for Na+/ Li+ countertransport and delta mu(H+)-induced Na+ influx, respectively. In contrast to the majority of nonepithelial cells and cells transfected with an ubiquitous isoform of Na+/H+ exchanger, both delta mu(H+)-induced Na+ influx and Na+/Li+ countertransport in human erythrocytes were completely insensitive to ethylisopropyl amiloride (20 micromol/L) and cell shrinkage. Thus, our data strongly suggest that human erythrocyte Na+/Li+ countertransport and delta mu(H+)-induced Na+/H+ exchange are mediated by the distinct transporters. Moreover, because the properties of these erythrocyte transporters and NHE-1 are different, it complicates the use of erythrocytes for the identification of the mechanism for activating the ubiquitous form of Na+/H+ exchanger in primary hypertension.

Inhibition of furosemide-sensitive cation transport and activation of sodium-lithium exchange by endogenous circulating factor(s) in Bartter's and Gitelman's syndromes

BACKGROUND

The nature of the cellular abnormality causing hypokalemia, hypotension, and hypovolemia in Bartter's and Gitelman's syndromes is still being debated. In fact, despite the recent descriptions of an array of nonconservative missense or point mutations in some ion transporters and in K+ channel, the lack of detectable defects in some patients suggests that other abnormalities of cell ion homeostasis may be involved in the pathophysiology of these syndromes. The study of the activity of cell ion transporters in patients with these syndromes using red blood cells (RBC) as a cellular model never investigated the role of plasma factor(s) affecting ion transport.

OBJECTIVE

To evaluate the effect of plasma from patients with these syndromes on furosemide-sensitive lithium efflux (FSLE) from lithium (Li+)-loaded RBC of healthy subjects in vitro.

METHODS

RBC of healthy controls were loaded with Li+ in the presence of nystatin and FSLE was evaluated in the presence of various concentrations of plasma from controls and patients with the two syndromes.

RESULTS

Plasma from controls did not affect FSLE (0.08 +/- 0.02 mmol/l cells per h with 1:4 vol:vol and 0.07 +/- 0.02 mmol/l cells per h with 1:2 vol:vol plasma dilution). In contrast, doubling concentrations of plasma from patients with either syndrome in the efflux solution halved FSLE (from 0.10 +/- 0.0 mmol/l cells per h with 1:4 vol:vol to 0.05 +/- 0.01 mmol/l cells per h with 1:2 vol:vol plasma dilution, P < 0.05). Na+/Li+ exchange was significantly greater for RBC from patients with either syndrome than it was for RBC from controls (0.373 +/- 0.06 versus 0.257 +/- 0.01 mmol/l cells per h, P < 0.01), but the kinetic properties of furosemide-sensitive Na+-K+-2Cl- cotransport were similar.

CONCLUSION

These data provide evidence for the hypothesis that plasma factor(s) affect ion transport in patients with these two syndromes. Since FSLE estimates Na+-K+-2Cl- cotransport the data suggest that plasma factor(s) contribute(s) to K+ wasting, hypokalemia, and hypotension by inhibiting cotransport in patients with these syndromes. The increase of Na+/Li+ exchange is most likely a secondary phenomenon associated with the hypermineralocorticoid state.