Abnormal net Na+ and K+ fluxes in erythrocytes of three varieties of genetically hypertensive rats

Tubulin pools in human erythrocytes: altered distribution in hypertensive patients affects Na+, K+-ATPase activity

The presence of tubulin in human erythrocytes was demonstrated using five different antibodies. Tubulin was distributed among three operationally distinguishable pools: membrane, sedimentable structure and soluble fraction. It is known that in erythrocytes from hypertensive subjects (HS), the Na(+), K(+)-ATPase (NKA) activity is partially inhibited as compared with erythrocytes from normal subjects (NS). In erythrocytes from HS the membrane tubulin pool is increased by ~150%. NKA was found to be forming a complex with acetylated tubulin that results in inhibition of enzymes. This complex was also increased in erythrocytes from HS. Treatment of erythrocytes from HS with nocodazol caused a decrease of acetylated tubulin in the membrane and stimulation of NKA activity, whereas taxol treatment on erythrocytes from NS had the opposite effect. These results suggest that, in erythrocytes from HS, tubulin was translocated to the membrane, where it associated with NKA with the consequent enzyme inhibition.

Membrane ion transport in erythrocytes of salt hypertensive Dahl rats and their F2 hybrids: the importance of cholesterol

The possible association of salt hypertension and altered lipid metabolism with abnormalities of particular systems transporting sodium and potassium has been studied in erythrocytes of Dahl rats and their F2 hybrids fed a high-salt diet since weaning. Our attention was paid to the Na(+)-K+ pump, Na(+)-K+ cotransport and especially to passive membrane permeability for Na+ and Rb+ (Na+ and Rb+ leak), because the Na+ leak was found to be dependent on the genotype, age and salt intake of Dahl rats, whereas the Rb+ leak was suggested to be a potential marker of salt sensitivity in Dahl and Sabra rats. Young male Dahl salt-sensitive (SS/Jr) and salt-resistant (SR/Jr) rats kept on a low-salt (0.3% NaCl) or high-salt diet (8% NaCl) were used for the progenitor study. The subsequent genetic study was based on 135 young male SS/Jr x SR/Jr F2 hybrids fed a high-salt diet since weaning. Ouabain (5 mmol/l) and bumetanide (10 micromol/l) were used to distinguish the contribution of the Na(+)-K+ pump, Na(+)-K+ cotransport and passive membrane permeability to measured net Na+ fluxes and unidirectional Rb+ (K+) movements. Compared to normotensive SR/Jr animals, salt-loaded SS/Jr rats had higher blood pressure (BP), elevated erythrocyte Na+ content, and increased Na+ and Rb+ leaks together with enhanced Na+ and Rb+ transport mediated by the Na(+)-K+ pump and Na(+)-K+ cotransport system. Salt hypertensive Dahl rats were also characterized by elevated plasma levels of total cholesterol and triglycerides, which were positively associated with BP of F2 hybrids (r=0.27 and 0.24, p< 0.01). In F2 hybrids, mean arterial pressure correlated significantly with erythrocyte Na+ content (r=0.24, p<0.01) and ouabain-sensitive Na+ extrusion, but not with the passive membrane permeability for Na+ or Rb+ (r=-0.02 and 0.06, not significant). Both of the above-mentioned significant associations could partially be ascribed to the dependence of erythrocyte Na+ content and ouabain-sensitive Na+ extrusion on plasma cholesterol (r=0.18 and 0.21, p<0.05). Our results support the idea that abnormal lipid metabolism and/or altered Na+,K(+)-ATPase function play an important role in the pathogenesis of salt hypertension in salt-sensitive Dahl rats.

alpha-Adducin 460Trp allele is associated with erythrocyte Na transport rate in North Sardinian primary hypertensives

Erythrocyte membrane alterations mirror those of vascular smooth muscle and renal tubular cell membrane. The interaction between adducin and Na-K pump is the most likely biochemical mechanism responsible for the increased tubular Na reabsorption and hypertension in Milan hypertensive strain (MHS) rats. To substantiate this hypothesis in humans, we tested to see if alpha-adducin Gly460Trp genotype is associated with erythrocyte sodium transport rate in a new cohort of n=268 never-treated North Sardinian primary hypertensives. Plasma renin activity and blood pressure response to hydrochlorothiazide were also measured to evaluate the relationship between sodium transport rate and two intermediate phenotypes with a higher degree of genetic complexity. Na-K pump, Na-K-Cl cotransport, and Li-Na countertransport at V(max) were faster (P<0.0001), whereas intracellular Na concentration was lower (P<0.0001) in patients carrying one or two 460Trp alleles. Such behavior was mirrored by opposite changes of intracellular Na concentration. Plasma renin activity and blood pressure response to diuretic treatment, on the other hand, showed a weaker association with the sodium transport rate. In conclusion, our findings are consistent with the hypothesis that the Gly460Trp alpha-adducin polymorphism may affect renal Na handling through an alteration in ion transport across the cell membrane mirrored by erythrocytes. These results may also have clinical relevance because the Gly460Trp alpha-adducin polymorphism may explain, at least in part, the variability of blood pressure response to diuretics in primary hypertensive patients.

Vegetable oils high in phytosterols make erythrocytes less deformable and shorten the life span of stroke-prone spontaneously hypertensive rats

Previous studies have shown that canola oil (CA), compared with soybean oil (SO), shortens the life span of stroke-prone spontaneously hypertensive (SHRSP) rats, a widely used model for hemorrhagic stroke. SHRSP rats are highly sensitive to dietary cholesterol manipulations because a deficiency of membrane cholesterol makes their cell membranes weak and fragile. Phytosterols, abundant in CA but not in SO, can inhibit the absorption of cholesterol and also replace a part of cholesterol in cell membranes. This study was performed to determine whether the high concentration of phytosterols in CA might account for its life-shortening effect on SHRSP rats. Male, 35-d-old SHRSP rats (n = 28/group) were fed semipurified diets containing CA, SO, CA fortified with phytosterols (canola oil + phytosterols, CA + P), SO fortified with phytosterols (soybean oil + phytosterols, SO + P), corn oil (CO), olive oil (OO) or a fat blend that mimicked the fat composition of a representative Canadian diet (Canadian fat mimic, CFM; 10 g/100 g diet). These fats provided 97, 36, 207, 201, 114, 27 and 27 mg phytosterols/100 g diet, respectively. Ten rats from each group were killed after 30-32 d for blood and tissue analyses. The remaining rats (18/group) were used for determination of life span. The life span of SHRSP rats fed the high phytosterol oils (CA, CA + P, SO + P and CO) was significantly (P<0.05) shorter than that of CFM- and SO-fed rats. At 30-32 d, the groups fed the high phytosterol oils had greater levels of phytosterols and significantly (P<0.05) higher ratios of phytosterols/cholesterol in plasma, RBC, liver and kidney, and a significantly (P<0.05) lower RBC membrane deformabilty index than the groups fed oils low in phytosterols (SO, OO and CFM). The mean survival times were correlated with RBC deformability index (r(2) = 0.91, P = 0.0033) and cholesterol concentration (r(2) = 0.94, P = 0.0016), and inversely correlated with RBC phytosterol concentration (r(2) = 0.58, P = 0.0798) and phytosterols/cholesterol (r(2) = 0.65, P = 0.0579), except in the OO group. This study suggests that the high concentration of phytosterols in CA and the addition of phytosterols to other fats make the cell membrane more rigid, which might be a factor contributing to the shortened life span of SHRSP rats.

Influence of sources of dietary oils on the life span of stroke-prone spontaneously hypertensive rats

In recent studies, the life span of stroke-prone spontaneously hypertensive (SHRSP) rats was altered by a variety of dietary fats. It was relatively shorter in rats fed canola oil as the sole source of fat. The present study was performed to find out whether the fatty acid profile and the high content of sulfur compounds in canola oil could modulate the life span of SHRSP rats. SHRSP rats (47 d old, n = 23/group) were matched by body weight and systolic blood pressure and fed semipurified diets containing 10% canola oil, high-palmitic canola oil, low-sulfur canola oil, soybean oil, high-oleic safflower oil, a fat blend that mimicked the fatty acid composition of canola oil, or a fat blend high in saturated fatty acids. A 1% sodium chloride solution was used as drinking water to induce hypertension. After consuming the diets for 37 d, five rats from each dietary group were killed for collection of blood and tissue samples for biochemical analysis. The 18 remaining animals from each group were used for determining their life span. The mean survival time of SHRSP rats fed canola oil (87.4+/-4.0 d) was not significantly different (P > 0.05) from those fed low-sulfur canola oil (89.7+/-8.5 d), suggesting that content of sulfur in canola oil has no effect on the life span of SHRSP rats. The SHRSP rats fed the noncanola oil-based diets lived longer (mean survival time difference was 6-13 d, P < 0.05) than those fed canola and low-sulfur canola oils. No marked differences in the survival times were observed among the noncanola oil-based groups. The fatty acid composition of the dietary oils and of red blood cells and liver of SHRSP rats killed after 37 d of treatment showed no relationship with the survival times. These results suggest that the fatty acid profile of vegetable oils plays no important role on the life span of SHRSP rat. However, phytosterols in the dietary oils and in liver and brain were inversely correlated with the mean survival times,indicating that the differential effects of vegetable oils might be ascribed, at least partly, to their different phytosterol contents.

Alterations in sodium metabolism as an etiological model for hypertension

An adequate matching for race, sex, stage of the menstrual cycle, family history of hypertension, and the amount of sodium and other electrolytes in the diet should be a prerequisite for valid conclusions when interpreting the erythrocyte concentration and fluxes of sodium in essential hypertensive patients in comparison with normal subjects. Alterations in intracellular sodium concentration and transmembrane sodium transport systems as causes of essential hypertension are postulated. This review article describes how this abnormal sodium and calcium metabolism translates into increased systemic vascular resistance through altered vasoactive responses and/or vasculature structural changes.

Red cell ion transport abnormalities in experimental hypertension

The original attractive hypothesis on the important role of elevated cell Na+ concentration in the pathogenesis of hypertension stimulated a search for generalized membrane defects and ion transport abnormalities in various easily accessible cells including erythrocytes. An attempt is made here to compare this hypothesis with the data on red cell ion transport alterations that were observed in experimental hypertension over the last 15 years. Several methodological (presence of extracellular Na+ in incubation media, kinetic approach to the evaluation of transport systems) and physiological problems (potassium depletion, age-dependent changes) are discussed in more detail because they can substantially modify the results obtained. Available data suggest a possible contribution of augmented Na+ leak to the development of both genetic and salt-dependent experimental hypertension. The role of alterations in the activity of the Na(+)-K+ pump or the Na(+)-K+ cotransport system still remains unclear.

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.

Erythrocyte sodium transport at twenty-year follow-up of childhood hypertension

Twelve-hour urinary sodium excretion, the number of sodium pump sites (Bmax) and the Na/K flux ratio in erythrocytes were measured in 13 previously diagnosed hypertensive subjects. At the time of the study, six subjects were still hypertensive and showed a significantly lower Na/K flux ratio in erythrocytes than the remaining seven subjects who were normotensive. Bmax was also lower in the hypertensive group compared to the normotensive group, although this was not statistically significant. Urinary Na excretion did not show any significant difference between the two groups. These findings suggest that a cell membrane sodium transport defect may have a role in the development of essential hypertension in adult life.

Augmented Na,K-ATPase gene expression in spontaneously hypertensive rat hearts

Abnormalities in cardiovascular Na,K-ATPase ion-transport function and regulation may play an important role in the pathogenesis of hypertension. However, it is not known whether these abnormalities are secondary to the effects of hypertension, such as increased pressure, or reflect an intrinsic abnormality in Na,K-ATPase gene expression and regulation. A genetic model of hypertension was used to address this issue. Na,K-ATPase alpha subunit gene expression in hearts was compared between spontaneously hypertensive rats (SHR) and normotensive Wistar-Kyoto rats (WKY). Pre-hypertensive, 4-week old SHR hearts exhibited an approximately 4 fold elevation in alpha 1 and 8 fold elevation in alpha 2 mRNA levels compared with age-matched WKY hearts. These SHR mRNA levels remained almost equivalent throughout the development of hypertension at 8 and 16 weeks of age. WKY alpha 1 and alpha 2 mRNA levels exhibited a progressive increase during the same time period. The neonatal alpha 3 mRNA isoform was detected only in pre-hypertensive (4-week) SHR hearts. We conclude that cardiac Na,K-ATPase alpha subunit gene expression is significantly altered in SHR even before the onset of hypertension. These findings suggest that an abnormality in cardiac Na,K-ATPase gene expression constitutes an early, if not primary, event in spontaneous hypertension.

Erythrocyte concentrations and transmembrane fluxes of sodium and potassium in essential hypertension: role of intrinsic and environmental factors

The intraerythrocyte sodium concentration is increased in the erythrocytes of Zaïrean Bantu with untreated hypertension, while the red blood cell potassium is not different from that of normotensive subjects. Compared with whites, normotensive healthy blacks have a higher intracellular concentration of sodium due to a depressed activity of the sodium-potassium pump. Normotensive healthy males with a positive familial background of hypertension display higher erythrocyte sodium and lower cotransport activity. None of the two measurements offer a clear-cut genetic marker of essential hypertension. In healthy women, the erythrocyte sodium concentration is lowered during the luteal as compared with the follicular phase of the menstrual cycle. This variability explains the difference observed between men and women. A low-sodium diet stimulates the activity of the sodium-potassium ATPase pump, which leads to a decrease in the erythrocyte sodium concentration. Both alterations reverse only slowly during sodium repletion. It is therefore suggested that an adequate matching for race, sex, stage of the menstrual cycle (in women), family history of hypertension, and the amount of sodium in the diet should be a prerequisite for valid conclusions when interpreting the erythrocyte concentration and fluxes of sodium.

Kinetics of red cell Na+ and K+ transport in Prague hypertensive rats

Kinetics of ouabain-sensitive, furosemide-sensitive (FS), bumetanide-sensitive (BS) and -resistant Na+ and K+ transport were studied in erythrocytes of Prague hypertensive rats (PHR) and Prague normotensive rats (PNR). Maximal transport rates (Vmax) and apparent affinities for either intracellular Na+ or extracellular K+ (replaced by Rb+) were determined in red cells in which Na+ content varied around the physiological range and that were incubated in Na+ media. No major differences between PHR and PNR were disclosed in the kinetics of ion transport mediated by the Na(+)-K+ pump or BS inward Na(+)-K+ cotransport. FS Rb+ uptake was higher (due to a greater Vmax) in red cells of PHR as compared to PNR. In cells with a lowered Na+ content this elevation of FS Rb+ uptake was largely due to an augmented K(+)-Cl- cotransport which exhibits a low affinity for Rb+o and is blocked by 1 mM furosemide but not by 10 microM bumetanide. Red cells of PHR and PNR strains did not differ in either Na+ or Rb+ leaks. A slight increase of red cell Na+ content in PHR was evaluated in terms of the pump-leak concept. The present study did not reveal any obvious kinetic abnormalities of red cell cation transport the presence of which in tissues involved in blood pressure regulation would favor the development or the maintenance of genetic hypertension in PHR.

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.

Red-cell sodium-lithium countertransport and fractional excretion of lithium in normal and hypertensive humans

To examine the relations between erythrocyte sodium-lithium countertransport and renal proximal tubular sodium handling, we measured countertransport, and then subjected 30 normal and 32 hypertensive subjects, both white and black, to provocative maneuvers of volume expansion and contraction. The fractional excretions of sodium and lithium were measured simultaneously. In agreement with previous studies, we found that countertransport in erythrocytes was elevated in hypertensive patients compared with normal subjects. We also observed that whites have a higher level of countertransport than blacks. In the basal state, we found that fractional sodium excretion of hypertensive patients was no different than in normal subjects, whereas the fractional lithium excretion of hypertensive persons was increased compared with normotensive values. Volume expansion with 2 1 0.9% saline administered intravenously during a 4-hour period provoked an exaggerated natriuresis and a greater increase in fractional lithium clearance in hypertensive patients compared with the control group. With volume expansion and contraction, fractional lithium clearance and countertransport were directly correlated. Our data suggest that hypertensive persons do not have increased proximal tubular sodium reabsorption compared with normal subjects. Further, the exaggerated natriuresis of hypertension is, in part, the result of increased distal solute delivery. The fact that our hypertensive patients were older may partially explain the discrepancies between this report and previous observations.

Endogenous digitalislike circulating substances in spontaneously hypertensive rats

Circulating digitalislike compounds have been proposed to be involved in some Na+-dependent types of experimental hypertension and in human essential hypertension. The level of circulating Na+-K+ pump inhibitor(s) was investigated in the spontaneously hypertensive rat of the Okamoto strain (SHR), its normotensive control, Wistar-Kyoto rat (WKY), and the regular Wistar rat using the following criteria: the ability of whole plasma to inhibit the total active Na+ efflux from Wistar rat erythrocytes and to cross-react with digoxin antibodies and the ability of plasma extracts to inhibit Na+,K+-adenosine triphosphatase (ATPase) activity of membranes from rat kidney. SHR plasma inhibited the net Na+ efflux from Wistar erythrocytes by up to 27% compared with WKY or Wistar plasma. For a given number of cells, the inhibition increased with the amount of available plasma. Cross-reactivity with digoxin antibodies was twice as high in SHR as in WKY or Wistar plasma. It was already enhanced in 3- to 4-week-old rats. Plasma extracts from SHR significantly inhibited Na+,K+-ATPase activity when compared with WKY extracts (75.6 +/- 2.6 vs 89.3 +/- 2.4 mumol Pi/mg/hr; p less than 0.01) but did not differ from Wistar plasma extracts. These results strongly suggest that circulating digitalislike compound(s) are present in elevated amounts in SHR as early as 3 to 4 weeks of age, but their exact participation in blood pressure elevation or maintenance remains to be clarified.

Electron spin resonance studies of erythrocytes from spontaneously hypertensive rats and humans with essential hypertension

The purpose of the present study was to investigate erythrocyte membrane abnormalities in hypertension by means of an electron spin resonance and spin-label technique. The erythrocytes from spontaneously hypertensive rats (SHR) and humans with untreated essential hypertension were examined and compared with their normotensive counterparts, and electron spin resonance spectra were obtained for a fatty spin-label agent (5-nitroxy stearate) incorporated into the erythrocyte membranes. The value of outer hyperfine splitting (2T' parallel) was significantly higher in erythrocytes of SHR and humans with essential hypertension than in erythrocytes of normotensive controls (at 37 degrees C: SHR, 56.14 +/- 0.51 gauss [G], n = 8; Wistar-Kyoto rats, 52.22 +/- 0.86 G, n = 4, p less than 0.01; humans with essential hypertension, 56.94 +/- 0.27 G, n = 11; normotensive subjects, 55.44 +/- 0.36 G, n = 8, p less than 0.01). The order parameter (S) was also increased in the hypertensive rats and humans compared to their respective normotensive controls. When calcium was loaded to erythrocytes with calcium ionophore A23187 (0.9 microM) and CaCl2 (1.0 mM), the parameters of the spectra were increased. These changes were more prominent in the hypertensive groups than in the normotensive controls. These results revealed that the erythrocyte membranes of the hypertensive subjects tolerated different spin motions than those of the normotensive controls in the electron spin resonance study and that membrane fluidity might be decreased in hypertension. Additionally, calcium loading to erythrocytes caused the reduction of membrane fluidity. Therefore, it is suggested that an abnormality of calcium handling at the cellular level might affect physical properties of the biomembranes in hypertension.

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 Na+-K+-ATPase activity and [3H]ouabain binding sites in various tissues of spontaneously hypertensive rats

Na+-K+-ATPase activity and [3H]ouabain binding were studied in cerebral cortex, kidney and heart isolated from spontaneously hypertensive rats (SHR) in both the prehypertensive (6 week old) and the hypertensive stages (14 week old). Na+-K+-ATPase activity of heart and kidney was found to be decreased by about 38 and 16% in the prehypertensive and hypertensive stages of SHR respectively; that of cerebral cortex decreased by 23.5% only in the hypertensive stages. Similar results were obtained by pretreatment of membranes with either 0.001% Triton X-100 or by increasing the K concentration from 4.7 to 12.7 mM in the Krebs solution. No significant differences in microsomal protein yield were noted between prehypertensive or hypertensive SHR and the age-matched WKY rats. The study of binding of [3H]ouabain to cerebral cortex, kidney and heart showed that the decreased Na+-K+-ATPase in hypertensive SHR was due to a 31.6, 21.8 and 41.3% reduction in the number of high affinity binding sites respectively, while the affinity constants (Kd) of ouabain binding sites on this enzyme in cerebral cortex, kidney and heart of the normotensive WKY rats were 26.5, 455.9 and 74.7 nM respectively and those from the hypertensive SHR were not altered. The plasma K concentration of the SHR in the prehypertensive and hypertensive stages was 4.07 and 4.13 mM, respectively, significantly less than that of the age-matched WKY rats. It appears that the decrease of plasma K and Na+-K+-ATPase activity in heart and kidney in SHR is derived from a genetic defect and may be related to the abnormal Na handling in this genetically hypertensive strain.

Is red cell sodium transport a function of pressure?

Sodium efflux from normal red cells was measured as a function of pressure to test whether abnormal sodium transport in hypertension is a direct consequence of the increased arterial pressure. Red cells were loaded with 22Na and sodium efflux was measured at 37 degrees C while the samples were in a bomb at constant pressures of 200 mmHg or 517 mmHg. Control samples were incubated concurrently at atmospheric pressure and the same temperature. The effect of preincubation of blood at 200 mmHg for 3.5 h on sodium efflux was also measured. 22Na efflux and first order efflux rate constants were similar in high and normal pressure samples in each case. These findings suggest that acute changes in pressure have no effect on erythrocyte sodium efflux, which in turn implies that abnormal membrane transport in hypertension is not a consequence of the raised arterial pressure.

Rat thymocyte sodium transport. Effects of changes in sodium balance and experimental hypertension

The wide range of membrane electrolyte transport abnormalities associated with experimental, genetic, and essential hypertension may either reflect an underlying global change in the cell membrane or may be directly related to the underlying disturbance that causes hypertension or to changes in sodium balance. To investigate this further, we studied sodium transport and intracellular electrolyte composition in the thymocytes of normal rats undergoing salt loading or depletion, and in rats with renovascular, mineralocorticoid, or spontaneous hypertension compared to appropriate age-matched normotensive control rats. In normotensive rats, although there was no significant difference between the blood pressures at the two extremes of sodium balance, sodium loading caused a nonsignificant rise in sodium transport, whereas sodium depletion was associated with a significant fall in sodium transport and intracellular sodium. When cells from salt-loaded or normal animals were incubated in a medium containing their own serum, sodium transport was slightly stimulated in both, but there was no significant difference in the sodium efflux-rate constant of thymocytes obtained from rats on the normal as opposed to the high salt intake. Compared to normotensive rats, there was no significant change in the sodium efflux-rate constant in any of the hypertensive rat models studied. However, the sodium efflux-rate constant fell with age in both the spontaneously hypertensive and Wistar-Kyoto normotensive rats. The present studies show that dietary sodium intake and aging had considerable effects on rat thymocyte sodium transport, but neither of these changes was related to a change in blood pressure.(ABSTRACT TRUNCATED AT 250 WORDS)

Centrally-induced vasopressor responses to ouabain are augmented in spontaneously hypertensive rats

Dihydroxyouabain (ouabain), 1.0-10 micrograms, per rat, injected intracerebroventricularly, produced dose-related vasopressor responses accompanied by corresponding increases in abdominal sympathetic nerve activity in 16 weeks old Wistar (NT) rats anesthetized with urethane. The heart rate then also increased, dose-dependently, to ouabain injected in doses up to 10 micrograms. However, 100 micrograms produced arrhythmia resulting in bradycardia. Pressor effects were appreciable within one minute after the ouabain injection, but did not become maximal until between 7-10 min later. Either the removal of sympathetic vasomotor tone by surgical section of the spinal cord or intravenous pretreatment with a vasopressin antagonist significantly reduced the vasopressor responses in the NT rat. Ouabain, 10 micrograms, injected intraventricularly in 16 weeks old Kyoto Wistar rats produced similar cardiovascular responses to those in the NT rat, but the magnitude of the blood pressure responses, along with the heart rate and sympathetic responses, was larger in SHR than in WKY. These results suggest that dihydroxyouabain acts centrally to elevate the blood pressure by increasing not only the sympathetic discharge but also, perhaps, the secretion of vasopressin. In light of previous studies showing that SHRs exhibit both sympathetic hyperactivity and hypersecretion of vasopressin, the present results suggest that their enhanced responsiveness to ouabain could result from both the sympathetic hyperactivity and an enhanced vasopressin release as a result of the centrally injected ouabain.

Increased osmotic fragility of erythrocytes in essential hypertension

The correlation between hypertension and the osmotic fragility of erythrocytes was examined. High osmotic fragility of erythrocytes was observed in patients with essential hypertension and normotensive subjects with family history of hypertension, compared with normotensive controls without family history of hypertension. In patients with secondary hypertension, the osmotic fragility of erythrocytes was not significantly different from that of normotensive controls without family history of hypertension. The membrane fragility had no correlation with the level of blood pressure or dietary salt intake. Thus, the osmotic fragility of erythrocytes might reflect functional or structural abnormalities of cell membranes, and could be one of the genetic markers of the hypertensive predisposition.

Intrinsic difference in erythrocyte membrane in spontaneously hypertensive rats characterized by Na+ and K+ fluxes

The goal of this study was to determine whether the elevated flux of sodium and potassium through the erythrocyte membrane of spontaneously hypertensive rats (SHR) is due to an intrinsic difference in the cell membrane or to a humoral factor present in the plasma. Isolated and washed erythrocytes from SHR and normotensive Wistar Kyoto (WKy) and Sprague-Dawley (SD) rats, were incubated in 1) a physiological salt solution, 2) WKy or SD plasma and 3) SHR plasma. Incubations were performed at 4 degrees C for 23 h. Erythrocytes from SHR incubated in physiological salt solution had significantly greater Na+ and K+ fluxes than those from normotensive WKy and SD rats (P less than 0.005). Plasma from any of the three strains of rats, as compared to physiological salt solution, increased Na+ influx in the following order: SD greater than WKy greater than SHR. Erythrocyte K+ efflux was not altered by plasma. We conclude that the elevated flux of Na+ and K+ in SHR erythrocytes is due to an intrinsic difference in the cell membrane. The greater Na+ influx in plasma from any strain of rats is not correlated with the blood pressure of the rat. The lesser increase in Na+ influx in erythrocytes incubated in plasma from SHR masks the greater intrinsic membrane permeability in the SHR erythrocyte when Na+ fluxes are studied in whole blood. The elevated flux of Na+ and K+ through the erythrocyte membrane of SHR may reflect a general membrane defect that underlies the pathogenesis of elevated arterial pressure.

Abnormal Na+,K+ cotransport function in a group of patients with essential hypertension

In 50 normotensive controls, the increase in erythrocyte Na+ concentration up to 12.4 +/- 2.0 mmol/l cells (mean +/- SD) ensures half-maximal stimulation of outward Na+,K+ cotransport fluxes. Forty-six out of sixty-five patients with essential hypertension required more than 16 mmol/l cells of internal Na+ concentration to obtain a similar effect, strongly suggesting an abnormal cotransport function. Seven out of fourteen hypertensive patients with normal Na,K cotransport function showed Na+,Li+ countertransport fluxes higher than the normal upper limit of 220 mumol (1 cells h)-1. Conversely, countertransport fluxes were normal in fourteen hypertensives with abnormal cotransport function. The above results indicate that the total population of patients with essential hypertension is heterogeneous and includes one subgroup of subjects with abnormal Na+,K+ cotransport function, and another with increased Na+,Li+ countertransport fluxes.

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.

Dissociation of genetic hyperactivity and hypertension in SHR

The Wistar Kyoto strain of spontaneously hypertensive rat (SHR) has been characterized as behaviorally hyperactive as well as hypertensive. The relationship between these two inbred traits remains uncertain, and their coexistence in the SHR has complicated studies of central nervous system mechanisms underlying the hypertensive process. A breeding program was initiated to examine the possible genetic linkage of these two traits which, if separable, would allow us to develop substrains of SHR that are hypertensive without being hyperactive, or hyperactive without being hypertensive. We crossed SHR males with Wistar Kyoto, normotensive (WKY) female rats and produced F1 hybrids which were then randomly inbred to produce an F2 population. When tested at 12 weeks of age, F2 rats exhibited the expected wide range of mean systolic blood pressures (BP), from 111 to 174 mm Hg, as determined using indirect tail plethysmography. The BP in the parental rats at the time of breeding (16 weeks) was 187 +/- 4.5 mm Hg (SHR males, n = 7) and 111 +/- 2.4 (WKY females, n = 7). Locomotor activity was determined in an automated activity cage in F1 and F2 rats at 12 weeks of age. These strains exhibited a wide range of phenotypic distribution of locomotor activity scores, and the mean scores were intermediary between those of SHR rats and WKY rats of the same age. Among individual rats of both the F1 and F2 hybrid strains, there was no correlation between the activity score and the level of the BP at 12 weeks of age. These findings indicated that the genes responsible for the hypertensive trait and those responsible for the hyperactivity trait were not tightly linked in the hybrid populations, suggesting that different genetic factors were involved in the transmission of each of these traits. Accordingly, it should be possible to separate the two traits by further selective, recombinant inbreeding procedures.

Achievements in hypertension: a 25 year overview

Only 25 years ago, the field of hypertension was challenged by retrospective clinical data and epidemiologic information suggesting that an elevated arterial pressure is a major risk factor for enhanced cardiovascular morbidity and mortality. Not only was antihypertensive therapy looked on by many as dangerous and fraught with severe and undesirable side effects, but its validity in reversing the course of disease was not yet demonstrated. This review discusses the dramatic new information amassed over the past 25 years that points to the new physiologic and clinical concepts concerning hypertension. It considers impressive new diagnostic techniques and methods designed to identify secondary forms of hypertension and target organ involvement. In summary, it outlines the feasibility of reversing overall (and cardiovascular) morbidity and mortality with an array of antihypertensive agents that provide the therapeutic ability to suppress most pathophysiologic pressor mechanisms of hypertensive disease. The lesson is clear: hypertension provides the greatest available challenge to the new era of preventive cardiology in the 21st century.

Erythrocyte membrane abnormalities in hypertension: a comparison between two animal models

Calcium-membrane interactions have been studied in two animal models of hypertension using the erythrocyte membrane as a model system. The Okamoto-Aoki strain of spontaneously hypertensive rats (SHR) was the first examined, and the activities of the Ca++/Mg++-ATPases in the membrane of SHR erythrocytes were found to be consistently higher than those of the normotensive controls (WKY), while other membrane enzymes such as Na+/K+-ATPase and acetylcholinesterase were not detectably altered. Erythrocyte membranes of the SHR also have a higher passive permeability to calcium as well as other functional and compositional differences when compared to those of the WKY. These findings suggest that the membrane alterations in the SHR may be related to an increased passive permeability to calcium, and a possibly compensatory increase in Ca++/Mg++-ATPase activity in these animals. The second animal model examined was the deoxycorticosterone/salt-induced hypertension (DOCA) in uninephrectomized rats. In DOCA rats with comparable degree of blood pressure elevation, none of the erythrocyte membrane abnormalities observed in the SHR were present, suggesting that the latter alterations are probably genetically determined and not a consequence of elevated arterial pressure.

Plasma protein changes in primary hypertension in humans and rats

To determine whether plasma protein changes may be associated with primary hypertension, we analyzed plasma proteins from essential hypertensive (EHT) patients and genetically hypertensive rats using two-dimensional electrophoresis. An additional plasma protein, having a molecular weight of 13,000 daltons and an isoelectric point of 4.5, was found in 82% of the patients with borderline or moderate hypertension (n = 29) and in all permanently hypertensive patients (n = 12). This protein was detected in 36% of normotensive (NT) subjects (n = 50). In the latter, the influence of family history, sex, and secondary hypertension were studied. Plasma proteins were also studied in spontaneously hypertensive rats (SHR). In all plasma from young male (n = 10) and female (n = 6) SHR, two additional proteins (molecular weight = 16,000 daltons, pHi = 4.7 and 5.1) were detectable. These plasma proteins were not detectable in male Wistar Kyoto rats (WKY) and in 50% of female WKY, and their frequency was 10% (n = 10) and 0% (n = 3) in normal male WKY and in male WKY rendered hypertensive by methylprednisolone, respectively. We conclude that these alterations of plasma proteins may be considered a biochemical feature of primary hypertension.

Reduced sodium-potassium dependent ATPase and its possible role in the development of hypertension in spontaneously hypertensive rats

Ouabain-sensitive Na+-K+-ATPase activity in red cell membranes, kidney cortical tissue, myocardium and adrenal glomerulosa tissue was examined in SHR and WKY rats at 6, 9, and 12 weeks of age. Red cell membrane enzyme activity was decreased (p less than 0.001) at 9 and 12 weeks of age in SHR. This activity was negatively correlated (r = -0.69, p less than .005) with blood pressure at 9 and 12 weeks. Kidney cortical enzyme activity was also decreased (p less than 0.001) in the SHR at 9 and 12 weeks of age. This decreased kidney enzyme activity was also inversely related to 9 and 12 week blood pressures (r = -0.71, p less than 0.001), urinary Na excretion (r = -0.62, p less than .005), and urinary Ca and K excretion. Myocardial enzyme activity was not decreased until 12 weeks in the SHR, and adrenal glomerulosa activity was not different in the SHR and WKY at any of the three ages that this enzyme was measured. Of the tissues examined decreased Na+-K+-ATPase activity in the erythrocyte membrane and in kidney cortical tissue appears to coincide best with the development of hypertension in the SHR. This study lends further support to the concept that alterations in membrane cation transport may be an important factor in the development of high blood pressure in SHR.

Ion transport in hypertension

A wide range of abnormalities of membrane sodium and potassium transport can be demonstrated in patients with essential hypertension, and in rats with genetic hypertension and with some forms of experimental hypertension. In the human red cell increased permeability to sodium and potassium, increased ouabain-sensitive sodium pumping, lithium-sodium counter-transport, and frusemide-sensitive co-transport have been described; by contrast, in the human leucocyte sodium pumping is reduced. In the spontaneously hypertensive rat and the rat with mineralocorticoid-induced hypertension, increased permeability to sodium and potassium, with increased ouabain-sensitive pumping, is shared by the red cell and the arterial smooth muscle. This abnormality is associated with decreased cell-membrane affinity for calcium and increased cell-membrane viscosity. It is proposed that in essential hypertension the decreased membrane affinity for calcium is a primary pathogenetic change giving rise to secondary changes in sodium and potassium transport.

Diffuse structural alterations in cell membranes of spontaneously hypertensive rats

Plasma membranes from heart, nerve endings, and liver were compared in 3-week-old male spontaneously hypertensive rats from the Okamoto substrain (SHR) and normotensive Wistar/Kyoto control rats (WKY) [systolic blood pressure 105 +/- 4 and 95 +/- 4 mm Hg, respectively (1 mm Hg = 133 Pa)] according to two criteria: calcium binding at physiological intracellular concentrations and polarization of an embedded fluorescent probe, 1,6-diphenyl-1,3,5-hexatriene. Whatever the tissue of origin, the density of high-affinity calcium binding sites was lower in SHR than in WKY plasma membranes, and the polarization of diphenylhexatriene fluorescence was constantly higher in SHR than in WKY membranes. These membrane abnormalities are similar to those previously described in the erythrocyte membrane from SHR. The presence of diffuse structural alterations in cellular membrane from young spontaneously hypertensive rats when blood pressure is still in the normotensive range suggests a genetic origin. Such inherited abnormalities may by themselves participate in the rise in blood pressure.

Analysis of calcium handling in erythrocyte membranes of genetically hypertensive rats

Calcium handling by erythrocyte membranes was compared in genetically hypertensive (SHR) and normotensive (WKR) rats by direct measurement of calcium binding, passive influx, and adenosine triphosphate (ATP)-dependent extrusion. The SHR erythrocyte membranes exhibited the following abnormalities: 1) the binding capacity of the high affinity Ca2+-binding sites located on the inner side of the membrane was 0.84 +/- 0.07 nmole/mg protein compared with 1.17 +/- 0.08 nmole/mg protein in WKR, 2) ATP-dependent Ca2+ extrusion, measured as the Ca2+ influx into inside-out vesicles, was also lower than the WKR, as was the La3+ -sensitive, Ca2+ -dependent hydrolysis, indicating reduced activity of the calcium pump; 3) the passive calcium influx into ATP-depleted red blood cells was slightly accelerated. these abnormalities in Ca2+ binding and transport probably enhanced intracellular Ca2+ concentration, and were observed under both prehypertensive an hypertensive conditions, in 3-week-old and adult SHR respectively. Similar membrane defects in excitable cells may help to explain the pathogenesis of hypertension, since they may increase vascular tone and/or catecholamine release.

Inheritance of abnormal erythrocyte cation transport in essential hypertension

Net fluxes of sodium and potassium ions were determined in sodium-loaded, potassium-depleted erythrocytes from 370 white subjects, 194 of whom had essential hypertension or had been born to parents with essential hypertension. Findings were compared with those in 86 controls who were normotensive and did not have a family history of hypertension. Compared with controls all patients with essential hypertension had a low sodium to potassium ratio secondary to a deficit in the sodium-potassium cotransport system. A similar abnormality was found in subjects born to parents with essential hypertension, the prevalences of a deficient cotransport system in such subjects being 53.6% (52 out of 97) among those with one hypertensive parent and 73.7% (14 out of 19) among those with two hypertensive parents. Both sexes were equally affected. Studies in 14 families over two or three generations showed the erythrocyte cation abnormality in one or more members of each consecutive generation. No close association was evident between the deficient erythrocyte sodium-potassium cotransport system and either blood groups ABO, Rh, Kidd, Duffy, P, and MNS or the major histocompatibility HLA antigens. Out of 90 consecutive unrelated and normotensive white blood donors, 36 showed a low erythrocyte sodium-potassium net flux ratio. It is concluded that in white people abnormal erythrocyte cation transport is a biochemical disorder characteristic of essential hypertension and transmitted by a dominant and autosomal mode expressing a single abnormal gene.

Furosemide and bumetanide-sensitive Na+ fluxes in erythrocytes from genetically hypertensive rats (SHR)

Na+ and K+ fluxes from erythrocytes of SHR and WKY control rats have been studied. Fluxes were measured in both fresh and sodium-loaded (after exposure to ouabain or PCMBS) erythrocytes. Pump activity appears higher and furosemide or bumetanide sensitive Na+ efflux lower in SHR compared to WKY suggesting a similar abnormality in SHR genetic hypertension as in human hypertension.

Genetic markers in spontaneously hypertensive rats

Establishment of various models for hypertensive diseases such as spontaneously hypertensive rats (SHR) and stroke-prone SHR (SHRSP) clarified the importance of genetic factors in the pathogenesis of these diseases and further accelerated studies on their genetic mechanisms. Although various biochemical abnormalities have been detected and can be used as biochemical markers in these models, they have not been always closely related to blood pressure in F2 generation obtained by cross breeding between SHR and normotensive Wistar-Kyoto rats. Recent studies indicate that biomembrane abnormalities detected in erythrocytes and other membranes of SHR and SHRSP may not only be biochemical markers but also related to the pathogenesis of hypertensive diseases.

A genetic approach to the geography of hypertension : examination of Na+ - K+ cotransport in Ivory Coast Africans

Outward Na+ - K+ cotransport in erythrocytes from essential hypertensive Caucasian subjects was found to be excessively low (Co -) compared to normotensives (Co +) carefully selected for their negative family history of hypertension. Since the frequency of essential hypertension varies widely among different populations and is particularly high in certain coloured peoples, we compared erythrocyte Na+ - K+ cotransport in normotensive and hypertensive subjects in Paris (France) and in Abidjan (Ivory Coast) to seen whether defective cotransport was related to high blood pressure in the African group as well. Of the 66 French unselected normotensives investigated, 26 (39%) were Co - whereas 14 of the 18 Ivory Coast unselected normotensives (79%) were Co -. 64 (80%) of the 80 essential hypertensives examined in France were Co -, but the proportion of Co - subjects among the Ivory Coast hypertensives was even higher. In addition, both hypertensives and normotensives in the African groups often had undetectable outward Na+ effluxes, a rare finding in the French subjects. We suggest that the high incidence of abnormal Na+ - K+ cotransport in the Ivory Coast series may reflect a genetic propensity to hypertension in this population, and that consequently, Na+ - K+ erythrocyte cotransport measurements might prove useful in defining geographic variations in hypertension.

Abnormal erythrocyte Na+ K+ cotransport system, a proposed genetic marker of essential hypertension

In erythrocytes, the extrusion of a cell sodium load is accomplished by the ouabain-sensitive sodium-potassium pump and by the furosemide-sensitive sodium-potassium cotransport, which operate against the passive sodium permeability. The precise characterization of these transport pathways requires the determination of the turnover rates of cation translocation and the affinities for substrates and effectors. The preliminary results of such kinetic study in essential hypertension is reported here. An abnormally low rate of net sodium extrusion by the sodium-potassium co-transport system was observed in essential hypertensive patients and in a high proportion of their young normotensive offspring. A normal cotransport system found in secondary hypertensive subjects devoid of familial history of hypertension confirmed that the abnormal cotransport system is not the consequence of high blood pressure per se. At the molecular level, the cotransport abnormality seems to be consecutive to a diminished apparent affinity for intracellular Na+. A 20-40% increase in the rate of net sodium extrusion by the sodium-potassium pump seems to compensate for the abnormal cotransport in erythrocytes from some young normotensive subjects born of essential hypertensive parents and from some benign essential hypertensive subjects. No difference could be detected between the passive sodium permeability of erythrocytes from hypertensive subjects and normotensive controls. In conclusion, essential hypertension seems to be associate with an inherited defect in the apparent affinity for intracellular Na+ of the sodium-potassium cotransport system. We propose therefore the laboratory study of this system for (i) the distinction between essential and secondary hypertension and (ii) the preventive investigation of young normotensive subjects in hypertensive families.

Cation fluxes and (Na+ + K+)-activated ATPase activity in erythrocytes of patients with essential hypertension

Various claims, partially conflicting, have been made in recent years for abnormalities in cation pumps and fluxes in erythrocytes of patients with essential hypertension. In view of the obvious significance of such abnormalities for diagnostic purposes, and possibly for our understanding of the pathophysiology of essential hypertension, we have investigated these claims. We have determined the following parameters of erythrocytes from essential hypertensives and normotensives: 1. (Na+ + K+)-ATPase activity and the Km values for Na+, K+ and ATP; 2. ouabain-sensitive fluxes of Na+ and K+ in Na+-enriched cells after cold treatment and after treatment with p-chloromercuribenzenesulphonate; 3. furosemide-sensitive, ouabain-insensitive cotransport efflux of Na+ + K+. No significant differences were observed, except for a slight decrease in the ouabain-sensitive K+ influx after cold treatment in hypertensives. Hence, we conclude that determination of these parameters in erythrocytes does not seem to be useful for the diagnosis of essential hypertension.