Erythrocyte anion exchanger activity and intracellular pH in essential hypertension

Carbonic Anhydrase Inhibitors suppress platelet procoagulant responses and in vivo thrombosis

Carbonic anhydrase (CA) inhibitors have a long history of safe clinical use as mild diuretics, in the treatment of glaucoma and for altitude sickness prevention. In this study, we aimed to determine if CA inhibition may be an alternative approach to control thrombosis. We utilized a high-resolution dynamic imaging approach to provide mechanistic evidence that CA inhibitors may be potent anti-procoagulant agents in vitro and effective anti-thrombotics in vivo. Acetazolamide and methazolamide, while sparing platelet secretion, attenuated intracellular chloride ion entry and suppressed the procoagulant response of activated platelets in vitro and thrombosis in vivo. The chemically similar N-methyl acetazolamide, which lacks CA inhibitory activity, did not affect platelet procoagulant response in vitro. Outputs from rotational thromboelastometry did not reflect changes in procoagulant activity and reveal the need for a suitable clinical test for procoagulant activity. Drugs specifically targeting procoagulant remodeling of activated platelets, by blockade of carbonic anhydrases, may provide a new way to control platelet-driven thrombosis without blocking essential platelet secretion responses.

Red blood cell sodium transport in patients with cirrhosis

Patients with advanced cirrhosis have abnormal sodium homoeostasis. The study was undertaken to quantify the sodium transport across the plasma membrane of red blood cells (RBC) in patients with cirrhosis. RBC efflux and influx of sodium were studied in vitro with tracer (22) Na(+) according to linear kinetics in 24 patients with cirrhosis and 14 healthy controls. The sodium efflux was modified by ouabain (O), furosemide (F) and a combination of O and F (O + F). RBC sodium was significantly decreased (4·6 versus control 6·3 mmol l(-1) , P<0·001) and directly related to serum sodium (r = 0·57, P<0·05). The RBC fractional sodium efflux was higher in patients with cirrhosis (+46%, P<0·01) compared to controls. Inhibition in both high (145 mmol l(-1) )- and low (120 mmol l(-1) )-sodium buffers showed that the F-insensitive sodium efflux was twice as high in cirrhosis as in controls (P = 0·03-0·007), especially the O-sensitive, F-insensitive efflux was increased (+ 225%, P = 0·01-0·006). Fractional F-sensitive transport was normal in cirrhosis. RBC sodium influx was largely normal in cirrhosis. In conclusion, RBC sodium content is reduced in patients with cirrhosis with a direct relation to serum sodium. Increased RBC sodium efflux is especially related to ouabain-sensitive, furosemide-insensitive transport and thus most likely due to upregulated activity of the sodium-potassium pump. The study gives no evidence to an altered intracellular/extracellular sodium ratio or to a reduced fractional furosemide-sensitive sodium transport in cirrhosis.

Utilizing hydrogen sulfide as a novel anti-cancer agent by targeting cancer glycolysis and pH imbalance

BACKGROUND AND PURPOSE

Many disparate studies have reported the ambiguous role of hydrogen sulfide (H2 S) in cell survival. The present study investigated the effect of H2 S on the viability of cancer and non-cancer cells.

EXPERIMENTAL APPROACH

Cancer and non-cancer cells were exposed to H2 S [using sodium hydrosulfide (NaHS) and GYY4137] and cell viability was examined by crystal violet assay. We then examined cancer cellular glycolysis by in vitro enzymatic assays and pH regulator activity. Lastly, intracellular pH (pHi ) was determined by ratiometric pHi measurement using BCECF staining.

KEY RESULTS

Continuous, but not a single, exposure to H2 S decreased cell survival more effectively in cancer cells, as compared to non-cancer cells. Slow H2 S-releasing donor, GYY4137, significantly increased glycolysis, leading to overproduction of lactate. H2 S also decreased anion exchanger and sodium/proton exchanger activity. The combination of increased metabolic acid production and defective pH regulation resulted in an uncontrolled intracellular acidification, leading to cancer cell death. In contrast, no significant intracellular acidification or cell death was observed in non-cancer cells.

CONCLUSIONS AND IMPLICATIONS

Low and continuous exposure to H2 S targets metabolic processes and pH homeostasis in cancer cells, potentially serving as a novel and selective anti-cancer strategy.

A nature-inspired betalainic probe for live-cell imaging of Plasmodium-infected erythrocytes

A model betalainic dye was semisynthesized from betanin, the magenta pigment of the red beet, and was effective for live-cell imaging of Plasmodium-infected red blood cells. This water-soluble fluorescent probe is photostable, excitable in the visible region and cell membrane-permeable, and its photophysical properties are not notably pH-sensitive. Fluorescence imaging microscopy of erythrocytes infected with Plasmodium falciparum, a causative agent of malaria in humans, showed that only the parasite was stained. Z-stacking analysis suggested that the probe accumulates proximal to the nucleus of the parasite. Indicaxanthin, one of the natural fluorescent betalains found in the petals of certain flowers, did not stain the parasite or the red blood cell.

Involvement of the Na+-independent Cl-/HCO3- exchange (AE) isoform in the compensation of myocardial Na+/H+ isoform 1 hyperactivity in spontaneously hypertensive rats

Enhanced activity of Na+/H+ isoform 1 (NHE-1) and the Na+-independent Cl-/HCO3- exchange (AE) is a feature of the hypertrophied myocardium in spontaneously hypertensive rats (SHR). The present study explored the possibility that sustained intracellular acidosis due to increased myocardial acid loading through AE causes NHE-1 enhancement. To this aim, SHR were treated for 2 weeks with a rabbit polyclonal antibody against an AE3 isoform that was recently developed and proven to have inhibitory effects on myocardial AE activity. We then compared the AE activity in the left ventricle papillary muscles isolated from untreated SHR with antiAE3-treated SHR; AE activity was measured in terms of the rate of intracellular pH recovery after an intracellular alkali load was introduced. AE activity was diminished by approximately 70% in SHR treated with the antiAE3 antibody, suggesting that the AE3 isoform is a major carrier of acid-equivalent influx in the hypertrophied myocardium. However, the antibody treatment failed to normalize NHE-1 activity that remained elevated in the myocardium of normotensive rats. The data therefore rule out the possibility that NHE-1 hyperactivity in hypertensive myocardium was due to sustained intracellular acidosis induced by increased AE activity that characterizes SHR myocardial tissue.

Chloride increases adrenergic receptor-mediated platelet and vascular responses

BACKGROUND

We postulated that increasing intracellular chloride concentration ([Cl-]i) in human platelets would potentiate alpha2 adrenergic receptor (A2AR)-mediated platelet aggregation, and that vascular reactivity would also be increased by raising [Cl-]i in blood vessels. We further hypothesized that ligands binding to the A2AR would increase [Cl-]i by stimulating carbonic anhydrase-dependent chloride/bicarbonate exchange. Because diuretics are potent inhibitors of carbonic anhydrase, we speculated that these agents inhibit platelet aggregation and vascular contractility through inhibition of chloride influx by decreasing carbonic anhydrase activity, and subsequently, chloride/bicarbonate exchange. The aim of this study was to test these hypotheses.

METHODS

Platelet aggregation was measured by determining changes in optical density of platelet-rich plasma. Contractile responses to A2AR agonists were recorded in isolated vascular smooth muscle. The substances [Cl-]i and intracellular pH (pHi) were measured using microfluorometric methods. Carbonic anhydrase activity and chloride/bicarbonate exchange were determined by an in vitro assay based on the Stewart cycle.

RESULTS

Increasing [Cl-]i potentiated platelet aggregation and vascular contractility, and epinephrine raised [Cl-]i by stimulating carbonic anhydrase-dependent chloride/bicarbonate exchange. Furthermore, diuretic-dependent inhibition of carbonic anhydrase activity decreased chloride/bicarbonate exchange.

CONCLUSIONS

Our data support the concept that diuretics inhibit carbonic anhydrase activity and chloride/bicarbonate exchange in platelets and vascular smooth muscle. The ensuing reduction in [Cl-]i that is induced by diuretics in these tissues could play a role in reducing the effect of catecholamines on precipitating thrombotic stroke or myocardial infarction.

In-vivo intracellular pH at rest and during exercise in patients with essential hypertension

BACKGROUND

Several studies in isolated cells have reported that intracellular pH (pHi) in individuals with essential hypertension may be relatively alkaline compared to normotensive individuals. Such an abnormality of pHi in hypertension would be consistent with enhanced sodium-hydrogen exchanger activity and may provide potential mechanisms by which hypertension and its complications could develop.

OBJECTIVES

To determine in-vivo intracellular pH of skeletal muscle at rest and during recovery from exercise-induced acidosis in hypertensive and normotensive subjects.

METHODS

Using 31-phosphorus magnetic resonance spectroscopy, pHi of the dominant flexor digitorum superficialis was measured in 20 Caucasian subjects (14 male) with essential hypertension and 20 normotensive controls matched for gender, age, race and body mass index. Measurements were made at rest and during the exercise and recovery periods of a stepped incremental maximal exercise protocol. The rate of pHi recovery from exercise-induced acidosis was calculated by linear regression over the first 210 s of recovery from the pHi time plots of respective subjects.

RESULTS

Mean resting pHi in the hypertensive (7.05 +/- 0.04) and normotensive groups (7.06 +/- 0.04) were not significantly different. There was a significant effect of gender on pHi: mean pHi was 7.07 +/- 0.03 in males and 7.02 +/- 0.03 in females, respectively (P < 0.0005). The mean intracellular pH achieved by exercise was 6.74 +/- 0.31 in hypertensive individuals and not significantly different in normotensive individuals (6.68 +/- 0.19; P = 0.4). The mean rate of pHi recovery in the hypertensives was 0.08 +/- 0.03 pH units/min and not significantly different in normotensives (0.08 +/- 0.02; P = 0.4).

CONCLUSIONS

These results contrast with previously documented abnormalities in the control of pHi in hypertension and demonstrate the absence of major in-vivo disturbances of pHi in skeletal muscle, both at rest and during recovery from exercise-induced acidosis, in essential hypertension. Therefore, it is possible that previously documented abnormalities of pHi and activity of the exchanger may be either specific to cell type or not present under in-vivo conditions.

Regulation of the Na+/H+ antiporter in patients with mild chronic renal failure: effect of glucose

BACKGROUND

The aim of this study was to determine the glucose-dependent regulation of the sodium-proton-antiporter (Na+/H+ antiporter) in patients with mild chronic renal failure (CRF).

METHODS

We measured plasma glucose concentrations, plasma insulin concentrations, plasma C peptide concentrations, arterial blood pressure, cytosolic pH (pHi), cellular Na+/H+ antiporter activity, and cytosolic sodium concentration ([Na+]i) in 19 patients with CRF and 41 age-matched healthy control subjects (control) during a standardized oral glucose tolerance test. Intracellular pHi, [Na+]i, and Na+/H+ antiporter activity was measured in lymphocytes using fluorescent dye techniques.

RESULTS

Under resting conditions, the pHi was significantly lower, whereas the Na+/H+ antiporter activity was significantly higher in CRF patients compared with controls (each P < 0.0001). The oral administration of 100 g glucose significantly increased the Na+/H+ antiporter activity in CRF patients from 13.35 +/- 1.26 x 10-3 pHi/second to 16.44 +/- 1.37 x 10-3 pHi/second after one hour and to 14.06 +/- 1.36 x 10-3 pHi/second after two hours (mean +/- SEM, P = 0.008 by Friedmans's two-way analysis of variance). In controls, the administration of 100 g glucose significantly increased the Na+/H+ antiporter activity from 4.23 +/- 0.20 x 10-3 pHi/second to 6.00 +/- 0.56 x 10-3 pHi/second after one hour and to 6.65 +/- 0.64 x 10-3 pHi/second after two hours (P = 0.0003). The glucose-induced enhancement of the Na+/H+ antiporter activity was more pronounced in CRF patients compared with controls (P = 0.011). Resting [Na+]i was not significantly different between the two groups.

CONCLUSIONS

CRF patients show an intracellular acidosis leading to an increased Na+/H+ antiporter activity. In addition, high glucose levels exaggerate the differences in Na+/H+ antiporter activity already present between cells from patients with mild CRF and those from control subjects.

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.

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.

Sodium-dependent Cl-/HCO3- exchange in patients with chronic renal failure: correlation with renal function

To investigate the effects of uremia on cellular function the activity of the sodium-dependent chloride-bicarbonate exchanger (sodium-dependent Cl-/HCO3- exchanger) and the sodium-independent chloride-bicarbonate exchanger (sodium-independent Cl-/HCO3- exchanger) were examined in lymphocytes from 25 patients with mild chronic renal failure, 9 patients with end-stage chronic renal failure on regular hemodialysis, and from 25 age-matched healthy control subjects. Cytosolic pH (pHi) and the activity of the sodium-dependent Cl-/HCO3- exchanger and the sodium-independent Cl-/HCO3- exchanger were measured spectrophotometrically using the pH-sensitive fluorescent dye 2'7'-bis-carboxyethyl-5 [6]-carboxyfluorescein acetoxy-methylester (BCECF-AM). The activation of the sodium-dependent Cl-/HCO3- exchanger by removal of extracellular chloride was prevented in the presence of 500 micromol/liter 4,4' diisothiocyanostilbene-2,2'-disulfonic acid (DIDS) or in the absence of extracellular sodium, but was not affected by the specific inhibitor of the sodium/proton exchanger, ethyl isopropyl amiloride (EIPA). The sodium-dependent Cl-/HCO3- exchangers were significantly different in lymphocytes from healthy control subjects, patients with mild chronic renal failure, and patients with end-stage chronic renal failure (X2 = 6.43, P = 0.040 by Kruskal-Wallis-test). The sodium-dependent Cl-/HCO3- exchanger was significantly lower in patients with end-stage chronic renal failure compared to patients with mild chronic renal failure or compared to healthy control subjects (each P < 0.05). In patients with chronic renal failure a significantly negative correlation between sodium-dependent Cl-/HCO3- exchanger and the serum creatinine concentration (r = -0.507; P = 0.0022) could be observed. On the other hand, resting pHi in lymphocytes and sodium-independent Cl-/HCO3- exchanger were not significantly different in lymphocytes from healthy control subjects, patients with mild chronic renal failure or patients with end-stage chronic renal failure. The present study suggests that the activity of the sodium-dependent Cl-/HCO3- exchanger is progressively impaired in chronic renal failure.

AE anion exchanger mRNA and protein expression in vascular smooth muscle cells, aorta, and renal microvessels

Intracellular pH (pHi) is an important regulator of vascular smooth muscle cell (VSMC) tone, contractility, and intracellular Ca2+ concentration. Among the multiple transport processes that regulate VSMC pHi, Na(+)-independent Cl-/HCO3- exchange is the major process that acidifies VSMCs in response to an alkaline load. Here, we characterize, in native and cultured VSMCs, the expression of the AE family of band 3-related anion exchangers, the best studied of these Cl-/HCO3- exchangers. A 4.2-kb AE2 mRNA was present in aorta and in all cultured VSMCs tested. Cultured VSMCs and aorta both expressed a approximately 165-kDa AE2 polypeptide, but a approximately 115-kDa polypeptide was the major AE2-related protein in aorta. AE3 mRNA levels in VSMCs and in arterial tissue were significantly lower than those for AE2, but AE3 or related polypeptides were readily detected by immunoblot and immunolocalization experiments. The approximately 125-kDa AE3 polypeptide was present in an immortalized aortic VSMC line, but the predominant AE3 epitope in aorta and most cultured cells was associated with a polypeptide of M(r) approximately 80 kDa. These data demonstrate the expression in native arteries and in VSMCs of products of the AE2 and AE3 genes, which may contribute to Na(+)-independent Cl-/HCO3- exchange activity in these tissues and cells.

pHi regulation in myocardium of the spontaneously hypertensive rat. Compensated enhanced activity of the Na(+)-H+ exchanger

To elucidate the mechanisms controlling pHi in myocardium of the spontaneously hypertensive rat (SHR), experiments were performed in papillary muscles (isometrically contracting at 0.2 Hz) from SHR and age-matched normotensive Wistar-Kyoto (WKY) rats loaded with the pH-sensitive fluorescent probe BCECF-AM. An enhanced activity of the Na(+)-H+ exchanger was detected in the hypertrophic myocardium of SHR. This conclusion was based on the following: (1) The myocardial pHi was more alkaline in SHR (7.23 +/- 0.03) than in WKY rats (7.10 +/- 0.03) (P < .05) in HEPES buffer. (2) SITS (0.1 mmol/L in HEPES buffer) did not alter pHi in the SHR (pHi 7.26 +/- 0.03 and 7.28 +/- 0.03 before and after SITS, respectively). (3) The fall in pHi observed after 20 minutes of Na(+)-H+ exchanger inhibition [5 mumol/L 5-(N-ethyl-N-isopropyl)amiloride (EIPA)] was greater in SHR (-0.16 +/- 0.01) than in WKY rats (-0.09 +/- 0.02, P < 0.05). (4) The velocity of pHi recovery from an intracellular acid load was faster in SHR than in WKY rats (0.068 +/- 0.02 versus 0.014 +/- 0.002 pH units/min at pHi 6.99, P < .05). (5) After EIPA inhibition, the rate of pHi recovery from the same acid load decreased to a similar value in both rat strains (0.0032 +/- 0.002 pH units/min in SHR and 0.0032 +/- 0.002 pH units/min in WKY rats). Under the more physiological HCO3(-)-CO2 buffer, no significant difference in steady state myocardial pHi was detected between rat strains (7.15 +/- 0.03 in SHR and 7.11 +/- 0.05 in WKY rats).(ABSTRACT TRUNCATED AT 250 WORDS)

Enhanced Na(+)-H+ exchanger activity and NHE-1 mRNA expression in lymphocytes from patients with essential hypertension

It has been demonstrated that the activity of the sodium-proton exchanger (NHE-1 isoform) is increased in lymphocytes and other blood cells from patients with essential hypertension. In the present study, we investigated whether an increased level of NHE-1-specific mRNA in lymphocytes from patients with essential hypertension would explain the enhanced transport activity. Twenty-two hypertensive patients and 21 normotensive subjects were studied. Basal cytosolic pH was measured by the pH-sensitive fluorescent probe 2',7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein. Maximal sodium-proton exchange activity was determined by acidifying cell pH and measuring the initial rate of the net sodium-dependent proton efflux driven by an outward proton gradient. The transcript level of NHE-1 was measured by reverse transcription-polymerase chain reaction in comparison with a constitutively expressed reference gene (beta-actin). Intracellular pH was lower in hypertensive patients than normotensive subjects (7.34 +/- 0.01 versus 7.39 +/- 0.01, mean +/- SEM, P < .001). The maximal activity of the sodium-proton exchanger was higher in hypertensive patients than in normotensive subjects (1262 +/- 100 versus 881 +/- 56 mmol/L cells per hour, P < .01). NHE-1 mRNA was increased in hypertensive patients compared with normotensive subjects (ratio of NHE-1 mRNA to beta-actin mRNA, 0.16 +/- 0.01 versus 0.12 +/- 0.02, P < .05). These data suggest that the increased sodium-proton exchange activity in essential hypertension may be related to the de novo synthesis of exchanger protein.

Abnormal Na+/H+ antiporter phenotype and turnover of immortalized lymphoblasts from type 1 diabetic patients with nephropathy

Cellular Na+/H+ exchanger (NHE) activity is elevated in type 1 diabetic patients with nephropathy and patients with essential hypertension. The characteristics of this NHE phenotype in hypertension (raised Vmax and a lowered Hill coefficient) are preserved in Epstein-Barr virus-transformed lymphoblasts from hypertensive patients. In this study, we have determined NHE kinetics in cultured lymphoblasts from diabetic patients with and without nephropathy, with nondiabetic controls, using fluorometry with the pH indicator 2,7'-bis-(carboxyethyl)-5,6-carboxyfluorescein and estimation of NHE isoform 1 (NHE-1) density with specific polyclonal antibodies. The Vmax of NHE was elevated significantly, and the Hill coefficient for internal H+ binding was lowered in cells from patients with diabetic nephropathy compared with both normal controls and normoalbuminuric diabetic patients. NHE-1 density as measured by Western blotting was similar in all groups. The turnover number of NHE-1 was thus elevated in cells from nephropathy patients. This phenotype in cells from diabetic nephropathy patients resembles that in essential hypertension and suggests that such patients may have a predisposition to hypertension. Moreover, as these changes persist in cultured lymphoblasts in vitro, these cells should provide a cell culture model to further define the basic mechanisms leading to NHE activation in diabetic nephropathy.