Further analysis of cell membrane changes in genetic hypertension in rats by diphenylhexatriene fluorescence polarization

Alterations in plasma membrane promote overexpression and increase of sodium influx through epithelial sodium channel in hypertensive platelets

Platelets are small, anucleated cell fragments that activate in response to a wide variety of stimuli, triggering a complex series of intracellular pathways leading to a hemostatic thrombus formation at vascular injury sites. However, in essential hypertension, platelet activation contributes to causing myocardial infarction and ischemic stroke. Reported abnormalities in platelet functions, such as platelet hyperactivity and hyperaggregability to several agonists, contribute to the pathogenesis and complications of thrombotic events associated with hypertension. Platelet membrane lipid composition and fluidity are determining for protein site accessibility, structural arrangement of platelet surface, and response to appropriate stimuli. The present study aimed to demonstrate whether structural and biochemical abnormalities in lipid membrane composition and fluidity characteristic of platelets from hypertensive patients influence the expression of the Epithelial Sodium Channel (ENaC), fundamental for sodium influx during collagen activation. Wb, cytometry and quantitative Reverse Transcription-Polymerase Chain Reaction (qRT-PCR) assays demonstrated ENaC overexpression in platelets from hypertensive subjects and in relation to control subjects. Additionally, our results strongly suggest a key role of β-dystroglycan as a scaffold for the organization of ENaC and associated proteins. Understanding of the mechanisms of platelet alterations in hypertension should provide valuable information for the pathophysiology of hypertension.

Impaired erythrocyte filterability of spontaneously hypertensive rats: investigation by nickel filtration technique

BACKGROUND

Deformability of erythrocytes plays a key role in the impairment of the microcirculation in hypertension. However, erythrocyte deformability in spontaneously hypertensive rats (SHR) during development of hypertension has not been fully investigated so far.

METHODS AND RESULTS

Erythrocyte filterability (whole cell deformability) was investigated in relation to blood pressure measured by the tail-cuff method in SHR and age-matched Wistar-Kyoto rats (WKY), using a highly sensitive and reproducible nickel mesh filtration technique. Impaired erythrocyte filterability was marked (37.0+/-17.5%) in prehypertensive young SHR (7 weeks of age) and sustained (51.6+/-13.3%) in hypertensive mature SHR (18 weeks of age), when compared with that of age-matched WKY (62.1+/-7.2% in 7 weeks of age, P<0.005, and 71.1+/-3.9% in 18 weeks of age, P<0.005, respectively). This impairment in SHR could not be explained by the mean corpuscular volume or mean corpuscular hemoglobin concentration of erythrocytes, but the erythrocyte count was significantly (P<0.005) greater in SHR than in the age-matched WKY.

CONCLUSIONS

Although the precise mechanisms remain to be elucidated, markedly impaired erythrocyte filterability in SHR is considered to contribute to the development and maintenance of genetic hypertension. (Circ J 2010; 74: 129 - 136).

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).

Normal thrombin binding leads to greater fibrinogen binding and increased platelet aggregation in spontaneously hypertensive rats

This study was conducted to determine the mechanisms of increased platelet reactivity to thrombin in hypertension. Thrombin induced significantly greater platelet aggregation in spontaneously hypertensive (SHR) than in normotensive (Wistar Kyoto, WKY) rats. Fibrinogen and thrombin binding to platelets was determined using [125I]-fibrinogen and [125I]-thrombin respectively. Increased platelet aggregation in SHR correlated with thrombin-induced greater binding of fibrinogen to SHR than to WKY platelets. However, the number of thrombin receptors (binding sites/platelet) in WKY (19,500 +/- 3,000) and SHR (23,100 +/- 3,000) as well as thrombin dissociation constants were statistically similar in WKY (1.17 +/- 0.2 microM) and SHR (1.62 +/- 0.27 microM) platelets. Fura 2/AM, a fluorescent calcium indicator, loaded platelets were used to quantify the platelet ionized calcium ([Ca2+]i). The [Ca2+]i in unstimulated SHR and WKY platelets was essentially the same. In a calcium poor medium, thrombin-induced a 35% greater increase in [Ca2+]i in SHR than in WKY platelets. These data, taken together with our earlier observations that thrombin induces a significantly greater hydrolysis of phosphoinositide (Thromb. Res. 49, 5-21, 1988), lead us to suggest that thrombin-induced increased generation of inositol 1,4,5-trisphosphate and diacylglycerol induces greater fibrinogen binding and consequently increased aggregation in SHR than WKY platelets. The finding that the thrombin binding isotherms are similar in WKY and SHR platelets suggests that increased platelet sensitivity to thrombin in hypertension may be due to altered signal transduction and not due to changes in the number or affinity of thrombin receptors.

Association of red blood cell sodium leak with blood pressure in recombinant inbred strains

Red blood cell Na+ content as well as ouabain-resistant Na+ and Rb+ (K+) transport (susceptible or resistant to inhibition by loop diuretics) were determined in spontaneously hypertensive rats (SHR) and normotensive Brown Norway (BN) rats the erythrocytes of which were incubated in either saline or Mg(2+)-sucrose medium. Elevated ouabain-resistant Na+ net uptake contrasted with slightly decreased red blood cell Na+ content in SHR compared with BN rats. Acceleration of furosemide- and bumetanide-sensitive Na+ fluxes contributed to enhanced ouabain-resistant Na+ influx into SHR erythrocytes in saline medium, whereas higher furosemide- or bumetanide-resistant Na+ efflux caused greater ouabain-resistant Na+ efflux in Mg(2+)-sucrose medium. Furosemide- and bumetanide-resistant Rb+ leaks were augmented in SHR erythrocytes. The association of the disclosed ion transport alterations with blood pressure was examined in 20 recombinant inbred strains derived from F2 SHR x BN hybrids. Ouabain-resistant Na+ uptake as well as furosemide- and bumetanide-resistant Na+ inward leaks (but not red blood cell Na+ content or furosemide- and bumetanide-sensitive Na+ net uptake) cosegregated with systolic and pulse pressures but not diastolic pressure of the recombinant inbred strains. In contrast, neither ouabain-resistant Na+ efflux nor any component of ouabain-resistant Rb+ uptake correlated positively with blood pressure of the recombinant inbred strains. Increased ouabain-resistant Na+ influx was compensated for by accelerated ouabain-sensitive Na+ extrusion because red blood cell Na+ content was not elevated in the hypertensive strains. Thus, high cell Na+ turnover rates might be related to genetic hypertension if an altered Na+ inward leak would be less effectively compensated for in tissues involved in cardiovascular regulation.

Lipid bilayer in genetic hypertension

Membrane microviscosity, phospholipid composition, and turnover were measured in cultured vascular smooth muscle cells isolated from mesenteric arteries of stroke-prone spontaneously hypertensive and age-matched, normotensive Wistar-Kyoto rats. Membrane microviscosity, measured with fluorescence polarization, revealed greater microviscosity (lower fluidity) of the membranes isolated from smooth muscle cells from hypertensive as compared with those isolated from normotensive rats (p less than 0.01). Preincubation of membranes from hypertensive rats with 5 mM calcium reduced membrane microviscosity in "core" and in "surface" regions of the bilayer toward values observed in Wistar-Kyoto rats. Phospholipid composition did not differ between intact aortas and cultured mesenteric cells or between those tissues obtained from normotensive and from hypertensive rats. The total lipid-associated radioactivity was significantly lower in cells from stroke-prone spontaneously hypertensive rats than in those from Wistar-Kyoto controls (p less than 0.01). Phosphatidylcholine incorporated 70% and phosphatidylinositol 16% of total lipid-associated radioactivity, with no difference between cells from hypertensive and normotensive animals. Turnover of phosphatidylethanolamine was greater in cells from Wistar-Kyoto rats (p = 0.02), whereas turnover of phosphatidylserine was greater in cells from stroke-prone spontaneously hypertensive rats (p = 0.04). The greater microviscosity of the lipid bilayer in hypertension is a generalized defect of the matrix in which the transport proteins function. We hypothesize that this defect is responsible for the multiple abnormalities of membrane transport systems that have been described in genetic 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.

Preliminary observations on abnormalities of membrane structure and function in essential hypertension

To test the hypothesis that structural abnormalities exist in the cell membrane in persons with essential hypertension and that these abnormalities affect membrane-related cellular functions, we examined several membrane-dependent phenomena and membrane lipid composition in the blood cells of subjects with essential hypertension. We analyzed platelet aggregability, membrane fluidity, membrane fatty acid composition, and erythrocyte deformability in four normolipidemic subjects with untreated essential hypertension and in five age-matched normotensive controls. As compared with the controls, the subjects with essential hypertension had platelets that aggregated at lower concentrations of adenosine 5'-diphosphate, platelet membranes that were less fluid, and erythrocytes that were more deformable. Lipid analysis of the membranes of platelets from the two study groups showed that although the cholesterol content was identical, the membranes from the essential hypertension group contained significantly less linoleic acid (18:2) than did those from the normotensive controls. Given the known effects of cis-unsaturated fatty acyl composition on membrane fluidity and membrane-related cellular functions, these data suggest that one factor contributing to essential hypertension is an inherent structural membrane abnormality that alters the physical and functional properties of the cell membrane.

Plasma catecholamine modulation of alpha 2 adrenoreceptor agonist affinity and sensitivity in normotensive and hypertensive human platelets

We measured alpha 2-adrenoreceptor density as well as affinity for and sensitivity to agonist on intact platelets of normotensive and hypertensive subjects before and after physiological increases in plasma catecholamines. In normotensives, posture-induced rises in plasma catecholamines correlated with reduced alpha 2-adrenoreceptor agonist affinity and fewer high affinity state receptors. Platelet aggregation and inhibition of adenylate cyclase by L-epinephrine also was reduced. Hypertensive subjects had similar rises in plasma catecholamines with upright posture, but showed no change in receptor affinity or sensitivity. No change in platelet alpha 2-adrenoreceptor number occurred in these studies. In vitro incubation with L-epinephrine revealed that platelets from hypertensives had slower desensitization than those from normotensives. Binding studies at different temperatures and with varying sodium concentrations found no thermodynamic or sodium-dependent differences between normotensive and hypertensive groups. These studies demonstrate that platelets from hypertensive subjects exhibit a defect in the ability of physiological concentrations of agonist to desensitize the alpha 2-adrenoreceptor.