Platelet cytosolic proton and free calcium concentrations in essential hypertension

Mitochondrial ion channels in cardiac function

Mitochondria have been recognized as key organelles in cardiac physiology and are potential targets for clinical interventions to improve cardiac function. Mitochondrial dysfunction has been accepted as a major contributor to the development of heart failure. The main function of mitochondria is to meet the high energy demands of the heart by oxidative metabolism. Ionic homeostasis in mitochondria directly regulates oxidative metabolism, and any disruption in ionic homeostasis causes mitochondrial dysfunction and eventually contractile failure. The mitochondrial ionic homeostasis is closely coupled with inner mitochondrial membrane potential. To regulate and maintain ionic homeostasis, mitochondrial membranes are equipped with ion transporting proteins. Ion transport mechanisms involving several different ion channels and transporters are highly efficient and dynamic, thus helping to maintain the ionic homeostasis of ions as well as their salts present in the mitochondrial matrix. In recent years, several novel proteins have been identified on the mitochondrial membranes and these proteins are actively being pursued in research for roles in the organ as well as organelle physiology. In this article, the role of mitochondrial ion channels in cardiac function is reviewed. In recent times, the major focus of the mitochondrial ion channel field is to establish molecular identities as well as assigning specific functions to them. Given the diversity of mitochondrial ion channels and their unique roles in cardiac function, they present novel and viable therapeutic targets for cardiac diseases.

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.

Membrane microviscosity, blood pressure and cytosolic pH in Dahl rats: the influence of plasma lipids

OBJECTIVE

To determine the relationships between blood pressure, membrane microviscosity, plasma lipids and cytosolic pH in Dahl rats susceptible or resistant to salt hypertension.

DESIGN AND METHODS

Blood pressure, plasma triglycerides and total cholesterol, platelet cytosolic pH (pHi) and the microviscosity of both outer membrane leaflet (TMA-DPH fluorescence anisotropy) and membrane lipid core (DPH fluorescence anisotropy) were studied in platelets and erythrocyte ghosts of Dahl salt-sensitive (SS/Jr) and salt-resistant (SR/Jr) rats fed either a low-salt diet (0.3% NaCl) until the age of 9, 15 or 24 weeks or a high-salt diet (4% NaCl) for 5 or 10 weeks after weaning.

RESULTS

At low salt intake, DPH but not TMA-DPH anisotropy increased with age in platelets of SS/Jr rats. Chronic high salt intake was accompanied by an increase of DPH anisotropy in platelets but not in erythrocyte ghosts of SS/Jr rats. Platelet DPH anisotropy correlated positively with blood pressure of salt-loaded SS/Jr rats. Chronic high salt intake also reduced pHi in platelets, the regulation of which seemed to be related to the changes in TMA-DPH anisotropy. This especially concerns the thrombin-induced pHi rise which was inversely related to basal pHi, plasma lipids and TMA-DPH anisotropy. Altered membrane lipid composition might be the underlying mechanism because both membrane microviscosity and platelet pHi regulation were reported to correlate significantly with plasma triglycerides and/or cholesterol.

CONCLUSIONS

Platelets of salt hypertensive Dahl rats are characterized by an increased microviscosity of membrane lipid core which correlated positively with blood pressure. The major influence of plasma triglycerides on DPH anisotropy should be taken into consideration when investigating the links between membrane microviscosity and blood pressure. On the other hand, the changes in microviscosity of the outer membrane leaflet might be involved in pHi regulation (probably through control of the Na+/H+ exchanger).

Cytosolic pH and calcium in Dahl salt-sensitive and salt-resistant rats: the relationship to plasma lipids

OBJECTIVE

To search for alterations of cytosolic pH and cell calcium handling in platelets and erythrocytes of Dahl rats susceptible and resistant to salt-induced hypertension.

DESIGN AND METHODS

Blood pressure, plasma lipids, platelet cytosolic calcium concentration ([Ca2+]i) and pH (pHi) together with thrombin-induced changes in these parameters as well as erythrocyte [Ca2+]i and 45Ca influx were determined in Dahl salt-sensitive (SS/Jr) and salt-resistant (SR/Jr) rats aged 9, 15 and 24 weeks, which were fed a low-salt diet (0.3% NaCl), and in animals fed high-salt diet (4% NaCl) for 5-10 weeks since weaning.

RESULTS

With a low salt intake platelet pHi was lower in SS/Jr than it was in SR/Jr rats, whereas basal platelet [Ca2+]i was similar in rats of both strains. The difference in basal pHi between SS/Jr and SR/Jr rats increased progressively with age of animals. A high salt intake from youth did not influence platelet [Ca2+]i in rats of either strain but it caused an earlier decrease in pHi in SR/Jr than it did in SS/Jr rats. Thrombin stimulation induced similar elevations of pHi and [Ca2+]i in rats of both strains, irrespective of age, salt intake and response of blood pressure to salt intake. Erythrocyte 45Ca influx and [Ca2+]i were greater for SS/Jr rats but only the latter parameter was correlated positively to blood pressure. Both regulation of platelet pHi and erythrocyte Ca2+ handling were significantly related to plasma lipid levels.

CONCLUSIONS

Platelets of SS/Jr rats fed a low-salt diet were characterized by a lower basal cytosolic pHi but unchanged [Ca2+]i relative to those of SR/Jr rats. Hypertension induced by high salt intake was associated with increased erythrocyte [Ca2+]i but not with elevation of platelet [Ca2+]i or alteration of response to stimulation with thrombin.

Platelet cytosolic calcium concentration in patients with liver cirrhosis. Relationship with hepatic and systemic hemodynamics

BACKGROUND/AIMS

Due to structural and functional similarities between platelets and vascular smooth muscle cells, platelet cytosolic calcium concentration ([Ca2+]i) has been suggested to be a useful tool to study regulatory mechanisms of peripheral vascular tone. The aim of the present study was to investigate platelet [Ca2+]i in patients with cirrhosis and whether this parameter is related with the systemic and splanchnic vasodilatation found in these patients.

METHODS

Seventeen patients with cirrhosis and eight age- and sex-matched controls were studied. Mean arterial pressure, cardiac output, femoral blood flow and basal and thrombin-stimulated platelet [Ca2+]i were measured. Cardiac output (thermal dilution), azygos blood flow, hepatic venous pressure gradient and hepatic blood flow were also measured in patients with cirrhosis.

RESULTS

Patients with cirrhosis had severe portal hypertension and a significantly higher cardiac output and femoral blood flow and a significantly lower systemic and femoral vascular resistance than controls. Patients with cirrhosis had a lower basal platelet [Ca2+]i than normal subjects. However, there was no relationship between platelet [Ca2+]i and any of the hemodynamic parameters that evaluate systemic or splanchnic vasodilatation.

CONCLUSIONS

This study shows that cirrhotic patients with portal hypertension have a significant reduction in platelet basal [Ca2+]i. The lack of correlation between platelet [Ca2+]i and hepatic and systemic hemodynamics does not support the use of platelet [Ca2+]i as a model to study mechanisms involved in the pathophysiology of the hyperdynamic circulation associated to portal hypertension.

Elaboration and use of nickel planar macrocyclic complex-based sensors for the direct electrochemical measurement of nitric oxide in biological media

We describe here the electrochemical detection of nitric oxide, NO, in biological systems by using chemically modified ultramicro carbon electrodes. In the first part of the paper, the different steps involved in the electrochemical preparation and characterization of the nickel-based sensor are described. This is illustrated by the use of nickel(II) tetrasulfonated phthalocyanine complex. The second part of the paper describes two examples of the direct electrochemical measurement of NO production in human blood platelets and endothelial cells from umbilical cord vein.

In vivo effect of insulin on intracellular calcium concentrations: relation to insulin resistance

Elevated intracellular calcium concentrations ([Ca2+]i) have been described in essential hypertension and other insulin-resistant states. Our aim was to explore the relationship between insulin resistance and abnormal Ca2+ metabolism. In 50 nondiabetic subjects, half of whom had untreated essential hypertension, we simultaneously measured the in vivo effect of insulin on glucose metabolism (by the insulin clamp technique) and on platelet [Ca2+]i (by the Fura-2 method). In each subject, [Ca2+]i measurements (both in Ca(2+)-free medium and, sequentially, following in vitro Ca2+ loading) were obtained in the fasting state and after 2 hours of euglycemic hyperinsulinemia. In the fasting state, no association was found between any measure of [Ca2+]i and gender, age, body mass index (BMI), blood pressure, or insulin sensitivity. In contrast, following in vivo insulin, platelet [Ca2+]i increased significantly (from 23 +/- 1 to 28 +/- 1 nmol/L in Ca(2+)-free medium, P < .01) in the whole group, and an insulin-induced increase in [Ca2+]i was associated with insulin resistance (r = .35, P = .01) but not with hypertension (r = .2, P = .17) and with impaired glucose storage (as determined by indirect calorimetry, r = .39, P = .01) but not with glucose oxidation. Thus, the 12 most insulin-resistant subjects were characterized by a cluster of abnormalities (mild overweight, higher blood pressure and prevalence of hypertension, higher serum triglycerides and insulin response to oral glucose, and reduced glucose storage) that included an insulin-induced increase in [Ca2+]i (9 +/- 2 nmol/L, P < .001 v basal). We conclude that insulin resistance, rather than hypertension, is associated with an abnormal in vivo effect of insulin on platelet [Ca2+]i.

Endothelin-3, Ca2+ mobilization and cyclic GMP content in human platelets

As previously described for endothelin-3, platelet exposure to cyclic GMP-elevating agents such as sodium nitroprusside and M&B-22948 (2-o-propoxyphenyl-8-azapurin-6-one), a cGMP phosphodiesterase inhibitor, lowered Ca2+ mobilization in response to thrombin. Interestingly, when cGMP phosphodiesterases were blocked, endothelin-3 produced a dose-dependent cGMP accumulation (P < 0.001). Since endothelin-3 has been proposed to decrease the activity of Ca2+ accumulating pumps, we examined whether this latter effect could be mediated by a rise in cGMP content. Cyclic GMP decreased in a dose-dependent manner the initial rate and plateau value of the ATP-dependent 45Ca2+ uptake in platelet membrane vesicles (P = 0.006 for each). Furthermore, combined treatment with endothelin-3 and M&B-22948 or a moderate concentration of Na(+)-nitroprusside further reduced the thrombin-evoked Ca2+ discharge (P = 0.004 and 0.01, respectively), suggesting that endothelin-3 pre-exposure had reduced the amount of mobilizable Ca2+. We propose that the depletion of platelet Ca2+ stores and the reduction of Ca2+ release evoked by endothelin-3 could be due, at least in part, to the elevation of cGMP content and to a decrease in Ca2+ accumulating pump activity.

Fluorescent indicators give biased estimates of intracellular free calcium change in aggregating platelets: implication for studies with human von Willebrand factor

The ratiometric fluorescent indicators Fura-2 and Indo-1 are considered optimal probes for monitoring intracellular free calcium concentration ([Ca2+]i). Unique problems arise, however, in studying [Ca2+]i changes induced in platelets by von Willebrand factor (vWF). Binding of native multimeric vWF causes extensive platelet aggregation, and is reported to evoke a gradual [Ca2+]i increase. the present investigation examined the reliability of platelet [Ca2+]i measurements in these circumstances. Ristocetin-mediated binding of vWF to human platelets promoted a slow rise in Fura-2 fluorescence ratio. Fura-2 extrusion contributed substantially to this rise, unless blocked by probenecid. Despite this precaution, the platelets were invariably contaminated slightly with extracellular indicator. As aggregation progressively reduced the number of platelets in the spectrofluorometer beam, through settling of the larger aggregates, such extracellular Fura-2 contributed proportionately more to the observed fluorescence. This extraneous signal accounted completely for the fluorescence ratio increase, and apparent [Ca2+]i rise, in response to native multimeric vWF. The same problem arose with Indo-1, whereas the single wavelength indicator Fluo-3 showed the opposite pattern of apparent [Ca2+]i changes. Thus, none of these indicators provides reliable data on [Ca2+]i signals in aggregating platelets. Use of a dimeric form of vWF eliminated the problem of platelet aggregates settling out of suspension, but also virtually abolished the [Ca2+]i increase. These observations may explain some of the inconsistencies among previous investigations of vWF-induced calcium signaling. Moreover, similar problems may arise in studies with other adhesive proteins.

Intraplatelet free calcium and calcium-regulating hormones in plasma are not related to the antihypertensive effect of nifedipine in hypertensive pregnancy

Intracellular free calcium regulates contraction-relaxation processes in vascular smooth muscle. We compared intraplatelet free calcium ([Ca2+]i) and pH ([pH]i) in hypertensive pregnant women to those in normotensive pregnant and non-pregnant women. Plasma parathormone and vitamin D metabolite were simultaneously assessed. In hypertensive pregnancy, [Ca2+]i tended to be lower than in normotensive pregnant (P = 0.08) and non-pregnant subjects (P = 0.06). In hypertensive pregnancy, 1,25, (OH)2 vitamin D in plasma was in the same range as in non-pregnant women and significantly lower than in normotensive pregnancy (p < 0.01). The other two vitamin D metabolites, parathormone and [pH]i were equal in the three groups. A five-day nifedipine treatment (10 mg t.i.d.) increased [Ca2+]i in hypertensive pregnant (P < 0.05) and normotensive non-pregnant subjects (P = 0.06), whereas [pH]i (P < 0.05) and 25 (OH) vitamin D (P < 0.05) decreased in the former and 24,25 (OH)2 vitamin D increased in the latter group (P < 0.05). Initial [Ca2+]i did not correlate with blood pressure in any group. The antihypertensive effect of nifedipine did not correlate with any variable measured. In conclusion, [Ca2+]i and calcium-regulating hormones seem not to be related to the antihypertensive effect of nifedipine in hypertensive pregnancy. In this type of hypertension, intraplatelet calcium may not reflect calcium balance in smooth muscle cells regulating vascular tone and blood pressure.

Endothelin influences pHi of human platelets through protein kinase C mediated pathways

Stimulation of human platelets with endothelin raises cytosolic pH (pHi). In order to determine whether this effect is mediated via protein kinase C and Na(+)-H+ linked pathways, the effects of staurosporine and calphostin C (protein kinase C inhibitors) and 5-(N,N-hexamethylene) amiloride (Na(+)-H+ exchange blocker) on endothelin-induced pHi responses in human platelets were assessed. In addition, platelet endothelin receptor subtypes were determined pharmacologically using the selective ETA receptor antagonist BQ-123 and the ETB receptor agonist sarafotoxin S6c. pHi was measured spectrofluorometrically using the fluorescent probe BCECF-AM in platelets obtained from 15 healthy subjects. Endothelin-1 at a fixed concentration of 10(-9) M significantly increased pHi from 7.11 +/- 0.01 ([H+]i = 77 +/- 0.9 nM) to 7.19 +/- 0.04 ([H+]i = 64 +/- 0.9 nM) (p < 0.01). The pHi-stimulating effect of endothelin-1 was inhibited by 10(-7) M staurosporine, calphostin C and 5-(N,N-hexamethylene) amiloride. BQ-123 (10(-7) M) abolished the pHi responses to endothelin-1, whereas sarafotoxin S6c had no effect on platelet pHi. These data suggest that in vitro effects of endothelin-1 on platelet pHi are receptor-mediated via a pathway involving protein kinase C. Platelet endothelin receptors appear to be of the ETA subtype.

Different effects of endothelin-3 on the Ca2+ discharge induced by agonists and Ca(2+)-ATPase inhibitors in human platelets

1. The present study demonstrates that endothelin-3 (ET-3), previously shown to attenuate thrombin-evoked aggregation of human platelets, delayed the dose-dependent aggregatory response to thapsigargin (Tg). As this Ca(2+)-ATPase inhibitor induces platelet activation in part through the depletion of internal Ca(2+)-stores, we examined the influence of ET-3 on Ca2+ discharge from internal pools. 2. Cytosolic Ca2+ concentration was evaluated with Fura-2 in the absence of Ca2+ influx. Platelet preincubation for 15 min with 5 x 10(-7) M ET-3 decreased the Ca2+ release evoked by thrombin and U46619, a thromboxane-mimetic. However, ET-3 did not affect Ca2+ movements induced by 1 microM ADP. Addition of Tg (0.5 to 5 microM) to resting platelets induced a cytosolic [Ca2+] rise with concentration-dependent increase of the initial rate and decrease of the time to reach the peak. ET-3 slowed down these dose-dependent effects with a more marked influence on the responses induced by low concentrations of Tg. 3. ET-3 did not modify the Ca2+ response to another Ca(2+)-ATPase inhibitor, 2,5-di-(tert-butyl)-1,4-benzohydroquinone(tBuBHQ). The thromboxane A2 receptor antagonist, SQ 29548, reduced by 53% the calcium signal evoked by 1 microM Tg, which became similar to that induced by 15 microM tBuBHQ. Under these conditions, the ET-3 effects were suppressed. A subsequent addition of thrombin induced a substantial further Ca2+ increase which was again sensitive to ET-3. 4. ET-3 attenuates Ca2+ mobilization from an internal pool dependent on the stimulation of thrombin and thromboxane A2 receptors and insensitive to the direct effect of Ca2+-ATPase inhibitors. The small but significant inhibitory effect of ET-3 leads us to propose that endothelin-3 acts as a modulator of platelet activation.

The lymphocyte Na+/H+ antiport: activation in primary hypertension and during chronic NaCl-loading

Increased activity of the Na+/H+ antiport may be a major abnormality in essential hypertension. The activity of this transport system was investigated in lymphocytes from 13 patients with untreated essential hypertension (Ht) and 13 normotensive control subjects (Nt) on an ad libitum (130-170 mmol d-1) NaCl intake. Furthermore, the effects of different states of NaCl balance on lymphocyte Na+/H+ antiport were evaluated in two groups of Nt volunteers receiving 20 vs. 300 mmol d-1 (n = 8) and 85 vs. 200 mmol d-1 (n = 14) of NaCl for 1 week each and in seven Ht patients (20 vs. 300 mmol NaCl d-1 for 1 week each). Additionally, during the 20 and 300 mmol/d NaCl intake red blood cell membrane transport was studied in eight subjects. For the determination of lymphocyte antiport activity, cells were loaded with the cytosolic pH (pHi) indicator bis-carboxyethyl carboxyfluorescein (BCECF-AM) and acidified by addition of different amounts of Na(+)-propionate (5-40 mM). Initial pHi-recovery was taken as the activity of the antiport system and plotted against pHi-values after acidification. Non-linear regression analysis yielded higher 'apparent' maximal transport rates in Ht than Nt (Nt: 2.00 +/- 0.22; Ht: (3.81 +/- 0.59) x 10(-3) s-1; P < 0.025). In contrast, baseline pHi-values and pHi-values at half-maximal activity (pK) were identical in Nt and Ht. In normotensive control subjects on an NaCl intake of 20, 85, 200 and 300 mmol d-1 for 7 d, 'apparent' maximal transport rates averaged 2.75 +/- 0.20, 2.89 +/- 0.17, 2.81 +/- 0.18 and (3.62 +/- 0.25) x 10(-3) s-1, respectively. Thus, antiport activity was significantly (P < 0.05) stimulated on the 300 mmol d-1 intake as compared to the three other NaCl intakes. The extreme intakes of NaCl (20 vs. 300 mmol d-1) in normotensive volunteers did not affect the erythrocyte Na+/K+ pump, Na+/K+ cotransport and Na+/Li+ countertransport. Our study supports the concept that a group of patients with primary hypertension exhibit an activated Na+/H+ antiport. Furthermore, our data demonstrate that a chronic high intake of NaCl is associated with an increase in lymphocyte antiport activity towards the high values observed in primary hypertension.

Intracellular calcium concentration and activation of the Na+/H+ exchanger in essential hypertension

To investigate the relationship between changes in intracellular calcium concentration ([Ca2+]i) and agonist-induced activation of the Na+/H+ exchanger in essential hypertension (EH), platelet [Ca2+]i and pHi were measured in 24 patients with EH (14 males) aged 48 +/- 2 years and 23 matched normotensive controls (NT) (12 males) aged 45 +/- 3 years. Measurements were done with spectrofluorimetry using the dyes Fura-2 for [Ca2+]i and BCECF for pHi. [Ca2+]i and pHi were measured in the resting condition and after stimulation in vitro with 0.1 U/ml human thrombin. The thrombin-induced rise in pHi was Na+ dependent and amiloride sensitive, indicating that it was mediated by the Na+/H+ exchanger. Unstimulated [Ca2+]i was higher in patients with EH than in NT (60 +/- 3 vs. 48 +/- 1 nmol/liter, P < 0.005), but there were no differences in resting pHi between both groups (7.16 +/- 0.01 vs. 7.16 +/- 0.008). In the presence of 1 mmol/liter external calcium (Ca2+o), thrombin-induced increment in [Ca2+]i was significantly greater in patients with EH than in NT (281 +/- 21 vs. 206 +/- 19; P < 0.05) as was the pHi increment (EH: 0.137 +/- 0.01; NT: 0.095 +/- 0.01; P < 0.05). Both agonist-induced increments in [Ca2+]i and in pHi were correlated with mean arterial pressure (MAP) only in the EH group (r = 0.58, P < 0.005 and r = 0.59, P < 0.005, respectively). The agonist-induced rise in pHi was positively correlated with the rise in [Ca2+]i both in the EH group (r = 0.65, P < 0.001) and in the NT (r = 0.55, P < 0.01).(ABSTRACT TRUNCATED AT 250 WORDS)

Dietary calcium and blood pressure in experimental models of hypertension. A review

More than 80 studies have reported lowered blood pressure after dietary calcium enrichment in experimental models of hypertension. The evidence presented here suggests that dietary calcium may act concurrently through a number of physiological mechanisms to influence blood pressure. The importance of any given mechanism may vary depending on the experimental model under consideration. Supplemental dietary calcium is associated with reduced membrane permeability, increased Ca(2+)-ATPase and Na,K-ATPase, and reduced intracellular calcium. These results suggest that supplemental calcium may limit calcium influx into the cell and improve the ability of the VSMC to extrude calcium. This could be a direct effect of calcium on the VSMC or an indirect effect mediated hormonally. The calcium-regulating hormones have all been found to have vasoactive properties and therefore may influence blood pressure. Furthermore, CGRP and the proposed parathyroid hypertensive factor are both vasoactive substances that are responsive to dietary calcium. Therefore, diet-induced variations in calcium-regulating hormones may influence blood pressure. Modulation of the sympathetic nervous system is another important way that dietary calcium can influence blood pressure. There is evidence of altered norepinephrine levels in the hypothalamus as a consequence of manipulations of dietary calcium as well as changes in central sympathetic nervous system outflow. Dietary calcium has also been shown to specifically modify alpha 1-adrenergic receptor activity in the periphery. In some experimental models of hypertension, dietary calcium may alter blood pressure by changing the metabolism of other electrolytes. For example, the ability of calcium to prevent sodium chloride-induced elevations in blood pressure may be attributed to natriuresis. However, natriuresis does not account for all of the interactive effects of calcium and sodium chloride on blood pressure. Sodium chloride-induced hypertension may be due in part to calcium wasting and subsequent elevation of calcium-regulating hormones. Chloride is an important mediator of this effect because it appears that sodium does not cause calcium wasting when it is not combined with chloride. More attention to the central nervous system effects of dietary calcium is needed. Not only can calcium itself influence neural function, but many of the calcium-regulating hormones appear to affect the central nervous system. The influence of calcium and calcium-regulating hormones on central nervous system activity may have important implications for blood pressure regulation and also may extend to other aspects of physiology and behavior.

Changes in Ca2+ mobilization in platelets from stroke-prone spontaneously hypertensive rats

Intracellular free Ca2+, [Ca2+]i, levels were measured in platelets from stroke-prone spontaneously hypertensive rats (SHRSP) and normotensive Wistar-Kyoto rats (WKY) using fura-2AM. In the presence of extracellular Ca2+ (1 mM), [Ca2+]i levels in unstimulated platelets of 2- and 9-month-old SHRSP were both significantly higher than those of the age matched WKY. In the absence of extracellular Ca2+, the levels in platelets from 9-month-old SHRSP were also higher than any other groups examined. Receptor-linked Ca2+ influxes of old SHRSP were smaller when thrombin or collagen was given to the platelets. Phorbol 12-myristate 13-acetate (TPA) enhanced more prominently the Ca2+ influx into old SHRSP platelets than into old WKY platelets. These results strongly suggest that the Ca2+ permeability across plasma membrane is increased in young as well as old SHRSP platelets, where the resting [Ca2+]i level is highly sustained because of an impaired Ca2+ uptake mechanism and possible enhancement of protein kinase C activity.

Effects of angiotensin II and endothelin-1 on platelet aggregation and cytosolic pH and free Ca2+ concentrations in essential hypertension

The aims of this study were to determine the relations between platelet free calcium concentrations ([Ca2+]i), intracellular pH (pHi), and aggregation and to assess the effects of angiotensin II (Ang II) and endothelin-1 on these platelet parameters in normotensive subjects and hypertensive patients. Seventeen normotensive subjects, 25 untreated hypertensive patients, and 34 treated hypertensive patients were studied. Platelet cytosolic free [Ca2+]i and pHi were measured spectrofluorometrically using specific fluorescent probes (fura 2-AM and BCECF-AM, respectively) in unstimulated and Ang II- and endothelin-1-stimulated platelets. Aggregation was measured by a turbidometric technique. Basal [Ca2+]i (141 +/- 11 nmol/L) and pH (7.16 +/- 0.01) were higher (P < .05) in the untreated hypertensive group compared with the normotensive (118 +/- 9 nmol/L, 7.11 +/- 0.01, respectively) and treated hypertensive (121 +/- 11 nmol/L, 7.12 +/- 0.01, respectively) groups. In the combined normotensive and hypertensive groups, there were significant correlations between [Ca2+]i and mean arterial pressure (r = .75, P < .01), pHi and mean arterial pressure (r = .72, P < .01), [Ca2+]i and pHi (r = .71, P < .01), [Ca2+]i and aggregation (r = .69, P < .02), and pHi and aggregation (r = .56, P < .05). Ang II stimulation significantly increased [Ca2+]i and pHi in the untreated hypertensive and normotensive groups. The net change in [Ca2+]i induced by Ang II was significantly higher (P < .05) in the untreated hypertensive group compared with the other groups (67 +/- 6 nmol/L for the untreated hypertensive group versus 54 +/- 5 and 29 +/- 8 nmol/L for the normotensive and treated hypertensive groups, respectively). In the presence of Ang II, thrombin-induced aggregatory responses were increased in all three groups, but the maximal response was significantly higher in the untreated hypertensive group compared with the other groups (P < .05). Endothelin-1 increased pHi through endothelin A-receptors (effect blocked by the specific antagonist BQ-123) but had no significant effect on [Ca2+]i or aggregation. However, endothelin-1 blunted thrombin-induced platelet aggregation in normotensive subjects but not in hypertensive patients. In conclusion, increased Ang II-stimulated [Ca2+]i and pHi in platelets of essential hypertensive patients may be associated with increased aggregatory responses. The stimulatory effect of endothelin-1 on pHi but not on [Ca2+]i or aggregation suggests that in platelets endothelin-induced signaling pathways other than phospholipase C may be involved.(ABSTRACT TRUNCATED AT 400 WORDS)

Erythrocyte anion exchanger activity and intracellular pH in essential hypertension

The present study was designed to examine the activity of the sodium-independent chloride-bicarbonate anion exchanger and the sodium-proton exchanger in erythrocytes of 30 normotensive and 35 hypertensive subjects and its relation to the previously reported decrease in erythrocyte pH. Erythrocyte cytosolic pH was measured by the pH-sensitive fluorescent probe 2'-7'-bis(2-carboxyethyl)- 5(6)-carboxyfluorescein. The activity of the anion exchanger was determined by acidifying cell pH and measuring the initial rate of the net sodium-independent, 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid-sensitive, bicarbonate influx driven by an outward proton gradient. The activity of the sodium-proton exchanger 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 activity of the anion exchanger was higher in hypertensive than control individuals (18,863 +/- 1081 vs 15,629 +/- 897 mmol/L cells per hour, P < .05). The activity of the sodium-proton exchanger was higher in hypertensive than control individuals (301 +/- 45 vs 162 +/- 23 mmol/L cells per hour, P < .005). Basal erythrocyte pH was lower in hypertensive than control individuals (7.27 +/- 0.02 vs 7.33 +/- 0.01, mean +/- SEM, P < .05). With the 100% confidence (lower) limit of the normotensive population as a cutoff point, a subgroup of 11 hypertensive patients had an abnormally low erythrocyte pH (< 7.19).(ABSTRACT TRUNCATED AT 250 WORDS)

Intracellular cation concentrations in essential hypertension and chronic renal failure

The aim of this study was to test basal and after treatment erythrocyte sodium and calcium concentrations, and calcium-ATPase activity and platelet cytosolic free calcium and pH in 20 normotensive controls, 20 hemodialysis-dependent chronic renal failure patients and in 18 essential hypertensives. Prior to treatment, essential hypertensive and uremic patients presented similar higher platelet calcium concentrations and lower pH than the normotensive control group. The erythrocyte sodium, calcium, and magnesium concentrations were only significantly elevated in chronic renal failure, with a significant decrease in the calcium-ATPase activity in the latter population. Hemodialysis partially reversed these intracellular ionic abnormalities with normalization of platelet pH. Significant correlations have been noted between weight loss and decreases in platelet calcium concentration (r = 0.60, p < 0.01) or in erythrocyte sodium (r = 0.50, p < 0.05). The systolic blood pressure decrease was only correlated to the increase in calcium-ATPase activity (r = 0.57, p < 0.05). Antihypertensive treatment (captopril and nifedipine) only tended to normalize the intracellular calcium concentration with correlation between the decrease of the latter and blood pressure decrease (r = 0.64 for the systolic blood pressure and 0.68 for the diastolic blood pressure, p < 0.01). Thus, in essential hypertension and in uremia, some cellular ionic abnormalities exist in platelets in baseline condition. Moreover, in uremia, erythrocyte presents abnormal ionic pattern. Some, but not all of these abnormalities could be corrected by treatment affecting blood pressure (cellular calcium) in essential hypertension or by hemodialysis (cellular sodium, calcium, and pH). In the latter treatment, the changes are linked to extracellular fluid modification. In essential hypertension, the intracellular calcium reduction was linked to blood pressure decrease.

Membrane sodium-proton exchange and primary hypertension

Recent studies have revealed that an enhancement of sodium-proton exchange is a frequently observed ion transport abnormality in essential hypertension. An altered antiport activity not only is measurable in blood cells of hypertensive subjects ex vivo but also is detectable in skeletal muscle in vivo. Several lines of argument suggest that the altered antiport activity is not an epiphenomenon of hypertension: 1) the increased activity is found only in a subgroup of patients with high blood pressure, 2) it is not tightly correlated to the severity or duration of hypertension, and 3) high sodium-proton exchange activity persists over time and is not affected by antihypertensive treatment. Available evidence suggests that enhanced sodium-proton exchange is associated with or a cause for the structural alterations found in resistance vessels of hypertensive individuals (media hypertrophy) and left ventricular hypertrophy. This review summarizes some of the physiological properties and roles of the sodium-proton exchanger and discusses its kinetic properties in essential hypertension. Furthermore, the reasons for the enhanced antiport activity and its potential implications regarding the pathogenesis of hypertension are discussed.

Regulation of the dynamic properties of platelet plasma membrane by intracellular sodium ions

Our previous experiments in human and rat platelets demonstrated that the absence of extracellular Na+ increased the fluorescence anisotropy of TMA-DPH (trimethylamino-diphenylhexatriene, probe preferentially incorporated into the outer leaflet of the plasma membrane). Here we investigated further the in vitro effects of Na+ ions on membrane dynamic properties. Na(+)-dependent changes were reversible and they required about 10-20 min to be induced. They were specifically located in the TMA-DPH environment because they were not observed with diphenylhexatriene (probe non-selectively incorporated into all hydrophobic domains of the cell). To evaluate the possible influence of the intracellular Na+, the effects of sodium replenishment, monensin, ouabain and thrombin on TMA-DPH anisotropy were measured. A rise in intracellular Na+ above the physiological level was associated with unchanged or slightly decreased TMA-DPH anisotropy whereas its decrease was accompanied by a pronounced rise in TMA-DPH anisotropy. Our data indicate that the changes in intracellular Na+ concentration, rather than those in extracellular Na+ concentration, are responsible for the alterations in platelet membrane fluidity probed by TMA-DPH.

In vitro inhibition by endothelins of thrombin-induced aggregation and Ca2+ mobilization in human platelets

1. The in vitro effects of endothelins (ET-1 and ET-3) on human platelets were investigated by measurement of the aggregatory responses of washed platelets to thrombin and by the determination of cytosolic pH (pHi) and free Ca2+ concentration ([Ca2+]i) determined with the fluorescent indicators, BCECF and Fura-2. 2. ET-1 and ET-3 at concentrations ranging from 10(-10) to 5 x 10(-7) M, did not promote platelet aggregation but inhibited in a dose-dependent manner the aggregation induced by 0.05 u ml-1 thrombin (P less than 0.002 and less than 0.001, respectively) with maximal effects reached at 10(-8) M (17 +/- 3 and 15 +/- 2%, n = 11, P = 0.002 for each). 3. Even at 5 x 10(-7) M, ET-1 and ET-3 did not cause a measurable change in basal [Ca2+]i and pHi. When tested in combination with thrombin, 5 x 10(-7) M ET-1 and ET-3 decreased the transient peak of [Ca2+]i by 17 +/- 7 and 28 +/- 7% (n = 7 and 11, P = 0.03 and P = 0.002). No effect on pHi variations was detected. In the virtual absence of external Ca2+, 5 x 10(-7) M ET-3 inhibited the peak of [Ca2+]i by 18 +/- 6% (n = 6, P = 0.02). 4. The anti-aggregating agents, prostacyclin (PGI2, 10(-8)-10(-7) M) and nitroprusside (NP, 10 ng-50 micrograms l-1) also induced a dose-dependent inhibition of the thrombin-induced [Ca2+]i peak (P = 0.001 for each).A combination of 10-9M PGI2 and 1O ng P' NP augmented the inhibitory effect of each drug(PGI2 alone 52 +/-11, plus NP 90 +/- 2; NP alone 26 +/- 4, plus PGI2 69 +/- 5% inhibition of [Ca2 ], peak, n = 6 for each, P <0.01 and P <0.001, respectively). Platelet preincubation with 5 x 10-7M ET-3 increased by 34+/-11% (n = 6, P = 0.0 14) the inhibitory effect of NP 1O ng without a significant influence on the PGI2 effect.5. In conclusion, endothelins ET-1 and ET-3 can reduce in vitro the aggregating response of human platelets to thrombin by a mechanism that is probably due to decrease Ca2+ mobilization.

Biochemical and functional alterations associated with hypercholesterolemia in platelets from hypertensive patients

Hypercholesterolemia and hypertension are two of the major risk factors associated with increased atherosclerotic vascular disease. An abnormal platelet function is one of the mechanisms proposed to participate in atherogenesis. This study was undertaken to find out whether hypercholesterolemia in hypertensive patients can change platelet lipid composition and reactivity. Twenty-nine untreated hypertensive patients were distributed into 3 age, body mass index and blood pressure-matched groups according to their plasma cholesterol levels (normal, borderline or elevated, group NC, BC and HC respectively). Their platelet lipid composition, cytosolic Ca2+ concentration, cyclic AMP content and aggregating response to ADP and collagen were determined. Platelet from group HC patients were characterized by reduced cyclic AMP content (evaluated in the presence and absence of a platelet phosphodiesterase inhibitor) and aggregating responses to ADP and collagen, increased palmitic acid content and decreased arachidonic, eicosapentaenoic and docosatetraenoic and pentaenoic acid content, resulting in a lowered polyunsaturated to saturated fatty acid ratio (P less than 0.001). In contrast, platelet cytosolic Ca2+ concentration, DPH steady-state anisotropy and cholesterol to phospholipid molar ratio were not significantly changed. This indicates that hypercholesterolemia is accompanied in hypertensive patients by marked changes in platelet fatty acid composition, cyclic AMP content and response to aggregating agents. These changes, which clearly differ from those induced by in vitro cholesterol loading, could reflect not only the balance between LDL and HDL stimulation but also an adaptation to hemodynamic perturbations.

Hypertension in the elderly with a special focus on treatment with angiotensin-converting enzyme inhibitors and calcium antagonists

Age-related changes (e.g., decrease in plasma renin activity and total body potassium, increase in plasma catecholamines, volume depletion) need to be taken into account when selecting an antihypertensive agent for the elderly patient. A number of large scale clinical trials (e.g., Systolic Hypertension in the Elderly Program, Veterans Administration Cooperative Study, European Working Party on High Blood Pressure in the Elderly) have demonstrated that antihypertensive therapy with diuretics substantially reduced cardiovascular mortality and stroke incidence. However, since diuretics, even potassium-sparing agents, may induce hypokalemia, newer antihypertensive agents (angiotensin-converting enzyme [ACE] inhibitors and calcium antagonists) may also be appropriate as first-line monotherapy for this patient population. ACE inhibitors are effective antihypertensive agents and are associated with a lower rate of adverse effects than diuretics, beta blockers, and centrally acting agents. Nevertheless, periodic monitoring of serum potassium, creatinine levels, and renal function is advisable. An important feature of calcium antagonists is that they lower blood pressure with no negative effect on serum lipids or glucose metabolism. Typically, they have few side effects, peripheral edema being the most commonly reported. A recent double-blind randomized study comparing a new sustained release nifedipine formulation and the ACE inhibitor lisinopril found the 2 drugs equivalent in efficacy with no differences in the rate of adverse events.

Control of the erythrocyte free Ca2+ concentration in essential hypertension

Since Ca2+ ions seem to directly participate in the control of erythrocyte membrane structure and deformability and because cell Ca2+ metabolism has been repeatedly proposed to be modified in hypertension, the intracellular calcium ion concentration ([Ca2+]i) was investigated in red blood cells from hypertensive and normotensive subjects. [Ca2+]i was measured by using the fluorescent Ca2+ chelator fura-2. Red blood cell [Ca2+]i was increased in hypertensive compared with normotensive subjects in the whole population and further increased when hypertensive were compared with age-matched normotensive subjects. An inverse relation between age and [Ca2+]i was observed when calculated with blood pressure adjusted. In hypertensive patients, high [Ca2+]i values were associated with a reduced erythrocyte deformability. The initial rate of 45Ca2+ uptake did not differ between the two blood pressure groups. Similarly, when the extracellular Ca2+ concentration was elevated from 1 to 2 mmol/l, [Ca2+]i increased by 16 +/- 4% (p less than 0.03) in red blood cells from both groups, thus maintaining a significant difference between hypertensive and normotensive subjects. Under these conditions, the addition of 10(-7) mol/l nicardipine, a dihydropyridine Ca2+ antagonist, decreased [Ca2+]i by 15 +/- 4% (p less than 0.05) and 7 +/- 5% in erythrocytes from hypertensive and normotensive subjects, respectively, thereby reducing the difference in [Ca2+]i observed between these two groups. This nicardipine effect was positively correlated to the initial [Ca2+]i. In the presence of 5 mumol/l W7, a calmodulin antagonist, [Ca2+]i increased significantly only in erythrocytes from hypertensive patients (26 +/- 6%, p less than 0.01).(ABSTRACT TRUNCATED AT 250 WORDS)

Na(+)- and Ca(2+)-dependent pH regulation in unstimulated human platelets

The influence of transmembrane Na+ and Ca2+ gradients on cytosolic pH (pHi) and free Ca2+ concentration ([Ca2+]i) have been examined in unstimulated human platelets with the aid of BCECF and Fura-2 fluorescent dyes. The removal of external Na+ (Na+o) acidified the cytosol in a pHo-dependent manner which was insensitive to EIPA and DIDS, the inhibitors of the Na+/H+ exchanger and bicarbonate transporters. Na+o removal also increased [Ca2+]i by 17 +/- 5%, but the amplitude of the concomitant acidification was independent on Ca2+ influx or cytosolic Ca2+ concentration. In contrast, in the presence of 145mM Na+o, a rise in external Ca2+ concentration from 1 to 2mM increased [Ca2+]i by 38 +/- 11% and acidified the cytosol by 0.16 +/- 0.04 pH units. These results indicated that, in resting human platelets, the transmembrane Na+ gradient is a major determinant of pHi. Two Na(+)-dependent processes have been found: one is triggered by an external acidification and the other activated by a rise in Ca2+ influx or cytosolic concentration.

Modulation of platelet Ca2+ homeostasis by hypertensive plasma factor(s) derived from patients with early-stage renal disease

To determine whether blood-borne factors in hypertension accompanying early-stage kidney disease might be responsible for altered cellular calcium homeostasis, we measured changes in cytosolic calcium before and after incubating platelets in plasma ultrafiltrates from normotensive and hypertensive renal patients. With the use of the chelating agent quin 2, we found the free-calcium concentrations in platelets to be higher in the hypertensive than in the normotensive group. When both groups of participants were combined, a direct correlation was found between arterial pressure and cytosolic calcium. The cytosolic calcium concentration in platelets of normotensive renal patients increased after incubation with plasma from patients with untreated renal hypertension, but it was unchanged after incubation with plasma from normotensive subjects. These data indicate that the total cell burden of calcium is increased in platelets of hypertensive patients with early-stage renal disease, and that plasma from these patients contains a substance that is capable of increasing the cytosolic calcium concentration in platelets. If the plasma factor (or factors) acts not only on platelets, but also on vascular smooth muscle cells, it may contribute to the increased peripheral vascular resistance associated with hypertension of renal origin.

Intracellular pH and sodium-proton exchange activity of lymphocytes in stroke-prone spontaneously hypertensive rats

1. At the age of 20 weeks, intracellular pH (pHi) of circulating lymphocytes suspended in HCO(3-)-free NaCl media was not significantly different between stroke-prone spontaneously hypertensive rats (SHRSP) and normotensive Wistar-Kyoto rats (WKY). 2. The initial recovery rate of pHi in lymphocytes tended to be greater in SHRSP than in WKY after the addition of 60 mmol/L or 120 mmol/L of NaCl, but there was no statistically significant difference. 3. The H+ equivalent efflux rate, which was a true reflection of Na(+)-H(+) activity, was significantly greater in SHRSP than in WKY (P less than 0.05). The difference in H+ equivalent efflux rate was not due to the difference in cellular buffering power between the two groups (P greater than 0.05). An increased Na(+)-H(+) exchange activity may play a partial role in the pathogenesis of hypertension.

Acute sodium-dependent changes in membrane dynamic properties

Na+ ions, which can play a pathogenic role in the development of high blood pressure, have been reported to regulate membrane enzymatic activities, receptor-ligand interaction and coupling of G-protein receptors to their effectors. This study was designed to investigate the in vitro effects of Na+ ions on membrane dynamic properties. The fluorescence anisotropy values of TMA-DPH (trimethylamino-diphenylhexatriene, probe selectively incorporated into the outer leaflet of the plasma membrane) was evaluated in platelets and erythrocytes of sodium-dependent hypertension-prone and -resistant rats of the Sabra Strain. Whereas no difference was observed between the 2 strains, TMA-DPH anisotropy was found to be strongly influenced in platelets by external Na+ ions. In the absence of external Na+, TMA-DPH anisotropy increased in human and rat platelets. In contrast, Na+ ions did not affect the anisotropy when the probe was inserted into erythrocyte ghosts. This indicates that Na+ ions can acutely regulate order parameter and microviscosity of platelet plasma membrane in the regions explored by the probe.

Changes in free calcium concentrations in platelets of SHRSP and WKY: its relationship to ATP releasing potencies and platelet aggregation activities upon stimulation of several reagents

To clarify the relationships between free calcium levels, ATP release and aggregation potencies of SHRSP platelets, we examined the platelets from 9-month-old SHRSP and WKY using fura-2AM and luciferine-luciferase. In the absence of extracellular Ca2+, each reagent elevated the free calcium level to the same extent in the samples of both SHRSP and WKY. With regard to ATP release, thrombin and collagen less potentiated the platelet action in SHRSP than WKY, and ATP release was not affected by extracellular Ca2+. Collagen and ADP induced aggregations showed lower activities in SHRSP than WKY. TPA caused higher Ca2+ influx and aggregation activity in SHRSP than WKY in the presence of extracellular Ca2+. These results indicate that Ca2+ release must be followed by ATP release, and ATP release may be less potentiated, while thrombin and TPA induced aggregation is likely to be stimulated in SHRSP platelets, because protein kinase C activity in SHRSP platelets appears to be high.

Direct characterization of the Na+/H+ exchanger in human platelets

The kinetic properties of the Na+/H+ exchanger in human platelets were investigated by direct measurements of pHi as detected with the fluorescent dye, BCECF. In acid-loaded cells, the antiporter displayed a hyperbolic dependence regarding external Na+ with an apparent Km of 38 +/- 4 mM (pHo 7.2 at 25 degrees C) whereas its pHi-dependent activation between 7.3 to 6.4 did not obey a Michaelian model. External acidification from 7.7 to 6.5 decreased significantly the initial rate of Na(+)-dependent H+ efflux. The amiloride derivative, ethylisopropylamiloride blocked this exchanger and exerted a non-competitive inhibition with respect to Na+o (Ki = 17 nM). The cation selectivity of the external site of the antiporter was Na+ greater than Li+ greater than K+ and choline. These results indicate that the BCECF technique allows to evaluate the main features of the Na+/H+ exchanger in human platelets, which possesses kinetic properties similar to those reported in other cell types.

Abnormal calcium handling by platelets of spontaneously hypertensive rats

There is controversy as to whether platelet intracellular free calcium ([Ca2+]i) is increased in spontaneously hypertensive rats (SHR) as compared with Wistar-Kyoto (WKY) rats. Discrepant results may be due to methodological problems including platelet activation during the collection process and leakage of intracellular dye used for [Ca2+]i measurement. To provide further insight into this problem, [Ca2+]i was estimated in fura-2-loaded platelets isolated from eight SHR and seven WKY rats at 12-14 weeks of age by using a two-syringe blood collection method and a correction method for fura-2 leakage. Basal [Ca2+]i was higher in SHR than in WKY rats (61.6 +/- 5.6 vs. 54.0 +/- 3.9 nM, p less than 0.02). However, the difference disappeared when a correction for fura-2 leakage was not used (109.7 +/- 18.4 vs. 94.9 +/- 9.2 nM, p less than 0.1). Thus, differences in [Ca2+]i between SHR and WKY rats may be obscured if dye leakage from platelets is not taken into account. Thrombin (0.1 units/ml) induced a rise in [Ca2+]i that was greater in SHR than WKY rats, both in the presence (491.4 +/- 31.6 vs. 377.5 +/- 21.7 nM, p less than 0.002) and absence (264.9 +/- 33.6 vs. 228.2 +/- 30.1 nM, p less than 0.05) of calcium in the media. These results indicate that thrombin-stimulated calcium influx as well as discharge of calcium from intracellular stores is increased in SHR platelets. Thus, under both basal and stimulated conditions, platelet calcium handling is abnormal in the SHR.