Role of Nitric Oxide in the Regulation of the Mechanical Properties of Peripheral Conduit Arteries in Humans

Abstract

Whether nitric oxide (NO) contributes to the regulation of the mechanical properties of large arteries in humans is not known. We measured the effect of local administration of the inhibitor of NO synthesis N ^G -monomethyl- l -arginine (L-NMMA; 1 and 4 μmol · L ⁻¹ · min ⁻¹ for 5 minutes) and acetylcholine (3 and 30 nmol · L ⁻¹ · min ⁻¹ for 3 minutes) on radial artery diameter and wall thickness in 11 healthy volunteers using an echo-tracking system coupled to a measurement of radial blood flow (Doppler) and arterial pressure. At the highest dose, L-NMMA reduced radial blood flow but surprisingly decreased incremental elastic modulus (from 1.36±0.22 to 1.00±0.22 kPa · 10 ³ ; P <.05) and increased arterial compliance (from 3.20±0.46 to 4.07±0.45 m ² · kPa · 10 ⁻⁸ , P <.05), without affecting radial artery internal diameter, wall thickness or midwall stress, thus reflecting a decrease in vascular tone. Acetylcholine decreased incremental elastic modulus (from 1.27±0.08 to 0.88±0.07 kPa · 10 ³ ; P <.05) and increased arterial diameter, radial blood flow, and compliance (from 2.82±0.16 to 5.30±0.62 m ² · kPa · 10 ⁻⁸ ; P <.05). These results demonstrate in vivo that NO is involved in the regulation of the mechanical properties of large arteries in humans. However, the effects of L-NMMA, ie, a decrease in arterial wall rigidity and an increase in arterial compliance, which occur in the absence of any changes in blood pressure or arterial geometry, suggest that inhibition of NO synthesis is associated in humans with a paradoxical isometric smooth muscle relaxation. This effect could be due to the development of compensatory vasodilating mechanisms after NO synthesis inhibition.