Threshold sodium excretory and renal blood flow effects of angiotensin converting enzyme inhibition

Objective: To probe the potential influence of a high renal tubular level of angiotensin II on sodium reabsorption using angiotensin converting enzyme inhibitors and a non-peptide angiotensin antagonist. Methods: Systemic arterial blood pressure, renal blood flow (RBF) and electrolyte and urinary excretion were measured in anesthetized uninephrectomized Wistar rats. Captopril (n = 12), ramiprilat (n = 10) or EXP 3174 (n = 9) was infused into the renal artery in graded doses to examine whether the threshold dose that increased RBF was lower than that which increased sodium excretion rate (Na⁺ excretion rate). Results: Whereas ramiprilat (0.5-4 μg/kg/min intra-arterially) and EXP3174 (0.5-4 μg/kg/min intra-arterially) decreased blood pressure in all but the lowest dose of 0.5 μg/kg/min, captopril (1-8 μg/kg/min intra-arterially) did not change blood pressure except for a slight effect with the two higher doses. These three agents increased RBF to about the same degree, between 6% and 18%, which was relatively large compared with the maximal vasodilator response achievable in the kidney with acetylcholine (30-37%). Captopril given intra-arterially had a more consistent effect on Na⁺ excretion rate than either ramiprilat or EXP3174. An increase in Na⁺ excretion rate occurred with captopril ranging from 44% to 78%, whereas no significant change was obtained with the other drugs. The dose of captopril that increased RBF was the same as that which increased Na⁺ excretion rate. In those experiments in which ramiprilat resulted in an increase in Na⁺ excretion rate (five of 10 experiments), the effective dose was the same as that which increased RBF. Conclusion: When administered by the intra-arterial route, captopril was more effective in increasing Na⁺ excretion rate than either ramiprilat or EXP 3174. Although the threshold dose of captopril and ramiprilat required to increase Na⁺ excretion rate and RBF was similar, suggesting blockade of a common angiotensin II pool, there was not a good correlation between these two effects.