Salt sensitivity and central dopaminergic activity in patients with essential hypertension

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.

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.

Salt restriction in hypertension

Salt has had a long and controversial history. It is not known when man began to use salt in his diet, but it is logical to believe that salt has always been a part of human diet though we do not have scientific proof for this. Restriction of salt in hypertensive patients has been a popular measure adopted world wide. This paper examines the usefulness of salt restrictions and the effect it would cause. Most cases of hypertension are symptomless--using a therapy which drastically affects a patients food habits is a poor strategy to ensure compliance. We conclude that salt restriction is not very useful and probably harmful in some patients. Additionally it reduces the quality of life.