Corcoran lecture: the case for or against salt in hypertension. Arthur Curtis Corcoran, MD (1909-1965). Tribute and prelude to Corcoran Lecture of 1988

Biochemical interaction of salt sensitivity: a key player for the development of essential hypertension

Worldwide, more than 1 billion people have elevated blood pressure, with up to 45% of adults affected by the disease. In 2016 the global health study report on patients from 67 countries was released in Lancet, which identified hypertension as the world's leading cause for death and disability-adjusted years since 1990. This paper aims to analyze the pathophysiological connection between hemodynamic inflammatory reactions through sodium balance, salt sensitivity, and potential pathophysiological reactions. Besides, we explore how sodium consumption enhances the expression of transient receptor potential channel 3 (TrpC3) mRNA and facilitates the release of calcium inside immune cell groups, together with elevated blood pressure in essential hypertensive patients.

Evaluation and management of hypertension in childhood

Hypertension, a relatively uncommon problem in childhood except in certain groups of children, is an important cardiovascular risk factor that can have significant health implications, especially the tendency for an elevated blood pressure in childhood to predict the development of adult hypertension. Common causes of childhood hypertension include renal and cardiac disease, as well as essential hypertension in adolescents. Given these factors, it is usually possible to evaluate the hypertensive child in a focused manner that should reveal not only the underlying cause of hypertension, but also its severity. Treatment should incorporate non-pharmacologic approaches as well as antihypertensive medications, and should take into account other cardiovascular risk factors such as hyperlipidemia. This review highlights these and other important issues in the evaluation and management of hypertensive children, and provides practical guidance to the practitioner involved in caring for such patients.

Dietary compliance among salt-sensitive and salt-insensitive normotensive adults

Little is known about the customary level of sodium intake by salt-sensitive people and the nature of obstacles they face in the adoption of a reduced-sodium diet. These issues were addressed with 12 salt-sensitive (SS) and 9 salt-insensitive (SI) normotensive adults. Information about sodium consumption, taste, and blood pressure and concerns about following a diet reduced in sodium were collected at baseline and monthly while participants followed a 100 mmol Na/day diet for 4 months. Mean sodium intakes of both groups were comparable at baseline and were reduced significantly during diet. The principal dietary concerns were reduced food availability, increased food costs, and reduced food palatability. There were no group differences. Ratings declined over time, but only the food palatability issue did so significantly because of a shift by the SI only. While the predictive value of SS classification remains uncertain, these data indicate that dietary change is feasible in SS subjects.

NaCl-induced renal vasoconstriction in salt-sensitive African Americans: antipressor and hemodynamic effects of potassium bicarbonate

In 16 African Americans (blacks, 14 men, 2 women) with average admission mean arterial pressure (MAP, mm Hg) 99.9+/-3.5 (mean+/-SEM), we investigated whether NaCl-induced renal vasoconstriction attends salt sensitivity and, if so, whether supplemental KHCO3 ameliorates both conditions. Throughout a 3-week period under controlled metabolic conditions, all subjects ate diets containing 15 mmol NaCl and 30 mmol potassium (K+) (per 70 kg body wt [BW] per day). Throughout weeks 2 and 3, NaCl was loaded to 250 mmol/d; throughout week 3, dietary K+ was supplemented to 170 mmol/d (KHCO3). On the last day of each study week, we measured renal blood flow (RBF) and glomerular filtration rate (GFR) using renal clearances of PAH and inulin. Ten subjects were salt sensitive (SS) (DeltaMAP >+5%) and 6 salt resistant (SR). In NaCl-loaded SS but not SR subjects, RBF (mL/min/1.73 m2) decreased from 920+/-75 to 828+/-46 (P<0.05); filtration fraction (FF, %) increased from 19. 4+/- to 21.4 (P<0.001); and renal vascular resistance (RVR) (10(3)xmm Hg/[mL/min]) increased from 101+/-8 to 131+/-10 (P<0.001). In all subjects combined, DeltaMAP varied inversely with DeltaRBF (r =-0.57, P=0.02) and directly with DeltaRVR (r = 0.65, P=0.006) and DeltaFF (r = 0.59, P=0.03), but not with MAP before NaCl loading. When supplemental KHCO3 abolished the pressor effect of NaCl in SS subjects, RBF was unaffected but GFR and FF decreased. The results show that in marginally K+-deficient blacks (1) NaCl-induced renal vasoconstrictive dysfunction attends salt sensitivity; (2) the dysfunction varies in extent directly with the NaCl-induced increase in blood pressure (BP); and (3) is complexly affected by supplemented KHCO3, GFR and FF decreasing but RBF not changing. In blacks, NaCl-induced renal vasoconstriction may be a pathogenetic event in salt sensitivity.

Salt sensitivity: concept and pathogenesis

Almost two decades ago, the existence of a subset of essential hypertensive patients, who were sensitive (according to the increase in blood pressure levels) to the intake of a diet with a high salt content, was described. These patients are characterized by an increase in blood pressure and in body weight when switched from a low to a high sodium intake. The increase in body weight is due to the incapacity of the kidneys to excrete the whole intake of sodium until renal perfusion pressure (mean blood pressure) attains a level that is able to restore pressure-natriuresis relationship to values that enable the kidney to excrete the salt ingested or administered intravenously. Salt sensitivity does not seem to depend on the existence of an intrinsic renal defect to handle sodium, but on the existence of subtle abnormalities in the regulation of the sympathetic nervous system, the renin-angiotensin system or endothelial function. It is also relevant that organ damage secondary to arterial hypertension, has been shown in animal models and in hypertensive humans sensitive to a high salt intake to be significantly higher when compared with that of salt-resistant animals or humans. Interestingly, in humans, salt sensitivity has been shown to correlate with microalbuminuria, an important predictor of cardiovascular morbidity and mortality, which correlates with most of the cardiovascular risk factors commonly associated with arterial hypertension. One of these factors is insulin resistance, that usually accompanies high blood pressure in overweight and obese hypertensives. Insulin resistance and hyperinsulinism are present in a significant percentage of hypertensive patients developing cardiovascular symptoms or death. For these reasons, therapy of arterial hypertension must be directed, not only to facilitate the lowering of BP level, but also, to halt the mechanisms underlying the increase in BP, when salt intake is increased. Furthermore, therapy must preferably improve the diminished insulin sensitivity present in salt-sensitive subjects that contribute independently to increased cardiovascular risk.

Sensitivity of blood pressure and renin activation during sodium restriction

The objective of the present study was to explore the interrelationships among cumulative sodium loss, renin activation, and blood pressure changes during sodium restriction in essential hypertensive patients. Specifically, we wanted to know whether the degree of sodium sensitivity of blood pressure depends on renin activation during steady state or on initial renin activation during the first days of sodium restriction. Sixty-seven untreated essential hypertensive patients were admitted to a metabolic ward for 8 days and put on a sodium restricted diet of 55 mmol/d from the second to the last day. Urinary excretions of sodium, potassium, and creatinine were determined along with mean arterial pressure and weight during 7 days. Besides measurements in steady state condition (after 7 days), active plasma renin concentration, aldosterone, and catecholamines were also assessed during the first 3 days of sodium restriction. Analyzable data are available for 55 patients. Baseline sodium excretion and the activation of renin during the first 3 days both appeared to be predictors of total sodium loss after 7 days. Changes in blood pressure were not related to changes in sodium balance, but they were to baseline blood pressure, baseline norepinephrine, and renin activation during the early phase of sodium restriction. In addition, blood pressure appeared to fall more when the normal relationship between sodium loss and early (but not late) activation of renin was disturbed. We conclude that sodium sensitivity of blood pressure during sodium restriction is associated with a relative unresponsiveness of the renin system during the early phase of sodium loss rather than to absolute renin levels during steady state.

Salt sensitivity of blood pressure in humans

A variety of different techniques have been used for the assessment of the blood pressure response to changes in salt and water balance in humans. These have generally been found to be reproducible and to yield congruent results. This review surveys the characteristics of subjects identified as salt sensitive and salt resistant by different investigators from demographic and physiological perspectives.

Does the renin-angiotensin system determine the renal and systemic hemodynamic response to sodium in patients with essential hypertension?

Many patients with essential hypertension respond to a high dietary sodium intake with a rise in blood pressure. Experimental evidence suggests that the renal hemodynamic response to sodium determines, at least partially, this rise in blood pressure. Our aim was to clarify the role of the renin-angiotensin system in the renal and systemic adaptation to a change in dietary sodium. We studied changes in mean arterial pressure (MAP) (millimeters of mercury), effective renal plasma flow (ERPF), body weight, and immunoreactive renin in 17 patients with essential hypertension and 15 normotensive control subjects, randomly crossing over between a 3-week sodium-restricted (50 mmol/24 h) and a sodium-replete (200 mmol/24 h) diet period. In addition, the effects of renin inhibition by remikiren (600 mg, single oral dose) were studied during the high sodium period. In normotensive control subjects, high sodium intake had no effect on MAP or body weight, whereas ERPF increased (490 +/- 19 to 535 +/- 21 mL/min, P < .05) and immunoreactive renin decreased (32 +/- 6 to 14 +/- 1 pg/mL). In hypertensive subjects, high sodium intake induced a heterogeneous response of MAP (median change, 2.6 mm Hg; range, -4.7 to +21.2; P = NS) and ERPF (median change, 21 mL/min; range, -33 to +98; P = NS). Body weight increased from 81.3 +/- 1.9 to 82.5 +/- 2.0 kg (P < .05), and immunoreactive renin decreased from 18 +/- 3 to 10 +/- 1 pg/mL (P < .05). Interestingly, the patients with a distinct rise in MAP showed a blunted ERPF response to high sodium intake (r = -.70, P < .01) and an increase in body weight (r = .76, P < .001). Moreover, the increase of ERPF was more pronounced in patients with a larger fall in immunoreactive renin (r = .77, P < .001). After administration of remikiren, a heterogeneous response in ERPF was observed: the patients with the blunted ERPF response to high sodium intake showed the largest ERPF rise (r = .70, P < .01). The remikiren-induced rise in ERPF correlated (r = .68, P < .01) with the fall in MAP (114 +/- 2 to 110 +/- 2 mm Hg). In conclusion, in patients with essential hypertension a rise in blood pressure in response to high sodium intake appears to partially be the result of insufficient renal vasodilatation. This seems to be due to an inadequate (intrarenal?) renin-angiotensin system response to increased sodium intake.

Development of hypertension in spontaneously hypertensive rats: role of milk electrolytes

1. Milk samples were collected from lactating spontaneously hypertensive rats (SHR) and Wistar-Kyoto (WKY) normotensive females at 8, 14 or 18 days postpartum. 2. Milk samples were later analysed for content of calcium (Ca++), sodium (Na+), potassium (K+), chloride (Cl-) and total protein. 3. Milk samples from SHR females had significantly higher concentrations of Na+ (at 8 and 14 days) and Cl- and significantly lower concentrations of Ca++, K+ (at 8 and 14 days) and total protein compared to milk samples from WKY females. 4. Preweanling dietary content of several electrolytes implicated in the development of hypertension differed dramatically between SHR and WKY strains. This altered diet early in life may serve as an environmental trigger for progressive age-related increases in arterial pressure in SHR.

Dietary sodium intake modulates systemic but not forearm norepinephrine release

INTRODUCTION

Sodium intake has profound effects on systemic and renal sympathetic activity, but its effects on sympathetic activity in skeletal muscle vascular beds, a site at which local regulatory mechanisms could alter vascular tone directly, are unclear.

METHODS

To determine the effect of dietary sodium intake on basal and isoproterenol-stimulated systemic and forearm norepinephrine kinetics, we studied seven healthy male volunteers twice, 4 weeks apart, while they were receiving a low-sodium (10 mmol sodium/24 hours) diet and a high-sodium diet (250 mmol sodium/24 hours). Forearm blood flow, measured by plethysmography, and systemic and forearm norepinephrine spillover, measured by radioisotope dilution, were determined before and after intra-arterial infusion of 60 and 400 ng/min isoproterenol.

RESULTS

Baseline (before isoproterenol) systemic norepinephrine spillover was higher when subjects received the low-sodium diet (448.1 +/- 55.7 ng/min) compared with the high-sodium diet (269.7 +/- 42.7 ng/min; p < 0.05). In contrast, sodium intake did not affect local forearm norepinephrine spillover, either at baseline (low-sodium diet, 2.05 +/- 0.48 ng/min versus high-sodium diet, 2.63 +/- 0.79 ng/min; p = 0.50) or after stimulation with isoproterenol in doses of 60 ng/min (low-sodium diet, 8.84 +/- 2.2 ng/min versus high-sodium diet, 6.1 +/- 1.9 ng/min; p = 0.38) or 400 ng/min (low-sodium diet, 16.4 +/- 4.5 ng/min versus high-sodium diet, 16.7 +/- 2.5 ng/min; p = 0.93).

CONCLUSIONS

Under conditions of low sodium intake, systemic norepinephrine spillover was increased but forearm norepinephrine spillover was not, suggesting that alteration in sodium intake may produce a differential effect on norepinephrine spillover in different tissues but that decreased local sympathetic activity in skeletal muscle is not the likely mechanism by which a low-sodium diet may lower blood pressure or attenuate stress-induced pressor responses.

Milk electrolyte content of Dahl hypertensive and normotensive rats

Milk samples were collected from lightly anesthetized lactating female rats of the Dahl hypertension-sensitive (SS/Jr) and Dahl hypertension-resistent (SR/Jr) inbred strains on postnatal days 8, 14 and 18. These milk samples were stored at 4 degrees C until analyzed for content of calcium (Ca++), magnesium (Mg++), chloride (Cl-), potassium (K+), sodium (Na+), and total protein. Our findings revealed that milk samples from females of the two strains were remarkably similar in content of electrolytes and total protein at each of the three sampling times. The only significant difference from among a total of 18 strain comparisons across the three sample times was that milk levels of Ca++ were significantly higher in SS/Jrs compared to SR/Jrs on postnatal day 14. These findings clearly indicate that these milk constituents do not differ between mothers of the two Dahl strains. Thus, milk concentrations of electrolytes and protein do not appear to serve as an environmental stimulus during the preweanling period for the dramatic age-related increases in arterial pressure characteristic of the SS/Jr strain.

Urinary kallikrein and salt sensitivity in essential hypertensive males

A strong influence of urinary kallikrein excretion on the salt sensitivity of blood pressure has been recently suggested in normotensive patients. To evaluate the relationship between kallikrein and salt sensitivity in essential hypertension, active kallikrein excretion, plasma renin activity, atrial natriuretic peptide and aldosterone levels were evaluated in 37 male hypertensives (mean age 43.3 +/- 4.7 years) after two weeks on a normal NaCl diet (120 mmol NaCl per day). After kallikrein determination, salt sensitivity was assessed in a randomized cross-over double-blind fashion by evaluating the blood pressure response to a high (240 mmol NaCl per day for two weeks) and a low (40 mmol NaCl per day for 2 weeks) NaCl intake. Blood pressure changes were evaluated considering as baseline blood pressure the measurement taken at the end of the 2 weeks under normal NaCl intake. Patients were classified as salt sensitive when a diastolic blood pressure change of 10 mm Hg or more occurred after both periods of low and high NaCl intake. At the end of the assessment of salt sensitivity, 19 hypertensive patients (mean age 43.0 +/- 4.6 years) were resistant. The urinary excretion of active kallikrein was significantly lower (P < 0.0001) in salt sensitive (0.51 +/- 0.36 U/24 hr) than in salt resistant patients (1.28 +/- 0.48 U/24 hr). Also, plasma atrial natriuretic peptide levels were higher in salt sensitive than in salt resistant hypertensives (P < 0.02), and a significant correlation between urinary kallikrein and plasma atrial natriuretic peptide was demonstrated in salt sensitive hypertensives (r = -0.691, P < 0.001).(ABSTRACT TRUNCATED AT 250 WORDS)

Salt sensitivity in hypertension. Renal and cardiovascular implications

The mechanisms responsible for the increase in blood pressure response to high salt intake in salt-sensitive patients with essential hypertension are complex and only partially understood. A complex interaction between neuroendocrine factors and the kidney may underlie the propensity for such patients to retain salt and develop salt-dependent hypertension. The possible role of vasodilator and natriuretic agents, such as the prostaglandins, endothelium-derived relaxing factor, atrial natriuretic factor, and kinin-kallikrein system, requires further investigation. An association between salt sensitivity and a greater propensity to develop renal failure has been described in certain groups of hypertensive patients, such as blacks, the elderly, and those with diabetes mellitus. Salt-sensitive patients with essential hypertension manifest a deranged renal hemodynamic adaptation to a high dietary salt intake. During a low salt diet, salt-sensitive and salt-resistant patients have similar mean arterial pressure, glomerular filtration rate, effective renal plasma flow, and filtration fraction. On the other hand, during a high salt intake glomerular filtration rate does not change in either group, and effective renal blood flow increases in salt-resistant but decreases in salt-sensitive patients; filtration fraction and glomerular capillary pressure decrease in salt-resistant but increase in salt-sensitive patients. Salt-sensitive patients are also more likely than salt-resistant patients to manifest left ventricular hypertrophy, microalbuminuria, and metabolic abnormalities that may predispose them to cardiovascular diseases. In conclusion, salt sensitivity in hypertension is associated with substantial renal, hemodynamic, and metabolic abnormalities that may enhance the risk of cardiovascular and renal morbidity.

Increased dietary salt sensitizes vasomotor neurons of the rostral ventrolateral medulla

Excess dietary sodium is a major contributing factor to the incidence and severity of hypertension. However, the precise mechanism or mechanisms by which salt contributes to the severity of hypertension are unknown. The region of the rostral ventrolateral medulla (RVLM) is a principal brain stem locus critical for the regulation of arterial blood pressure by the sympathetic nervous system. The purpose of this study was to determine if excess dietary sodium chloride might alter the function or responsiveness of neurons in the RVLM. Male Sprague-Dawley rats were given either tap water or 0.9% sodium chloride solution to drink for 10 to 14 days. Excess sodium chloride did not affect baseline blood pressure. However, when neurons of the RVLM were stimulated by microinjections of L-glutamate, evoked increases in arterial pressure were potentiated in rats given sodium chloride. Augmented pressor responses could not be accounted for by increased vascular reactivity because both groups responded similarly to intravenously administered phenylephrine and norepinephrine. Additionally, electrical stimulation of descending spinal sympathoexcitatory axons produced identical pressor responses in both groups, indicating that altered synaptic transmission at central or peripheral neuroeffector junctions distal to the RVLM could not explain enhanced pressor responses produced by direct stimulation of RVLM cell somata. Finally, impaired arterial baroreceptor reflexes could not account for augmented RVLM pressor responses, as depressor and bradycardic responses produced by electrical stimulation of aortic baroreceptor afferents were not reduced in rats given excess dietary sodium chloride.(ABSTRACT TRUNCATED AT 250 WORDS)

Obesity and hypertension in blacks

In the United States, obesity and hypertension are more common in blacks than in whites, but that general statement hides some important sex differences. Thus, in black women the prevalences of both obesity and hypertension are greater than in white women, whereas in men, although there is no racial difference in obesity, in blacks hypertension is more common and more severe than in whites. For white people, there is a well-documented causal relationship between obesity and hypertension, however, results from the second National Health and Nutrition Examination (NHANES II) suggest that this relationship is not so strong for blacks. Obesity is also an important risk factor for diabetes, which in itself is associated with hypertension. The mechanism of obesity-associated hypertension appears to be an inadequate vasodilation in the face of the increased blood volume and cardiac output, which are the natural consequences of an increased body mass. This defect in control of vascular resistance has been attributed to increased activity of the sympathetic nervous system, abnormal renin-angiotensin-aldosterone relationships, and insulin resistance. However, none of these attributes has been found to be the exclusive characteristic of obese hypertensive as compared with normotensive obese subjects.

Effects of non-pharmacologic therapy

Whereas there is no doubt that non-drug treatment of hypertension, and modifications in life-style to prevent an age-associated rise in arterial pressure are attractive concepts, opinions vary as to what advice should be offered to hypertensive patients and to populations at large. No modification in diet or life style has been demonstrated to reduce the complications of hypertension. Review of recent reports suggests that moderate exercise, weight reduction in the obese, and moderation of alcohol intake are usually effective in reducing arterial pressure and are unaccompanied by adverse effects. On the contrary, restriction of dietary sodium, or supplementation with potassium, calcium or magnesium have little or variable antihypertensive effects, are not readily accepted by a high percentage of the population, and could have adverse consequences. Other dietary modifications may reduce blood pressure, but additional research is needed to define the relevant component in the diet.