Contribution of the Renin-Angiotensin System and of the Sympathetic Nervous System to Blood Pressure Homeostasis During Chronic Restriction of Sodium Intake

In a previous paper, we showed that during a long-term, moderate restriction in sodium intake, sympathetic nervous system activity was only transiently stimulated, whereas a sustained rise of plasma renin activity occurred. However, the contribution from stimulation of the renin-angiotensin system in the maintenance of blood pressure homeostasis during a low-salt diet is still unclear. To investigate this issue, in eight normal subjects blood pressure, heart rate, plasma catecholamines, renin activity, and aldosterone were measured during normal sodium intake (150 mEq/d), after converting-enzyme inhibition (enalapril 20 mg/d po), and during one month of sodium intake restriction (50 mEq/d) associated with chronic inhibition of converting-enzyme (CEI). During a low-salt diet with CEI, plasma renin activity rose significantly as a result of CEI. In addition, even in the presence of a marked and sustained increase in plasma norepinephrine and in upright heart rate, a decrease in blood pressure was observed in all the measurements performed during the course of the study as compared to the control values. Our findings support the hypothesis that renin-angiotensin system activation plays an important role in the maintenance of blood pressure homeostasis during a low-salt diet. In fact, in the presence of an effective blockade of angiotensin II formation, blood pressure is decreased, despite the concurrent, sustained stimulation of the sympathetic nervous system.