Impaired flow-mediated dilation with age is not explained by L-arginine bioavailability or endothelial asymmetric dimethylarginine protein expression

Aging is associated with a decline in vascular endothelial function, manifesting in part as impaired flow- mediated arterial dilation ( FMD), but the underlying mechanisms are uncertain. Impaired FMD may be mediated in part by a decrease in synthesis of nitric oxide by endothelial nitric oxide synthase, and in clinical populations this has been attributed to competitive inhibition of L-arginine binding sites by asymmetric dimethylarginine ( ADMA). If this mechanism is involved in the age- associated decline in FMD, increasing L-arginine concentration may swing the competitive balance in favor of L-arginine binding, restoring nitric oxide synthesis, and enhancing FMD in older humans. To test this hypothesis, we measured FMD ( brachial ultrasound) in 10 younger ( 21 +/- 1 yr) and 12 older healthy men and women ( 60 +/- 2 yr) following infusion of vehicle or vehicle + L-arginine. Baseline FMD in the older subjects was only similar to 60% of that in the younger subjects ( P = 0.002). L-Arginine did not significantly increase FMD in either group despite 23-fold ( older) and 19-fold ( younger) increases in plasma L-arginine concentrations ( P < 0.0001 vs. control). Protein expression ( immunofluorescence) in vascular endothelial cells showed that ADMA and the enzyme isoform that controls its degradation, dimethylarginine dimethylaminohydrolase II, were not different in the younger and older subjects. Endothelium-independent vasodilation ( sublingual nitroglycerine) was not different between age groups or conditions. We conclude that acutely increasing plasma concentrations of L- arginine do not significantly improve brachial artery FMD in healthy older subjects and thus does not restore the age- associated loss of FMD. Together with the finding that endothelial cell ADMA protein expression was not increased in older adults, these findings suggest that competitive inhibition of L- arginine binding sites on endothelial nitric oxide synthase by ADMA is not an important mechanism contributing to impaired conduit artery endotheliumdependent dilation with aging in healthy humans.