Increased erythrocyte permeability to Li and Na in the spontaneously hypertensive rat

Molecular origin of Na(+)/Li(+) exchanger: Evidence against the involvement of major cloned erythrocyte transporters

Numerous studies have demonstrated heightened Na(+)/Li(+) countertransport (NLCT) activity in erythrocytes of patients with essential hypertension or diabetic nephropathy. The same carrier also contributes to the therapeutic action of lithium salt, widely used in the treatment of psychiatric disorders. However, the molecular origin of NLCT remains unknown. This study examined the role of major ion transporters in NLCT by comparative analysis of its activity and that of ion transporters providing inwardly directed (86)Rb, (22)Na and (32)P fluxes. NLCT was below the detection limit in rat erythrocytes and ∼50-fold higher in rabbits compared to humans. Unlike NLCT, the activities of Na(+),K(+)-ATPase, Na(+),K(+),2Cl(-) cotransporter and anion exchanger were somewhat similar in the erythrocytes of these species, whereas Na(+),P(i) cotransport was in 1:2:6 proportion in rats, humans and rabbits, respectively. Loading of erythrocytes with Li(+) for NLCT measurement did not affect the activity of Na(+),P(i) cotransporter. Keeping in mind that NLCT is much higher in rabbits vs humans and rats, we compared the set of membrane proteins in these species using 2-dimensional gel electrophoresis. This approach revealed 174 common spots, whereas 132 proteins were detected only in human and rabbit erythrocyte membranes. Among these proteins, we found 17 spots whose expression was higher by more than 5-fold in rabbit compared to human erythrocytes. Thus, our results argue against the involvement of major ion transporters in NLCT. They also show that comparative proteomics is a potent tool to identify the molecular origin of this carrier.

Decreased NKCC1 activity in erythrocytes from African Americans with hypertension and dyslipidemia

BACKGROUND

Recent studies demonstrated a key role of ubiquitous isoform of Na+,K+,2Cl- co-transport (NKCC1) in regulation of myogenic tone and peripheral resistance. We examined the impact of race, gender, and plasma lipid on NKCC1 activity in French Canadians and African Americans with hypertension and dyslipidemia.

METHODS

NKCC and passive erythrocyte membrane permeability to K+, measured as ouabain-resistant, bumetanide-sensitive, and (ouabain+bumetanide)-resistant 86Rb influx, respectively, were compared in 111 French-Canadian men, 107 French-Canadian women, 26 African-American men, and 45 African-American women with essential hypertension and dyslipidemia.

RESULTS

The African-American men and women were 7 years younger and presented twofold decreased plasma triglycerides compared to their French-Canadian counterparts (P < 0.01) whereas body mass index (BMI), total cholesterol, low-density lipoprotein, and high-density lipoprotein (HDL) were not different. NKCC was respectively 50 and 38% lower in the African-American men and women than in the French Canadians (P < 0.005) without any differences in passive erythrocyte membrane permeability for K+. We did not observe any impact of age on NKCC in all groups under investigation, whereas plasma triglycerides correlated positively with the activity of this carrier in the French-Canadian men only.

CONCLUSIONS

NKCC1 activity is lower in erythrocytes of African Americans with essential hypertension and dyslipidemia than in Caucasian counterparts. We suggest that decreased NKCC1 may contribute to the feature of the pathogenesis of salt-sensitive hypertension seen in African Americans.

Role of dietary salt in hypertension

Certain things have not changed since my colleague and I last reviewed the role of dietary salt in hypertension [Haddy, F.J., Pamnani, M.B., 1995. Role of dietary salt in hypertension. Journal of the American College of Nutrition 14, 428-438]. Over half of hypertensives are still salt sensitive, i.e., they respond to a high NaCl intake with a rise in blood pressure. This can be ameliorated by restricting NaCl intake, supplementing potassium intake, and consuming diuretics. Some things have changed. We now have more insight into mechanism; we suspected that volume expansion and endogenous Na(+),K(+)-ATPase inhibitors were the connection between excessive salt intake and the hypertension, but we were not certain as to the nature of the inhibitors. Now it appears that the inhibitors are steroids released from the adrenal gland and are members of the cardenolide family, e.g., ouabain, and the bufadienolide family, e.g., marinobufagenin. This presents new possibilities in therapy, including antibodies to these agents and competitive inhibitors to their binding to Na(+),K(+)-ATPase.

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.

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.

Kinetics of Na+ and K+ transport in red blood cells of Dahl rats. Effects of age and salt

Blood pressure response to chronic high salt intake and kinetics of red blood cell Na+ and K+ (Rb+) transport were studied in salt-sensitive (DS) and salt-resistant (DR) Dahl rats fed a high salt diet (8% NaCl) for 7 weeks from the fifth (young), 12th (adult), or 23rd (old) week of age. The kinetics of ouabain-sensitive Rb+ uptake and Na+ extrusion were determined in Na+ media as a function of both intracellular Na+ (Na+i, 2-8 mmol/l cells) and extracellular Rb+ (Rb+o). In addition, the kinetics of furosemide-sensitive Rb+ uptake (related to Rb+o) and the magnitude of the Na+ and Rb+ leaks were assessed. High salt induced hypertension in young and adult but not in old DS rats although red blood cell Na+ was slightly increased in all age groups of DS rats fed a high salt diet. The kinetic parameters of the Na(+)-K+ pump were similar in DS and DR rats fed a low salt diet. Ouabain-sensitive transport rates were not suppressed in erythrocytes of salt hypertensive Dahl rats. Maximal velocities of the Na(+)-K+ pump (related to Na+i) decreased significantly with age in all groups except in DS rats fed a high salt diet. This was compensated by an age-dependent increase in the affinity for Na+i so that no substantial differences in transport rates between young and old rats were seen at physiological cell Na+ and plasma K+ levels.(ABSTRACT TRUNCATED AT 250 WORDS)

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)

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.

Effect of 6-iodoamiloride in various models of experimental hypertension

6-Iodoamiloride, an analogue of the sodium channel blocker amiloride, is a vasodilator-depressor, diuretic-natriuretic, and antikaliuretic agent. In these experiments we intravenously infused 6-iodamiloride (0.38 mg/100 g body weight) over a 10- to 11-minute period into rats with reduced renal mass-saline hypertension or one-kidney, one clip hypertension. The infusion produced a prompt but transient fall in blood pressure. These findings are in contrast to those in spontaneously hypertensive rats (SHR), in which the same infusion of 6-iodoamiloride produced a prompt, pronounced, and sustained fall in blood pressure. Studies from a number of laboratories suggest that vascular smooth muscle cells from the SHR have increased permeability to sodium whereas vascular smooth muscle cells from the other two models do not. Thus, 6-iodoamiloride may have potential both as a diagnostic probe and a therapeutic agent for hypertension characterized by increased vascular smooth muscle cell permeability to sodium.

Pharmacologic agents for the in vivo detection of vascular sodium transport defects in hypertension

Anatagonists to angiotensin, catecholamines, aldosterone, and vasopressin have long been used to help determine agonist roles in hypertension. We here call attention to a possible extension of this approach to detect, evaluate, and treat vascular sodium transport defects in hypertension. Two basic types of transport defects have been identified in the blood vessels of hypertensive animals, increased sodium permeability and decreased sodium pump activity. Intravenous injection of 6-iodo-amiloride, a sodium channel blocker and vasodilator, produces an immediate and sustained decrease in blood pressure in two genetic models of hypertension characterized by increased permeability of the vascular smooth muscle cell membrane to sodium (Okamoto spontaneously hypertensive rat, Dahl salt sensitive rat), whereas it produces only a transient fall in arterial pressure in two renal models of hypertension having normal sodium permeability in vascular smooth muscle cells (reduced renal mass-saline rat, one-kidney, one clip rat). Canrenone, a metabolic product of spironolactone which can compete with oubain for binding to Na+,K+-ATPase at the digitalis receptor site, decreases blood pressure in a low renin, volume expanded model of hypertension which has been shown to have depressed sodium pump activity in arteries and increased sodium pump inhibitor in plasma (reduced renal mass-saline rat) but has no effect on blood pressure in a genetic model of hypertension which has been shown to have increased sodium pump activity secondary to increased sodium permeability (spontaneously hypertensive rat). Thus, a sodium channel blocker and a competitor to ouabain binding can detect and determine the functional significance of sodium transport defects in the blood vessels of intact hypertensive animals. Studies in red and white blood cells suggest that similar defects may exist in the blood vessels of hypertensive humans. Thus, this approach, probing for vascular transport defects in the intact animal, may ultimately also be useful in the clinical setting.

Phospholipase C activation and diacylglycerol kinase inactivation lead to an increase in diacylglycerol content in spontaneously hypertensive rat

Activities of three kinases, phosphatidylinositol (PI), phosphatidylinositol 4-phosphate (PIP), and diacylglycerol (DG) kinases, and phospholipase C were measured in erythrocyte ghosts from spontaneously hypertensive rats (SHR) and normotensive Wistar Kyoto rats (WKY). PI kinase activity was significantly higher in SHR than WKY but there was no significant difference in PIP kinase activity between SHR and WKY. The activity of phospholipase C, which hydrolyzes PIP2, was also increased in SHR. However, DG kinase activity was, on the contrary, decreased in SHR. These results suggest that there is a tendency to accumulate DG in SHR. Indeed, DG content in erythrocytes of SHR increased 1.7-fold compared to that of WKY. Such DG accumulation may cause the sustained activation of protein kinase C in SHR, since DG is a physiological activator for protein kinase C.

Erythrocyte membrane sialic acids in primary and secondary hypertension in man and rat

Sialic acids of erythrocyte membranes (erythrocyte 'ghosts') and blood plasma were studied in patients with essential or chronic renal hypertension, and in spontaneously hypertensive rats (SHR). The total content of sialic acids in erythrocyte membranes (determined by thiobarbituric-acid assay) was around 100 mumol/g protein in both the hypertensive patients and controls, there being no difference between the three groups. Similarly total sialic-acid content of plasma did not differ between the hypertensive patients and controls, being around 2 mmol/l. Although total membrane sialic acid was unchanged, the sialic-acid content of glycolipids extracted from erythrocyte membranes was 17% greater (P less than 0.001) in patients with essential hypertension than in renal hypertensive patients and controls (22.1 +/- 0.5 mumol/g protein v. 18.2 +/- 0.8 and 18.9 +/- 0.8, respectively). Sialic acid in plasma glycolipid did not differ between the patient groups. The animal study revealed no differences between total--or glycolipid--sialic-acid content in plasma and erythrocyte membrane in SHR and normotensive rats. The finding of an increase in the sialic-acid content of erythrocyte membrane glycolipid in essential hypertension is in agreement with recent studies demonstrating structural abnormalities in hydrophobic regions of erythrocyte membrane, and is considered a manifestation of membrane glycolipid alteration in primary hypertension.

The erythrocyte membrane in essential hypertension. Characterization of the temperature dependence of lithium efflux

Erythrocytes of most patients with essential hypertension are distinguished by a typical pattern of temperature-dependence of Li efflux. In the present study we have attempted to characterize this unique temperature response. Measurements of Li efflux into Na medium and Lii-Nao countertransport were conducted simultaneously at finely spaced temperature intervals with increments of 1 to 2 degrees C in the range of 10-40 degrees C. The Arrhenius plots for the efflux in Na medium and for Lii-Nao countertransport in erythrocytes of both normotensives and hypertensives were biphasic with slopes representing apparent energies of activation of about 28 and 8 kcal/mol below and above the 'break', respectively. However, the 'break' in the Arrhenius plot appeared at distinctly different temperatures: 30 degrees C for normotensives and 20 degrees C for hypertensives. The Li efflux was resolved into N-ethylmaleimide-sensitive and -insensitive components. The sensitive component exhibited a typical biphasic temperature response, with the characteristic 'break': at 30 degrees C for normotensives and at 20 degrees C for hypertensives. In contrast, the N-ethylmaleimide-insensitive component was alike in normotensives and hypertensives. It is concluded that: (a) the unique temperature dependence of Li efflux in erythrocytes of hypertensives results from a localized modification in the membrane; (b) the N-ethylmaleimide-sensitive component represents a protein moiety which distinguishes between the erythrocyte membrane of normotensives and hypertensives; (c) the expression of the temperature dependence as judged by the sharp transition in slope (within 1 to 2 degrees C), apparently reflects the cooperative involvement of membrane lipids, associated with the Li efflux system.

Cell membrane changes after in vivo acute Na+ load in normotensive and spontaneously hypertensive rats

Our previous observation of a greater increase in erythrocyte Na+ in SHR than in WKY after an acute Na+ load may result either from a genetic membrane property or from a specific plasma influence. In order to elucidate this question, membrane characteristics were compared with or without an acute Na+ load. Na+ transport was measured in Ringer and in plasma on Na+ enriched and K+ depleted red cells. Platelet microviscosity was measured as an index of membrane structural changes. After acute Na+ load a similar reduction of net Na+ extrusion and of K+ influx was observed in both strains. This indicates an inhibition of the Na+,K+-pump. Platelet microviscosity was similarly increased in SHR and WKY. Thus an acute Na+ load induced alterations of membrane properties in both SHR and WKY. The higher erythrocyte Na+ content in SHR stems rather from their intrinsic membrane properties than from a plasma factor.

Essential hypertension: improved differentiation by the temperature dependence of Li efflux in erythrocytes

Kinetic and thermodynamic properties of red cell lithium (Li) efflux were examined in patients with essential hypertension; the maximal rate of Li efflux as affected by temperature was measured at the range of 12 degrees to 42 degrees C. Fifty-two patients with essential hypertension and 22 normotensives were studied. The mean Li efflux, both into sodium (Na) medium and Li-Na countertransport, was higher in hypertensive than in normotensive persons, but the distinction between the two groups was limited by extended scatter and overlap. The distinction could be markedly improved by determining the effect of temperature on Li efflux. While all the normotensives exhibited Arrhenius plots of Li efflux with a change in slope ("break") around 30 degrees C, the corresponding "break" for most (75%) of the hypertensives was about 20 degrees C. Consideration of both the rate and the temperature dependence of Li efflux further improved the differentiation of hypertensive patients. Analysis of normotensive offspring of hypertensives and of patients with secondary hypertension indicates that the temperature dependence of Li efflux may serve as a genetic marker for essential hypertension.

Sodium and potassium ion transport accelerations in erythrocytes of DOC, DOC-salt, two-kidney, one clip, and spontaneously hypertensive rats. Role of hypokalemia and cell volume

Sodium (Na+) and potassium (K+) transport by the furosemide-sensitive Na+-K+ transport system, the Na+-K+ pump, and the cation leak(s) were studied in erythrocytes from DOC-water, DOC-salt, two-kidney, one clip (Sprague-Dawley), and spontaneously hypertensive rats (Wistar-Kyoto). Rubidium (Rb+) was used as a tracer for K+. After 4 weeks of DOC-salt hypertension, inward K+ (Rb+) transport by the furosemide-sensitive system was increased threefold, and the inward Na+ leak and the red cell Na+ content were elevated by about 50%. The rise in cell Na+ accelerated K+ inward and Na+ outward transport by the Na+-K4 pump, DOC-water hypertension caused similar but less pronounced changes. In two-kidney, one clip hypertension, the Na+ leak and the Na+-K+ pump rates were slightly elevated, and furosemide-sensitive Rb+ uptake tended to be increased. In spontaneously hypertensive rats, furosemide-sensitive Rb+ uptake was accelerated by 50%. The marked hypokalemia in DOC-water and DOC-salt hypertension was associated with a slight loss of red cell K+ and an increase in mean cellular hemoglobin content (MCHC), indicative of cell shrinkage. Hypokalemia induced by dietary K+ deficiency caused alterations in red cell cation transport, content, and cell volume which were qualitatively similar but more pronounced than those seen in DOC-salt hypertension. Osmotic shrinkage in vitro induced a severalfold acceleration of furosemide-sensitive Rb+ uptake, similar to that observed in rat erythrocytes shrunken in vivo in K+-deficient states. It is concluded that the acceleration of furosemide-sensitive K+ (Rb+) transport in erythrocytes of mineralocorticoid hypertensive rats is largely caused by the hypokalemia and consecutive red cell K+ loss and shrinkage, respectively. Mean cellular hemoglobin content (MCHC) is thus a parameter that must be considered in studies on Na+ and K+ transport across the membrane of rat erythrocytes.

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.

Differences in transmission through the dorsal column nuclei in spontaneously hypertensive and Wistar Kyoto rats

The medical lemniscal evoked potential in response to a range of footshock intensities was recorded in spontaneously hypertensive (SHR) and Wistar Kyoto rats (WKY). Input-output (I-O) relationships were constructed as the percent of maximum response at each intensity. The SHR had a steeper I-O relationship than did the WKY. This difference was also evident when SHR maintained normotensive from weaning with hydralazine were compared with identically treated WKY. The treatment itself steepened the I-O relationship of the SHR while leaving that of the WKY unchanged. These results indicate an inherent hyperresponsiveness in the SHR dorsal column nuclei and an inhibitory effect of elevated blood pressure on transmission through these nuclei.

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.

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.

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.

The relation of genetics to essential hypertension: a review

Essential hypertension is fundamentally a genetic disease which emerges because of environmental impact. The genetic factors involve intracellular abnormalities which affect calcium metabolism within smooth muscle cells and possibly within the heart and sympathetic nervous system. These abnormalities are influenced by one or more proteins of which calmodulin is a candidate. Sodium pump abnormalities may be primary but may be secondary feedback effects. Homeostasis keeps the blood pressure and cardiac function normal, but eventually becomes less effective. Such homeostasis is produced by negative feedback, but positive external factors also influence the eventual results. Gross homeostasis is provided by the baroreceptors, the renin-angiotensin-aldosterone system, etc. The malfunction of any one or any combination of these systems may produce or accelerate hypertension. Hyperplasia and hypertrophy of arteriolar smooth muscle play their role but also multiply the genetic cellular defects. It is possible that in the early history of man essential hypertension had an advantage which now has become obsolete because normotensive man's potential lifespan has been increased by modern civilization.

Evidence for an enhanced transmembrane sodium (Na+) gradient induced by aldosterone in the incubated rat tail artery

Aldosterone is known to stimulate Na+ transport as measured in terms of current-carrying capacity of epithelial sheets or of Na+ - K+ ATPase activity in cells. The possibility that this is reflected in an altered steady-state transmembrane Na+ distribution in vascular smooth muscle was here examined directly. Transmembrane Na+ and K+ gradients were first dissipated by overnight incubation in K-free physiological salt solution (PSS) at 10 degrees C and then reestablished by 3 hours in normal PSS at 37 degrees C. The addition of d-aldosterone (but not corticosterone) to these media significantly reduced cell Na. This involved only free cell Na which was reduced by about 20% of 3 mmole/kg dry wt. No significant change in membrane permeability measured in terms of net Li uptake at 3 degrees C or at 37 degrees C was observed. The lowest effective aldosterone concentration was 2.8 x 10(-9) M. These results are consistent with the observed enhancement of net Na+ transport in incubated arteries in DOCA-induced hypertension and in the SHR but do not account for the increased Na+ permeability observed in these states.

Lithium treatment decreases blood pressure in genetically hypertensive rats

Chronic lithium treatment was examined for effects on the blood pressure of spontaneously hypertensive rats (SHRs). Treated rats were allowed continuous access to a pelleted lithium diet (1.7 g LiCl (40 mmol)/kg diet) for 21 days. Control SHRs were fed a similar diet lacking the lithium. Two groups of control rats were examined. One group had continuous access to control diet (ad lib controls); a second group was pair-fed a daily ration of control diet such that their mean body weight remained similar to the lithium treated group (pair-fed controls). Heart rate, systolic, diastolic and mean arterial blood pressures were recorded on day 21 from freely moving conscious rats. Systolic, diastolic and mean arterial blood pressure was significantly lower (approximately 15 mmHg) in the lithium treated SHRs as compared to either control group; the pair-fed and ad lib controls had similar blood pressures at the end of the treatment periods. Heart rate was increased in the lithium treated animals. All rats gained weight during the 21 days of treatment, although the lithium treated group and the pair-fed control group did not gain weight as much or as rapidly as the control group that had continuous access to control diet. Plasma and brain lithium levels were in the moderate range (0.3--0.4 meq/l) and all rats appeared healthy at the end of the experiment. These results suggest that chronic treatment may have clinically relevant effects on 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.

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.

Net efflux rate of norepinephrine from platelets in normotensive individuals belonging to families with a heavy accumulation of essential hypertension

Platelets have been used as a model of sympathetic neurons to study the storage of norepinephrine in normotensive individuals who all had a first degree relative with essential hypertension and had hypertension in the family for at least two generations. The initial rate of efflux of norepinephrine, k, was determined in 44 young relatives (mean age 29.2 years), in 18 middle-aged relatives (mean age 46.7 years) and in 31 young controls with no known family history of essential hypertension (mean age 29.8 years). From the material of relatives all those with definite hypertension had been a priori excluded. k was significantly higher in the young relatives (22.7 +/- 7.9) than in the middle-aged relatives (17.7 +/- 6.4) (p less than 0.05). 27.3% of the relatives had higher k-values than any of the controls. A significant correlation was found between k-value and diastolic blood pressure in controls but not in young relatives.

Increased sodium permeability of erythrocytes in spontaneously hypertensive rats

1. The Na permeability of red cells from spontaneously hypertensive rats (SHR) has been studied. In the Japanese Wister-Kyoto strain, a significant increase of 30% in 22Na influx for male SHR was found compared with normotensive male controls. In contrast the Na influx for genetically hypertensive and normotensive New Zealand rats did not differ. Normotensive Wister-Kyoto rats rendered hypertensive by unilateral renal artery clipping, did not show any difference in Na influx from control animals. 2. These results suggest that the Japanese strain of SHR has a genetically determined increase in erythrocyte Na permeability.

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

Net Na+ and K+ fluxes were measured in Na+-loaded and K+-depleted erythrocytes of three varieties of genetically hypertensive rats. In Okamoto spontaneously hypertensive rats (4 and 10-12 weeks of age), Na+ extrusion was reduced as compared to normotensive controls (Wistar/Kyoto). Na+ extrusion was also reduced in the hypertension-prone substrain of the Hebrew University Sabra rats as compared to the Na+-resistant substrain. K+ fluxes were similar in both groups. In both Okamoto spontaneously hypertensive rats and the hypertension-prone substrain, hypertension was severe and developed rapidly. In the Lyon spontaneously hypertensive rats, in which the blood pressure elevation is less severe than in other genetically hypertensive rats, erythrocyte net Na+ extrusion was the same as in normotensive controls, but net K+ gain was slightly increased. These erythrocyte abnormalities, observed in three varieties of genetically transmitted hypertension of the rat, are in several aspects similar to those previously described in accelerated and benign human essential hypertension. Erythrocyte Na+ and K+ net flux alterations may thus represent biochemical markers of primary hypertension.

Laboratory distinction between essential and secondary hypertension by measurement of erythrocyte cation fluxes

An abnormally low sodium-potassium net flux ratio in erythrocytes was recently described in human essential hypertension. We have confirmed this finding in 65 patients with essential hypertension who were compared with 33 normotensive controls born of normotensive parents. In 23 other subjects with documented secondary hypertension and normotensive parents, the sodium-potassium net flux ratio was found to be similar to that in the controls. The erythrocyte test thus appears to be of interest in distinguishing between essential and secondary hypertension. Severe renal failure itself reduces the flux ratio and would therefore distort the results of this test.

Evidence for enhanced sodium transport in the tail artery of the spontaneously hypertensive rat

Transmembrane Na+ and K+ gradients in the rat tail artery were dissipated by overnight incubation in K-free PSS at 10 degrees C and then allowed to recover in normal physiologic salt solution (PSS) at 37 degrees C. The active extrusion of Na+ and uptake of K+ during the recovery period was monitored with Na+ and K+ selective glass electrodes. Passive exchanges were differentiated by re-admitting K+ at 3 degrees C, or in the presence of 1 mM ouabain at both 3 degrees C and 37 degrees C. Active exchange was switched on by an abrupt transfer of the tissue from 3 degrees C to 37 degrees C. Active exchange, measured in perfused, superfused, or sequentially incubated arteries, was distinctly enhanced in young (16-, 20- and 26-week-old) spontaneously hypertensive rats (SHR) of the Okamoto strain compared with age-matched Wistar-Kyoto normotensive (WKY) controls. No such difference was observed in rats with hypertension of 7 or 12 weeks' duration and equal severity induced by unilateral constriction of the renal artery. Steady-state Nai and Ki were measured after washing the tissues for 45 minutes at 3 degrees C in lithium-substituted medium to exchange extracellular sodium with lithium. Cell sodium in these tissues was further partitioned into a free component proportional to [Na]0 and an independent, constrained component. Cell potassium was found to be distinctly elevated in 2- and 4-month-old SHR, while free cell sodium remained normal, despite increased cell permeability demonstrable in a significant exchange of lithium for cell potassium and sodium even at 3 degrees C.

Overview of cardiovascular reflexes in hypertension

The problem of cardiovascular reflexes in hypertension poses several questions. The first is whether alterations in cardiovascular reflexes can initiate a persistent increase in arterial pressure. Another is whether it is an alteration in depressor or in pressor reflexes that matters in hypertension. Other questions concern the type and nature of the reflex alterations that actually occur in hypertension and the place of resetting of baroreflexes in the sequence of physiologic events leading to increased blood pressure.

Decrease of calcium binding by the red blood cell membrane in spontaneously hypertensive rats and in essential hypertension

Ca binding in the red blood cell (RBC) membrane of spontaneously hypertensive rats (SHR) and of patients with essential hypertension was studied. Under conditions of physiological concentration of free Ca in the incubation medium of RBC the outer part of the membrane binds 393 +/- 32 and 435 +/- 30 nmole of Ca per ml of RBC in rats and humans, respectively, without essential differences in the amount of Ca in hypertensive individuals as compared to the normotensive controls. The membrane of red blood cell ghosts (RBCgh) at concentrations of free Ca corresponding to its intracellular concentration binds 4.28 +/- 0.39 and 3.53 +/- 0.15 nmole of Ca per mg of protein of RBCgh in rats and humans, respectively. This part of membrane-bound Ca pool (most probably related to the inner part of the red blood cell membrane) is reduced by 48% in SHR and by 28% in patients with essential hypertension as compared to normotensive controls. It is suggested that the decrease of Ca binding ability of the RBC membrane in both types of hypertension studied may be a pattern of a more widespread cell membrane defect.

A new test showing abnormal net Na+ and K+ fluxes in erythrocytes of essential hypertensive patients

A new and simple laboratory test for measuring net Na+ and K+ fluxes in Na+-loaded/K+-depleted human erythrocytes was developed and applied to hypertension. Moderate essential hypertension (10 patients) was characterised by a constant increase in net K+ influx, possibly related to higher Na+, K+-pump activity. In more severe cases (8 patients) net Na+ efflux from erythrocytes dropped. The ratio of Na+/K+ net fluxes was therefore reduced in all essential hypertensive patients. Conversely, Na+ and K+ erythrocyte fluxes were normal in hypertension of renal origin (5 patients). Erythrocyte K+ influx was normal in young normotensive people born of normotensive parents (17 cases), but was increased in 5 of 8 young normotensive people born of essential hypertensive parents, in families where blood-pressure has been recorded for three generations. This result, which seems to indicate genetic transmission, suggests that measurement of Na+ and K+ erythrocyte fluxes may help to detect subjects liable to high blood-pressure.

Relationship of aortic wall and baroreceptor properties during development in normotensive and spontaneously hypertensive rats

We studied the relationship between aortic baroreceptor function and aortic wall properties in normotensive (NTR) and spontaneously hypertensive (SHR) rats 10-20 weeks old. Baroreceptor discharge, static pressure-volume (P-V), and pressure-radius relationships were measured in excised aortic segments. Histological studies of wall thickness and receptor numbers also were made. Circumferential wall stress and strain were calculated, as was the incremental elastic modulus (EINC). EINC in NTR's at 100 mm Hg was similar to values reported for in vivo human, dog, and rat aortas. At 10 weeks, SHR's had significantly elevated blood pressure, but SHR and NTR aortas had similar relationships among pressures, volumes, strains, and EINC's. Differences arose subsequently and, at 20 weeks, NTR aortas had larger volumes, larger strains, and smaller EINC's at equivalent pressures, whereas SHR aortas were unchanged. Thus the reduced distensibility of SHR relative to NTR aortas, rather than being due to retrogressive changes from normal, appeared to result from a failure to pass through a phase of increased distensibility. At 10 weeks, SHR baroreceptors showed resetting in both pressure-response and strain-response curves, and it was concluded that early hypertensive baroreceptor resetting was due to primary changes in the receptors. At 20 weeks, the order of the strain-response curves for NTR and SHR baroreceptors was reversed due to a reduction in strain sensitivity of NTR baroreceptors. Resetting of NTR baroreceptors during development may have important implications as a mechanism of blood pressure control in development.

Phosphoinositide content in the erythrocyte membrane of rats with spontaneous and renal hypertension

The increase of the content of triphosphoinositide in the erythrocyte membrane in rats with spontaneous and renal hypertension and decrease of phosphatidylinositol at spontaneous hypertension were revealed.

Sodium ions, calcium ions, blood pressure regulation, and hypertension: a reassessment and a hypothesis

An attempt is made to elucidate the cellular mechanisms which may account for the well-documented correlation between sodium metabolism and peripheral vascular resistance. As a starting point, the evidence that the Na electrochemical gradient across the vascular smooth muscle cell plasma membrane (sarcolemma) plays an important role in cell calcium regulation is reviewed. Because there is significant resting tension ("tone") in most resistance vessels, the ionized Ca2+ level ([Ca2+]1) in the smooth muscle fibers in these vessels must be maintained above the contraction threshold. Consequently, the Ca transport system in the sarcolemma, presumably an Na-Ca exchange mechanism, must be set so as to hold [Ca2+]1 at this suprathreshold level. Any change in the Na gradient will then be reflected as a change in [Ca2+]1 and, therefore, in steady vessel wall tension and peripheral resistance. The correlation between Na metabolism and hypertension could then be accounted for if a circulating agent, perhaps the "natriuretic hormone," affects the Na gradient (across the sarcolemma) and, therefore, [Ca2+]1 and tension.