Tissue cations and water in arterial hypertension

The Interaction of Kidneys and Gut in Development of Salt-Sensitive Hypertension

The incidence of salt-sensitive hypertension is quite common and varies between 30-60% in hypertensive patients. Regarding the causal role of high salt intake in the development of salt-sensitive hypertension, recent evidence has demonstrated that the gut through its microbiota plays a significant role in its genesis. Besides the gut, the kidneys also play important role in salt-sensitive hypertension and there is clinical and experimental evidence of an interrelationship between the gut and the kidneys in the development of salt-sensitive hypertension through the so-called "gastro-renal axis." The gut besides being an absorptive organ, it is also a hormonal secretory organ involving the secretion of gastrin, dopamine, norepinephrine, angiotensin, and aldosterone which through their action with the kidneys are involved in the development of salt-sensitive hypertension. In addition, the kidneys exert a protective role against the development of hypertension through the secretion of prostaglandins and their vasodilatory action. To assess the current evidence on the role of high salt intake and the interplay of the gut and kidneys in its development, a Medline search of the English literature was contacted between 2012 and 2022, and 46 pertinent papers were selected. These papers together with collateral literature will be discussed in this review.

Mechanisms of sodium-mediated injury in cardiovascular disease: old play, new scripts

There is a strong association between salt intake and cardiovascular diseases, particularly hypertension, on the population level. The mechanisms that explain this association remain incompletely understood and appear to extend beyond blood pressure. In this review, we describe some of the 'novel' roles of Na⁺ in cardiovascular health and disease: energetic implications of sodium handling in the kidneys; local accumulation in tissue; fluid dynamics; and the role of the microvasculature, with particular focus on the lymphatic system. We describe the interplay between these factors that involves body composition, metabolic signatures, inflammation and composition of the extracellular and intracellular milieus.

Does Excess Tissue Sodium Storage Regulate Blood Pressure?

Purpose of Review

The regulation of blood pressure is conventionally conceptualised into the product of “circulating blood volume” and “vasoconstriction components”. Over the last few years, however, demonstration of tissue sodium storage challenged this dichotomous view.

Recent Findings

We review the available evidence pertaining to this phenomenon and the early association made with blood pressure; we discuss open questions regarding its originally proposed hypertonic nature, recently challenged by the suggestion of a systemic, isotonic, water paralleled accumulation that mirrors absolute or relative extracellular volume expansion; we present the established and speculate on the putative implications of this extravascular sodium excess, in either volume-associated or -independent form, on blood pressure regulation; finally, we highlight the prevalence of high tissue sodium in cardiovascular, metabolic and inflammatory conditions other than hypertension.

Summary

We conclude on approaches to reduce sodium excess and on the potential of emerging imaging technologies in hypertension and other conditions.

Interstitial fluid-solid interaction within aneurysmal and non-pathological human ascending aortic tissue under translational sinusoidal shear deformation

The interaction shear force between internal interstitial fluid motion and the solid circumferential-longitudinal medial lamellae helps generate the shear stress involved in dissection of human ascending aorta aneurysmal or non-pathologic tissue. Frequency analysis parameters from the total shear stress versus time response to translational 1 Hz sinusoidal shear deformation over 50 cycles measure the interaction with respect to the three factors: tissue type, sinusoidal deformation amplitude and direction of the shear deformation. Significant 1, 3, and 5 Hz components exist in this order of descending magnitude for shear deformation amplitudes of either 25% or 50% of the specimen length. Evaporation tests indicate that the amount of free water in both aneurysmal and non-pathological tissue is nearly the same. The interstitial fluid-solid interaction under shear deformation is visible in the shoulders of the total shear stress versus time response curve that are caused by the 3 Hz component. During a single deformation cycle, the ratio of the amplitudes of the 3 Hz and the 1 Hz components measures the normalized amount of interaction. Under translational sinusoidal shear deformation at 25% amplitude, this interaction ratio is statistically smaller in non-pathologic than in aneurysmal human ascending aortic tissue in the circumferential direction. The frequency analysis parameters provide evidence that the structural changes in aneurysmal tissue induce an increase in the interstitial fluid-medial solid interaction shear force which contributes to the propensity for aneurysmal rupture. STATEMENT OF SIGNIFICANCE: Circumferential shear force between the interstitial fluid and medial lamellae within the human ascending aortic wall is demonstrably greater in aneurysmal than non-pathologic tissue. This force likely increases with medial elastin degeneration and may facilitate the dissection propensity in aneurysmal tissue. The 3 Hz component in frequency analyses of the total shear stress versus time curve produced by 1 Hz sinusoidal translational shear deformation measures the fluid-solid interaction shear force that is otherwise difficult to isolate. This non-standard examination of the interstitial fluid interaction helps clarify clinical mechanical implications of structural differences between aneurysmal and non-pathologic human ascending aortic tissue. The aneurysmal dissection susceptibility does not appear to depend on the amount of interstitial fluid or the wall thickness compared to non-pathologic tissue.

Tissue sodium excess is not hypertonic and reflects extracellular volume expansion

Our understanding of Na⁺ homeostasis has recently been reshaped by the notion of skin as a depot for Na⁺ accumulation in multiple cardiovascular diseases and risk factors. The proposed water-independent nature of tissue Na⁺ could induce local pathogenic changes, but lacks firm demonstration. Here, we show that tissue Na⁺ excess upon high Na⁺ intake is a systemic, rather than skin-specific, phenomenon reflecting architectural changes, i.e. a shift in the extracellular-to-intracellular compartments, due to a reduction of the intracellular or accumulation of water-paralleled Na⁺ in the extracellular space. We also demonstrate that this accumulation is unlikely to justify the observed development of experimental hypertension if it were water-independent. Finally, we show that this isotonic skin Na⁺ excess, reflecting subclinical oedema, occurs in hypertensive patients and in association with aging. The implications of our findings, questioning previous assumptions but also reinforcing the importance of tissue Na⁺ excess, are both mechanistic and clinical.

Na⁺ has been suggested to accumulate in tissues, particularly skin, in a hypertonic manner and to exert local pathogenic effects. Here, we reappraise this phenomenon which is systemic in nature and reflects isotonic changes in the relative extracellular volume in tissues, e.g. subclinical oedema; as such, it occurs in human hypertension and aging.

The Role of Na, K-Atpase Inhibitors in Hypertension and End-Stage Renal Disease

Objective

To review the role of Na, K-ATPase inhibitors in the pathogenesis of essential hypertension and hypertension associated with end-stage renal disease.

Data Sources

MEDLINE search, 1966 to 1997.

Results

There is a suggestive physiologic and epidemiologic relationship between Na, K-ATPase inhibition and hypertension. However, clearance data cannot support the hypothesis that differential metabolism of this family of compounds explains the improved hypertensive control seen in patients on peritoneal dialysis compared to those on hemodialysis.

Conclusions

As a result of the complex methodologies involved, it is unclear whether Na, K-ATPase inhibitors playa significant role in the hypertension of endstage renal disease in general and peritoneal dialysis in particular.

Features of endothelial dysfunction and morphofunctional changes of the uteroplacental complex in experimentally induced pre-eclampsia

BACKGROUND

Pre-eclampsia is considered to be a severe complication of pregnancy. Theoretical investigation of its etiology and pathogenesis, development of strategies for its prevention and treatment are conditioned by the development of appropriate experimental models of this pathology.

METHODS

The study involved Wistar rat lines weighing 220-240g. Experimental pre-eclampsia was modeled by replacing drinking water consumed by pregnant female rats with 1.8% NaCl solution throughout gestation. Arterial pressure, protein concentration in urine and tissue hydration extent were measured on the 1st and 21st days of gestation. Uteroplacental blood flow, vasodilating and antithrombotic endothelial functions were also assessed. For pathomorphological and immunohistochemical investigation murine monoclonal antibodies against vascular endothelial growth factor (VEGF), polyclonal rabbit antibodies against inducible and endothelial NO-synthases were used.

RESULTS

Replacing drinking water with 1.8% NaCl solution in female rats throughout gestation elevates arterial pressure, causes proteinuria and edema, impairs vasodilating and antithrombotic endothelial properties, and suppresses uteroplacental blood circulation. A morphological examination of the animals revealed the signs of focal duodenitis, spasms of myometrium arteries with no invasion of syncytiotrophoblast into its walls which also involved a raised VEGF and reduced eNOS expression in the endothelium of myometrial vessels, as well as cytoplasmic expression of iNOS in the cells of inflammatory infiltrate.

CONCLUSIONS

These findings make it possible to conclude that replacing drinking water with 1.8% NaCl solution causes a number of changes typical of pre-eclampsia and, therefore, can be regarded as an experimental model of this pathologic condition.

Rubidium uptake of mononuclear leukocytes from normotensive and borderline hypertensive first degree relatives to patients with essential hypertension

Uptake of 86Rubidium of mononuclear leukocytes (MNL) was used as a measure of cellular sodium-potassium pump activity. 86Rb-uptake was determined with the pump stimulated mainly from inside the cells by sodium as well as with a combined stimulation from inside by sodium and from outside by Rb. In the first case there was an increased pump activity in MNL from borderline hypertensive offspring of hypertensive patients (BHO), and this may be related to an increased number of pump sites observed previously (10). Estimation of maximal pump activity (Vmax) of MNL suggested that Vmax of each pump site in MNL from BHO may be decreased compared to control value of MNL from healthy normotensive subjects.

The use of diuretics in modern antihypertensive therapy

A review is made to sum up the indications when diuretics may be the drug of choice in the treatment of hypertension. Advantages from the combined treatment of thiazide diuretics and other antihypertensive agents are also emphasized. Adverse effects from diuretic treatment, i.e. hypokalemia, hyperuricemia, impaired glucose tolerance and risks associated with unfavourable serum lipoprotein patterns are discussed. It is concluded that from a hemodynamic point of view thiazide diuretics can be a good therapeutic alternative for most patients, excepting those with hyperkinetic circulation. It is recommended to use lower doses of thiazide diuretics than previously and the monitoring of S-potassium is necessary in all patients.

The dietary guideline for sodium: should we shake it up? Yes!

The dietary guidelines established under the auspices of public health policy are intended to promote healthy diets in the general public. The current recommendations for sodium intake stem from studies and publications that are older than much of the public they are designed to benefit. The past 2 decades have seen a dramatic increase in our knowledge of nutritional science, particularly our understanding of the role of sodium in blood pressure regulation. With a myriad of data from observational studies and randomized, controlled trials, we have the information to finally put sodium into its correct context in terms of its role in the regulation of blood pressure and hypertension. Not the sole and pervasive dietary villain it was once believed to be, sodium is but one factor in the complex interplay of multiple, inextricably related regulatory systems of which hypertension is the end result. With the data now available concerning dietary sodium, including the minimal and specific blood pressure effects of sodium in normotensive adults and both the benefits and risks of sodium reduction, future public health recommendations can be based on carefully acquired, consistent, and rational science.

Platelet sodium pump and sodium potassium cotransport activity in nonpregnant, normotensive, and hypertensive pregnant women

OBJECTIVE

To determine ouabain-sensitive sodium pump and bumetanide-sensitive sodium potassium cotransport activity in platelets from nonpregnant and normotensive pregnant women and from women with pregnancy-induced hypertension (PIH).

METHODS

Blood was collected from 9 normotensive nonpregnant subjects, 24 normotensive pregnant subjects in both second and third trimesters, 9 subjects who developed proteinuric PIH, and 9 subjects who developed moderate nonproteinuric PIH. Platelet sodium pump activity was determined by the difference in the uptake of rubidium-86 in the presence and absence of ouabain; sodium potassium cotransport (SPC) activity is that component that is inhibitable by bumetanide.

RESULTS

SPC activity was similar in normotensive subjects in the second [median (range) 78 mmol Rb/h/mg protein (18-140)] and third trimesters [85 (39-134)] but was significantly (p < 0.001) higher than in nonpregnant subjects [22 (4-107)]. In addition, SPC was significantly (p < 0.001) lower in subjects with nonproteinuric [42 (4-67)] or proteinuric PIH [59 (33-102)] compared to those who remained normotensive. Sodium pump activity was significantly higher (p < 0.05) in nonpregnant subjects [263 (188-430)] compared with the other groups of subjects. Total rubidium uptake was significantly higher (p < 0.05) in third-trimester normotensive subjects [471 (243-560)] compared with second-trimester subjects [405 (278-608)].

CONCLUSION

Our results suggest that the lower SPC activity in both nonproteinuric and proteinuric PIH may be an early sign of abnormality in the transport of sodium and potassium across the vascular smooth-muscle cell membrane, which is responsible for the maintenance of blood pressure.

Electron-probe X-ray Microanalysis of Magnesium and Sodium Ion Content in Vascular Smooth Muscle Cells from Spontaneously Hypertensive and Normotensive Rats

Whereas in blood cells decreased magnesium concentrations and increased sodium concentrations in essential hypertension have often been described, only sparse data exist on cellular magnesium or sodium content in vascular smooth muscle cells. Therefore, in aortic smooth muscle cells from seven spontaneously hypertensive rats (SHR) of the Münster strain and seven normotensive Wistar-Kyoto rats (WKY), the intracellular magnesium and sodium content were measured by electron-probe X-ray microanalysis. Measurements were performed in aortic cryosections 3 µm thick. The magnesium ion content was 0.93 +/- 0.17 g/kg dry weight in SHR vs 1.14 +/- 0.12 g/kg dry weight in WKY (p < 0.05). Vascular smooth muscle sodium ion content was measured at 6.85 +/- 0.59 g/kg dry weight in WKY and 12.47 +/- 1.62 g/kg dry weight in SHR (p < 0.05). In conclusion, aortic smooth muscle cells from SHR are characterized by a markedly lowered intracellular magnesium ion content and increased sodium ion concentrations compared with normotensive cells. The results may be due to genetically determined disturbances in transmembrane magnesium and sodium ion transport. Cellular magnesium and sodium handling may be disturbed in SHR aortic smooth muscle as it is in hypertensive blood cells.

Effect of thiazide diuretics on plasma volume, body electrolytes, and excretion of aldosterone in hypertension

A study has been made at the Mayo Clinic of the results of treatment in 28 hypertensive patients receiving thiazide diuretics. The use of these agents produced a significant but very temporary reduction in plasma volume in most patients. The plasma volume reverted to or toward normal after the first week of treatment; therefore, hypovolemia could not explain the prolonged hypotensive effect of these drugs. Administration of thiazide diuretics did not deplete the body of its sodium stores, as measured by isotope studies in five patients on an unrestricted diet. Therapy with thiazides did not produce a persistent increase in urinary aldosterone excretion in the nine patients studied for this possibility.

Systems behavior, feed-back loops, and high blood pressure research

A major challenge to those investigating the etiology and progress of hypertension, as well as of other diseases, results from the fact that the control and regulation of most bodily functions are comprised of multiple feedback loops involving many bodily "systems." Thus, mechanisms which seem to offer clues as to the etiology of an abnormality, when investigated as an isolated function, fail to explain the etiology when sought in the intact experimental animal or patient.

Examples of simple closed-loop systems are described to illustrate the effects of feed-back. A simplified diagram of several factors known to play a role in the control and regulation of the characteristies of the blood vessel wall is described to illustrate the multiplicity of the feed-back pattern which probably occurs. Furthermore, it is evident that almost none of the factors which constitute the loops have been evaluated sufficiently to permit characterization of the system. It is not surprising that apparently promising clues turn out to be disappointing and to become lost in the system.

The medical investigator must learn to deal with methods of analyzing systems containing multiple factors and functions as well as to characterize the properties and behavior of the parts of the system.

Red blood cell sodium and potassium concentration and blood pressure. The Gubbio Population Study

The relations of red blood cell sodium (RBC Na) and potassium (RBC K) concentrations to blood pressure and prevalence of hypertension were assessed for 1805 men and women, aged 25 to 74 years, who participated in the baseline examination of the Gubbio Population Study in north central Italy. In men, in univariate analyses, RBC Na concentration was not significantly related to systolic or diastolic blood pressure, while RBC K concentration was significantly and inversely related to blood pressure. In women RBC Na values correlated significantly and directly with systolic and diastolic pressure, but RBC K concentration was not significantly related to blood pressure. Results of the multivariate analyses indicated in men a significant independent and inverse relationship of RBC K concentration with hypertension and blood pressure, and in women a significant positive association of RBC Na concentration with hypertension. RBC Na did not relate independently to either systolic or diastolic blood pressure in men or women. Age-specific analyses suggested that the relationships between RBC K level and blood pressure in men and the relationship between RBC Na level and hypertension in women were stronger in older (age 55 to 74 years) compared to younger participants (25 to 54 years). These findings indicate that the associations of RBC Na and K concentrations and hypertension may be sex and age specific. The nature of these gender- and age-specific associations remains to be clarified. Prospective data are also needed for further clarification of the relation of intracellular Na metabolism to the etiology of hypertension.

The lymphocyte Na+/H+ antiport: activation in primary hypertension and during chronic NaCl-loading

Increased activity of the Na+/H+ antiport may be a major abnormality in essential hypertension. The activity of this transport system was investigated in lymphocytes from 13 patients with untreated essential hypertension (Ht) and 13 normotensive control subjects (Nt) on an ad libitum (130-170 mmol d-1) NaCl intake. Furthermore, the effects of different states of NaCl balance on lymphocyte Na+/H+ antiport were evaluated in two groups of Nt volunteers receiving 20 vs. 300 mmol d-1 (n = 8) and 85 vs. 200 mmol d-1 (n = 14) of NaCl for 1 week each and in seven Ht patients (20 vs. 300 mmol NaCl d-1 for 1 week each). Additionally, during the 20 and 300 mmol/d NaCl intake red blood cell membrane transport was studied in eight subjects. For the determination of lymphocyte antiport activity, cells were loaded with the cytosolic pH (pHi) indicator bis-carboxyethyl carboxyfluorescein (BCECF-AM) and acidified by addition of different amounts of Na(+)-propionate (5-40 mM). Initial pHi-recovery was taken as the activity of the antiport system and plotted against pHi-values after acidification. Non-linear regression analysis yielded higher 'apparent' maximal transport rates in Ht than Nt (Nt: 2.00 +/- 0.22; Ht: (3.81 +/- 0.59) x 10(-3) s-1; P < 0.025). In contrast, baseline pHi-values and pHi-values at half-maximal activity (pK) were identical in Nt and Ht. In normotensive control subjects on an NaCl intake of 20, 85, 200 and 300 mmol d-1 for 7 d, 'apparent' maximal transport rates averaged 2.75 +/- 0.20, 2.89 +/- 0.17, 2.81 +/- 0.18 and (3.62 +/- 0.25) x 10(-3) s-1, respectively. Thus, antiport activity was significantly (P < 0.05) stimulated on the 300 mmol d-1 intake as compared to the three other NaCl intakes. The extreme intakes of NaCl (20 vs. 300 mmol d-1) in normotensive volunteers did not affect the erythrocyte Na+/K+ pump, Na+/K+ cotransport and Na+/Li+ countertransport. Our study supports the concept that a group of patients with primary hypertension exhibit an activated Na+/H+ antiport. Furthermore, our data demonstrate that a chronic high intake of NaCl is associated with an increase in lymphocyte antiport activity towards the high values observed in primary hypertension.

Smooth muscle hypertrophy and arterial remodelling in deoxycorticosterone acetate-salt hypertension

Morphometric analyses were made of medial-intimal cross-sectional area and lumen diameter in transverse sections of large arteries and small arterioles from normal and deoxycorticosterone acetate-salt hypertensive animals whose vasculatures were perfusion fixed at their in vivo mean arterial pressures. During the borderline and established phases of hypertension, changes in medial cross sectional area and lumen diameter were not detected in any vessel. During chronic mineralocorticoid hypertension, significant (p < 0.05) medial hypertrophy and an increased lumen diameter were found in the abdominal aorta whereas these parameters were normal in other large conduits, small arteries, and arterioles examined. These results indicate that medial hypertrophy is a late consequence of chronic hypertension in this model and is confined primarily to the abdominal aorta. Further, remodelling without medial hypertrophy may explain the normal lumen diameters of small arteries and arterioles in this model of hypertension.

Erythrocyte sodium ion transport system in DOC-salt, Goldblatt, and spontaneously hypertensive rats

Altered erythrocyte Na+ transport has been observed in relation to the pathogenesis of essential hypertension. In the present study, intracellular Na+ and K+ levels, Na(+)-K+ pump activity, Na(+)-K+ cotransport, and Na+ passive permeability were measured in erythrocytes of DOC-salt hypertensive (DSH) rats, two-kidney, one clip Goldblatt hypertensive (2KH) rats, and spontaneously hypertensive rats (SHR). The results were as follows: 1. In comparison with the control groups, no change in the erythrocyte Na+ level was noted in the DSH and 2KH groups, whereas a significant increase was seen in the SHR group. 2. Although no change was noted in the erythrocyte K+ level in the 2KH and SHR groups when compared with the control groups, a significant decrease was seen in the DSH group. 3. Na(+)-K+ pump activity of erythrocytes was not changed in the DSH and 2KH groups when compared with the control group, but a significant increase was noted in the SHR group. 4. Na(+)-K+ cotransport of erythrocytes was not changed in any hypertensive rats when compared with the controls. 5. Na+ passive permeability in the erythrocyte membrane was not changed in the DSH and 2KH groups when compared with the control groups, but a significant increase was noted in the SHR group. These findings suggest that increased erythrocyte Na+ levels in SHR are due to increased Na+ passive permeability of the erythrocyte membrane, and increased Na(+)-K+ pump activity may be compensating for the increased intracellular Na+ concentration in erythrocytes. Furthermore, the increase in Na+ passive permeability observed in SHR might not result from hypertension itself but from abnormalities in the erythrocyte cell membrane, because no increase in Na+ passive permeability was noted in either DSH or 2KH rats.

Oxygen free radicals in essential hypertension

Membrane abnormalities in essential hypertensives (EH) are well known. The respiratory burst enzyme, NADPH oxidase is located in the cell membrane of the neutrophil (PMNLs) and its activity is important in generation of oxygen derived free radical (OFR). Recently OFR have been implicated in vascular changes in variety of conditions. An attempt was made to delineate the status of OFR and antioxidants in EH. Ten, age and sex-matched, healthy controls (GpI) and 26 untreated EH (Gp IIA mild-8, Gp IIB Moderate-8, Gp IIC Severe-10) were studied. After clinical examination and basic laboratory evaluation of subjects, neutrophils isolated from their blood were studied. Chemiluminescence (CL) emitted by PMNLs after stimulation was measured (counts/min) in a luminometer and was taken as measure of OFR production and thereby of NADPH oxidase activity. The levels of antioxidants, superoxide dismutase (SOD) and reduced glutathione (GSH), were also estimated. Chemiluminescence was increased significantly (p less than 0.01) in Gp IIC (243.04 +/- 24.9 x 10(3) counts per minute) as compared to Gp IIA (2.80 +/- 1.87), Gp IIB (34.54 +/- 30.24) and Gp I (0.52 +/- 0.15) and SOD was reduced significantly (p less than 0.05) in all EH (Gp IIA 3.9 +/- 0.3 units per mg protein, Gp IIB 3.5 +/- 0.3 and Gp IIC 3.12 +/- 0.3) as compared to controls (4.1 +/- 0.2). Similarly GSH was reduced (p less than 0.05) in EH (Gp IIA 11.2 +/- 1.7 mg per gm protein, Gp IIB 8.5 +/- 1.1 and Gp IIC 6.6 +/- 0.3) as compared to Gp I (13.5 +/- 2.5).(ABSTRACT TRUNCATED AT 250 WORDS)

Acute effects of angiotensin-converting enzyme inhibitor on erythrocyte sodium ion transport in essential hypertension

The acute effects of angiotensin-converting enzyme inhibitor, captopril, on sodium ion transport systems were investigated in essential hypertensive and normotensive subjects. The passive sodium efflux through the erythrocyte membrane was significantly higher and erythrocyte sodium-potassium cotransport was lower in patients with essential hypertension when compared with normal subjects. However, sodium-potassium pump activity and sodium-lithium countertransport did not differ significantly between the hypertensive patients and the normal subjects. Immediately after captopril administration, erythrocyte passive sodium efflux and sodium-potassium cotransport returned to normal levels in the hypertensive subjects. Although the plasma renin activity and plasma aldosterone concentration were altered by captopril, they did not correlate with changes in any sodium transport system. These results suggest that the changes in sodium transport systems which occur immediately after captopril administration may contribute, at least in part, to its antihypertensive action.

Sodium depresses arterial baroreceptor reflex function in normotensive humans

Sodium may contribute to the pathogenesis of hypertension by impairing arterial baroreceptor reflex function. The objectives of this study were to 1) determine whether a high sodium diet depresses arterial baroreceptor reflex function in normotensive humans, and 2) determine whether alterations in baroreceptor reflex function are related to changes in arterial compliance. Seventeen normotensive men, aged 30 +/- 2 years, received 10 and 200 meq sodium per day diets, each for 5 days, in a randomized crossover trial. Carotid baroreceptor reflex function was assessed by measuring the blood pressure response to sequential neck suction (0, -10, -20, and -30 mm Hg) and neck pressure (0, +10, +20, and +30 mm Hg). Forearm vascular resistance was determined by venous occlusion plethysmography. Arterial compliance was evaluated by calculating the quotient of the diastolic blood pressure decay time constant and forearm vascular resistance. Blood pressure averaged 124 +/- 3/62 +/- 2 mm Hg on the low sodium diet and 122 +/- 3/60 +/- 2 mm Hg on the high sodium diet (p = NS). Baroreceptor reflex slopes representing the systolic and diastolic blood pressure responses to changes in neck chamber pressure were steeper in the subjects when randomly assigned to low sodium diet than to high sodium diet. Diastolic blood pressure decay time and forearm arterial compliance were similar during low and high sodium intake. We conclude that short-term exposure to a high sodium diet depresses carotid baroreceptor reflex function in normotensive humans. This observation cannot be attributed to changes in the arterial compliance.

Ionic mechanisms regulating sodium entry into vascular smooth muscle

1. Na+/H+ exchange, an ethylisopropylamiloride (EIPA)-sensitive Na+ and HCO3- dependent system and a diisothio-cyanatostilbenedisulphonic acid (DIDS)-sensitive Na+ and HCO3- transporter, contribute to sodium influx and intracellular pH (pHi) regulation in vascular smooth muscle. 2. In cultured cells from the human internal mammary artery, Na+/H+ exchange and the EIPA-sensitive Na+ and HCO3- dependent system contribute about 80% to basal sodium influx. The residual Na+ influx is both EIPA and DIDS-insensitive. 3. Sodium influx via these mechanisms influences the ability of vascular smooth muscle to synthesize protein late in the G1 phase of the mitotic cell cycle. This, in turn, affects DNA biosynthesis. 4. These Na+ exchanges/transporters have the capability to facilitate the development of vascular hypertrophy in hypertension.

Characteristics of the limb circulation in hypertension

The aim of this study was to investigate the effect of hypertension on the regulation of limb circulation. The study group consisted of 50 patients suffering from hypertension; in 21 cases the peripheral circulation was intact, while 29 patients had peripheral obliterative arterial disease (POAD) (clinical symptoms: intermittent claudication or rest pain). The control group consisted of 67 normotensive patients. In 33 subjects of the control group the peripheral circulation was intact, while 34 patients suffered from POAD (clinical symptoms: intermittent claudication or rest pain). The total limb blood flow (LBF) was measured by using venous isotope dilution technique. In hypertensive patients the limb vascular resistance (LVR) was markedly elevated, but the LBF remained in the normal range. In patients suffering from both hypertension and POAD the LVR was pathologically elevated and the LBF markedly diminished. After acute vasodilator therapy the LBF significantly increased, despite the drop in blood pressure owing to the extensive decrease of the LVR. It appears that in hypertension the elevation of the LVR is the most characteristic change of the limb circulation. The elevation of the arterial blood pressure has no favorable effect on the limb circulation, and the LBF compared with the blood pressure is relatively low.

Effects of ouabain and low sodium on contractility of human resistance arteries

Earlier work with rat arteries has resulted in a widely held assumption that resistance artery smooth muscle will not contract on exposure to a reduced transplasmalemmal sodium gradient. In view of the well-recognized low sensitivity of rat tissue to cardiac glycosides, we have investigated the effects of altering the transplasmalemmal sodium gradient on vascular smooth muscle tone by using human resistance arteries. Incubation of arteries in low sodium or in ouabain to inhibit active sodium efflux for 1 hour increased the contractile response to caffeine stimulation; this finding indicated enhanced calcium buffering by the sarcoplasmic reticulum. Prolonged incubation in ouabain in the presence of phentolamine or diltiazem resulted in a concentration-dependent increase in the tone of resting human resistance arteries. Reduction of the transplasmalemmal sodium gradient by incubation in low sodium buffer effected an increase in tone similar to that obtained in the presence of ouabain. These results suggest that alteration of the transplasmalemmal sodium gradient may increase the vascular smooth muscle tone of human resistance arteries by altering intracellular calcium handling. This is a new finding in human resistance arteries and may involve inhibition and, indeed, reversal of sodium-dependent calcium efflux. A concentration-dependent potentiation of tone was found after the addition of ouabain to submaximally activated arteries. Sodium-calcium exchange may also play a pivotal role in this mechanism.

Leukocyte isotopically exchangeable intracellular sodium fractions in lean and overweight hypertensives

Leukocyte intracellular sodium, as measured by flame photometry, is increased in essential hypertension, especially when associated with a body mass index greater than 27 kg.m-2. A triple isotope method for measuring the isotopically exchangeable pool of intracellular sodium was used to assess if this pool was increased in hypertension. No significant differences in the isotopically exchangeable intracellular sodium concentration were found between lean and overweight hypertensives compared with normotensive controls. Lean hypertensives with systolic blood pressures below the median had significantly lower exchangeable intracellular sodium concentrations than lean normotensives, whereas those with systolic blood pressures above the median had raised exchangeable intracellular sodium concentrations. The obese hypertensives did not show this trend. The exchangeable intracellular sodium concentration was correlated to systolic (r = .53, P less than .001) and diastolic (r = 0.39, P less than .01) blood pressure in hypertensives. We conclude that the increase in total cellular sodium content (as measured by flame photometry) in hypertensives described in previous studies is not associated with any increase in the isotopically exchangeable pool of intracellular Na+, except in those lean hypertensives with systolic blood pressures above the median. By implication, there may be an increased slowly exchangeable pool of intracellular Na+ in leukocytes from most hypertensives.

Mineralocorticoid-induced increase in beta-adrenergic receptors of cultured rat arterial smooth muscle cells

We have investigated the effect of mineralocorticoids on beta-adrenergic receptors in cultured arterial smooth muscle cells. Mineralocorticoid (aldosterone) treatment resulted in a significant increase in beta-adrenergic receptors measured by [3H]dihydroalprenolol (DHA) binding. This effect required at least 20 hours of incubation with aldosterone and was completely blocked by cycloheximide (10 micrograms/ml), indicating protein synthesis was required for this response. Aldosterone at the concentration range of 10(-8)-10(-6) M increased [3H]DHA binding, but was ineffective at 10(-9) M. Scatchard analysis of [3H]DHA binding revealed that the observed significant increase in binding was due to an increased number of binding sites (P less than 0.05), and that the affinity was unchanged. The aldosterone (1 x 10(-8) M) effect was completely blocked by the combination of RU 38486 (10(-6) M) and spironolactone (10(-7) M), but not by the glucocorticoid antagonist RU 38486 alone. While basal c-AMP levels were not changed by aldosterone (10(-6) M) treatment, the isoproterenol (10(-6) M) stimulated level of c-AMP was significantly higher in cells treated with aldosterone (P less than 0.05). We conclude that aldosterone, acting through the mineralocorticoid receptor, has a direct effect on arterial smooth muscle cells mediated through modulation of beta-adrenergic receptors of these cells.

Aortic hypertrophy and "waterlogging" in the development of coarctation hypertension

To study the mechanisms and roles of vascular structural changes during the development of hypertension, we coarcted or sham-coarcted the abdominal aorta of rats. At intervals of 3 to 56 days later, we obtained standardized segments of thoracic and abdominal aortas for measurement of dry weight, water content, and amino acid content. Carotid arterial pressure was elevated by day 5 in coarcted rats and remained elevated. Femoral and tail arterial pressures remained normal. Cardiac ventricular weight and dry weight of the thoracic aorta, normalized for body weight, rose rapidly over 3-10 days in coarcted rats, remaining constant at 50-60% above levels in sham-coarcted rats thereafter. In contrast, water content of thoracic aorta in coarcted rats peaked at 123% of control values on day 7 (p less than 0.001), falling rapidly thereafter to levels about half of peak. Increments in dry weight and water content of the normotensive abdominal aortic segments were of far lesser magnitude and occurred 1 to 2 weeks later, probably reflecting the effects of initial hypotension of the hindquarters. Percent hydroxyproline of intima-media segments of the thoracic aorta remained normal during the 8-week period, indicating that increases in aortic dry weight did not represent disproportional fibrosis and thus are attributable to muscular hypertrophy. These results provide support for the hypothesis that arterial wall "waterlogging" is primarily an early manifestation of the hypertensive process. The greatest magnitude of waterlogging coincides with the rapid early increase in aortic dry weight, representing hypertrophy, which suggests common mechanisms, such as activation of Na+-H+ antiport.

Leucocyte sodium content and sodium pump activity in overweight and lean hypertensives

Leucocyte sodium content and sodium pump activity was studied in overweight and lean hypertensive subjects and normotensive controls, all in the fasting state. In lean subjects (body mass index less than 27 kg m-2), hypertensives did not have altered leucocyte sodium content or pump activity. In the overweight (mostly obese) subjects, the leucocyte sodium content was higher in hypertensive than in normotensive subjects (median (range) 56.1 (42.0-84.1) vs 32.0 (18.2-59.4) mmol kg-1, P less than 0.001). This raised sodium content in overweight hypertensives was associated with a lower (ouabain-sensitive) 22Na efflux rate constant (2.25 (1.15-3.01) vs 2.64 (1.98-3.61) h-1, P less than 0.05) and a higher passive (or ouabain-insensitive) 22Na efflux rate constant (0.90(0.53-1.18) vs 0.63 (0.21-1.09) h-1, P less than 0.01). The systolic and diastolic blood pressures were significantly correlated to intracellular Na+ in the overweight group (r = 0.41 and 0.56, P less than 0.02 and 0.001 respectively). Thus, hypertension in the overweight subjects is associated with accumulation of intracellular sodium that may be due to abnormalities of the active sodium pump, though changes in ouabain-insensitive mechanisms also occur.

Erythrocyte water, Na+-K+ cotransport, and forearm vascular function in humans

We examined the relationships between erythrocyte (RBC) composition (Na+, K+, and water content) and ouabain-insensitive transports (Na+-K+ cotransport, Li+-Na+ countertransport) and forearm vascular hemodynamics under standardized basal conditions and during vasoconstriction (intra-arterial infusion of graded doses of norepinephrine and angiotensin II) and vasodilation (intra-arterial phentolamine and postischemic exercise). RBC water content correlated positively and significantly (r = 0.53, p = 0.001) with minimum forearm vascular resistance, a measure of vascular structural change, and negatively with maximal forearm blood flow (r = -0.55, p less than 0.001). Similar correlations with forearm vascular resistance and blood flow were observed under all experimental conditions. RBC Na+-K+ cotransport correlated positively and significantly (r = 0.43, p = 0.01) with the change in forearm blood flow produced by phentolamine, a functional measure of alpha-adrenergic tone, and was as strong an independent predictor of phentolamine-induced blood flow change as was arterial norepinephrine concentration. RBC Na+-K+ cotransport was also significantly positively correlated with residual forearm blood flow and resistance after phentolamine administration, where nonadrenergic influences predominate. RBC water correlated negatively with Li+-Na+ countertransport (r = -0.33, p less than 0.05) and Na+-K+ cotransport (r = -0.44, p less than 0.01). We propose that RBC water is a marker for a vascular structural property that contributes to vascular reactivity. RBC Na+-K+ cotransport seems to relate most strongly to the sympathetically mediated control of forearm blood flow and may also be linked to the intrinsic myogenic tone of the forearm vasculature.(ABSTRACT TRUNCATED AT 250 WORDS)

Salt sensitivity and systemic hypertension in the elderly

Aging in industrialized societies is accompanied by increases in the incidence and prevalence of hypertension, with a disproportionately greater increase occurring among aging blacks than among aging whites. This geriatric hypertension is generally of a salt-sensitive nature with a disproportionate frequency of isolated systolic hypertension. Although salt-taste acuity declines with age, salt sensitivity among the elderly does not appear to result from a compensatory increase in salt intake. Rather, age-related increases in salt sensitivity result, in part, from a reduced ability to appropriately excrete a salt load, which is due to a decline in renal function and to a reduced generation of natriuretic substances such as prostaglandin E2 and dopamine. Age-associated declines in the activity of membrane sodium/potassium-adenosine triphosphatase (Na/K-ATPase) may also contribute to geriatric hypertension because this results in increased intracellular sodium that may cause reduced sodium-calcium exchange and thereby increase intracellular calcium and vascular resistance. Reductions in cellular calcium efflux due to reduced calcium-ATPase activity may similarly cause an increase in intracellular calcium and vascular resistance. Increasing dietary calcium intake may represent an effective nonpharmacologic treatment for some salt-sensitive persons because it appears to reduce intracellular calcium by (1) suppressing parathyroid hormone-mediated calcium influx, (2) increasing Na/K-ATPase activity, and (3) reducing intravascular volume due to calcium-induced natriuresis.

Functional versus structural changes of forearm vascular resistance in hypertension

Structural changes in resistance vessels have been considered an important factor in triggering and maintaining chronic hypertension in humans and in experimental animals. To determine whether the increased forearm vascular resistance observed following vasodilator maneuvers in hypertensive patients is predominantly due to structural or to functional changes, we examined the influence of different vasodilator stimuli on forearm blood flow and blood pressure in 22 male patients with established essential hypertension and in 22 age-matched normotensive men (age range, 28-52 years). Blood pressure was measured directly, and blood flow was measured by venous occlusion plethysmography. The maneuvers applied were 1) arterial occlusion combined with handgrip exercise and local heating, 2) intra-arterial infusion of the calcium entry blocker nifedipine, 3) intra-arterial infusion of the nonspecific vasodilator sodium nitroprusside, 4) arterial occlusion initiated after intra-arterial infusion of nifedipine. Vascular resistance during vasodilation induced by arterial occlusion or infusion of nifedipine or sodium nitroprusside remained significantly higher in the hypertensive than in the normotensive subjects. However, the maximal vasodilation achieved by the combination of arterial occlusion and nifedipine resulted in a similar resistance in both groups (1.6 +/- 0.2 in the hypertensive vs 1.4 +/- 0.2 mm Hg/ml/min/100 ml tissue in the normotensive subjects. These data suggest that there is an important functional component of the elevated resistance in patients with essential hypertension.

Cell membrane in hypertension

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

The effects of short-term hypertensive left ventricular hypertrophy on intracellular pH and intracellular electrolytes in rats

Experiments were performed to assess the effect of left ventricular hypertrophy (induced by experimental hypertension) on intracellular pH (pHi) and intracellular electrolytes in left ventricular tissue. They were undertaken on: (1) hypertensive rats (hypertension being induced by either: (a) subdiaphragmatic aortic constriction, (b) unilateral renal artery clipping, or (c) unilateral renal artery clipping with contralateral nephrectomy); (2) sham-operated rats for the above 3 subgroups; and (3) control (unoperated) rats. Intracellular pH and intracellular electrolytes were measured in left ventricular, right ventricular and skeletal muscle tissue from these animals. Intracellular pH control was assessed by exposing a number of animals in each group to an acute respiratory acidosis (by varying the concentration of inspired PCO2). In association with left ventricular hypertrophy (secondary to hypertension), left ventricular pHi became significantly alkaline in all experimental hypertensive groups compared with control values; pHi control (in response to an acidosis) was also significantly improved. There was no change in resting levels of pHi or pHi control in right ventricular or skeletal muscle tissue in any hypertensive group. There was no change in resting levels of pHi or pHi control in left ventricular, right ventricular or skeletal muscle tissue from sham-operated animals. This suggests that these changes are the result of hypertrophy per se, rather than due to a generalised mechanism secondary to hypertension and operating on all tissues. There was no change in intracellular electrolyte concentration or content in association with hypertension in any tissue or group studied.

A role of calcium in altered sodium ion transport of hypertensives?

Research on the etiology of essential hypertension has led to many reports of altered ion transport in cells from hypertensive patients and animal models. Abnormalities in sodium and calcium ion gradients and transport in vascular smooth muscle, neuronal tissue, cardiac muscle as well as erythrocytes have been extensively investigated. It is not clear whether these abnormalities are of primary or secondary nature. The current knowledge of sodium and calcium ion transport in essential hypertension is briefly reviewed here. Furthermore, evidence is presented which suggests a role of calcium in the regulation of sodium transport activity.

Decreased hindquarters venous distensibility during high salt intake in stroke-prone spontaneously hypertensive rats

This study was done to examine whether high salt intake decreases venous distensibility in stroke-prone spontaneously hypertensive rats (SHR-sp). Ten weeks old male SHR-sp and normotensive Wistar-Kyoto rats (WKY) were fed either high (8%) or pressure-volume curves were obtained by infusing Krebs-Henseleit solution retrograde into the inferior vena cava at a rate of 12 or 2.1 ml/min. After 3 weeks of dietary treatment, high salt intake shifted the venous pressure-volume curves at maximal venodilatation caused by nitroprusside toward the pressure axis in SHR-sp but not in WKY. The venous pressure-volume curve during interruption of infusion was also shifted toward the pressure axis in SHR-sp on high salt diet as compared with that in SHR-sp on normal salt diet. Water content and thickness of the smooth muscle layer of the venous wall were not different between the two groups of SHR-sp. These results suggest that high salt intake for 3 weeks decreased hindquarters venous distensibility at maximal venodilatation in SHR-sp but not in WKY. The salt-induced decrease in venous distensibility in SHR-sp might be related to changes in interstitial space compliance or the adventitia since water content or thickness of the smooth muscle layer of the venous wall was not altered.

Epidemiological studies of sodium transport and hypertension

Red blood cell membrane cation transport was measured in five population-based surveys and two randomized, controlled, dietary intervention studies to examine its associations with demographic, biological, and dietary variables in free-living individuals. A total of 508 individuals, 255 with high blood pressure, were studied. Both sexes, blacks and whites, and several age groups were represented. The intervention studies included short-term dietary sodium restriction in normotensive adolescents, and a 4-year multifactorial trial on weight, sodium, and alcohol in hypertensive adults. The findings from these surveys and intervention studies are summarized in this report. Sodium-stimulated lithium countertransport was significantly related to diastolic blood pressure in white adults (r = 0.28, p less than 0.001), and to systolic blood pressure in black children (r = 0.50, p less than 0.005) and white adolescents (r = 0.31, p less than 0.05). Lithium countertransport was related to sex and race, but not age. Body mass index had an independent relationship with lithium countertransport in some age groups. Lithium countertransport was lower in normotensive adults than in both younger and older hypertensive adults. Lithium countertransport did not differ significantly between subjects with hypertension treated with antihypertensive medications and those with untreated hypertension. Short-term dietary sodium restriction did not influence lithium countertransport in normotensive adolescents. Long-term dietary intervention was associated with low lithium countertransport in hypertensive adults able to maintain blood pressure control without medication. These findings indicate that lithium countertransport is related to blood pressure and hypertension among free-living individuals.

Quantitative X-ray microanalysis of bulk hydrated specimens: a method using gelatine standards

A method of preparing and using gelatine blocks as standards for quantitative X-ray microanalysis of bulk hydrated tissue is described. Each series of standards had several constant and one variable element. The average deviations in the X-ray counts of the constant elements from the series means were used to correct the recorded count of the variable element in each block. This led to an improved linear relationship between X-ray counts and concentration. The elements tested, sodium, phosphorus, and potassium, were chosen as representatives of different parts of the X-ray spectrum. Sodium cannot be measured reliably below 50 mmol kg-1 wet weight, although phosphorus and potassium can be detected at concentrations of 5 mmol kg-1 wet weight. It is argued that this sensitivity of analysis is sufficient for studying electrolyte changes in certain pathological processes.

Increased platelet Na+-H+ exchange rates in essential hypertension: application of a novel test

Enhanced sodium-proton exchange may play a role in the pathogenesis of hypertension. Na+-H+ exchange was measured indirectly in platelets as the rate of amiloride-sensitive and sodium-dependent volume gain of cells suspended in sodium-propionate; the cytoplasmic acidification induced by the permeant propionic acid activated the exchanger and the volume changes coupled to Na+ uptake were measured by cell sizing with a Coulter counter and 'Channelyzer'. The test was rapid, simple, and reproducible. 20 normotensives; 8 normotensives with a family history of hypertension; 15 patients with essential hypertension receiving medication; and 7 hypertensives who had not received any antihypertensive drugs were studied. The exchange rate constants of these groups were (mean [SE] in s-1 X 10(-3)) 13.1 (0.6); 15.5 (0.7); 18.4 (0.9); and 25.6 (2.8), respectively. The differences between hypertensives and normotensives were significant. Measurement of sodium-proton exchange in platelets may help to clarify the pathogenesis of hypertension.

Cellular calcium metabolism in primary hypertension

Several disturbances of cellular Ca2+ metabolism have been described in essential hypertension and in the spontaneously hypertensive rat. Possibly the elevation of intracellular free Ca2+ concentration in arterial smooth muscle cells is one important step in the pathogenesis of primary hypertension. In most studies a decreased energy-dependent Ca2+ transport has been proposed as a mechanism. However, disturbances in cellular Ca2+ metabolism, which can be exclusively ascribed to essential hypertension, have not yet been found. The cause of altered cellular Ca2+ transport in primary hypertension may either be a genetically determined defect of membrane transport or a still-unidentified humoral factor.