Role of Endogenous Nitric Oxide Generation in the Regulation of Vascular Tone and Reactivity in Small Vessels as Investigated in Transgenic Mice Using Synchrotron Radiation Microangiography

Abstract

Nitric oxide (NO) produced by endothelial NO synthase (eNOS) plays a central role in regulation of vascular tone and reactivity. The purpose of this study is to clarify the basal tone and microvascular reactivity in eNOS-overexpressing transgenic (Tg) mice in vivo with a microangiography system using monochromatic synchrotron radiation (SR). The mouse femoral artery was cannulated, nonionic contrast media was injected, and microangiography was performed in hindlimbs of mice. Serial images of the small blood vessels (diameter <200 μm) were recorded by the SR microangiography system. At basal conditions, the diameter of tibial arteries in eNOS-Tg mice was larger than that of wild-type mice (179 ± 8 versus 132 ± 8 μm; P < 0.01). l-NAME treatment decreased the vessel diameter and canceled the difference in vessel diameters between two genotypes. Acetylcholine- and sodium nitroprusside-induced relaxations of small vessels were significantly reduced in Tg mice compared with wild-type mice (35.0 ± 9.4 versus 61.6 ± 6.7%, 85.0 ± 10.2 versus 97.3 ± 6.7% of the maximum relaxation, respectively). Our data provide the evidence that overproduced NO from endothelium reduces vascular tone and plays a pivotal role in regulation of vascular tone in small vessels. Furthermore, the reduced NO-mediated relaxation in small vessels of eNOS-Tg mice is demonstrated for the first time in vivo. SR microangiography allows us to evaluate the reactivity in small-sized vessels and appears to be a powerful tool for assessing the microvascular circulation in vivo.