In vivo shear flow and erythrocyte membrane fluidity in hypertensive patients

1. To evaluate the response of red blood cells subjected to the shear flow in hypertension, the relationships between wall shear phenomena determined in vivo in the brachial artery of hypertensive patients and the modifications of the membrane dynamics measured in vitro in erythrocyte ghosts of 32 patients were investigated. 2. Two fluorescent probes, diphenylhexatriene (DPH) and its trimethylamino-derivative (TMA-DPH), localized respectively in the lipid membrane core and at the lipid-water interface, were used. 3. Shear rate, shear stress and blood velocity were positively correlated with TMA-DPH anisotropy (P = 0.015, 0.005 and 0.026, respectively), but not with that of DPH. This indicates that wall shear forces were associated with the microviscosity of the outer part of the cell membrane. 4. The changes in wall shear forces and erythrocyte membrane microviscosity probed by TMA-DPH or DPH were observed to vary in parallel under nitrendipine therapy. 5. These results suggest that in vivo shear forces participate in the control of erythrocyte membrane fluidity or that erythrocytes adapt their membrane properties to blood flow conditions.

Acute membrane effects of trimetazidine in human platelets

The mechanisms by which trimetazidine (1-[2,3,4-trimethoxybenzyl]-piperazine) exerts its cytoprotective action have not been identified. This study was designed to investigate in human platelets and erythrocyte ghosts a possible perturbation of membrane dynamics by trimetazidine. Its effects on the steady-state anisotropies of two fluorescent probes, trimethylamino-diphenyl-hexatriene (TMA-DPH) and diphenylhexatriene (DPH) were compared. The effects on the aggregatory responses to collagen and ADP, and on platelet cAMP content were also investigated. In platelets, trimetazidine dose-dependently raised TMA-DPH anisotropy but not that of DPH. It reduced cAMP content (in the presence of Ro 15-2041, a phosphodiesterase inhibitor) and the aggregation responses to collagen and ADP. This suggests that trimetazidine decreases the 'fluidity' of the outer part of the plasma membrane, the adenylyl cyclase activity and some steps involved in platelet activation. In erythrocyte ghosts, the fluorescence anisotropy of TMA-DPH was not modified by trimetazidine. The membrane effects reported here could participate in the protection of cell metabolism afforded by a long-term treatment with trimetazidine.

Plasma lipids and platelet membrane fluidity in essential hypertension

Essential hypertension is often associated with high levels of plasma cholesterol or triglycerides. The relationships between plasma lipids and platelet lipids, membrane fluidity and functions in untreated hypertensive patients were investigated by measuring the fluorescence anisotropies of two fluorescent dyes (DPH and its cationic derivative, TMA-DPH, with different subcellular localization), cytosolic Ca2+ and pH, cyclic AMP content and aggregation to ADP and collagen. Hypercholesterolemia was found to be accompanied by a rise in platelet cholesterol content without changes in TMA-DPH or DPH anisotropies whereas hypertriglyceridemia was associated with a decreased cholesterol to phospholipid molar ratio, a decreased DPH anisotropy and a tendency of the cytosol to alkalinize. These results point out the differences between the effects of an acute cholesterol load and those of chronic hypercholesterolemia on platelet membrane microviscosity and aggregation. They demonstrate a strong association between plasma triglyceride levels and platelet membrane structure.

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 vivo and in vitro effects of isradipine on cytosolic Ca2+ concentration in erythrocytes from spontaneously hypertensive rats

Cytosolic Ca2+ concentration ([Ca2+]i) was investigated in erythrocytes from spontaneously hypertensive rats (SHR) and their normotensive controls (WKY), after an acute treatment with the Ca2+ antagonist isradipine. Blood samples were obtained from conscious rats and [Ca2+]i measured with fura-2. The [Ca2+]i was higher in SHR than in WKY erythrocytes (P < .05). Isradipine administration had no effect on WKY [Ca2+]i, but reduced that of SHR to WKY levels after 1 h. In vitro, isradipine dose-dependently decreased [Ca2+]i only in SHR (P = .006). The reduction by isradipine of the elevated [Ca2+]i in SHR suggests the presence of a greater dihydropyridine-sensitive Ca2+ influx in the SHR erythrocyte.

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.

Further investigation of platelet cytosolic alkalinization in essential hypertension

OBJECTIVE

To verify that platelet cytosolic pH is altered in essential hypertension and to investigate the mechanisms involved.

METHODS

Cytosolic pH was determined in unstimulated platelets by the fluorescent indicator 2,7-bis-carboxyethyl-5(6)-carboxyfluorescein (BCECF). Membrane microviscosity was evaluated by the fluorescence anisotropies of diphenylhexatriene (DPH) and its cationic derivative trimethylamino-diphenylhexatriene (TMA-DPH).

RESULTS

The cytosolic alkalinization previously observed in platelets from untreated hypertensive patients was confirmed. The buffering capacity appeared unaltered and the cytosolic pH was not modified by 50 mumol/l N-5-ethylisopropylamiloride, a specific inhibitor of the Na(+)-H+ exchange. Exposure to external Na(+)-free media produced an intracellular acidification that was similar in hypertensive and normotensive donors and maintained the cytosolic pH difference between the two groups. In the two blood pressure groups platelet cytosolic pH varied inversely with the steady-state anisotropy of TMA-DPH but not with that of DPH. Experimentally induced acidification of the cytosol by Na+ removal with or without nigericin treatment was accompanied by rises in TMA-DPH anisotropy.

CONCLUSIONS

This study of platelet intracellular pH in essential hypertension confirms cytosolic alkalinization and demonstrates its association with changes in the dynamic properties of the platelet plasma membrane.

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.

Cytosolic pH in cultured cardiac myocytes and fibroblasts from newborn spontaneously hypertensive rats

Newborn spontaneously hypertensive rats (SHR) develop cardiac hypertrophy before a rise in blood pressure. Cytosolic pH (pHi) has been discovered to modulate cell growth and proliferation; therefore, we have investigated pHi in myocytes and fibroblasts from 3- to 4-day-old SHR and normotensive Wistar (W) and Wistar-Kyoto controls (WKY). The ratio of heart to body weight was higher in SHR than in W and WKY (7.56 +/- 0.10 v 6.21 +/- 0.10 and 5.98 +/- 0.14 mg/g in 10, 5, and 7 groups of 20 to 40 animals; P less than .001 for both). Cytosolic pH, determined with the fluorescent probe BCECF, was measured from the sixth to the eighth day in culture on confluent cells. The mean pHi was higher in myocytes from SHR than in those from W or WKY rats (7.19 +/- 0.03, N = 30, v 7.09 +/- 0.03 and 7.11 +/- 0.02, N = 25 and 30; P = .008 and .024, respectively). In contrast, pHi was similar in fibroblasts from the three strains (7.21 +/- 0.03, 7.18 +/- 0.03, and 7.19 +/- 0.02, N = 15, 15, and 14, in SHR, W, and WKY rats, respectively). External acidification induced similar decreases in pHi from SHR and WKY myocytes, maintaining higher pHi values in SHR myocytes along the entire external pH (pHo) range studied. The inhibition of Na+/H+ exchange by the amiloride derivative, ethylisopropylamiloride, decreased the steady-state pHi of myocytes independently of the initial pHi values. This study demonstrated a cytosolic alkalinization in contractile cardiac cells from SHR before a significant rise in blood pressure and in the absence of hemodynamic influences and specific plasma factors.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Plasma levels of an endogenous Na(+)-K+ pump inhibitor in relation to haemodynamic data in cardiopathic patients

1. Despite the fact that numerous studies provide evidence for the existence of an endogenous inhibitor of the cell membrane Na(+)-K+ pump in plasma, little is known about the relationships between this factor and the main haemodynamic parameters. 2. In order to shed some light on this, we attempted to correlate haemodynamic parameters, measured during heart catheterization in a group of 22 cardiopathic subjects, with plasma digitalis-like activity levels, determined by two different procedures. 3. The ability of plasma to inhibit a human renal Na(+)-K(+)-ATPase showed an inverse correlation with cardiac index and a direct correlation with peripheral resistance. Plasma cross-reactivity with digoxin antibodies correlated directly with left atrial pressure. 4. These results furnish confirmation of a number of theoretical assumptions which attribute to the digitalis-like factor the ability to modify the contractility of the cardiovascular system.

Alterations of platelet membrane microviscosity in essential hypertension

1. The metabolism of blood platelets, taken as an accessible model of excitable cells, has been reported to be altered in hypertension. Most of the identified alterations concern the functions of various plasma membrane constituents. 2. A possible modification of membrane microviscosity was investigated by 1,6-diphenyl-1,3,5-hexatriene and 1-[4-(trimethylamino)phenyl]-6-phenyl-1,3,5-hexatriene fluorescence depolarization. In order to determine whether or not the membrane structures probed by these indicators were related to platelet physiological functions, the cytosolic free Ca2+ concentration was determined in parallel. 3. At physiological temperature, the fluorescence anisotropy of 1-[4-(trimethylamino)phenyl]-6-phenyl-1,3,5-hexatriene was decreased in untreated hypertensive patients (0.276 +/- 0.002 versus 0.288 +/- 0.002, n = 23 and 22, P less than 0.001), indicating a lowered microviscosity at the lipid-water interface of cell membrane. It correlated inversely with blood pressure (P less than 0.001) and cytosolic free Ca2+ concentration (P less than 0.030). On the contrary, 1,6-diphenyl-1,3,5-hexatriene fluorescence anisotropy was observed to vary with sex but not with blood pressure. 4. These results suggest that structural membrane modifications may participate in the various functional abnormalities observed in platelets from hypertensive patients.

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.

Modulation by external Ca2+ and nicardipine of Ca2+ influx and cytosolic concentration in human erythrocytes

Variations of Ca2+ influx (evaluated by the initial rate of 45Ca2+ uptake) and cytosolic free Ca2+ concentration ([Ca2+]i, measured with fura-2) were investigated in human erythrocytes. When external Ca2+ concentration ([Ca2+]o) rose from 1 to 2 mM, the initial rate of Ca2+ influx nearly doubled whereas [Ca2+]i increased only by 15%. Nicardipine dose-dependently decreased both initial rate of Ca2+ influx and [Ca2+]i (up to 53 and 18%. respectively at 10(-6) M). The less marked changes in [Ca2+]i than in Ca2+ influx indicate a partial adjustment of the Ca2+ extruding-pump activity to of Ca2+ influx. In vivo administration of nicardipine reduced [Ca2+]i only when its initial value exceeded 80 nM and prevented the rise in [Ca2+]i induced by the increase in [Ca2+]o. Our results indicate that nicardipine may reduce Ca2+ influx in human erythrocytes and participate in the control of [Ca2+]i when elevated.

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.

Dynamic association between artery shear flow condition and platelet cytosolic free Ca2+ concentration in human hypertension

1. Blood cells and vascular endothelial cells are subjected to a wide range of haemodynamically generated shear stress forces. In vitro, membrane stretching or shear stress have been observed to activate ion channels and cell metabolism and to facilitate erythrocyte and platelet aggregation. 2. The present study was designed to evaluate the participation of shear stresses in the control of apparent platelet cytosolic free Ca2+ concentration in hypertensive patients. 3. Shear conditions and platelet cytosolic free Ca2+ concentration in vitro were studied after a dynamic perturbation induced by 3 months of double-blind treatment with one of two beta-antagonists, carteolol and atenolol. Brachial artery wall shear rate and stress were estimated by means of a pulsed Doppler velocimeter, and blood viscosity was measured by a co-axial viscometer at a shear rate of 96 s-1. Platelet cytosolic free Ca2+ concentration was simultaneously measured by using the Quin-2 fluorescent chelator. The direct effect of atenolol and carteolol on platelet cytosolic free Ca2+ concentration in vitro was also measured after addition of the beta-blockers to platelet-rich plasma. 4. Atenolol and carteolol decreased blood pressure similarly but their effects on shear rate (P less than 0.02), shear stress (P less than 0.01) and platelet cytosolic free Ca2+ concentration (P less than 0.05) differed after 3 months of therapy. In contrast, neither of the drugs significantly altered platelet cytosolic free Ca2+ concentration, in vitro per se. 5. In the overall population, strong positive correlations existed not only between changes in platelet cytosolic free Ca2+ concentration and those in shear rate (r = 0.81, P less than 0.001) and shear stress (r = 0.83, P less than 0.001), but also between their absolute values, suggesting a possible haemodynamic shear-dependent modulation of transmembrane Ca2+ transport.

Plasma digitalis-like activity and cytosolic Ca2+ in essential hypertension

Digitalis compounds are known to increase Ca2+ influx in various cells. As platelet cytosolic free [Ca2+] and plasma digitalis-like activity have been reported independently to be higher in some untreated hypertensive patients than in normotensive subjects, we have investigated a possible relationship between these two parameters. Platelet cytosolic free [Ca2+], determined using Quin-2, the capacity of plasma extracts to inhibit renal Na+, K(+)-ATPase activity and ouabain binding on human erythrocytes were measured in parallel in 25 untreated hypertensive patients and 11 normotensive subjects. Enhanced values for all 3 parameters were observed in the same hypertensive patient. Platelet cytosolic free [Ca2+] was positively correlated to the plasma digitalis-like activity, which was evaluated by the inhibition of Na+, K(+)-ATPase activity and ouabain binding (r = 0.430, P = .010 and r = 0.448, P = .006, respectively). These relationships were independent of age and blood pressure. These results indicate that endogenous digitalis-like compounds may participate in the control of cytosolic free [Ca2+], in agreement with their proposed hypertensive role.

Increase in plasma digitalis-like activity during percutaneous transluminal coronary angioplasty in patients with coronary stenosis

Despite the fact that numerous studies have been published regarding the possible presence in plasma of an endogenous Na-K pump inhibitor with a digitalis-like structure in essential hypertension, very little is known about this factor in heart disease in general, and in situations characterized by low cardiac output. We measured the ability of plasma obtained from the femoral vein to inhibit a human renal Na(+)-K+ ATPase before and immediately after percutaneous transluminal coronary angioplasty (PTCA) in 6 patients suffering from angina pectoris and severe coronary stenosis. Intraerythrocyte sodium and potassium concentrations were also measured simultaneously. Na(+)-K+ ATPase inhibition proved significantly greater after angioplasty as compared to basal activity (percentage inhibition: 31.5 +/- 7.8 vs 16.1 +/- 12.2). No significant changes in intraerythrocyte sodium and potassium were detected. Though we are not in a position to define the mechanism underlying the increase in the digitalis-like factor, a plausible hypothesis may be that the reduction in cardiac output during PTCA by raising cardiac pressures may stimulate the production of a factor of compensatory inotropic significance.

Platelet cyclic AMP in essential hypertensive and normotensive offspring

Essential hypertension is accompanied by several modifications to platelet metabolism suggesting hyper-reactivity to various aggregating agents. As the platelet response is mediated by both cytosolic free calcium, which is stimulatory, and cyclic (c)AMP, which is inhibitory, this hyper-reactivity may be caused by a modification in cAMP metabolism. We therefore determined cAMP in unstimulated platelets from 19 patients with essential hypertension and 27 age-matched normotensive subjects, nine with and 19 without a family history of hypertension. The platelet cAMP content was reduced in the essential hypertensives and in the normotensives with a positive family history by 37.5% and 42%, respectively (P less than 0.001 for both). Platelet cAMP was inversely correlated with diastolic blood pressure (P = 0.036). After prostaglandin (PG) E1 stimulation, the platelet cAMP content remained lower in the patients with essential hypertension than in the normotensive subjects, whatever their hypertensive heredity. The rises in cAMP caused by inhibition of phosphodiesterase by 7-bromo-1,5-dihydro-3,6-dimethylimidazo-[2,1-b]quinazolin-2[ 3H]-one (Ro 15-2041) were similar in the three groups. These results indicate that cAMP, the platelet inhibitory messenger, is reduced in hypertensive patients and in their normotensive offspring and may affect the various platelet abnormalities previously described in this disease.

[Essential hypertension and platelet cytosolic concentrations of H(+) and Ca2(+)]

Essential hypertension is associated with various cell abnormalities, including alterations in the metabolism of intracellular messengers, such as cytosolic free Ca2+ and cyclic AMP or IP3. Intracellular pH is implicated in the regulation of number vital functions, including cell metabolism, division and response to various stimuli. We have measured cytosolic H+ concentration ([H+]i) in human platelets and investigated a possible relationship with free Ca2+ concentrations ([Ca2+]i) in hypertensive patients. [H+]i was determined with the pH sensitive fluorescent probe BCECF in platelets from 15 normotensive subjects and 15 patients with mild to moderate essential hypertension, free from medication, il any, for at least two weeks. Donor characteristics are indicated in the table. (table; see text) [H+] cytosolic from hypertensive patients was significantly lowered by 21 p. 100 compared to normotensive values (table). Cytosolic free Ca2+ concentrations, measured with the Ca2+ fluorescent probe Fura2, were significantly increased by 19 p. 100 in platelets from hypertensive patients when compared to those of normotensive donors. Taken all together, [H+]i and [Ca2+]i varied inversely (r = -0.421, p = 0.02) in these thirty donors and tends to be correlated in the essential hypertensive patients (r = 0.490, p = 0.06). This correlation remained significant at constant age systolic and diastolic blood pressures. The simultaneous rise in [H+]i and [Ca2+]i in platelets from essential hypertensive patients are compatible with their observed enhanced sensitivity to several aggregating agents. This alkalinisation, if present in the vascular smooth muscle cells, may also reflect facilitated cell proliferation and increased sensitivity to stimulating agents, two parameters implicated in the rise of arterial blood pressure.

Platelet cyclic AMP in essential hypertension

Various abnormalities in platelet metabolism, including increased sensitivity to several aggregating agents, have been described in essential hypertension. Platelet response is controlled by Ca2+ and cyclic AMP-dependent mechanisms (stimulatory and inhibitory, respectively) which oppose one another. In the present study, the cyclic AMP contents of unstimulated platelets were measured by radio-immunoassay and observed to be lower in hypertensive than in normotensive subjects, either in the basal state or after prostaglandin E1 (PGE1) stimulation. In the presence of 7-bromo-1,5,dihydro-3,6-dimethylimidazo [2,1-b] quinazolin-2(3H)-one (Ro 15-2041), a specific inhibitor of phosphodiesterase, the increases in cyclic AMP content were similar in platelets from both groups, indicating that this enzyme was not responsible for the alterations in cyclic AMP metabolism observed in hypertension. Low external Ca2+ reduced basal and PGE1-stimulated cyclic AMP content in both normotensive and hypertensive groups but cyclic AMP levels remained lower in hypertensive patients than in normotensive subjects, indicating that Ca2+ influx is not responsible for this altered metabolism of cyclic AMP in hypertension. These data suggest that the reduced platelet cAMP content may participate in the hyperreactivity to various aggregating agents previously reported to accompany essential hypertension.