Biochemical aspects of salt-induced, pressure-independent left ventricular hypertrophy in rats

Salt and high blood pressure

HBP (high blood pressure) is the leading risk of death in the world. Unfortunately around the world, blood pressure levels are predicted to become even higher, especially in developing countries. High dietary salt is an important contributor to increased blood pressure. The present review evaluates the association between excess dietary salt intake and the importance of a population-based strategy to lower dietary salt, and also highlights some salt-reduction strategies from selected countries. Evidence from diverse sources spanning animal, epidemiology and human intervention studies demonstrate the association between salt intake and HBP. Furthermore, animal studies indicate that short-term interventions in humans may underestimate the health risks associated with high dietary sodium. Recent intervention studies have found decreases in cardiovascular events following reductions in dietary sodium. Salt intake is high in most countries and, therefore, strategies to lower salt intake could be an effective means to reduce the increasing burden of HBP and the associated cardiovascular disease. Effective collaborative partnerships between governments, the food industry, scientific organizations and healthcare organizations are essential to achieve the WHO (World Health Organization)-recommended population-wide decrease in salt consumption to less than 5 g/day. In the milieu of increasing cardiovascular disease worldwide, particularly in resource-constrained low- and middle-income countries, salt reduction is one of the most cost-effective strategies to combat the epidemic of HBP, associated cardiovascular disease and improve population health.

Cirrhotic cardiomyopathy

Cirrhotic cardiomyopathy is a recently recognized condition in cirrhosis consisting of systolic incompetence under condition of stress, diastolic dysfunction related to altered diastolic relaxation, and electrophysiological abnormalities in the absence of any known cardiac disease. It can be diagnosed by using a combination of electrocardiograph, 2-dimensional echocardiography, and various serum markers such as brain natriuretic factor. The underlying pathogenetic mechanisms include abnormalities in the beta-adrenergic signaling pathway, altered cardiomyocyte membrane fluidity, increased myocardial fibrosis, cardiomyocyte hypertrophy, and ion channel defects. Various compounds for which levels are elevated in cirrhosis such as nitric oxide and carbon monoxide can also exert a negative inotropic effect on the myocardium, whereas excess sodium and volume retention can lead to myocardial hypertrophy. Various toxins can also aggravate the ion channel defects, thereby widening the QRS complex causing prolonged QT intervals. Clinically, systolic incompetence is most evident when cirrhotic patients are placed under stress, whether physical or pharmacological, or when the extent of peripheral arterial vasodilatation demands an increased cardiac output as in the case of bacterial infections. Acute volume overload such as immediately after insertion of a transjugular intrahepatic portosystemic shunt or after liver transplantation can also tip these cirrhotic patients into cardiac failure. Treatment of cirrhotic cardiomyopathy is unsatisfactory. There is some evidence that beta-blockade may help some cirrhotic patients with baseline prolonged QT interval. Long-term aldosterone antagonism may help reduce myocardial hypertrophy. Future studies should include further elucidation of pathogenetic mechanisms so as to develop effective treatment strategies.

Dietary sodium and cardiovascular outcomes: a rational approach

Hypertension, the leading risk factor for mortality in the world, affects nearly one in four Canadians. There is substantive evidence that high dietary sodium contributes to hypertension. Animal studies consistently demonstrate increased blood pressure and cardiovascular morbidity and mortality with high dietary sodium intake. Evidence of the adverse health effects in humans associated with increased sodium intake is accumulating rapidly. Previously, limitations on sodium consumption were recommended only for those identifiable groups of people shown to be at higher risk. With the lifetime risk of developing hypertension being more than 90% in an average lifespan, the need for a population-based approach to reducing hypertension is clear. The present paper reviews the evidence of sodium and cardiovascular disease, resulting in the 2007 Canadian Hypertension Education Program recommendation of daily intake of less than 100 mmol of sodium in both normotensive and hypertensive adults.

Plasma sodium and hypertension

Dietary salt is the major cause of the rise in the blood pressure with age and the development of high blood pressure in populations. However, the mechanisms whereby salt intake raises the blood pressure are not clear. Existing concepts focus on the tendency for an increase in extracellular fluid volume (ECV), but an increased salt intake also induces a small rise in plasma sodium, which increases a transfer of fluid from the intracellular to the extracellular space, and stimulates the thirst center. Accordingly, the rise in plasma sodium is responsible for the tendency for an increase in ECV. Although the change in ECV may have a pressor effect, the associated rise in plasma sodium itself may also cause the blood pressure to rise. There is some evidence in patients with essential hypertension and the spontaneously hypertensive rat (SHR) that plasma sodium may be raised by 1 to 3 mmol/L. An experimental rise in sodium concentration greater than 5 mmol/L induces pressor effects on the brain and on the renin-angiotensin system. Such a rise can also induce changes in cultured vascular tissue similar to those that occur in the vessels of humans and animals on a high sodium diet, independent of the blood pressure. We suggest that a small increase in plasma sodium may be part of the mechanisms whereby dietary salt increases the blood pressure.

Control of Hypertension with Captopril Affords Better Renal Protection as Compared with Irbesartan in Salt-Loaded Uremic Rats

BACKGROUND/AIM

Hypertension induced by exaggerated sodium consumption accelerates the progression of renal failure. We investigated the effects of a high-sodium (HS) diet on the progression of renal failure in rats maintained normotensive by angiotensin-converting enzyme inhibition or AT-1 blockade.

METHODS

In 70 Sprague-Dawley rats, renal failure was induced by five-sixths nephrectomy. They were fed isocaloric normal-sodium (NS), low-sodium (LS), or HS diets. HS rats prone to develop hypertension were divided into three subgroups: treated to normotension by irbesartan (HS-1) or captopril (HS-2) or left untreated (HS-0).

RESULTS

All HS animals developed significant proteinuria which strongly correlated with the 24-hour sodium excretion. HS-0 rats demonstrated severe hypertension, rapid deterioration of the renal function, and 100% mortality after 3 weeks. In irbesartan-treated HS-1 rats, mortality and decline of the glomerular filtration rate were similar to those of normal- or low-sodium-fed animals (100% mortality after week 12). In captopril-treated HS-2 rats, glomerular filtration rate decline and mortality were significantly blunted as compared with all other groups (50% mortality after week 12).

CONCLUSIONS

(1) In five-sixths-nephrectomized uremic rats maintained normotensive by either irbesartan or captopril, the rate of deterioration of the renal function was not aggravated by exaggerated sodium consumption. (2) In this experimental setting, captopril treatment yielded a better survival outcome as compared with irbesartan, despite the similar hypotensive effect.

Sodium and blood pressure

The human race is genetically programmed to consume less than 1 g of salt per day. In most human populations, the diet contains 6 to 12 g of salt per day and, in contrast to populations that consume less than 3 g of salt per day, their blood pressure rises with age. Independent of the rise in blood pressure, a high-salt diet also increases left ventricular mass, incidence of strokes, stiffness of conduit arteries, and activity of resistance arteries. In populations with high salt intake, a modest reduction in salt intake lowers blood pressure and diminishes cardiovascular disease and mortality.

Harmful effects of dietary salt in addition to hypertension

In addition to raising the blood pressure dietary salt is responsible for several other harmful effects. The most important are a number which, though independent of the arterial pressure, also harm the cardiovascular system. A high salt intake increases the mass of the left ventricle, thickens and stiffens conduit arteries and thickens and narrows resistance arteries, including the coronary and renal arteries. It also increases the number of strokes, the severity of cardiac failure and the tendency for platelets to aggregate. In renal disease, a high salt intake accelerates the rate of renal functional deterioration. Apart from its effect on the cardiovascular system dietary salt has an effect on calcium and bone metabolism, which underlies the finding that in post-menopausal women salt intake controls bone density of the upper femur and pelvis. Dietary salt controls the incidence of carcinoma of the stomach and there is some evidence which suggests that salt is associated with the severity of asthma in male asthmatic subjects.

Dietary-sodium-induced cardiac remodeling in spontaneously hypertensive rat versus Wistar-Kyoto rat

OBJECTIVE

To study the effects of short-term and long-term high sodium intake on cardiac mass and design in sodium-sensitive spontaneously hypertensive rats versus sodium-resistant Wistar-Kyoto rats.

METHODS

Young spontaneously hypertensive rats and Wistar-Kyoto rats were randomly allocated to control diet, 2 or 8% dietary sodium for 2-12 weeks and changes in resting hemodynamics, cardiac angiotensin II level, sympathetic activity and cardiac structure evaluated. Sympathetic activity was assessed by measuring levels of plasma catecholamines, responses of blood pressure to ganglionic blockade, and rates of cardiac turnover of norepinephrine.

RESULTS

High sodium intake for 4 weeks increased left ventricle weight of Wistar-Kyoto rats aged 4 weeks (by 11 and 25% for 2 and 8% NaCl diets, respectively). This hypertrophic response was temporary, however, had already diminished after 6 weeks, and was absent after 12 weeks of a high sodium intake. However, after prolonged exposure concentric remodeling occurred (i.e. left ventricle wall thickness : radius ratio increased with no change in left ventricle mass). High sodium intake did not affect resting blood pressure, cardiac index, cardiac angiotensin II level, and general sympathetic activity of Wistar-Kyoto rats. Short-term high sodium intake did not increase left ventricle mass of young spontaneously hypertensive rats, unless sodium intake was so high (8% NaCl) that blood pressure and general sympathetic activity increased, too. However, a prolonged moderate (2%) increase in sodium intake also caused concentric remodeling in spontaneously hypertensive rats without increasing left ventricle mass, blood pressure, cardiac index, and general and cardiac sympathetic activities.

CONCLUSIONS

The blood pressure in young Wistar-Kyoto rats is sodium-insensitive but the heart structure is sodium-sensitive and high dietary sodium intake causes an early hypertrophic response, and then concentric remodeling. In contrast, hypertrophic response appears to occur after the response of blood pressure in spontaneously hypertensive rats, whereas the remodeling is similar to that in Wistar-Kyoto rats.

Influence of age on cardiovascular effects of increased dietary sodium and angiotensin-converting enzyme inhibition in normotensive Wistar rats

Recent studies have shown that increased intake of dietary sodium chloride produces blood pressure-independent increase in cardiac and renal mass even in young normotensive rats. With advancing age the harmful cardiovascular effects of increased dietary sodium are not so well known. In the present study the influence of advancing age on the cardiovascular effects of increased intake of sodium (control diet, 0.3% and high-sodium diet, 2.6% sodium in the chow) were examined in young and aged (3 and 18 months old, respectively, at the beginning of the experiment) male normotensive Wistar rats in a six-week study. Moreover, the potential role of renin-angiotensin system in ageing during normal and a high-sodium intake was studied using a pharmacological tool, angiotensin converting enzyme (ACE) inhibitor ramipril. Ageing did not significantly modify basal systolic blood pressure measured by the tail cuff method. A high intake of sodium chloride increased blood pressure significantly only in aged rats, while in young rats it increased renal weight. Left ventricular weight was not affected by high-sodium diet in either age group. The ACE inhibition during control diet lowered blood pressure and decreased left ventricular weight in young rats only and these effects were completely blocked by a high-sodium diet. The maximal vascular contraction force of mesenteric arterial rings to noradrenaline was decreased with ageing while endothelium-dependent and -independent relaxation responses were unaltered with ageing. The sensitivity to sodium nitroprusside was impaired by the high-sodium diet in young rats. In both age groups the urinary excretion of calcium was increased during the high-sodium diet. In conclusion, the increased intake of sodium produced different changes in cardiovascular function in normotensive rats depending on age. With advancing age, the sensitivity to sodium-induced increase in blood pressure was increased. In aged rats a high intake of dietary sodium elevated blood pressure, while in young rats it increased renal mass without increase in blood pressure. In both age groups sodium did not affect left ventricular hypertrophy. Both high-sodium intake and ageing attenuated or even abolished the cardiovascular effects of ACF inhibition.

Antihypertensive drugs reduce noradrenaline-induced hypertrophy of cultured myocardial cells

The cellular mechanisms of cardiac hypertrophy are still largely unknown. In-vivo studies have demonstrated that antihypertensive drugs can regress hypertrophy independently of reductions in blood pressure. The antihypertrophic effects of metoprolol, propranolol, felodipine, verapamil and captopril were studied in neonatal cardiac myocyte culture. Prazosin was used as a positive control. Hypertrophy was defined as an increase in protein content measured by [3H]leucine incorporation. Noradrenaline induced a 1.5-fold increase in protein synthesis over 48 h. Prazosin prevented the hypertrophic effect of noradrenaline. Adrenergic beta-receptor blocking agents and calcium antagonists reduced myocyte hypertrophy in a dose-dependent manner. The angiotensin-converting enzyme inhibitor captopril was ineffective. These results indicate that adrenergic beta-receptor blockers and calcium antagonists may have direct nonhaemodynamic effects on the growth of cultured cardiac myocytes.

Deleterious effects of salt intake other than effects on blood pressure

1. Salt intake is not only known to play an important role in determining blood pressure (BP) but has been shown to have other deleterious effects independent of BP. 2. Epidemiological and animal studies have provided evidence that salt intake may have an adverse effect on stroke mortality independent of BP. 3. Significant correlation between sodium excretion (as a measure of salt intake) and left ventricular (LV) hypertrophy has been shown in many clinical studies. Salt restriction has also been found to produce a significant reduction in LV mass. 4. In animal studies, salt restriction in uninephrectomized spontaneously hypertensive rats retarded renal glomerular injury and suppressed compensatory growth independent of hypertension. Moreover, a high sodium diet accelerated cerebral arterial disease even when no increases in BP could be detected. 5. Epidemiological data have shown an association between asthma mortality and regional purchases of table salt. Furthermore, dietary salt restriction in asthmatic patients results in improvement of symptomatology with lower consumption of bronchodilators. 6. Patients with essential hypertension are known to have increased urinary calcium excretion, and hypertension may be one factor that may increase the likelihood of osteoporosis. High salt intake is also associated with increased hydroxyproline excretion indicating increased resorption of bone. Sodium restriction reduces calcium excretion and may reduce bone demineralization and hip fractures in a similar manner to that seen with diuretics.

Beneficial effects of a potassium- and magnesium-enriched salt alternative

The effects on blood pressure and the development of cardiac hypertrophy of sodium chloride (regular salt) and a novel potassium-, magnesium-, and l-lysine-enriched salt alternative, which in a previous study prolonged the life span of hypertensive rats nearly threefold as compared with the animals receiving regular salt, were compared both in spontaneously hypertensive rats and their hypertension-resistant genetic controls. In particular, the possible protective effect of increased intakes of potassium, magnesium, and l-lysine during a high intake of sodium chloride was examined. Therefore, the salt alternative was added at 1.75 times higher levels to produce the same dietary levels of sodium chloride in the regular salt and the salt alternative groups. Regular salt produced a remarkable left ventricular hypertrophy in both rat strains, but as compared with the respective control groups, it induced an increase of blood pressure only in the spontaneously hypertensive rats. The salt alternative did not induce a rise in blood pressure in either of the rat strains, nor did it produce left ventricular hypertrophy in the hypertension-resistant rats and, in the spontaneously hypertensive animals, significantly less hypertrophy than regular salt. The salt alternative appeared to prevent the sodium chloride-induced volume load since plasma levels of atrial natriuretic peptide were increased in the regular salt groups but remained normal in the salt alternative groups. Therefore, potassium, magnesium, and/or l-lysine of the salt alternative produced a powerful protection against the harmful effects of sodium chloride.