Ultrastructural study of perivascular nerve fibres and endothelial cells of the rat basilar artery immunolabelled with monoclonal antibodies to neuronal and endothelial nitric oxide synthase.

The ultrastructural distribution of neuronal (type I) and endothelial (type III) isoforms of nitric oxide synthase in perivascular axons and endothelial cells was examined in the Wistar rat cerebral basilar artery, employing monoclonal antibodies specific either to type I or type III of nitric oxide synthase in pre-embedding peroxidase-antiperoxidase immunocytochemistry. Both neuronal and endothelial nitric oxide synthase were localized in similar proportions of perivascular axons (25.2% +/- 2.8 and 28.7% +/- 6.5, respectively) but in different proportions in vascular endothelial cells (6.2% +/- 0.9 and 27.1% +/- 2.4, respectively). The intracellular distribution of immunoreactivity to neuronal and endothelial nitric oxide synthase was similar both in axons and endothelial cells; e.g. the labelling of the membranes of mitochondria and synaptic/cytoplasmic vesicles. However, the intensity of immunoreactivity was most prominent in profiles positive for endothelial nitric oxide synthase. The neuronal and endothelial nitric oxide synthase-positive axon varicosities were characterized by the presence of small spherical agranular vesicles; agranular vesicles in nerve varicosities positive for neuronal nitric oxide synthase were significantly larger than those in nerve varicosities positive for endothelial nitric oxide synthase (43.2 nm +/- 0.5 versus 37.7 nm +/- 0.8; p < 0.001). Varicosities positive for endothelial nitric oxide synthase also contained large granular vesicles (116.6 nm +/- 5.9) with labelled cores. In conclusion, the present data demonstrate that monoclonal antibodies to neuronal and endothelial nitric oxide synthase immunoreact with subpopulations of both perivascular axons and endothelial cells of rat basilar artery. The significance of neuronal and endothelial isoforms of nitric oxide synthase for the basilar artery is discussed.