(Hypertension. 1996;27:919-925.)
© 1996 American Heart Association, Inc.
Articles |
Erythrocyte Sodium Transport, Intraplatelet pH, and Calcium Concentration in Salt-Sensitive Hypertension
From the Hypertension Unit, Departments of Internal Medicine and Nephrology (E.P.), Hospital Clínic, University of Barcelona (Spain).
Correspondence to Alejandro de la Sierra, MD, Department of Internal Medicine, Hospital Clínic, Villarroel 170, 08036-Barcelona, Spain.
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Abstract |
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Abstract We evaluated changes in erythrocyte sodium transport systems, platelet pH, and calcium concentration induced by low and high salt intakes in a group of 50 essential hypertensive patients classified on the basis of their salt sensitivity. Patients received a standard diet with 20 mmol NaCl daily for 2 weeks supplemented in a single-blind fashion by placebo tablets the first 7 days and NaCl tablets the following 7 days. Salt sensitivity, defined as a significant rise (P<.05) in 24-hour mean blood pressure obtained by ambulatory blood pressure monitoring, was diagnosed in 22 (44%) patients. The remaining 28 (56%) were considered to have salt-resistant hypertension. In the entire group of hypertensive patients, high salt intake promoted a significant increase (P<.05) in the maximal rate of erythrocyte Na+-Li+ countertransport (from 271±19 to 327±18 µmol/(L cells/h) and of the Na+-dependent HCO3--Cl- exchanger (from 946±58 to 1237±92 µmol/L cells/h) as well as in platelet pH (from 7.15±0.01 to 7.19±0.02) and calcium concentration (from 49±2 to 57±2 nmol/L). Depending on salt sensitivity, high salt intake promoted opposing changes in some of the sodium transport systems studied. Salt-sensitive patients increased the maximal rate of the erythrocyte Na+-K+ pump (from 7.0±0.4 to 8.8±0.4 mmol/(L cells/h), Na+-K+-Cl- cotransport (from 416±37 to 612±41 µmol/(L cells/h), and Na+-Li+ countertransport (from 248±20 to 389±17 µmol/(L cells/h) at the end of the high salt period. Conversely, salt-resistant patients decreased the Na+-K+ pump (from 8.0±0.4 to 6.9±0.3 mmol/(L cells/h) and Na+-K+-Cl- cotransport (from 578±53 to 481±43 µmol/(L cells/h). We conclude that modulation of erythrocyte sodium transport systems by high salt intake depends on salt sensitivity. The Na+-K+ pump, Na+-K+-Cl- cotransport, and Na+-Li+ countertransport increase in salt-sensitive patients, whereas the activity of these sodium transport systems tends to decrease in salt-resistant patients. Independent of salt sensitivity, high salt intake promotes a significant increase in the erythrocyte Na+-dependent HCO3--Cl- exchanger, platelet pH, and calcium concentration in essential hypertensive patients.
Key Words: hypertension, sodium-dependent • sodium, dietary • ions • sodium-potassium pump • calcium
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Introduction |
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High dietary salt intake is associated with primary hypertension in both humans and experimental animals. However, the response to excess NaCl is heterogeneous in hypertensive individuals, a phenomenon known as salt sensitivity.1 The underlying mechanisms of this salt sensitivity are not well understood. It has been postulated that alterations in cellular sodium and calcium metabolism may be related to this responsiveness to NaCl.
Several studies have examined the changes in cellular sodium and calcium contents, as well as in the transmembrane movements of these cations, with different levels of salt intake. The results obtained differ with the cellular model used, the transport system analyzed, the level of salt intake, and the salt-sensitivity status. Morgan et al2 found that high salt intake reduced erythrocyte Na+-K+ pump activity, measured as the rate constant of ouabain-sensitive 22Na efflux. However, these results were observed only when erythrocytes were incubated with autologous plasma. Canessa et al3 reported significant increases of the maximal rate (Vmax) of the erythrocyte Na+-K+-Cl- cotransport induced by high salt intake. Finally, Weder4 showed that salt-sensitive patients presented increased values of erythrocyte Na+-Li+ countertransport compared with salt-resistant patients.
Intracellular Ca2+ and H+ concentrations have also been correlated with the amount of NaCl intake. Oshima et al5 found a correlation between the increase in intralymphocytic free calcium concentration and the increase in mean blood pressure (BP) with a high salt diet. Likewise, Alexiewicz et al6 reported an increase in intracellular free calcium concentration with a high salt diet only in salt-sensitive hypertensive patients, and Batlle et al7 demonstrated an increase in intralymphocytic pH with a high salt intake in salt-sensitive rats.
Given these previous findings, the aim of the present study was to evaluate salt-induced changes in erythrocyte sodium transport systems as well as in intraplatelet pH and calcium concentration with low and high salt intakes in essential hypertensive patients classified on the basis of their salt sensitivity.
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Methods |
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Patient Selection
Sixty-three essential hypertensive patients were originally recruited from the Hypertension Clinic of the Hospital Clinic, Barcelona, Spain. The diagnosis of essential hypertension was considered on the basis of the fact that no known cause of high BP could be detected after complete clinical, biochemical, and radiological examinations. None of the patients had renal impairment, papilloedema, evidence of cardiac failure, or coronary or cerebrovascular diseases. All patients had at least three office BP measurements above 140/90 mm Hg after 4 weeks of an unrestricted salt diet and without antihypertensive medication. Ambulatory BP monitoring was performed during the week before their inclusion in the study. Only 50 patients (23 men, 27 women) aged 25 to 72 years (mean±SE, 52.3±1.7 years) whose mean 24-hour diastolic BP was more than 90 mm Hg were finally included in the study.
Study Protocol
The study was approved by the Ethics Committee of the hospital
and the Spanish Health Authority (protocol F.I.S. 93/0177). Written
informed consent was obtained from all participants. Essential
hypertensive patients were admitted into a metabolic ward
throughout the study. A low salt diet containing 20 mmol sodium daily
was given to all participants for 14 days. The diet was prepared by the
dietary kitchen of the hospital and contained 62 g protein, 234 g
carbohydrates, 108 g fat, 60 mmol potassium, and 20 mmol calcium. The
amount of calories remained constant for the entire study period and
was slightly modified for individual needs. Patients were advised to
drink 2 L of water per day.
This baseline diet was supplemented in a single-blind fashion by placebo tablets during the first 7 days (low salt period) and by NaCl tablets (240 mmol daily) during the following 7 days (high salt period). Consequently, the total NaCl intake during the high salt period was 260 mmol daily.
Compliance with the diet was assessed by daily measurement of 24-hour urinary Na+ excretion throughout the study.
BP Measurements
On the last day of both the low and high salt periods, 24-hour
ambulatory BP monitoring was performed with an automated, noninvasive
oscillometric device (SpaceLabs 90207). The appropriate cuff was placed
on the nondominant arm, and BP was registered automatically at
15-minute intervals for 24 hours. The following parameters
were obtained from each record in the 24-hour period, as well as in
the daytime (8 AM to 11 PM) and nighttime (11
PM to 8 AM) periods: mean and SE of
systolic, mean, and diastolic BPs as well as of
heart rate. The day-night ratio was obtained by dividing daytime by
nighttime BP values.8
Definition of Salt Sensitivity
Unedited records obtained from ambulatory BP monitoring
performed during either low salt or high salt periods were individually
analyzed by means of BMDP statistical software. The normal
distribution of 24-hour BP values for individual records was tested
by the Shapiro-Wilk test. If both records of the same subject were
normally distributed, they were compared by Student's t
test. If one or both records of the same subject were not normally
distributed, they were compared by the nonparametric
Mann-Whitney test.
The BP change in the entire group of essential hypertensive patients studied followed a gaussian distribution, ranging from a decrease of 9 mm Hg to an increase of 18 mm Hg. There was no clear cutoff in the BP change that allowed us to define patients as having salt-sensitive or salt-resistant hypertension. Thus, the selection of a minimal increase in 24-hour BP would have been completely arbitrary. To eliminate this arbitrariness, we chose the mathematical criteria of the statistical significance of the variation. Thus, salt sensitivity was defined when the increase in 24-hour mean BP from the low salt period to the high salt period was statistically significant (P<.05).9
Simultaneous Measurement of
Vmax of Erythrocyte
Na+-K+ Pump,
Na+-K+-Cl- Cotransport,
Na+-Li+ Countertransport, and Rate Constant of
Na+ Leak
The Vmax of the
Na+-K+ pump,
Na+-K+-Cl- cotransport, and
Na+-Li+ countertransport and the rate constant
of Na+ leak were measured in sodium-loaded erythrocytes
by previously described methods.10 11 12 13 At the end of both
low and high salt periods, 10 mL of venous blood was collected in
heparinized tubes. Plasma and the buffy coat were removed, and
erythrocytes were washed twice with cold KCl (150 mmol/L). The internal
Na+ content was modified to values of 39.82±2.54 mmol/L
cells with the nystatin technique (see Reference 13 for details). At
the end of this procedure, Na+ efflux was measured in red
blood cells washed five times in cold MgCl2 (110 mmol/L)
and resuspended (duplicates) at a final hematocrit of 0.05 in four
different Na+-free media containing (mmol/L)
MgCl2 75, sucrose 85, MOPS-Tris 10, and glucose 10, plus
the following additions (mmol/L): KCl 2 (medium 1); ouabain 0.1 (medium
2); ouabain 0.1 and bumetanide 0.02 (medium 3); and ouabain 0.1,
bumetanide 0.02, and LiCl 10 (medium 4). They were incubated at 37°C
for 30 (medium 1) or 60 (media 2, 3, and 4) minutes.
At the end of incubation, tubes were transferred to ice and centrifuged at 1750g for 4 minutes at 4°C. The supernatant was removed, avoiding pellet contamination, and stored for Na+ concentration measurement by flame photometry. The Na+ flux depending on the Na+-K+ pump (ouabain-sensitive Na+ efflux) was estimated by the difference of Na+ efflux in the presence (medium 2) and absence (medium 1) of ouabain. The Na+ flux depending on Na+-K+-Cl- cotransport (bumetanide-sensitive Na+ efflux) was obtained by the difference of Na+ efflux in media containing ouabain with (medium 3) or without (medium 2) bumetanide. The Na+ flux depending on Na+-Li+ countertransport (Li+-stimulated Na+ efflux) was estimated by the difference of Na+ efflux in media containing ouabain and bumetanide in the presence (medium 4) and absence (medium 3) of LiCl. The Na+ efflux in medium 3 (ouabain- and bumetanide-resistant Na+ efflux) was assumed as the Na+ leak.
Vmax values of the Na+-K+ pump are expressed as millimoles per liter of cells per hour. Vmax values of Na+-K+-Cl- cotransport and Na+-Li+ countertransport are expressed in micromoles per liter of cells per hour. Values of Na+ leak are expressed by its rate constant (ouabain- and bumetanide-resistant Na+ efflux divided by intraerythrocyte Na+ content) (10-3 per hour).
Measurement of Li+ Influx by
HCO3--Cl- Anion Exchanger
(Na+-Dependent)
4,4'-Diisothiocyanatostilbene-2,2'-disulfonic acid
(DIDS)–sensitive Li+ influx in erythrocytes incubated in
Li2CO3 medium was used for measurement of the
activity of the Na+-dependent
HCO3--Cl- anion
exchanger.14 Briefly, red blood cells washed twice with
cold NaCl (150 mmol/L) were resuspended (duplicates) in an
Li2CO3 medium containing (mmol/L)
Li2CO3 20, NaCl 110, CaCl2 1,
MgCl2 1, MOPS-Tris 10 (pH 7.4 at 37°C), glucose 10,
ouabain 0.1, and bumetanide 0.02, with and without the addition of DIDS
at a final concentration of 20 µmol/L. Red blood cells were incubated
for 60 minutes in this influx medium. At the end of incubation, tubes
were transferred to ice and centrifuged at 4°C. The
supernatants were removed, and the erythrocyte pellet was washed twice
with cold NaCl (150 mmol/L) and hemolyzed with 4 mL
double-distilled water. Li+ content was measured in
supernatants and corrected with the hemoglobin content per liter of
cells. The activity of the Na+-dependent
HCO3--Cl- anion
exchanger was calculated as the difference between Li+
influx in cells with and without DIDS. Values are expressed as
micromoles per liter of cells per hour.
Measurement of Platelet Cytosolic pH and Calcium
Concentration
For measurement of platelet cytosolic pH and calcium
concentration,15 20 mL of venous blood was drawn into 20%
(vol/vol) CCD (2.5 g sodium citrate, 1.5 g citric acid, and 2.0 g
dextrose in 100 mL distilled water) and centrifuged at
200g for 10 minutes at 20°C to obtain
platelet-rich plasma. Platelet-rich plasma was
centrifuged at 500g for 20 minutes at 20°C in the
presence of adenosine (0.28 g/dL) and theophylline (1 g/dL)
(50% vol/vol) dissolved in CCD, and the pellet was resuspended in
HEPES-buffered saline containing (mmol/L) NaCl 145, KCl 5,
MgCl2 1, glucose 6, and HEPES 10, as well as 0.2 mg/mL
bovine serum albumin. The pH was adjusted to 7.4 at 37°C. The
platelet suspension was incubated for 35 minutes at 37°C in a
shaking water bath with either 2 µmol/L fura 2–acetoxymethyl ester
(fura 2–AM) for calcium measurements or 0.75 µmol/L
2',7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein
tetraacetoxymethyl ester (BCECF-AM) for pH measurements. The labeled
platelets were washed by centrifugation and
resuspension in HEPES-buffered saline, with platelet count adjusted
to 0.5x108/mL by a Coulter counter (Technikon H-1
system). After 15 minutes of equilibration at 37°C in the presence of
1 mmol/L CaCl2, 2 mL of the platelet suspension
was placed in a quartz cuvette for fluorescence measurements,
which were carried out in a spectrofluorometer (Hitachi F-2000). The
fluorescence signal was obtained once every second with
alternate excitation wavelengths, and platelet cytosolic calcium
concentration ([Ca]i) and platelet cytosolic pH
(pHi) were estimated by the ratio of the excitation
wavelengths 340/380 and 500/440 nm, respectively. The emission
wavelength was 510 nm for fura 2–AM and 530 nm for BCECF-AM.
For calculation of [Ca]i, the relationship between the fluorescence ratio at 340/380 nm and [Ca2+]i as well as the calibration were obtained according to Grynkiewicz et al,16 using an effective dissociation constant (Kd) of fura 2–AM of 224 nmol/L, as reported previously.15 16
For calculation of pHi, the fluorescence signal of BCECF-AM was calibrated to pH at the end of every single experiment. Platelets were lysed with 50 µmol/L digitonin, and the fluorescence signal was recorded at known pH values from 6 to 8 and simultaneously monitored by a combined microelectrode inserted directly into the cuvette. The pHi estimated for this calibration was corrected for the shift in the excitation maximum of intracellular and extracellular BCECF-AM by the method of Thomas et al17 as described previously.15
Statistical Analysis
Values are expressed as mean±SE. The statistical method for the
diagnosis of salt sensitivity is described above. Differences between
salt-sensitive and salt-resistant hypertensive patients
during low and high salt periods were analyzed by paired and
unpaired Student's t tests. The relationship between
changes in intracellular ions and sodium transport systems and changes
in BP induced by high salt intake was examined by Pearson's
correlation coefficient. Statistical analysis was performed
with the aid of the BMDP statistical package.
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Results |
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BP Changes and Salt-Sensitive Hypertension
Salt-sensitive hypertension was diagnosed in 22 patients (44%) by the above-mentioned criteria. Their mean 24-hour BP increased 6.5±0.8 mm Hg (from 109.7±2.5 mm Hg at the end of the low salt period to 116.2±2.6 mm Hg at the end of the high salt period). Salt-resistant hypertension was diagnosed in 28 patients (56%). Their mean 24-hour BP decreased 0.6±0.8 mm Hg (from 110.2±2.3 to 109.6±2.6 mm Hg). As shown in Table 1
,
the changes in both 24-hour systolic and diastolic
BPs were statistically significant only in salt-sensitive patients.
High salt intake significantly increased both daytime and nighttime BPs
in salt-sensitive patients. In salt-resistant patients,
daytime BP tended to decrease (P<.05 for
diastolic BP), and nighttime BP tended to increase
(P<.05 for systolic and mean BPs). Closely related
to this, the day-night ratio significantly decreased in
salt-resistant patients with high salt intake, whereas this
ratio was not modified in salt-sensitive patients at both extremes
of salt intake.
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Changes in Erythrocyte Sodium Transport Systems,
Intraplatelet pH, and Calcium Content Induced by High
Salt Intake
Table 2 shows mean values of the erythrocyte sodium
transport systems studied as well as intraplatelet pH and
calcium content during low and high salt intakes in the entire group of
essential hypertensive patients. As shown, high salt intake promoted a
significant increase in the Vmax of
Na+-Li+ countertransport (P=.0086),
Na+-dependent
HCO3--Cl- exchanger
(P=.0065), intraplatelet pH (P=.0451),
and free calcium concentration (P=.0006). The
Na+-K+ pump,
Na+-K+-Cl- cotransport, and
the rate constant of Na+ leak were not significantly
affected by the level of salt intake in the entire group of essential
hypertensive patients studied.
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Differences Between Salt-Sensitive and Salt-Resistant
Hypertensive Patients
Table 3 shows mean values of the different
parameters studied during low and high salt intakes in
essential hypertensive patients classified on the basis of their salt
sensitivity. During low salt intake, salt-sensitive patients
exhibited lower values of
Na+-K+-Cl- cotransport
(P=.0163) and intraplatelet pH (P=.0220)
compared with salt-resistant hypertensive patients.
Conversely, during high salt intake, salt-sensitive patients
presented significantly higher values of the
Na+-K+ pump (P=.0001),
Na+-K+-Cl- cotransport
(P=.0348), and Na+-Li+
countertransport (P=.0006). As shown in Fig 1, Na+-Li+ countertransport
values observed during high salt intake were clearly different between
salt-sensitive and salt-resistant patients. In fact, 21
of 22 salt-sensitive patients presented increased values
(>325 µmol[L cells/hr]) of this transport system.
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A high salt intake promoted opposing changes in some of the transport
systems studied, depending on salt sensitivity. Compared with
salt-resistant patients, salt-sensitive hypertensive
patients significantly increased Vmax of the
Na+-K+ pump (P<.001),
Na+-K+-Cl- cotransport
(P<.001), and Na+-Li+
countertransport (P<.001) with a high salt intake. In fact,
we observed a significant direct correlation between changes in the
Na+-K+ pump (r=.387,
P=.005, Fig 2),
Na+-K+-Cl- cotransport
(r=.414, P=.003, Fig 3), and
Na+-Li+ countertransport (r=.491,
P<.001, Fig 4) and changes in mean BP
induced by a high salt intake.
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indicates salt-sensitive
patients; 
, salt-resistant patients.
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Discussion |
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The present article shows the changes in erythrocyte sodium transport systems, intraplatelet pH, and calcium concentration promoted by low and high salt intakes in salt-sensitive and salt-resistant hypertensive patients. Compared with a low salt intake, high salt promoted a significant increase in the Vmax of the erythrocyte Na+-K+ pump, Na+-K+-Cl- cotransport, and Na+-Li+ countertransport in salt-sensitive hypertensive patients, whereas the Vmax of the Na+-K+ pump and Na+-K+-Cl- cotransport was significantly reduced in salt-resistant patients. Na+-Li+ countertransport was not significantly modified in this group of patients. In addition, high salt intake significantly increased Li+ influx, depending on the HCO3--Cl- exchanger as well as intraplatelet pH and calcium concentration, in the entire group of essential hypertensive patients studied irrespective of their salt sensitivity. Vmax values of Na+-Li+ countertransport obtained during high salt intake were clearly associated with salt sensitivity. Twenty-one of 22 salt-sensitive patients presented increased values (>325 µmol[L cells/hr]) of this erythrocyte sodium transport system, whereas only 9 of 28 salt-resistant patients showed this abnormality.
In the past two decades, a great amount of evidence has linked abnormalities of transmembrane sodium transport systems and hypertension. Changes in these transport pathways induced by different levels of salt intake have been extensively investigated. Morgan et al2 found that high salt intake reduced erythrocyte Na+-K+–pump activity, measured as the rate constant of ouabain-sensitive 22Na efflux. These results have been confirmed by several authors, although negative results have also been published (see Reference 4 for review). Our results do not show differences in the erythrocyte Na+-K+ pump at low or high salt intake in the entire group of essential hypertensive patients. However, when hypertensive patients were classified by salt sensitivity, we found that the erythrocyte Na+-K+ pump was increased in salt-sensitive patients with high salt intake and decreased in salt-resistant patients. The increase in the erythrocyte Na+-K+ pump was directly correlated with the increase in BP with high salt intake.
In the present study, changes in Na+-K+-Cl- cotransport induced by salt intake paralleled those observed in the Na+-K+ pump. High salt intake did not significantly modify Na+-K+-Cl- cotransport in the entire group of essential hypertensive patients. However, opposing changes were observed in salt-sensitive and salt-resistant patients, with a significantly increased Vmax of erythrocyte Na+-K+-Cl- cotransport in the former and significantly decreased rate in the latter. Consequently, the Vmax of Na+-K+-Cl- cotransport was significantly higher in salt-sensitive compared with salt-resistant hypertensive patients during high salt intake. In 1985, Dagher et al18 reported a suppression of outward Na+-K+-Cl- cotransport in salt-loaded normotensive subjects. On the contrary, in a carefully performed kinetic study, Canessa et al3 showed that whereas normotensive subjects did not modify this transport system at different levels of salt intake, essential hypertensive patients significantly increased Vmax and Kd values for internal sodium when shifted from a low to high salt intake. Our results are in accordance with those of Canessa et al, although only salt-sensitive essential hypertensive patients increased erythrocyte Na+-K+-Cl- cotransport with high levels of salt intake in the present study.
We do not have a satisfactory explanation for the causes and meaning of the differences in Na+ transport systems observed at both levels of salt intake between salt-sensitive and salt-resistant hypertensive patients. Changes in membrane ouabain-sensitive and bumetanide-sensitive Na+ efflux might be related to the presence of several circulating modulators, such as putative natriuretic hormones exhibiting ouabain-like19 20 or bumetanide-like21 22 activities as well as the renin-angiotensin axis and catecholamines. In this sense, it has been reported that compared with salt-resistant hypertensive patients or normotensive control subjects, salt-sensitive patients present a blunted response in plasma renin activity23 and a paradoxical increase in plasma catecholamines24 with high salt intake. These divergent changes in plasma modulators might help to explain the differences in the activity of the Na+-K+ pump and Na+-K+-Cl- cotransport between salt-sensitive and salt-resistant patients at low and high salt intakes as well as the divergent changes observed when patients switched from a low to high salt diet.
Several studies have reported no differences in the Vmax of Na+-Li+ countertransport at low or high salt intake.3 4 25 However, Redgrave et al26 described an increased prevalence of high Na+-Li+ countertransport in the nonmodulated subset of normal and high-renin hypertensive patients, who have been suggested to be salt-sensitive in other studies.27 Weder4 studied erythrocyte Na+-Li+ countertransport in a small subset of essential hypertensive patients classified according to their salt sensitivity. Although no differences were observed during low or high salt intake, salt-sensitive individuals showed significantly higher values of Na+-Li+ countertransport compared with salt-resistant patients. The author suggested that elevated Na+-Li+ countertransport might be a biological marker of salt sensitivity. Our results showed that Na+-Li+ countertransport increased with high salt intake in the entire group of essential hypertensive patients. However, this was due to the increase in the subset of salt-sensitive patients, because this transport system was not modified in salt-resistant individuals. Values of Na+-Li+ countertransport obtained at high salt intake were significantly different between the groups. Moreover, a positive correlation between changes in erythrocyte Na+-Li+ countertransport and changes in BP induced by high salt intake was observed in the hypertensive patients studied. Despite these differences, individual values of Na+-Li+ countertransport showed considerable overlap between salt-sensitive and salt-resistant patients. Thus, the usefulness of the measurement of this transport parameter as a biological marker of salt sensitivity seems uncertain.
Irrespective of salt sensitivity, high salt intake significantly increased Li+ influx depending on the HCO3--Cl- exchanger as well as intraplatelet pH and calcium content in the entire group of essential hypertensive patients studied. The activity of the Na+-dependent HCO3--Cl- anion exchanger previously has been found to be increased in essential hypertensive patients with the use of Li+ influx.14 However, with the more sensitive methodology of changes in intracellular pH,28 the kinetic properties of this exchanger were not found to be appreciably different between spontaneously hypertensive rats and their normotensive controls. Intracellular pH has been found to be normal,29 increased,30 and decreased31 in essential hypertensive patients. However, our data could not address this issue directly because we did not study a group of normotensive control subjects. In a study from Batlle et al,7 Dahl salt-sensitive rats showed a decreased intralymphocytic pH compared with Dahl salt-resistant rats. Likewise, high salt diet promoted a significant increase in intralymphocytic pH. Our results are in agreement with those from Batlle et al. In the entire group of patients, intraplatelet pH was significantly higher during high salt intake, although there were no differences between salt-sensitive and salt-resistant patients.
Finally, intraplatelet free Ca2+ concentration significantly increased with high salt intake. This increase was slightly more pronounced in salt-sensitive compared with salt-resistant patients, although differences were not statistically significant. Erne et al32 reported an elevation of intraplatelet free Ca2+ content in essential hypertensive patients, and Oshima et al5 later observed that high salt intake increased intracellular free calcium content only in salt-sensitive hypertensive patients. These results were confirmed by Alexiewicz et al6 in lymphocytes and Resnick et al33 in erythrocytes from salt-sensitive hypertensive patients. Our data are in agreement with those previously reported, although we did not find a clear difference between salt-sensitive and salt-resistant patients.
In conclusion, we have observed that high salt intake promotes an increase in the erythrocyte Na+-dependent HCO3--Cl- exchanger as well as in intraplatelet pH and calcium content. The erythrocyte Na+-K+ pump and Na+-K+-Cl- cotransport increased only in salt-sensitive patients. Finally, erythrocyte Na+-Li+ countertransport increased in salt-sensitive patients with high salt intake, but individual values exhibited considerable overlap between salt-sensitive and salt-resistant hypertensive patients. Thus, the usefulness of this transport measurement as a biological marker of salt sensitivity seems uncertain.
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Acknowledgments |
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This work was supported in part by a grant from the Fondo de Investigaciones Sanitarias (F.I.S. 93/0177).
Received July 21, 1995; first decision September 5, 1995; accepted December 5, 1995.
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