High Dietary Sodium Blunts Effects of Angiotensin-converting Enzyme Inhibition on Vascular Angiotensin I–to–Angiotensin II Conversion in Rats

High sodium intake blunts the efficacy of angiotensin (Ang)-converting enzyme (ACE) inhibition (ACEi), but the underlying mechanism is incompletely characterized. High sodium has been reported to increase vascular expression and vascular activity of ACE. To investigate whether high-dietary sodium-induced effects on vascular conversion of Ang I might be involved in the sodium-induced blunting of the response to ACEi, the authors studied the vasoconstrictor responses to Ang I and Ang II of isolated aortic rings from healthy rats on low dietary sodium (LS: 0.05% NaCl) and high dietary sodium (HS: 2.0% NaCl) after 3 weeks of ACEi (lisinopril 75 mg/L) or vehicle (CON). Blood pressure was similar in LS and HS in CON, but HS blunted the blood pressure response to ACEi. Functional conversion of Ang I was assessed as the difference in dose-response curves to Ang I and Ang II in parallel aortic rings. Sodium intake did not affect the dose-response curves to Ang I and Ang II in CON. In the ACEi groups, a significant difference was present between the curves for Ang I and Ang II on LS (deltaEC50, 6.7 nM; range, 2.2-13 nM; P < 0.01) but not on HS (deltaEC50: 1.3 nM; range, 0.0-4.1 nM, median [interquartile range], NS). Thus, HS blunts the ACEi-induced reduction of functional vascular Ang I conversion compared with LS. Whether the blunted functional vascular conversion is causally related to the blunted blood pressure response remains to be elucidated.