The role of nitric oxide in the cerebral vasodilatation during hypercapnia

In order to analyze the role of nitric oxide (NO) in cerebral blood flow, we observed microvessels on the brain surface by using a needle-probe CCD intravital microscope with a relay lens and a triple CCD. The spatial and temporal resolutions of this system are 2.5 μm and 33 ms, respectively. Mongrel dogs were anesthetized with intravenous pentobarbital, and a large piece of one side of the skull was removed. A needle probe of CCD videomicroscope was placed gently on the surface of the brain. Changes in the microvessels were observed during hypercapnia (10% CO₂, 7 min). The microvessels on the brain surface showed diffuse dilation. After intravenous injection of L-NAME, the microvessels showed a mixture of segmental constriction and dilation. Then, the changes in the microvessels were observed during hypercapnia. The microvessels that constrict by L-NAME were showed vasodilatation. The vasodilator portion by L-NAME did not show more dilation. That vasodilatory response by hypercapnia after L-NAME injection was smaller than the response before L-NAME injection. We conclude that NO regulates basal tonus of the brain arteriole and in part play a role in the cerebral vasodilatation caused by CO₂