Increased activity of the renin–angiotensin and sympathetic nervous systems is required for regulation of the blood pressure in rats fed a low‐protein diet

New Findings • What is the central question of this study? Is the increase in blood pressure observed in rats fed a low protein diet due to overactivation of sympathetic and renin angiontensin systems? • What is the main finding and its importance? The data show an increase in the expression of angiotensin II type 1 receptors and an incrased sympathetic activity in this experimental model, suggesting that both systems are contributing to the high blood pressure observed in these animals. Previous studies have shown that postweaning protein restriction induces changes in the sympathetic nervous system in rats, leading to alterations in cardiovascular parameters. In addition, the renin–angiotensin system is also affected in these animals. Here, we hypothesized that adjustments in the interaction between the RAS and SNS underlie the cardiovascular adaptations observed in rats fed a low‐protein diet. Thus, we evaluated the alterations in the mean arterial pressure (MAP) and heart rate of Fisher rats fed a protein‐deficient diet before and after systemic administration of the angiotensin‐converting enzyme inhibitor enalapril and the angiotensin II (Ang II) type 1 (AT1) receptor antagonist losartan alone or in combination with the α1‐adrenergic receptor antagonist prazosin. Administration of enalapril or losartan decreased the MAP only of rats under protein restriction. Prazosin injection after the infusion of losartan caused a further decrease in the MAP of malnourished rats. In contrast, only the administration of prazosin elicited a reduction in the MAP of control animals. When the sequence of administration of the antagonists was inverted, infusion of prazosin in animals fed the standard or the low‐protein diet induced a reduction in the MAP that was further decreased by the subsequent injection of losartan. Importantly, in both protocols the responses of malnourished animals to losartan were markedly greater when compared with the control group. Moreover, these animals presented lower levels of circulating Ang II and a reduced responsiveness to Ang II. In contrast, the expression of AT1 receptors in the aorta of malnourished animals was increased. Thus, our data suggest that the renin–angiotensin system is an important factor supporting blood pressure in rats fed a low‐protein diet and that the sympathetic nervous system activity in these animals is under strong influence of Ang II acting via AT1 receptors.