Lymphocytic Na(+)-H+ exchange increases after an oral glucose challenge

Sustained increase of extracellular calcium concentration causes arterial vasoconstriction in humans

BACKGROUND

Vascular tone is affected by extracellular electrolytes.

OBJECTIVE

To evaluate whether a sustained increase in the extracellular calcium concentration may produce vasoconstriction in humans.

DESIGN

Cross-sectional data of a cohort study of 65 patients with end-stage renal failure.

MEASUREMENTS

Arterial tone was obtained from radial artery waveforms. Intracellular stored calcium and sarcoendoplasmic reticulum Ca(2+)-ATPase activity were measured in mononuclear leukocytes using fluorescent dye techniques and oxalate-supported calcium uptake.

RESULTS

During the haemodialysis sessions the extracellular calcium concentration increased significantly from 2.28 +/- 0.03 to 2.63 +/- 0.03 mmol/l (n = 65; mean +/- SEM; P < 0.001) and arterial tone increased from 31 +/- 2 to 44 +/- 3 mmHg/ml (n = 65; P < 0.001). Multivariate analysis showed that pre and postdialysis extracellular calcium and pre and postdialysis body weight were the only independent predictors of arterial tone during haemodialysis. Intracellular stored calcium in mononuclear leukocytes significantly declined from 5.1 +/- 1.2 arbitrary units at the start to 2.3 +/- 0.6 arbitrary units at the end of haemodialysis (n = 10; P = 0.01). The activity of the sarcoendoplasmic reticulum Ca(2+)-ATPase significantly decreased from 13.6 +/- 2.6 nmol calcium/mg protein per 5 min at the start to 9.2 +/- 1.6 nmol calcium/mg protein per 5 min at the end of haemodialysis (n = 28; P = 0.01). On the other hand, when a low dialysate calcium concentration was used, the increase in artery tone and reduction in sarcoendoplasmic reticulum Ca(2+)-ATPase activity were reversed.

CONCLUSION

A sustained increase in the extracellular calcium concentration causes arterial vasoconstriction in humans. In addition, a reduction of sarcoendoplasmic reticulum Ca(2+)-ATPase activity and intracellular stored calcium in mononuclear leukocytes was observed.

Increased plasma phenylacetic acid in patients with end-stage renal failure inhibits iNOS expression

NO prevents atherogenesis and inflammation in vessel walls by inhibition of cell proliferation and cytokine-induced endothelial expression of adhesion molecules and proinflammatory cytokines. Reduced NO production due to inhibition of either eNOS or iNOS may therefore reinforce atherosclerosis. Patients with end-stage renal failure show markedly increased mortality due to atherosclerosis. In the present study we tested the hypothesis that uremic toxins are responsible for reduced iNOS expression. LPS-induced iNOS expression in mononuclear leukocytes was studied using real-time PCR. The iNOS expression was blocked by addition of plasma from patients with end-stage renal failure, whereas plasma from healthy controls had no effect. Hemofiltrate obtained from patients with end-stage renal failure was fractionated by chromatographic methods. The chromatographic procedures revealed a homogenous fraction that inhibits iNOS expression. Using gas chromatography/mass spectrometry, this inhibitor was identified as phenylacetic acid. Authentic phenylacetic acid inhibited iNOS expression in a dose-dependent manner. In healthy control subjects, plasma concentrations were below the detection level, whereas patients with end-stage renal failure had a phenylacetic acid concentration of 3.49 +/- 0.33 mmol/l (n = 41). It is concluded that accumulation of phenylacetic acid in patients with end-stage renal failure inhibits iNOS expression. That mechanism may contribute to increased atherosclerosis and cardiovascular morbidity in patients with end-stage renal failure.

Troglitazone reduces activity of the Na+/H+ exchanger in fructose-fed borderline hypertensive rats

Activation of the Na+/H+ exchanger (NHE) is known to be related to elevated blood pressure in hyperinsulinemia. We previously demonstrated that a fructose-enriched diet induced hyperinsulinemia and hypertriglyceridemia, elevated NHE activity, increased intracellular calcium concentrations ([Ca2+]i), and increased blood pressure in borderline hypertensive rats (BHR). This study examines whether pharmacologically reducing plasma triglyceride or insulin concentrations lowers blood pressure and reduces NHE activity in fructose-fed BHR. Eicosapentaenoic acid (EPA), bezafibrate (BEZ), and troglitazone (TRO) were administered to treat hypertriglyceridemia and/or hyperinsulinemia. Rats were fed a 60% fructose diet or a control diet for 4 weeks, followed by a diet with either vehicle, EPA, BEZ, or TRO for 4 weeks. Intracellular pH (pHi) was measured in platelets by fluorescent dye. Platelet NHE activity was evaluated by the recovery of pHi following addition of sodium propionate (Vmax). [Ca2+]i in platelets were measured fluorometrically. In fructose-fed rats, EPA prevented further increase in blood pressure, and reduced triglyceride concentration and [Ca2+]i without affecting Vmax or plasma insulin concentrations. BEZ reduced triglyceride concentrations without affecting blood pressure, Vmax, [Ca2+]i, or insulin concentrations. TRO prevented an increase in blood pressure, and reduced Vmax, [Ca2+]i, and insulin, but not triglycerides. Plasma insulin and Vmax were positively correlated. In conclusion, improvement of hyperinsulinemia can decrease NHE activity and blood pressure in fructose-fed BHR.

Time-dependent effect of statins on platelet function in hypercholesterolaemia

BACKGROUND

Reduction of platelet activity induced by statins has been described as a positive effect exerted by such molecules on vascular thrombotic events. However, the relations among cholesterol (LDL-C) reduction, the timing of the antiplatelet effect, the involved mechanisms and the doses of each statin able to reduce platelet function are not actually well known. The aim of our study was to evaluate the impact of simvastatin (20 mg day-1), atorvastatin (10 mg day-1), fluvastatin (40 mg day-1) and pravastatin (40 mg day-1) on platelet function in hypercholesterolaemic subjects with relation to (LDL-C), oxidized-LDL (ox-LDL) and antiport mechanism modifications.

MATERIALS AND METHODS

Sixteen subjects were assigned to each treatment (40 males, 24 females, mean age 48.7 +/- 13.4, LDL-C 5.13 +/- 0,23 mmol L-1) and evaluated for platelet surface P-selectin (P-sel), lipid profile, ox-LDL, platelet-associated ox-LDL (Pox-LDL), platelet cholesterol content, antiport mechanisms, and intracellular and systemic NO synthase every 7 days for one month.

RESULTS

Our data show a strong relation between enhanced P-sel and Pox-LDL (r = 0.68, P < 0.01). Simvastatin, atorvastatin, fluvastatin and pravastatin reduce platelet activity after 1, 2, 3 and 4 weeks of treatment, respectively (P < 0.001, P < 0.001, P < 0.01, P < 0.05). Pox-LDL are modulated early by simvastatin, atorvastatin and fluvastatin Pox-LDL (r = 0.66, 0.65 and 0.52; P < 0.001, 0.001 and 0.01, respectively) whereas LDL-C and ox-LDL reductions associated to modifications of antiport activity act later. Moreover, they are the most relevant finding in pravastatin-related subjects.

CONCLUSIONS

Our data suggest a different impact of several statins on platelet function, which is initially related to interference with Pox-LDL rather than LDL-C reduction.

The Effect of Coronary Perfusion with an Oxygenated Celsior Solution on 12-hour Cardiac Preservation

Celsior is a new extracellular-type cardiac preservation solution. We recently developed an apparatus for preservation using low-pressure continuous coronary perfusion. The purpose of this study was to investigate the efficacy of coronary perfusion with an oxygenated Celsior solution using the new apparatus for prolonged cardiac preservation. Adult mongrel dogs weighing 9-13 kg were divided into two groups: the coronary perfusion group (CP; n = 5) and the simple immersion group (SI; n = 7). The coronary vascular beds were washed out with a 4 degrees C Celsior solution following cardiac arrest using the same solution, and their hearts were excised. In the CP group, the graft was immersed in a 4 degrees C Celsior solution and perfused with the same oxygenated solution. In the SI group, the graft was simply immersed in a 4 degrees C Celsior solution. beta-adenosine triphosphate (beta-ATP), phosphocreatine (Pcr), inorganic phosphate (Pi) levels and myocardial pH (pHi) were measured immediately after excising the heart, and at 3, 6, and 12 hours after preservation. beta-ATP, Pcr, and Pi values were expressed as a percentage of control values, which were measured immediately after excising the heart. beta-ATP/Pi and Pcr/Pi levels were significantly higher in the CP group than in the SI group at 6 and at 12 hours after preservation. The pHi levels during preservation were significantly higher in the CP group than in the SI group. Low-pressure hypothermic coronary perfusion with an oxygenated Celsior solution is effective for long-term heart preservation. </hea

Increased sodium-proton antiporter activity in patients with obstructive sleep apnoea

Cytosolic pH (pH(i)) and the activity of the sodium-proton antiporter (Na(+)/H(+) antiporter) were measured in lymphocytes from 22 patients with obstructive sleep apnoea and from 24 age-matched healthy subjects (Controls). The cellular Na(+)/H(+) antiporter was measured spectrophotometrically using a pH-sensitive fluorescent dye after intracellular acidification using sodium propionate. Resting pHi was similar in lymphocytes from patients with obstructive sleep apnoea and from controls (7.36 +/- 0.20, n=22; vs. 7.35 +/- 0.19, n=24; mean +/- SD). The Na(+)/H(+) antiporter activity was significantly higher in patients with obstructive sleep apnoea than in controls (11.87 +/- 3.26 x 10(-3) pH(i)/s vs. 4.38 +/- 1.40 x 10(-3) pH(i)/s; P < 0. 0001). The apparent affinity of the Na+/H+ antiporter was not significantly different between the groups (6.90 +/- 0.23 vs. 6.87 +/- 0.20). In patients with obstructive sleep apnoea the activity of the Na(+)/H(+) antiporter remained stable during the night. The activity of the Na(+)/H(+) antiporter was 13.49 +/- 4.80 x 10(-3) pH(i)/s at 20.00 and 13.26 +/- 6.13 x 10(-3) pH(i)/s at 02.00. From the present results it is concluded that an increased cellular Na(+)/H(+) antiporter activity may be a genetic marker for patients who are predisposed to obstructive sleep apnoea.

D609-phosphatidylcholine-specific phospholipase C inhibitor attenuates thapsigargin-induced sodium influx in human lymphocytes

Previously, we reported that the phosphatidylcholine-specific phospholipase C (PC-PLC) inhibitor tricyclodecan-9-yl xanthogenate (D609) potentiates thapsigargin-induced Ca(2+) influx in human lymphocytes. In the present study we examined the effect of D609 on the thapsigargin-induced Na(+) entry. We found that the early phase of the thapsigargin-induced increase in the intracellular Na(+) concentration (approx. 1-2 min after stimulation) was attenuated after preincubation of lymphocytes with D609. By contrast, thapsigargin-induced Na(+) influx was not affected in the presence butan-1-ol, which inhibits phosphatidylcholine-specific phospholipase D (PC-PLD). The thapsigargin-induced Na(+) influx could be mimicked by PC-PLC exogenously added to the lymphocyte suspension, whereas addition of PC-PLD had no effect. In addition, thapsigargin stimulated formation of the physiological PC-PLC products, diacylglycerol. Cell-permeable diacylglycerol analogue, dioctanoyl-glycerol (DOG), produced time- and concentration-dependent increase in the intracellular Na(+) concentration. Both thapsigargin- and DOG-induced Na(+) increases were not affected in the presence of Na(+)/H(+) antiport inhibitor, HOE609, or Na(+)/Ca(2+) antiport inhibitor, dimethylthiourea, as well as in the presence of Co(2+) and Ni(2+), which block store-operated Ca(2+) entry. By contrast, markedly reduced thapsigargin- and DOG-induced Na(+) influx were noted in the presence of flufenamic acid, which blocks the non-selective cation current (I(CRANC)). In conclusion, our results suggest that diacylglycerol released due to the PC-PLC activation contributes to the thapsigargin-induced Na(+) entry.

Phospholipase A(2) is involved in thapsigargin-induced sodium influx in human lymphocytes

Previously, we reported that emptying of intracellular Ca(2+) pools with endoplasmatic Ca(2+)-ATP-ase inhibitor thapsigargin leads to the Na(+) influx in human lymphocytes (M. Tepel et al., 1994, J. Biol. Chem. 269, 26239-26242). In the present study we examined the mechanism underlying the thapsigargin-induced Na(+) entry. We found that the thapsigargin-induced increase in Na(+) concentration was effectively inhibited by three structurally unrelated phospholipase A(2) (PLA(2)) inhibitors, p-bromophenacyl bromide, 3-(4-octadecyl)-benzoylacrylic acid (OBAA), and bromoenol lactone (BEL). The thapsigargin-induced Na(+) influx could be mimicked by PLA(2) exogenously added to the lymphocyte suspension. In addition, thapsigargin stimulated formation of arachidonic acid (AA), the physiological PLA(2) product. AA induced Na(+) entry in a time- and concentration-dependent fashion. Both, thapsigargin-induced Na(+) influx and AA liberation were completely inhibited in the presence of tyrosine kinase inhibitor genistein but not in the absence of extracellular Ca(2+). Collectively, these data show that thapsigargin-induced Na(+) entry is associated with tyrosine kinase-dependent stimulation of PLA(2).

Effect of dexamethasone on the lymphocytic Na+/H+ antiporter activity

OBJECTIVE

The effects of short-term administration of dexamethasone on sodium/proton antiporter (Na+/H+ antiporter) in human lymphocytes were investigated in vitro.

METHODS

Cytosolic pHi in lymphocytes was measured spectrophotometrically using the pH-sensitive fluorescent dye 2'-7'-bis-carboxyethyl-5(6)-carboxyfluorescein at 530 nm with excitation wavelengths of 440 and 530 nm. The Na+/H+ antiporter activity was determined after intracellular acidification using 100 mmol/l propionic acid.

RESULTS

The addition of 1 micromol/l dexamethasone significantly reduced cytosolic pHi from 7.44+/-0.03 to 7.25+/-0.05 (mean +/- SEM; P<0.01). Incubation with dexamethasone for 40 min significantly reduced the Na+/ H+ antiporter activity from (9.19+/-0.61)x10(-3) pHi/s (n = 22) to (7.23+/-0.49)x10(-3) pHi/s (n = 22; P<0.01). The effect of dexamethasone was time and concentration dependent The apparent affinity of the Na+/H+ antiporter was not significantly different in the absence or presence of dexamethasone. The inhibition of the Na+/H+ antiporter by dexamethasone was abolished in the presence of the glucocorticoid receptor blocker, mifepristone.

CONCLUSION

Dexamethasone directly inhibits the Na+/H+ antiporter using a receptor-dependent pathway. This effect may be important for pharmacological side effects such as glucocorticoid-induced hypertension.

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.

G proteins and hypertension: an alternative candidate gene approach

Hypertension affects approximately 20 to 30% of individuals in industrialized countries, and is commonly believed to develop on the basis of both genetic and environmental factors. The identification of genes susceptible to the most frequent form of hypertension, commonly referred to as "essential" hypertension, is hampered by the fact that blood pressure is a poorly defined phenotype that is modulated by multiple factors, such as gender, race, body mass etc., and that the definition of hypertension depends on a rather arbitrarily chosen cut-off value. Hence, more progress has been made in the identification of genes responsible for rare autosomal dominant forms of hypertension, such as Liddle's disease. This review focuses on an experimental approach that attempts to define candidate genes for essential hypertension using immortalized cells from well characterized normotensive and hypertensive subjects. From the presently available results, one attractive speculation is that an increased intracellular signal transduction caused by an enhanced reactivity of Gj-type G proteins represents a genetically fixed trait that renders affected individuals susceptible to essential hypertension.

Phosphatidylcholine-specific phospholipase C regulates thapsigargin-induced calcium influx in human lymphocytes

The involvement of phosphatidylcholine-specific phospholipase C (PC-PLC) and D (PC-PLD) in the regulation of the thapsigargin-induced Ca2+ increase was investigated. Pretreatment of human lymphocytes with the PC-PLC inhibitors D609 or U73122 enhanced the thapsigargin-induced Ca2+ influx. By contrast, no effect was observed in the presence of phospholipase D inhibitor butanol. Addition of exogenous PC-PLC but not PC-PLD to lymphocytes prestimulated with thapsigargin led to a decrease of intracellular Ca2+. In addition, thapsigargin was shown to release diacylglycerol (DAG) from cellular phosphatidylcholine pools. The thapsigargin-induced DAG formation was inhibited by U73122 and D609 but not by butanol. Moreover, no formation of the PC-PLD activity marker phosphatidylbutanol was detected. Thapsigargin-induced DAG formation was dependent on the Ca2+ entry, as it was abolished in the absence of extracellular Ca2+ or in the presence of Ni2+. Further investigations demonstrated that the inhibition of the cellular DAG target, protein kinase C (PKC), enhanced thapsigargin-induced Ca2+ increase, whereas direct PKC activation had an inhibitory effect. Taken together, our results reveal the involvement of PC-PLC in the regulation of the thapsigargin-induced Ca2+ increase and point to the existence of a physiologic feedback mechanism activated by Ca2+ influx and acting via consecutive activation of PC-PLC and PKC to limit the rise of intracellular Ca2+.

Hyperglycemia counterbalances the antihypertensive effect of glutathione in diabetic patients: evidence linking hypertension and glycemia through the oxidative stress in diabetes mellitus

Diabetes mellitus is associated with hypertension. An antihypertensive effect of the antioxidant glutathione has been recently demonstrated. It has been suggested that hyperglycemia may contribute to the pathophysiology of hypertension in diabetes by generating an oxidative stress. In this study, three different tests were performed in ten hypertensive and ten nonhypertensive diabetic subjects: (1) an oral glucose tolerance test, (2) glutathione i.v. administration (1 g/m2 bolus + 1 g/m2 in 2 h), and (3) oral glucose tolerance test + glutathione administration. At -15', 0', 30', 60', 90', 120', and 180' systolic and diastolic blood pressure, plasma glucose, and insulin were measured. Variations in plasma glucose and insulin levels were not different during each test in the two groups of patients and in test (1) compared to (3). Glutathione administration reduced systolic and diastolic blood pressure in both hypertensive and nonhypertensive diabetic subjects from 30' to 120'. This phenomenon was abolished as glycemia increased after oral glucose loading. In hypertensive, but not in nonhypertensive diabetic subjects, a significant increase of systolic and diastolic blood pressure was observed at 90' and 120' of the oral glucose tolerance test (p < 0.01). These data show that hyperglycemia can counteract the hypotensive effects of the antioxidant glutathione, suggesting that glucose may impair arterial relaxation by producing free radicals. Also, it appears that hypertension in diabetic patients is aggravated by high glucose plasma levels.

Na+/H+ exchange in hypertension and in diabetes mellitus--facts and hypotheses

An enhancement of Na+/H+ exchange (NHE) in blood cells of selected patients with essential hypertension and with diabetic nephropathy has been described by various investigators. Recent studies have shown that enhanced NHE activity persists in immortalized lymphoblasts from these patients after prolonged cell culture and, thus, appears to be under genetic control. Available evidence strongly argues against a mutation in the encoding gene or an overexpression of the NHE. Immortalized cells from hypertensive patients with enhanced NHE activity display two-fold enhanced agonist-induced rises of the cytosolic free Ca2+ concentration and the underlying reason was identified as an increased activation of pertussis toxin (PTX)-sensitive G proteins. The molecular mechanism(s) of this phenomenon have not yet been elucidated. It appears likely that similar changes contribute to the enhanced NHE activity phenotype in diabetic nephropathy, although experimental evidence for this is still lacking. An enhanced activation of PTX-sensitive G proteins could explain many of the hitherto unexplained phenomena in essential hypertension, e.g. inheritance, increased vasoconstriction, hypertrophy of remodeling of arterial blood vessels and the heart, enhanced platelet aggregation etc. In diabetes the same defect could provide the basis for the susceptibility to nephropathy, e.g. by enhancing the deleterious effects of autocrine and paracrine growth factors. Thus, the experimental approach of immortalizing blood cells from patients with essential hypertension and diabetic nephropathy has opened new horizons in the identification of genetically fixed abnormalities in intracellular signal transduction which could contribute to both pathologies and which can now be studied without the confounding influences of the diabetic or hypertensive in vivo milieu.