Functional consequences of endothelial nitric oxide synthase uncoupling in congestive cardiac failure

Impaired endothelium-mediated vasodilatation (EMVD) in congestive cardiac failure (CCF) has been linked to decreased nitric oxide (NO) bioavailability because of its interaction with vascular superoxide (O2*-), derived predominantly from NAD(P)H-dependent oxidases. When uncoupled from essential cofactors, endothelial nitric oxide synthase (eNOS) produces O2*-. We studied the functional consequences of eNOS uncoupling in relation to EMVD in patients with CCF.

We employed the platelet as a compartmentalized ex-vivo model to examine O2*- and NO production. When eNOS is functioning normally, incorporation of Nomega-Nitro-L-Arginine methyl ester (L-NAME, 1 mmol/L), results in increased O2*- detection, as inhibition of NO production prevents NO scavenging of O2*-. This was observed in controls and 9 of the CCF patients, in whom O2*- detection increased by 63% and 101%, respectively. In the remaining 9 CCF patients, incorporation of L-NAME reduced O2*- production by 39%, indicating O2*- production by eNOS uncoupling. Detection of platelet-derived NO was significantly greater in eNOS-coupled platelets compared with the uncoupled group (2.8+/-1.4 versus 0.9+/-0.4 pmol/108 platelets, P=0.04). Endothelium-dependent and -independent vasodilator responses to acetylcholine and sodium nitroprusside recorded using venous occlusion plethysmography were significantly impaired in patients exhibiting eNOS uncoupling.

This study provides first evidence that platelet eNOS can become uncoupled in human CCF. Impaired endothelium-dependent and -independent vasodilator responses and diminished platelet-derived NO production occurred in association with enzyme uncoupling.