Diffuse structural alterations in cell membranes of spontaneously hypertensive rats

Vascular impairment of adenosinergic system in hypertension: increased adenosine bioavailability and differential distribution of adenosine receptors and nucleoside transporters

Adenosinergic system regulates vascular tonicity through the complex system of adenosine, adenosine receptors (ARs) and nucleoside transporters. This work aimed at evaluating the impact of hypertension on adenosine bioavailability and expression/distribution profile of AR subtypes (A₁, A_2A, A_2B, A₃) and equilibrative nucleoside transporters (ENT1, ENT2, ENT3, ENT4). Adenosine was measured in vascular tissue extracts by HPLC (fluorescence detection); immunoreactivities (ARs/ENTs) in mesenteric arteries/veins from normotensive Wistar Kyoto (WKY) and spontaneously hypertensive rats (SHR) were analyzed by histomorphometry. Significantly higher adenosine bioavailability occurred in arteries than in veins. Adenosine bioavailability was even more increased in SHR vessels. Expression/distribution of ARs and ENTs observed in all vascular layers (intima, media, adventitia), with more intensified expression in arteries than in veins. In SHR arteries, a downregulation of all ENT along with downregulated and punctuated distribution of A₁ and A_2B receptors occurred comparatively to WKY arteries. By contrast, expressions of ARs and ENTs were unaltered, exception for an A_2A receptor upregulation, and ENT2 downregulation in SHR veins relatively to WKY veins. Our data evidenced clear alterations of adenosinergic dynamics occurring in hypertension, particularly in arterial vessels. An increased adenosine bioavailability was observed, for the first time, in hypertensive vascular tissues.

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.

Composition en acides gras des hémisphères cérébraux de rats spontanément hypertendus allaités par des femelles Wistar

Total lipid fatty acid composition was investigated in brain hemispheres of male Spontaneously Hypertensive Rats (SHR), compared with normotensive Wistar Kyoto rats (WKY) used as controls. Both strains were suckled by adoptive Wistar mothers, and then fed a standard diet after weaning. No difference was observed between the two hemispheres of WKY killed either at 10 or 30 days. In SHR killed at 10 days, the two hemispheres showed differences, SHR left hemispheres exhibiting greater fatty acid composition changes than those of WKY, phenomenon that toned down at 30 days. Hence, SHR pups showed a different total lipid fatty acid composition of their brain hemispheres when compared with their WKY controls, though the two strains received the same diet. Genetically programmed hypertension might be, directly or not, involved in these changes.

Desaturase activities are depleted before and after weaning in liver microsomes of spontaneously hypertensive rats

In the present study, we have investigated the microsomal linoleic acid desaturation steps into arachidonic acid in 10- and 30-day-old spontaneously hypertensive rats (SHR), as compared to their normotensive control rats, Wistar Kyoto (WKY). Suckled by adoptive Wistar normotensive female, the SHR and WKY were fed the same diet. Our results show lower Delta 6 and Delta 5 desaturase activities (the limiting steps in the bioconversion of linoleic acid into arachidonic acid) in the young SHR, as compared to the WKY normotensive rats. The fatty acid composition of liver microsomal total lipids evidences a higher proportion of linoleic acid in SHR than in WKY, in agreement with the partially depleted desaturase activities. Such a loss of desaturase activities may be under the control of hormones involved in the regulation of SHR blood pressure.

Coexisting independent sodium-sensitive and sodium-insensitive mechanisms of genetic hypertension in spontaneously hypertensive rats (SHR)

Some essential hypertensive patients and genetic hypertensive rat strains have less than the normal levels of Mg2+ tightly bound to the plasma membranes of their erythrocytes and other cells, i.e., the magnesium binding defect (MgBD). This binding defect appears to cause increased passive permeability of the membrane to Na+ and thereby its increased intracellular concentration, particularly if the Na+-extrusion enzyme systems of the cell are also defective. The Na+-Ca2+ exchange system in the cell membrane exports Na+ and imports Ca2+, increasing the tone of the smooth muscle cell and thus producing hypertension (HTn). This HTn is Na+-sensitive. Evidence supporting this postulate was obtained by determining the intraerythrocyte total concentrations of Na+, Ca2+, K+, and Mg2+ in two strains of spontaneously hypertensive rats (SHR and SS/Jr rats, having the MgBD together with the other requisites of the Na+-sensitive pathway) and their respective controls (WKY and SR/Jr rats, in which this complete pathway is absent). The Na+ and Ca2+ concentrations in the hypertensive rats were increased, and that of K+ was decreased. The concentrations of these cations were very similar in the two hypertensive strains. The level of membrane tightly bound Ca2+ in SHR erythrocyte membranes was significantly higher than those in the other three rat strains, which were not statistically different from each other. These results support previously reported evidence of the existence of a novel HTn-generating mechanism in the SHR rat, in which the intracellular Ca2+ concentration is increased as the result of the enhanced diffusion of this ion into the cell and the accompanying deficiency of the Ca2+ extrusion enzyme systems. This pathway is therefore Na+-insensitive, i.e., Ca2+-sensitive.Key words: essential hypertension, Na+-sensitive hypertension, Na+-insensitive hypertension, Ca2+-sensitive hypertension.

Inhibition of nitric oxide synthesis increases erythrocyte membrane fluidity and unsaturated fatty acid content

Changes in the lipid composition of the membrane affect its fluidity and function. These variables are altered in various forms of hypertension. Our hypothesis was that the rapid increase in blood pressure (BP) caused by inhibition of nitric oxide production would lead to alterations in membrane fluidity similar to those observed in genetic hypertension. We used Nomega-nitro L-arginine methyl ester (L-NAME) and vehicle-treated (3 weeks) Wistar-Kyoto rats to study the effects of nitric oxide synthase (NOS) inhibition on membrane fluidity and lipid composition. Erythrocyte membrane fluidity was measured by fluorescence anisotropy. Membrane lipids were separated using Sep-Pak and thin-layer chromatography. Fatty acid methyl esters were produced and analyzed by gas chromatography-mass spectrometry. Nomega-nitro L-arginine methyl ester treatment increased BP and erythrocyte membrane fluidity. The phospholipid and unsaturated fatty acid levels in the membranes from the L-NAME-treated rats were consistent with the increase in fluidity (ie, more unsaturated fatty acid, in particular, arachidonic and docosahexaenoic acid) and a reduction in membrane sphingomyelin content. Fatty acid analysis of individual lipid groups suggested the changes in membrane fatty acid composition may be asymmetric, with the majority of the changes occurring in the outer leaflet. Inhibition of NOS results in changes in membrane composition that may explain the concurrent changes in fluidity. The increased membrane fluidity observed here contrasts with the reduced fluidity observed in genetic hypertension or unchanged fluidity in secondary hypertension. The effects could be related to NOS inhibition or may be a direct effect of L-NAME.

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).

Liver microsomal membrane fluidity and microsomal desaturase activities in adult spontaneously hypertensive rats

OBJECTIVE

The purpose of the present study was to investigate liver microsomal membrane fluidity simultaneously with membrane fatty acid composition and desaturase activities in spontaneously hypertensive rats (SHR).

DESIGN AND METHODS

The membrane fluidity was determined, after electron spin resonance (ESR) measurement, in SHR compared with normotensive Wistar-Kyoto (WKY) rats, by calculating the order parameter S from ESR spectra of 5-nitroxide stearate and 10-nitroxide stearate, used as spin-labelled fatty acids. Desaturase activities were measured by incubating SHR and WKY rat liver microsomes with [14C]-radiolabeled fatty acids as substrates for desaturation reactions. The fatty acid composition of liver microsomal membranes was determined by gas-liquid chromatography.

RESULTS

Whereas no significant difference between S of 5-nitroxide stearate was observed for SHR and WKY rats, S of 10-nitroxide stearate was significantly lower in SHR than it was in WKY rat microsomal membrane, indicating that the core microsomal membrane fluidity was higher in SHR. Significant differences between fatty acid compositions were observed for SHR and WKY rat microsomal membranes. Delta9 and n-6 delta6 microsomal desaturase activities were significantly lower in SHR.

CONCLUSION

These results suggest that the higher liver core microsomal membrane fluidity observed in SHR might be dependent on the increased proportion of mono-unsaturated fatty acids. Such observed modifications and the alterations in delta9 and n-6 delta6 desaturase activities suggest that an impaired polyunsaturated fatty acid biosynthesis is related to changes in microsomal membrane fluidity in hypertension.

Lipid composition and fluidity in the jejunal brush-border membrane of spontaneously hypertensive rats. Effects on activities of membrane-bound proteins

The lipid composition and fluidity of jejunal brush-border membrane vesicles (BBMV) have been studied in spontaneously hypertensive rats (SHR) and normotensive Wistar Kyoto (WKY) rats. The activities of both Na(+)-dependent D-glucose cotransport and Na(+)-H+ antiport have also been determined. A significant increase in the level of free cholesterol was observed in jejunal BBMV from SHR compared to WKY rats. Since phospholipid values did not change in either group of animals, a significant enhancement in the free cholesterol/phospholipid ratio was observed in SHR. A decrease in the levels of phosphatidylethanolamine together with an increase in the values of phosphatidylserine was observed in hypertensive rats. Although the content of phosphatidylcholine (PC) and sphingomyelin (SM) was not significantly altered in SHR, the ratio PC/SM significantly increased in these animals when compared to WKY rats. The major fatty acids present in bursh-border membranes prepared from SHR and WKY rats were palmitic (16:0), stearic (18:0), oleic (18:1, n-9) and linoleic (18:2, n-6), and the fatty acid composition was not modified by the hypertension. A decreased fluorescence polarization, i.e., increased membrane fluidity, was observed in SHR, which was not correlated to the increased ratio of cholesterol/phospholipid found in the brush-border membrane isolated from these animals. These structural changes found in SHR were associated to an enhancement in both Na(+)-dependent D-glucose transport and Na(+)-H+ antiport activity in the jejunal BBMV of SHR.

Functional beta-adrenoceptors in the left atrium of normotensive and hypertensive rats

1. The aim of the study was to characterize the functional beta 1-and beta 2-adrenoceptors of the rat left atrium and to investigate how these functional beta-adrenoceptor responses were altered in hypertension. The contractile responses of the left atrium from Wistar-Kyoto (WKY) and spontaneously hypertensive (SH) rats to isoprenaline, T-0509 and procaterol were characterized. Subsequently, the effects of selective beta 1-(bisoprolol) and beta 2 (ICI 118,551)-adrenoceptor antagonists were investigated on these responses. 2. The maximal combined contractile responses of the rat left atrium to cardiac stimulation and CaCl2, isoprenaline, T-0509 or procaterol were not altered by hypertension. 3. The sensitivities to CaCl2 (pD2 on WKY left atrium = 2.99), isoprenaline (8.82) and T-0509 (8.84) were not altered by hypertension. There was an increase in sensitivity to procaterol from a pD2 value of 7.21 to 7.61 in the left atrium of the SH rat. 4. The basal tension induced by cardiac stimulation alone was inhibited by bisoprolol at > or = 10(-8) M and by ICI 118,551 at > or = 10(-7) M and this inhibitory effect is probably due to membrane stabilizing activity. 5. The pKB values for bisoprolol against isoprenaline, T-0509 and procaterol on the WKY were 8.43, 8.68 and 8.18, respectively, and were not different from SH rat left atrium. 6. The pKB value for ICI 118,551 against isoprenaline was increased from 7.06 on the WKY to 7.44 on the SH rat left atrium. The pKB values for ICI 118,551 against T-0509 and procaterol on the WKY were 7.18 and 8.14, respectively and were not significantly different from the SH rat left atrium values. 7. These results suggest that: (a) procaterol stimulates the beta 1-, in addition to, the beta 2-adrenoceptors of the rat left atrium; (b) functional beta 1-adrenoceptors are not altered in hypertension, and (c) there is probably an increase in the affinity of procaterol and isoprenaline for the beta 2-adrenoceptors which underlies the small increase in the functional beta 2-adrenoceptor response in hypertension.

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.

Many membrane abnormalities in hypertension result from one primary defect

Evidence has been presented that: 1.) Changes in lipid bilayer alter the function of integral membrane proteins. 2.) There is less calcium bound to the plasma membrane in hypertension. 3.) Structural and functional abnormalities of the lipid bilayer have been reported in genetic hypertension. We hypothesize that multiple abnormalities of membrane transport systems in hypertension are secondary to an inherent abnormality of the lipid bilayer in which these transport proteins reside.

Alterations of phosphoinositide-specific phospholipase C and protein kinase C in the myocardium of spontaneously hypertensive rats

In order to determine whether phosphoinositide metabolism is altered in hypertensive cardiac hypertrophy, phospholipase C (PLC) and protein kinase C activities were measured in hearts from 4- and 20-week-old spontaneously hypertensive rats (SHR) and age-matched, normotensive Wistar-Kyoto rats (WKY). PLC activities were assayed using phosphatidylinositol (PI) and phosphatidylinositol-4,5-bisphosphate (PIP2) as substrates to assess the substrate specificity. PI-hydrolyzing PLC activity (PI-PLC) was predominantly located in the cytosol, and its activity was similar in both strains. Membrane-bound PIP2-hydrolyzing PLC activity (PIP2-PLC) was significantly lower in 20-week-old SHR than in WKY, but there was no significant difference in soluble PIP2-PLC. Protein kinase C activity was significantly elevated in 20-week-old SHR and Ca2(+)-phospholipid-dependent phosphorylation was observed in the proteins of molecular weight 26, 32, 43, and 95 KDa. In 4-week-old prehypertensive SHR, there were no significant differences in PI-PLC, PIP2-PLC, or protein kinase C activities as compared with age-matched WKY. These data demonstrated that protein kinase C and membrane-bound PIP2-PLC are altered during the period of hypertension development. These alterations may have important roles in the development or maintenance of hypertensive cardiac hypertrophy in SHR.

Dietary linoleic acid prevents the development of deoxycorticosterone acetate-salt hypertension

The aim of this study was to elucidate the effect of dietary variations of linoleic acid on the development of deoxycorticosterone acetate (DOCA)-salt hypertension in rats. All rats were divided into three groups and fed one of the following isocaloric diets with 8% NaCl: a high linoleic acid (HLA) (20% sunflower oil), a moderate linoleic acid (5% lard oil + 15% sunflower oil), or a low linoleic acid (DLA) (20% lard oil). After 4 weeks of feeding, we determined intraerythrocyte sodium, potassium, and magnesium concentrations, intra-aortic and lymphocyte magnesium content, and erythrocyte ouabain-sensitive 22Na efflux rate constant. Cytoplasmic free calcium concentration of lymphocytes from thymus was also determined with quin-2 as a fluorescent indicator. In the HLA group, the elevation of systolic blood pressure was significantly attenuated, and intraerythrocyte sodium concentration was significantly lower than in the DLA group. There were greater intraerythrocyte potassium and magnesium concentrations, intra-aortic and lymphocyte magnesium contents, and erythrocyte ouabain-sensitive 22Na efflux rate constant in the HLA group as compared with other groups. Cytoplasmic free calcium concentration in the HLA group was significantly lower than in other groups. Systolic blood pressure significantly correlated negatively with intraerythrocyte and intra-aortic magnesium concentrations and intraerythrocyte potassium concentration, and correlated positively with cytoplasmic free calcium concentration. Erythrocyte ouabain-sensitive 22Na efflux rate constant significantly correlated positively with intraerythrocyte magnesium concentration. These findings suggest that dietary linoleic acid can attenuate the development of DOCA-salt hypertension.(ABSTRACT TRUNCATED AT 250 WORDS)

Calcium binding capacity of erythrocyte membrane in human hypertension

The cell membrane calcium binding capacity of genetically hypertensive rats is reduced when measured in the presence of the submicromolar calcium concentrations proper of intracellular environment. The present work, performed as an ancillary study to an epidemiological survey on an entire population, aimed to investigate the existence of a similar abnormality in human hypertension. Calcium binding to the erythrocyte membrane was measured in clinically healthy normotensive (n = 12) and hypertensive individuals (n = 24). For this purpose, a filtration technique was used, based on the determination of 45Ca bound to the erythrocyte membrane in the presence of free calcium concentrations (40 nmol/l and 1 mumol/l), which are similar to those of the intracellular environment. The intra-assay technical error was determined on 35 duplicate samples and, when expressed as percent of the mean, was 24.1 at the 40 nmol/l concentration and 16.8 at the 1 mumol/l concentration. Membranes of untreated hypertensive patients, at both calcium concentrations, bound significantly less calcium than the control group. Treated and untreated hypertensive individuals had comparable values of membrane calcium binding capacity. Membranes of the treated hypertensive group bound less calcium than those of the normotensive group at both calcium concentrations, but the difference was statistically significant only in the presence of 40 nmol/l free calcium. A significant positive correlation was observed between the calcium binding capacity at 40 nmol/l concentration and that at 1 mumol/l in the treated and untreated hypertensive groups (r = 0.73 and 0.75, respectively; 0.51 for the normotensive group). These findings support the hypothesis that a cell membrane abnormality is detectable in some hypertensive patients.(ABSTRACT TRUNCATED AT 250 WORDS)

Norepinephrine release and reuptake by hypothalamic synaptosomes of spontaneously hypertensive rats

We compared the overflow of endogenous norepinephrine during electrical field stimulation, the norepinephrine content, and the rate of initial neuronal uptake of [3H]norepinephrine in synaptosomes isolated from hypothalamus and brainstem of spontaneously hypertensive rats (SHR) and Wistar-Kyoto (WKY) rats at 7 and 13 weeks of age. The synaptosomes of two rats, a SHR and a WKY rat control, were simultaneously processed and subjected to the same electrical field stimulation. The overflow of endogenous norepinephrine during electrical stimulation (2 Hz, 2 minutes) in the hypothalamic synaptosomes of 7-week-old SHR was significantly greater, whereas the overflow of 13-week-old SHR was equivalent to the age-matched WKY rat. The norepinephrine content of synaptosomes was about the same in SHR and age-matched controls. There was also significantly enhanced [3H]norepinephrine uptake in the hypothalamic synaptosomes of young SHR, but neither the hypothalamic nor the brainstem samples of 13-week-old SHR showed any significant difference in their rate of [3H]norepinephrine uptake. These data are similar to those we observed (unpublished observations) in perfused mesenteric artery system in which norepinephrine release was significantly elevated during periarterial nerve stimulation only in young SHR. Thus, these results suggest that a parallel enhancement of norepinephrine release in hypothalamus with that of peripheral nervous system may play an important role during development of hypertension in young SHR.

Cell membrane in hypertension

Several alterations in membrane transport systems are observed in rat and human hypertension. Na+ flux changes are numerous, and cellular homeostasis to Na+ loading is impaired. Transmembrane Ca2+ movements are also numerous but clearly defined by a reduction in Ca2+ binders, a hypersensitivity of membrane phospholipase C, possible increased Ca2+ leak and reduced sensitivity of the Ca2+-pump to calmodulin. The resulting Ca2+ increase within arterial cells can be responsible for increased contractility and tone, leading to hypertension. These functional alterations in membrane transport can be secondary to a few well-defined membrane defects of genetic origin or to a diffuse structural perturbation in membranes involving lipid changes.

Experimental hypertension in young and adult animals

The susceptibility of immature and adult animals to various environmental factors often differs because the response of the young organism can only involve those regulatory mechanisms that are available at the particular stage of development. Increased sensitivity to certain (e.g., hypertensive) stimuli may be limited to a relatively short age period that is usually characterized by the maturation of some important physiological functions. High salt intake seems to influence the animals especially during the weaning period and prepuberty, in the course of which profound developmental changes of circulation, electrolyte metabolism, and neurohumoral regulation have been demonstrated. Indeed, salt-dependent forms of experimental hypertension are more severe when they are induced in immature animals. Moreover, substantial differences in hemodynamics, distribution of body fluids, and involvement of pressor and natriuretic agents indicate that the mechanisms of salt hypertension need not be the same in immature and adult animals. For this reason, increased attention should be paid to developmental factors in the study of induced forms of experimental hypertension.

Decreased content of integral membrane calcium-binding protein (IMCAL) in tissues of the spontaneously hypertensive rat

Prior studies report that plasma membranes of the spontaneously hypertensive rat (Okamoto-Aoki strain) bind less calcium than do the corresponding preparations from Wistar Kyoto controls. The possibility that the differences result from a decrease in the content of integral membrane calcium-binding protein (IMCAL) was explored by the application of immunoassays with polyclonal antisera and a mouse monoclonal antibody. IMCAL binds calcium with relatively high affinity, and its content in many rat tissues is regulated by vitamin D and the level of dietary calcium. Immunoassays of tissue IMCAL demonstrate significant reductions in content in the erythrocyte ghost, intestinal mucosa, kidney, heart, testis, and liver of the spontaneously hypertensive rat as compared to the control strains. The decreases are observed both at 4-5 weeks of age, before the onset of severe hypertension and at 8-9 weeks in the presence of severe hypertension. Moreover, the magnitude of the decrease in erythrocyte IMCAL can account for much of the decrease in the calcium-binding capacity of erythrocyte membranes reported by others. The results are especially significant because an abnormality in the membrane binding of calcium and in the regulation of cytosolic calcium ion concentration could underlie the pathogenesis of the hypertension.

Lymphocyte abnormalities in three types of hypertension in the rat

Lymphocyte number and weight and their sodium and potassium contents and net passive fluxes were measured in spontaneously hypertensive stroke-prone rats, deoxycorticosterone acetate-treated rats, and two-kidney, one clip renal hypertensive rats. Wistar-Kyoto rats were used as controls for the spontaneously hypertensive stroke-prone rats, and normal intact Sprague-Dawley rats were used as controls for the others. Blood lymphocyte count was higher and lymphocyte weight was lower in the hypertensive rats. Intralymphocytic sodium content (millimoles per kilogram of dry weight) was elevated in the three forms of hypertension as compared with control values (spontaneously hypertensive stroke-prone rats, 43.0 +/- 1.7 vs Wistar-Kyoto rats, 37.3 +/- 1.3; deoxycorticosterone acetate-treated rats, 44.4 +/- 3.1 vs Sprague-Dawley rats, 36.1 +/- 1.7; one-kidney, one clip rats, 50.5 +/- 3.7 vs Sprague-Dawley rats, 38.9 +/- 2.0). Intralymphocytic potassium content was not significantly altered in any of the forms of hypertension. Lymphocytes from spontaneously hypertensive stroke-prone rats and deoxycorticosterone acetate-treated rats exhibited elevated net sodium fluxes (millimoles per kilogram of dry weight per hour) as compared with those of controls (spontaneously hypertensive stroke-prone rats, 7.00 +/- 0.99 vs Wistar-Kyoto rats, 4.89 +/- 0.63; deoxycorticosterone acetate-treated rats, 7.58 +/- 0.97 vs Sprague-Dawley rats, 5.6 +/- 0.64). Net potassium fluxes were significantly elevated only in the spontaneously hypertensive stroke-prone rats (14.07 +/- 1.70 vs 8.23 +/- 1.04 in Wistar-Kyoto rats). Sodium and potassium fluxes in lymphocytes from two-kidney, one clip rats and Sprague-Dawley rats were not significantly different.(ABSTRACT TRUNCATED AT 250 WORDS)

Mn++-stimulated 3H-inositol incorporation in cultured vascular smooth muscle cells: deficiency in cells from spontaneously hypertensive rats

Incubation of cultured dissociated aortic smooth muscle cells from Wistar-Kyoto rats (WKY) with Mn++ resulted in a 10-fold stimulation of 3H-myo-inositol incorporation into membrane phospholipid. The stimulation was temperature and energy dependent. The Mn++ EC50 was 1 mM and maximum stimulation occurred at 10 mM Mn++. Other cations were ineffective. In cells from spontaneously hypertensive rats (SHR), the Mn++ EC50 was unchanged whereas the maximum stimulation of 3H-inositol incorporation was reduced by 50% compared to cells from parallel WKY cultures. These results suggest that in SHR cultured vascular smooth muscle cells there is a deficiency in an enzymatic process mediating exchange of free inositol with the headgroup inositol of membrane phosphatidylinositol.

Altered turnover of polyphosphoinositides in the erythrocyte membrane of the spontaneously hypertensive rat

The metabolism of inositol phospholipids of the erythrocyte membrane was compared in normotensive Wistar-Kyoto (WKY), spontaneously hypertensive (SHR), and stroke-prone SHR (SHR-SP) rats. This was performed on isolated ghost membranes by measuring the incorporation of 32P from [ gamma-32P ] adenosine triphosphate (ATP) into the diphosphoinositides (DPI) and the triphosphoinositides (TPI) which were the only 32P-labeled phospholipids. 32P-labeling of TPI was altered in adult and 3-week-old SHR as well as in SHR-SP compared to WKY controls; the radioactivity associated with TPI in hypertensive rats was about 30% lower than that associated with TPI in age-matched normotensive controls. By contrast, the radioactivity associated with DPI was similar in both hypertensive and normotensive rats. Measurement of the phosphoinositide distribution in both SHR and WKY indicates that the change observed in 32P-TPI could not be accounted for by a reduced phosphatidylinositol content in SHR membrane. Measurement of the Mg2+-activated TPI-phosphomonoesterase and of the Ca2+-activated polyphosphoinositide phosphodiesterase activities did not show any significant difference between SHR and WKY. It thus appears that the altered phosphoinositide metabolism observed in hypertensive rats was a consequence of some alteration in the activity of kinases which are responsible for the conversion of phosphatidylinositol into DPI and TPI. These results also suggest that the defect in phosphoinositide metabolism observed in genetically hypertensive rats was not a consequence of the blood pressure elevation and could be related to the pathogenesis of hypertension.

Modulation of Na+ transport systems in Wistar rat erythrocytes by excess dietary Na+ intake

Plasma Na+, erythrocyte Na+ content and the activity of Na+ transport systems of red cells were measured in Wistar rat fed a normal or high Na+ diet. Net Na+ and K+ fluxes of erythrocytes were also measured in the presence of plasma of rats fed with excess Na+. Na+-K+ cotransport and passive Na+ permeability were increased. Erythrocyte Na+ content was increased after 2 months but not after 8 days of high Na+ intake. No significant difference in plasma Na+ and pump activity could be detected after such a diet. No factor acting in Na+ extrusion was found to be present in plasma of salt loaded rats. These results indicate that Na+ intake may modulate Na+ transport systems, namely passive permeability and Na+-K+ cotransport and that increased Na+ erythrocyte content is not a causative factor.