Endothelium‐Derived Nitric Oxide Synthase Inhibition: Effects on Cerebral Blood Flow, Pial Artery Diameter, and Vascular Morphology in Rats

Background and Purpose We determined the effects of inhibiting the production of cerebral endothelium- derived nitric oxide on pial artery diameter, cortical blood flow, and vascular morphology. Methods An inhibitor of endothelium-derived nitric oxide synthesis, NG-nitro-L-arginine methyl ester hydrochloride (L-NAME), or an equivalent volume of 0.9% saline was infused into rats intra-arterially in a retrograde fashion via the right external carotid artery at a rate of 3 mg/kg/min to a total dose of 190 mg/kg or intravenously at 1 mg/kg/min to a total dose of 15 mg/kg. Large pial arteries were continuously visualized through an operating microscope, and cortical cerebral blood flow was monitored by laser-Doppler flowmetry. To localize areas of morphological interest, the protein tracer horseradish peroxidase was injected 15 minutes before termination of the L-NAME infusion and the rats were perfusion-fixed 15 minutes later for light and electron microscopic analysis. Results Infusion of L-NAME significantly raised arterial blood pressure at both doses (for 190 mg/kg, from 103.2±3.4 to 135±3.4 mm Hg; for 15 mg/kg, from 125±2.8 to 144.4±4.0 mm Hg). Pial arteries constricted within 10 minutes after the start of the intracarotid infusion to 40% of the preinfusion diameter, while cortical cerebral blood flow decreased to an average of 72.5% of that at baseline. Morphological abnormalities in the experimental rats included microvascular stasis and focal areas of blood–brain barrier disruption to protein. Ultrastructural examination of cortical leaky sites revealed constricted arterioles with many endothelial pinocytotic vesicles and microvilli. Conclusions These observations suggest that inhibition of endothelium-derived nitric oxide synthesis affects the relation between cerebral arterial diameter and cerebral blood flow and can lead to subtle cerebral vascular pathological changes consistent with focal brain ischemia.