Vasodilation to bradykinin is mediated by an ouabain-sensitive pathway as a compensatory mechanism for impaired nitric oxide availability in essential hypertensive patients

Abstract

Background - In essential hypertension, endothelium-dependent vasodilation is impaired because of reduced nitric oxide (NO) availability, which is mainly caused by oxidative stress. The present study was designed to identify the mechanism(s) responsible for NO-independent vasodilation to bradykinin in patients with essential hypertension. Methods and Results - In 16 healthy subjects (49.5±5.8 years; 118.6±3.5/78.9±2.9 mm Hg) and 16 patients with essential hypertension (47.9±4.8 years; 154.6±4.5/102.9±3.2 mm Hg), we measured modifications in forearm blood flow (strain-gauge plethysmography) during intrabrachial infusion of bradykinin (5, 15, or 50 ng/100 mL of forearm tissue per minute) in the presence of saline, N(ω)- monomethyl-L-arginine (L-NMMA; used to inhibit NO synthase; 100 μg/100 mL of forearm tissue per minute), and ouabain (to block Na+K+/ATPase and prevent hyperpolarization; 0.7 μg/100 mL of forearm tissue per minute). In healthy subjects, vasodilatation to bradykinin was significantly blunted by L-NMMA and unaffected by ouabain. In hypertensive patients, vasodilatation to bradykinin was not modified by L-NMMA, but it was significantly reduced by ouabain. In an adjunctive group of 8 hypertensive patients (49.9±3.8 years; 155.9±5.5/103.7±3.9 mm Hg), the response to bradykinin was repeated during the administration of intrabrachial vitamin C (a scavenger for oxygen free radicals; 8 mg/100 mL of forearm tissue per minute). In these patients, L- NMMA-induced inhibition of vasodilation to bradykinin was restored, and ouabain was no longer effective. In a final group of 6 normotensive controls (45.9±4.1 years; 115.1±2.9/79.3±2.1 mm Hg), vasodilation to bradykinin residual to L-NMMA blockade was further inhibited by simultaneous ouabain infusion. Conclusions - Vasodilation to bradykinin is impaired in essential hypertensive patients because of an NO-system alteration caused by oxidative stress, and it is mediated by an alternative pathway, possibly involving endothelium-dependent hyperpolarization.